Distillation Process

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Introduction To Distillation Process

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Process File Reference: CHE20501

For additional information on this subject, contact D. J. Clarke on 873-9817

Engineering Encyclopedia

Process Introduction to Distillation Process

Contents

Pages

Distillation, Fractionation, And Key Components .......................................... 1 Column Sections, Reflux............................................................................... 3 Major Equipment ........................................................................................... 4 Auxiliary Facilities.......................................................................................... 5 Calculating Vapor And Liquid Compositions In Ideal Mixtures...................... 7 Ideal And Nonideal Gases.................................................................. 7 Real-Gas Equations ........................................................................... 7 Ideal Mixtures - Dalton's, Raoult's Laws............................................. 9 Equilibrium K-Values ........................................................................ 10 Two Component Example...................................................... 12 Mixtures Approximated As Ideal ............................................ 13 Equilibrium Diagram ......................................................................... 14 Nomenclature .............................................................................................. 17 Subscripts......................................................................................... 17 Work Aid 1:

Formulas And Guidelines For Calculating Vapor And Liquid Compositions In Ideal Mixtures ................................ 18

Formulas........................................................................................... 18 Ideal Gas Law ........................................................................ 18 Dalton's Law .......................................................................... 18 Ideal Mixture Relationship...................................................... 18 Guidelines......................................................................................... 18 Glossary ...................................................................................................... 20

Saudi Aramco DeskTop Standards



Addendum A................................................................................................ 28 Introduction ....................................................................................... 28 Vapor-Liquid Equilibrium (Vle) Relationships ................................... 28 Ideal And Nonideal Gases ..................................................... 28 Vapor Pressure...................................................................... 29 Ideal Mixtures - Dalton's, Raoult's Laws................................ 31 Real-Gas Equations............................................................... 33 Fugacity ................................................................................. 34 Equilibrium K-Values.............................................................. 37 Relative Volatility.................................................................... 37 Nonideal Liquids .................................................................... 39 Equations Of State................................................................. 39 Vle Calculations................................................................................ 41 Equilibrium Diagram............................................................... 41 Vapor-Liquid Phase Diagrams............................................... 43 Bubble Point And Dew Point.................................................. 44 Equilibrium Flash Separation ................................................. 45 Bubble And Dew Point Calculations ...................................... 50 Flash Calculations: One Main Component Plus Gas ........... 51 Physical Properties........................................................................... 53 Physical Property Sources .................................................... 53 Average Boiling Point............................................................. 54 Characterization Factor ......................................................... 55 Inspection Properties ............................................................. 56




Petroleum Fraction Distillations ........................................................ 57 15/5 Distillation....................................................................... 57 Astm Distillations.................................................................... 59 Gas Chromatographic Distillation (Gcd) ................................ 60 Equilibrium Flash Vaporization (Efv)...................................... 60 Distillation Curve Relationships ............................................. 61 Crude Assays ........................................................................ 62 Computer Simulations............................................................ 64 Nomenclature ................................................................................... 68 Addendum B................................................................................................ 69 Example 1......................................................................................... 69 Example 2......................................................................................... 71 Example 3......................................................................................... 73 Example 4......................................................................................... 75 Addendum C ............................................................................................... 77 Addendum D ............................................................................................... 91 Addendum E.............................................................................................. 100




DISTILLATION, FRACTIONATION, AND KEY COMPONENTS Distillation is the separation of the constituents of a liquid mixture by partial vaporization of the mixture, followed by separate recovery of the vapor and liquid. The more volatile (light) constituents of the mixture are obtained in increased concentration in the vapor, while the less volatile (heavy) are components concentrated in the liquid residue, also called the bottoms. The vapor is most frequently condensed by cooling and is called the distillate or overhead product. Fractionation is a term commonly used in the petroleum industry for a distillation in which the vapor is contacted continuously and countercurrently with a condensed portion of the vapor. In most petroleum processing plants, continuous fractionation is the only type of distillation used, so the terms distillation and fractionation are used interchangeably. An example of a simple distillation unit, the Ras Tanura Plant 10 Depropanizer, is shown in Figure 1. The feed to the unit is a mixture of light hydrocarbons, mostly C3 through C5 paraffins. The objective of the unit is to remove the propane and lighter components (overhead product) while keeping the butanes and heavier components in the bottoms. Frequently in multicomponent distillation, a light component that must be recovered in the distillate is also present in the residue in important amounts, while components lighter than this component are present only in small amounts. This component is called the light key. In the case of the Ras Tanura depropanizer, the light key is the propane, which has a concentration in the residue of 0.7%. Similarly, a heavy component present in the distillate in important amounts is called the heavy key. If more than one of the heavy components is present in the distillate in important amounts, then the more volatile component is the heavy key. In the Ras Tanura Depropanizer, where both the isobutane and the n-butane are present in the distillate in important amounts, the heavy key is the isobutane. Key components are used in shortcut distillation calculations and in some tray and packing efficiency calculations. Frequently product specifications are based on the concentration of key components, for example, 0.7% maximum propane in the depropanizer bottoms.


1



C2 0.4 mole % C3 97.2 mole % C4 + 2.4 mole %

Sour Propane to ADIP Treaters Condenser

270 psig

Reflux Drum 43

130ÞF

4078 gpm

NG L

15 psig STM 29 Depropanizer

C 2 0.2 mole % C 3 44.7 mole % i - C 4 6.7 mole % n - C 4 21.5 mole % i - C 5 6.7 mole % n - C 5 9.7 mole % C 6 6.2 mole % C 7+ 4.3 mole %

1

Data from Dwg. NA-637118 Sh. 1 Rev. 1 - Summer

Reboiler 150 psig STM Cond.

C3 0.7 mole % 276ÞF C4+ 99.3 mole %

Bottoms to Debutanizer

Ras Tanura Plant 10 Depropanizer Simplified Process Flow Diagram Figure 1


2



COLUMN SECTIONS, REFLUX The feed to the Ras Tanura depropanizer is partially vaporized in a steam heater and fed to the distillation tower (Figure 2). When the feed enters the depropanizer, the vapor portion of the feed rises in the column while the liquid portion of the feed descends in the column. As the vapor portion of the feed, along with vapor from the bottom section of the tower, rises in the column, it contacts the descending liquid. The section of the distillation column above the feed is called the rectifying (or enriching) section. In the rectifying section, the concentration of the light components increases toward the top of the tower; that is, the light products are enriched. The section of the column below the feed is the stripping section. Here, the light components are stripped out of the liquid as they descend the column. Part of the liquid condensed from the vapor leaving the column top is returned to the column as reflux. The reflux provides the liquid for the contact with the vapor in the rectifying section of the column. The important role of reflux in distillation will be discussed in later sections. Condenser Reflux Drum

Reflux Rectifying Section

Steam Heater

Sour Propane to ADIP Treaters

Vapor

Liquid

Stripping Section

Bottoms to Debutanizer

Distillation Process in the Ras Tanura Depropanizer Figure 2


3



MAJOR EQUIPMENT The column or tower is the main piece of equipment in a distillation unit. It contains vaporliquid contacting devices, trays in most cases, packing less frequently. The Ras Tanura Plant 10 Depropanizer column contains 43 trays, 29 below the feed, 24 above the feed. In this unit, the section below the feed has a larger diameter, which is needed to accommodate the heavier liquid load below the feed. Typical major auxiliary equipment of a column includes the condenser, the condenser separator, and the reboiler. The condenser condenses the reflux and the part of the distillate that is removed as liquid. The condenser separator separates any vapor distillate from the liquid and provides surge capacity for the reflux and the distillate. The Ras Tanura Plant 10 Depropanizer has a total condenser; that is, all the column overhead vapor is condensed, and the distillate product is completely liquid. The reboiler vaporizes part of the liquid that leaves the bottom tray of the column. This vapor, playing a role similar to that of the reflux, contacts the descending liquid and strips the lighter components. The most common types of reboilers are thermosyphon and kettle. In thermosyphon reboilers, the driving force for the circulation of the liquid is the difference in hydrostatic head between the column of liquid feeding the reboiler and the column of mixed liquid and vapor leaving the reboiler. In the kettle reboilers, only vapor returns to the tower. The heat source of the reboiler may be steam, or a process fluid; the reboiler may also be a gas- or fuel-fired furnace. Fired reboilers are generally forced circulation type, that is, a pump is used to circulate the fluid through the furnace.


4



AUXILIARY FACILITIES Figure 3 is a more detailed process flow diagram of the Ras Tanura Plant 10 Depropanizer. It provides more information on auxiliary facilities such as pumps, piping, valves, instruments, and controls. A typical distillation tower has pumps for feed, reflux/distillate, and, if required, bottoms. In the depropanizer, bottoms pumps are not required because the downstream unit, the debutanizer, is at a lower pressure. Control valves are used to maintain stable rates (e.g., feed and reflux rates) and stable liquid levels in the distillate and bottoms surge vessels. Instruments such as flow meters and pressure, temperature, and level indicators are used to monitor and control the operation of the tower. Instruments such as gas chromatography analyzers are used to monitor the quality of separation by identifying the concentration of important components. In the depropanizer, for example, an analyzer in the liquid distillate identifies the concentrations of ethane, isobutane, and n-butane. These are components related to the propane product specifications. Various control schemes are used to achieve objectives such as stable feed rate, on-spec products, low utility consumption, and stable operation. The Ras Tanura Plant 10 Depropanizer, for example uses the following three control schemes: • • •

Feed temperature control to assure stable feed vaporization levels. Reboiler steam rate control, related indirectly to the concentration of propane in the bottoms via a tray temperature. Tower pressure control which is necessary for the stable operation of the tower.


5



10-C-1 Depropanizer 16'/24'-6" ID-99'-6"TT DP = 380 psig o DT = 350 F

10-E-2A/B Depropanizer Reboiler CAP. MM BTU/HR: S: 200.0 W: 199.0

10-E-3 Depropanizer Condenser CAP. MM BTU/HR: S: 140.1 W: 148.6

10-G-2 A/B/C Deprop. Reflux Pump 5720 gpm 86 psi∆ p 412 bhp.

10-D-3 Depropanizer Reflux Drum 11'-0" ID x 32'-0" T-T DP = 320 psig DT = 370 oF To FC at ADIP Unit Dwg. 637118 Sh. 3

To ADIP Treating Dwg. NA. 637118 Sh. 3 Chromatograph C2, iC4, nC4 AR

PC

Split Range

Condenser

134o F S: 282 psia W: 290 psia

PC

FC 10-E-3

15 psig Steam

NNF

43

Depropanizer

To Fuel Gas

FR

TC 276 psia

Steam Heater

FR

36

TC

LC

10-D-3

10-E-1 LC

30

Feed

Reflux Drum

29

TC

To Sour Water Stripper in Plant 45

130 F o

10-G-2 A/B/C 10-C-1 TC

Condensate FC

FC

S: 287 psia W: 295 psia

LC

10-E-2 A/B

150 psig Steam Condensate

Feed From Plants 25,45

Reboiler FR

S : Summer Conditions W: Winter Conditions Data from Dwg. NA-637118. Sh. No. 8, Rev 1

o 276 F (S) o 274 F (W)

To Debutanizer 10-C-2

Ras Tanura Plant 10 Depropanizer Process Flow Diagram Figure 3


6



CALCULATING VAPOR AND LIQUID COMPOSITIONS IN IDEAL MIXTURES A vapor-liquid system is considered to be in equilibrium when there are no longer any detectable changes occurring in the system. Generally, a system is assumed to be in equilibrium when the mass, energy, and composition of each phase remain constant with time. An example of a system in equilibrium is a mixture of water and air in a closed vessel. After some time, there will be no change in temperature, in the amount of water in the vapor phase, or in the number of gas molecules dissolved in the water. The system is in equilibrium. Equilibrium also applies to systems that are not static. We may have equilibrium in an overhead condenser separator of a distillation column. The vapor and liquid leaving the separator are in equilibrium, and their compositions can be described by relationships for systems in equilibrium. Ideal and Nonideal Gases Ideal gases are those whose behavior can be described by the ideal gas law, which is stated mathematically as: PV = nRT or

PV = 1.0 nRT

The ideal gas law indicates that the product of pressure P times volume V is proportional to the number of molecules of the component, n, times the absolute temperature T. R is an ideal gas proportionality constant. The values of R in various units are given in Work Aid 1. Gases tend to behave as ideal gases at temperatures higher than their critical temperature and pressures well below their critical pressure. Real-Gas Equations If the ideal gas equation is applied to situations with elevated pressures, significant errors may result. Deviations from the ideal gas law at high pressure can be attributed to the assumptions inherent in the law's derivation, namely, that all molecules are hard spheres that do not interact with one another and that occupy negligible volume. Therefore, the ideal gas law is independent of the composition of the gas. For example, the ideal gas law implies that one mole of any gas will occupy the same volume as one mole of any other gas at the same temperature and pressure. In this sense, it implies that all gases are identical on a molar basis. This assumption is not correct because different gases have radically different molecular and chemical structures. As an example, take the specific volumes of hydrogen sulfide, propane, and nitrogen at 400 psia and 180_F. From the ideal gas law and n = 1, V = RT/P = [10.73 psia-ft3/lb-mole-°R x (180 + 460)°R]/400 psia


7



= 17.17 ft3/lb-mole


8



The experimental molar volumes for these three gases are as follows: Component

Molar Volume

Propane Hydrogen sulfide Nitrogen

11.44 ft3/lb-mole 16.28 ft3/lb-mole 16.82 ft3/lb-mole

Thus, although the ideal gas law provides a qualitative measure of the behavior of gases, it does not predict PVT behavior accurately for most gases and cannot be used for liquids. The compressibility factor Z expresses the deviation from the ideal gas equation. It can be used to predict real gas properties. The compressibility factor is the ratio of the real gas volume to that of the ideal gas at the same temperature and pressure: PV = ZnRT or PV = Z nRT For an ideal gas, the compressibility factor is 1.0. The compressibility factor Z can be obtained from generalized graphs such as those in Maxwell, pages 148-153 or the GPSA Engineering Data Book, Chapter 16. Ideal Mixtures - Dalton's, Raoult's Laws Ideal mixtures, gas or liquid, consist of components that do not interact with each other chemically or physically. The concept of ideal mixtures has formed the basis for many quantitative relationships describing equilibrium. Of particular interest are Dalton's law of partial pressures and Raoult's law relating the pressure exerted by a component in the vapor phase to its concentration in the liquid phase. Dalton's law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the mixed gases. Thus, PT = _PPi = PP1 + PP2 + PP3 + ... Dalton also postulated that the partial pressure of an ideal gas in a gas mixture is proportional to its mole fraction, that is, the relative number of molecules of that gas in the mixture. Thus, PPi = yi PT


9



Raoult's law, relating the partial pressure in the vapor phase to the liquid phase composition, is expressed as: PPi = xi VPi Combining Dalton's and Raoult's laws results in an expression describing mixtures of ideal vapors and liquids in equilibrium. PT = _PPi = _yi PT = _xiVPi and for component i, yi = xi (VPi/PT) Equilibrium K-Values The definition of equilibrium K-value, also called K factor or distribution coefficient, of component i in a mixture is given in the following equation: Ki =

yi xi

The K-value is simply the ratio of the vapor to the liquid mole fraction of i. This ratio has no special thermodynamic significance, but has found extensive use in high-pressure VLE work. For ideal systems where Raoult's law applies, it can be expressed as: y VP Ki = xi = i PT i Equilibrium K values can be obtained from graphs or nomographs like the De Priester nomograph, Figure 4. K values are a function of temperature and pressure. For nonideal mixtures, K values are also a function of composition.


10



De Priester Nomograph Figure 4 "Light Hydrocarbon Vapor-Liquid Distribution Coefficients", C. L. De Priester, Chemical Engineering Symposium Series Vol. 49, No. 7, pp 1-43 (1953) Reproduced by permission of the American Institute of Chemical Engineers.


11



Two Component Example Let's assume that we have an ideal mixture of propane and n-butane at 100°F. The vapor pressures of the two components at 100°F are: • •

Propane 13 atm = 191 psia (Component 1). n-Butane 3.5 atm = 52 psia (Component 2).

The total pressure, PT, of the mixture can be calculated from, PT = PP1 + PP2 = x1VP1 + x2VP2 PT = 191x1 + 52(1-x1) = 52 + 139x1 This last equation indicates that the total pressure of an ideal binary mixture is a linear function of the composition. This relationship is illustrated in Figure 5, which shows that the total pressure is the sum of partial pressures and is a straight line between the vapor pressure of n-butane (x1 = 0) and propane (x1 = 1.0). 200

Vapor Pressure of Propane, 191 psia

180 100o F 160 140 120 100 80 60 Vapor Pressure of n-Butane 40 52 psia 20 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Mole Fraction Propane in Liquid, x 1

Ideal Mixtures Propane-n-Butane System Pressure Figure 5


12



Mixtures Approximated as Ideal The mixtures that can be approximated as ideal must satisfy the following requirements: • • • •

Total pressure of the system must be below 200 psia. The components must be chemically similar, for example, butane and pentane, both paraffins. A mixture of an aromatic component and a paraffin such as benzene and hexane cannot be approximated as ideal. The components must be close boiling, that is, they must have similar boiling points. The system pressure and temperature must not be near the critical pressure and temperature of the mixture.

Using ideal mixture correlations in calculations results in approximate compositions or conditions (P, T). The error may be acceptable for a simple operation, such as a flash drum separation. The same correlations used in a superfractionator, where tray-to-tray calculations compound the error, may produce unacceptable results.


13



Equilibrium Diagram Figure 6 depicts a simple flash separation. The feed consists of two components, propane and n-butane. The feed temperature and composition vary. The table in Figure 6 lists vapor and liquid concentrations of propane and distribution coefficients (K1 and K2) for propane and nbutane, of the two components for five temperatures. Pressure is fixed at 100 psia. At 70°F, the mole fraction of propane in the liquid phase is 0.746. Its mole fraction in the vapor phase is higher, 0.907, since propane is the more volatile of the two components. The distribution coefficient K1 for propane is equal to the ratio y1/x1 = 0.907/0.746 = 1.22. As the temperature increases, the K values increase by a factor greater than two. From 70°F to 140°F, the value of the relative volatility, however, changes by only about 25%. The small effect of temperature on relative volatility is the reason for using relative volatility in shortcut distillation calculations. Relative volatility data for only two or three points in the column provide results of acceptable accuracy.

F V

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Vapor-Liquid Equilibrium Figure 6


14



Figure 7 is an equilibrium diagram for the propane/n-butane system using the data from Figure 6 at 100 psia. The horizontal axis indicates the mole fraction of the more volatile component, propane, in the liquid phase. The vertical axis indicates its mole fraction in the vapor phase. The equilibrium line connects all the (x1, y1) points. Given the mole fraction in the liquid phase, the equilibrium line can be used to obtain the mole fraction in the vapor phase. Given the mole fraction in the vapor phase, the mole fraction in the liquid phase can be found. Propane - n-Butane 1.0 C3 - nC4 at 100 psia

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Mole Fraction Propane in Liquid, x1

Equilibrium Diagram Figure 7 Figure 7 contains a second line, the reference line. It is simply the diagonal of the diagram: for all the points on the reference line, x = y. The reference line makes it easier to see the differences between the vapor and the liquid phase compositions. Since by convention the horizontal axis represents the composition of the more volatile component, y1 is larger than x1. Therefore, the equilibrium line is above the reference line. Large differences in y1, x1 mole fractions indicate large differences in the volatility of the two components. Accordingly, equilibrium lines bulging away from the reference line are indicative of mixtures that are easy to separate by successive vaporization and condensation steps, that is, by multistage distillation. Saudi Aramco DeskTop Standards

15



For some mixtures, there is a reversal in relative volatilities and the equilibrium line intersects the x = y reference line. Because the vapor and liquid fractions at that point are equal, these mixtures cannot be separated by distillation. Such mixtures are called azeotropes.


16



NOMENCLATURE C1, C2 Hydrocarbons with 1, 2 … number of carbons K

Distribution coefficient (K = y/x), also called K factor or equilibrium K.

L

Liquid rate, mole/hr

n

Next to a hydrocarbon name, it indicates a normal (paraffin) isomer

n

Number of moles

P

Pressure, absolute

PP

Partial pressure

PT

Total pressure

R

Gas constant. For values, see Figure 9.

T

Temperature, absolute

TB

Boiling point

V

Vapor rate, mole/hr

V

Volume

x

mole fraction in the liquid phase

y

Mole fraction in the vapor phase

Z

Compressibility factor

Subscripts 1,2

Value refers to component 1, 2, ... or measurement 1, 2, ...

i

Value refers to component i


17



WORK AID 1: FORMULAS AND GUIDELINES FOR CALCULATING VAPOR AND LIQUID COMPOSITIONS IN IDEAL MIXTURES This Work Aid is designed to assist the Participant in calculating vapor and liquid compositions in ideal mixtures. Formulas Ideal Gas Law PV = nRT where P V n R T

= = = = =

the absolute pressure (psia) the volumetric rate (ft3/hr) the molar rate (lb-mole)/hr the ideal gas constant (see Figure 9) the absolute temperature (°R), T (°R) = T (°R) + 459.7

Dalton's Law PPi = yi PT Ideal Mixture Relationship yi = xi (VPi/PT) Guidelines a.

Use the Ideal Gas Law to find the feed and vapor molar rates (n = PV/RT). Select the value of R (see Figure 9) that corresponds to the appropriate P, V, T units, e.g., (psia) (ft3) / (lb-mole) (°R). Calculate the liquid molar rate by subtracting the feed molar rate from the vapor molar rate.

b.

Calculate the partial pressure of each component in the vapor phase by using Dalton's Law.

c.

Solve for the Ideal Mixture Relationship for xi for both propane and n-butane.


18



Ideal Gas Constant Figure 9


19



GLOSSARY 15/5 distillation

A standard distillation analysis using a column with 15 trays and a 5:1 reflux ratio.

absolute pressure

Pressure measured with respect to zero pressure, as distinct from pressure measured with respect to some standard pressure.

API gravity

An arbitrary scale expressing the gravity or density of liquid petroleum products. The measuring scale is calibrated in degrees API. It is calculated by the following formula: Deg API =

141.5 - 131.5 Sp Gr 60°F/60°F

assay

A procedure for determining the distillation characteristics and other properties of a crude oil.

ASTM

American Society for Testing and Materials. Organization standardizes specifications and methods of testing for engineering materials. Most of the data that describe, identify, or specify petroleum products are determined in accordance with ASTM test methods.

ASTM distillation

A distillation made in accordance with an ASTM distillation procedure.

average boiling point

For component mixtures see definitions in text. For hydrocarbon fractions, unless otherwise indicated it is the sum of the ASTM distillation temperatures from the 10% point to the 90% point, inclusive, divided by 9. Sometimes half the initial and half the maximum distillation temperatures are also added, and the sum is then divided by 10.

azeotrope

Liquid mixture of two or more components that boils at a temperature either higher or lower than the boiling point of any of the individual components. In refining, if the components of a solution are very close in boiling point and cannot be separated by conventional distillation, a substance can be added that forms an azeotrope with one component, modifying its boiling point and making it separable by distillation.

barrel

Standard unit of measurement in the petroleum industry, equivalent to 42 standard U.S. gallons.


20



binary distillation

Distillation of a mixture containing two components.

boiling range

The range of temperature, usually determined at atmospheric pressure in standard laboratory apparatus, over which the boiling or distillation of an oil begins, proceeds, and finishes.

bottoms

The bottom (heavy) product of a distillation column. A synonymous term is residue.

Btu

British thermal unit. The quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit, at 60°F and at a pressure of 1 atmosphere.

bubble point

The temperature and pressure at which a liquid is in equilibrium with an infinitesimal amount of vapor.

bubble point curve

The temperature and pressure conditions at which an infinitesimal amount of vapor (first bubble of vapor) is in equilibrium with vapor. For a pure component, this curve is the same as the vapor pressure curve.

butane

Gaseous paraffinic hydrocarbon (C4H10), usually a mixture of iso- and normal butane. Also called, along with propane, liquefied petroleum gas (LPG).

cetane

Colorless liquid hydrocarbon, C15H34, used as a standard in determining diesel fuel ignition performance. See Cetane Number.

cetane number

Measure of the ignition quality of a diesel fuel, expressed as the percentage of cetane that must be mixed with liquid methylnaphthalene to produce the same ignition performance as the diesel fuel being rated, as determined by the test method ASTM D 613. A high cetane number indicates shorter ignition lag and a cleaner burning fuel.

characterization factor

Factor that expresses variations in physical properties with change in character of the stock. The ratio of the cube root of the molal average boiling point, TB, in degrees Rankine (°R = °F + 460), to the specific gravity at 60°F/60°F: 3

Kw = TB / Sp Gr It ranges from 12.5 for paraffinic stocks to 10.0 for aromatic stocks. Also called Watson factor or Watson K or UOP K.


21



chromatography (gas)

A method of separation based on selective adsorption capable of identifying individual compounds. An analytical technique for separating mixtures of volatile substances. The mixture is inserted into the chromatographic column and washed down with an inert gas. The column is packed with absorbent materials that selectively retard the components of the sample.

cloud point

Temperature at which a cloud or haze of wax crystals appears at the bottom of a sample of lubricating oil in a test jar, when cooled under conditions prescribed by test method ASTM D 2500. Cloud point is an indicator of the tendency of an oil to plug filters or small orifices at cold operating temperatures.

column

A vertical vessel containing contacting devices such as trays or packing, used to perform separations such as distillation or extraction. A synonymous term is tower.

compressibility factor

The ratio of the actual volume occupied by a vapor to the theoretical volume occupied by the same quantity of vapor under identical conditions of temperature and pressure.

condenser

A cooler that condenses all (total condenser) or part (partial condenser) of the overhead vapor of a column.

condenser separator

A vessel that separates any vapor distillate from the liquid and provides surge capacity for the reflux and the distillate.

countercurrent flow

A system in which one fluid flows in one direction and another fluid flows in the opposite direction.

critical point

See Critical State.

critical pressure

The pressure necessary to condense a gas at the critical temperature. Above the critical temperature the gas cannot be liquefied, no matter what pressure is applied.

critical state

The pressure and temperature at which liquid or gaseous phases reverse at the slightest change in conditions.

critical temperature

The maximum temperature at which a gas can be liquefied by pressure (critical pressure); above this temperature the gas cannot be liquefied, no matter what pressure is applied.

crude oil

Unrefined or unprocessed liquid hydrocarbons.


22



cut

A fraction obtained by a separation process. (See also Fractional Distillation.)

dew-point

The temperature and pressure at which a vapor is in equilibrium with an infinitesimal amount of liquid.

dew-point curve

The temperature and pressure conditions at which an infinitesimal amount of liquid (first drop of liquid) is in equilibrium with vapor. For a pure component, this curve is the same as the vapor pressure curve.

DGA

Diglycolamine. An amine used in the removal of H2S and CO2 from gases.

diesel fuel

The portion of crude oil that distills out in the temperature range 392°F to 698°F. Diesel fuel is close in boiling range and composition to lighter heating oils.

distillate

The overhead (light) product of a distillation column. It may be vapor, liquid, or both.

distillation

The separation of the constituents of a liquid mixture by partial vaporization of the mixture, followed by separate recovery of the vapor and liquid residue.

distillation curve

Curve plotting the percentage of petroleum products distilled versus the temperature.

distillation test

Methods for determining the volatility characteristics of a hydrocarbon liquid by progressively boiling off a sample under controlled heating. If the boiling range is small, the fluid is narrow cut, that is, having components with similar volatilities. If the boiling range is wide, the fluid is wide cut.

dry point

In a distillation test, the temperature at which the last drop of petroleum fluid evaporates.

end point (EP)

In the distillation of liquids, the maximum temperature that occurs during the test. Also called final boiling point, FBP.

enriching section

The section of the distillation column above the feed.

equilibrium

The state of a system under a constant environment when the intensive properties remain unchanged with time. Net fluxes of mass, energy, and chemical reactions in the system are zero.


23



flash point

Lowest temperature at which the vapor of a combustible liquid can be made to ignite momentarily in air, as distinct from fire point. Flash point is an important indicator of the fire and explosion hazards associated with a petroleum product. There are a number of ASTM tests for flash point, e.g., Cleveland open cup, Pensky-Martens closed tester, Tag closed tester, Tag open cup.

flooding

Overloading of the tray interspace with liquid.

fractionation

Distillation in which the vapor is contacted continuously and countercurrently with a condensed portion of the vapors.

freezing point

A specific temperature that can be defined in two ways, depending on the ASTM test used. In ASTM D 1015, which measures the freezing point of high-purity petroleum products (such as nitration-grade toluene), freezing point is the temperature at which a liquid solidifies. In ASTM D 2386, which measures the freezing point of aviation fuel, freezing point is that temperature at which hydrocarbon crystals formed on cooling disappear when the temperature of the fuel is allowed to rise.

fuel oil

Term encompassing a broad range of distillate and residual fuels identified by ASTM grades 1 though 6.

fugacity

The tendency of a substance to escape or disappear from the phase in which it is present.

gas chromatography

An analytical technique that can identify concentrations of components in a gas or vaporized liquid.

gauge pressure

The pressure as shown by a pressure-registering instrument (gauge). The gauge pressure, in pounds per square inch, is approximately equal to the absolute pressure minus 14.7.

GCD

Gas Chromatography Distillation. A chromatographic technique that produces results approximating 15/5 distillations.

grids

Countercurrent contacting devices fabricated in panels and installed in an ordered manner. In contrast to structured packing, grids provide wide clearances. See the figures in the text.


24



heavy key

A heavy component that must be recovered with the residue and is present in the distillate in important amounts.

heterogeneous system

One in which intensive properties are uniform from point to point and thereby constitute a single phase.

initial boiling point (IBP)

In a distillation test, the fluid temperature at which the first drop falls into a graduated cylinder.

jet fuel

Fuel meeting the required properties for use in jet engines and aircraft turbine engines.

kerosene

Relatively colorless light distillate.

kettle reboiler

A type of reboiler acting as a vaporizer and a separator. The kettle reboiler produces a vapor stream that is sent to the tower and a liquid stream that is in equilibrium with the vapor. The liquid is the tower bottom product.

light ends

Low-boiling-point hydrocarbons having up to five carbon atoms; including butanes, butenes, pentanes, pentenes. Also, any extraneous low-boiling fraction in a refinery process stream.

light key

A light component that must be recovered with the distillate and is present in the residue in important amounts.

mid-boiling point (MBP)

In a distillation test, the temperature at which 50% of the fluid has collected in the cylinder.

middle distillate

One of the distillates obtained between kerosene and lubricating oil fractions in the refining process. Includes light fuel oils and diesel fuel.

multicomponent distillation

Distillation of a mixture containing more than two components.

naphtha

Generic, loosely defined term covering a range of light petroleum distillates with boiling range of 90° to 475°F. Includes gasoline blending stocks, mineral spirits, and a broad selection of petroleum solvents.

NGL

Natural gas liquids.


25



normal boiling point

Boiling point is the temperature at which a substance boils. It varies with pressure. Normal boiling point is the boiling point at atmospheric pressure.

overhead

The vapor leaving the top of the column.

overhead product

The top product of a distillation column. It may be vapor, liquid or both.

overlap

In adjacent fractions, the temperature interval between the initial boiling point of the higher boiling fractions and the end point of the lower boiling fractions.

packing

Devices that provide countercurrent vapor-liquid contact in distillation columns.

partial condenser

A condenser that condenses part of the vapor.

phase

A homogeneous portion of a system. A gas or a mixture of gases, a liquid or a liquid solution, and a solid are examples of phases.

plates

See stages.

pour point

The lowest temperature at which oil will pour or flow when it is chilled without disturbance under definite conditions. These conditions are prescribed in ASTM Method D 97.

reboiler

A heater vaporizing part of the liquid leaving the bottom of the distillation column. The vapor that returns to the column provides the stripping action in the bottom section.

recirculating reboiler

A type of reboiler that sends both the vapor and liquid phases to the distillation tower. Recirculating reboilers operate either by natural circulation (thermosyphon) or forced circulation.

rectifying section

The section of the distillation column above the feed.

reflux

Condensed overhead vapor that is returned to the top tray of the distillation column.

Reid Vapor Pressure (RVP)

A measure of the vapor pressure of a sample of gasoline at 100°F. The results are reported in pounds. This test is usually carried out in accordance with ASTM Method D 323.


26



residue

The bottom (heavy) product of a distillation column. A synonymous term is bottoms.

stages

Contact points of the vapor and liquid in a column, such as on column trays. The term theoretical stage is used to indicate that equilibrium is reached at the contact point between the vapor and the liquid. The actual stages reflect the obtained tray efficiency. A synonymous term is plates.

stripping section

The section of the distillation column below the feed.

TBP

True Boiling Point of a component.

total condenser

A condenser that condenses all the vapor.

tower

See column.

trays

Horizontal devices providing crossflow vapor-liquid contact in distillation columns.

weeping

Undesirable liquid flow through the tray openings (sieve holes, valves, etc.).


27



ADDENDUM A Introduction In distillation, the separation of a mixture of materials to obtain one or more desired products is achieved through a series of countercurrent vapor and liquid contacts called stages. In each stage, a difference in the relative concentration of hte components in the two phases is attained. When the two phases are in equilibrium, the difference in concentrations between the two phases is at its maximum; therefore, it is desirable to reach equilibrium in each stage. A stage in which equilibrium is reached is called a theoretical stage. Because equilibrium represents a theoretical boundary that is universal and easy to define, most design methods are based on calculations using theoretical stages. The deviation from equilibrium is then considered by including a stage efficiency when the equivalent actual stages are calculated. This module examines the concept of vapor-liquid equilibrium (VLE) and the basic relationships that apply to VLE. The module provides guidance for selecting BLE methods in computer simulations. It also includes a brief review of physical properties related to distallation and distillation processes. Vapor-Liquid Equilibrium (VLE) Relationships A vapor-liquid system is considered to be in equilibrium when there are no longer any detectable changes occurring in the system. Generally, a system is assumed to be in equilibrium when the mass, energy, and composition of each phase remain constant with time. An example of a system in equilibrium is a mixture of water and air in a closed vessel. After some time, there will be no change in temperature, in the amount of water in the vapor phase, or in the number of gas molecules dissolved in the water. The system is in equilibrium. Equilibrium also applies to systems that are not static. We may have equilibrium in an overhead condenser separator of a distillation column. The vapor and liquid leaving the separator are in equilibrium, and their compositions can be described by relationships for systems in equilibrium. Ideal and Nonideal Gases Ideal gases are those whose behavior can be described by the ideal gas law, which is stated mathematically as: PV = nRT or PV = 1.0 nRT The ideal gas law indicates that the product of pressure P times volume V is proportional to the number of molecules of the component times the absolute temperature T. R is an ideal gas proportionality constant. The values of R in various units are given in Work Aid 1.


28



Gases tend to behave as ideal gases at temperatures higher than their critical temperature and pressures well below their critical pressure. Vapor Pressure The vapor pressure of a pure component at a given temperature is the pressure that is exerted by the component when it is in the liquid phase. Vapor pressure is a unique property, and it is a direct function of temperature. A material having a higher vapor pressure at the same temperature than another is said to be more volatile. Vapor pressure and temperature are often related by means of the Antoine equation: Log (VP) =

A-

B T+C

where A, B, C are constants for a particular compound over a relatively narrow temperature range, usually not over 100°C. Values of these constants for various compounds and the temperature ranges for which the constants apply appear in a number of references. The Antoine equation is often plotted in charts with the horizontal axis in a reverse absolute temperature scale and a vertical axis in a logarithmic scale. Vapor pressures for various components can be obtained from Maxwell, Data Book on Hydrocarbons, Section 4. An example for propane and propylene is shown in Figure 10.


29



Vapor Pressure of Propane And Propylene Figure 10


30



Ideal Mixtures - Dalton's, Raoult's Laws Ideal mixtures, gas or liquid, consist of components that do not interact with each other chemically or physically. The concept of ideal mixtures has formed the basis for many quantitative relationships describing equilibrium. Of particular interest are Dalton's law of partial pressures and Raoult's law relating the pressure exerted by a component in the vapor phase to its concentration in the liquid phase. Dalton's law states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the mixed gases. Thus, PT = ∑PPi = PP1 + PP2 + PP3 + ... Dalton also postulated that the partial pressure of an ideal gas in a gas mixture is proportional to its mole fraction, that is, the relative number of molecules of that gas in the mixture. Thus, PPi = yi PT Raoult's law, relating the partial pressure in the vapor phase to the liquid phase composition, is expressed as: PPi = xi VPi Combining Dalton's and Raoult's laws results in an expression describing mixtures of ideal vapors and liquids in equilibrium. PT = ∑PPi = ∑yi PT = ∑xi VPi and for component i, yi = xi (VPi/PT) Two Component Example - Let's assume that we have an ideal mixture of propane and nbutane at 100°F. The vapor pressures of the two components at 100°F (Maxwell, pages 29 and 30) are: • •

Propane 13 atm = 191 psia (Component 1). n-Butane 3.5 atm = 52 psia (Component 2).

The total pressure, PT, of the mixture can be calculated from, PT = PP1 + PP2 = x1VP1 + x2VP2 PT = 191x1 + 52(1-x1) = 52 + 139x1


31



This last equation indicates that the total pressure of an ideal binary mixture is a linear function of the composition. This relationship is illustrated in Figure 11, which shows that the total pressure is the sum of partial pressures and is a straight line between the vapor pressure of n-butane (x1 = 0) and propane (x1 = 1.0).

Ideal Mixtures Propane - N-Butane System Pressure Figure 11


32



Mixtures Approximated as Ideal - The mixtures that can be approximated as ideal must satisfy the following requirements: • • • •

Total pressure of the system must be below 200 psia. The components must be chemically similar, for example, butane and pentane, both paraffins. A mixture of an aromatic component and a paraffin such as benzene and hexane cannot be approximated as ideal. The components must be close boiling, that is, they must have similar boiling points. The system pressure and temperature must not be near the critical pressure and temperature of the mixture.

Using ideal mixture correlations in calculations results in approximate compositions or conditions (P, T). The error may be acceptable for a simple operation, such as a flash drum separation. The same correlations used in a superfractionator, where tray-to-tray calculations compound the error, may produce unacceptable results. Real-Gas Equations If the ideal gas equation is applied to situations with elevated pressures, significant errors may result. Deviations from the ideal gas law at high pressure can be attributed to the assumptions inherent in the law's derivation, namely, that all molecules are hard spheres that do not interact with one another and that occupy negligible volume. Therefore, the ideal gas law is independent of the composition of the gas. For example, the ideal gas law implies that one mole of any gas will occupy the same volume as one mole of any other gas at the same temperature and pressure. In this sense, it implies that all gases are identical on a molar basis. This assumption is not correct because different gases have radically different molecular and chemical structures. As an example, take the specific volumes of hydrogen sulfide, propane, and nitrogen at 400 psia and 180ϒF. From the ideal gas law and n = 1, V = RT/P = [10.73 psia-ft3/lb-mole-°R x (180 + 460)°R]/400 psia = 17.17 ft3/lb-mole


33



The experimental molar volumes for these three gases are as follows: Component

Molar Volume

Propane Hydrogen sulfide Nitrogen

11.44 ft3/lb-mole 16.28 ft3/lb-mole 16.82 ft3/lb-mole

Thus, although the ideal gas law provides a qualitative measure of the behavior of gases, it does not predict PVT behavior accurately for most gases and cannot be used for liquids. The compressibility factor Z expresses the deviation from the ideal gas equation. It can be used to predict real gas properties. The compressibility factor is the ratio of the real gas volume to that of the ideal gas at the same temperature and pressure: PV = ZnRT or PV = Z nRT For an ideal gas, the compressibility factor is 1.0. The compressibility factor Z can be obtained from generalized graphs such as those in Maxwell, pages 148-153 or the GPSA Engineering Data Book, Chapter 16. Fugacity The vapor-liquid equilibrium of an ideal mixture can be described by: PPi = yi PT = xi VPi To improve the accuracy of prediction the pressures can be replaced by analogous fugacities: fPPi = yifPT = xi fVPi

where fPP i = fVP i = fP T

=

Fugacity of i in either phase of the system. Fugacity of i as a pure saturated liquid (or vapor) at its vapor pressure corresponding to the equilibrium temperature of the system. Fugacity of i as a pure vapor at the equilibrium temperature and total pressure of the system.


34



Generalized correlations have been developed for the ratio of fugacity to pressure for pure hydrocarbons as a function of reduced temperature and reduced pressure. A correlation of this type was used in conjunction with the vapor pressure charts to develop the fugacity function charts for individual hydrocarbons. The fugacity function given by these charts is defined as: Fi = fVPi PT / fPT = PT yi /xi The fugacity function Fi may be considered a corrected vapor pressure and used in place of vapor pressure in any equation pertaining to liquid-vapor equilibrium. Values for fugacity functions can be obtained from Maxwell, Section 5. An example for propane is reproduced in Figure 12. Fugacities for petroleum fractions can be obtained from a generalized graph on pages 62-63 of Maxwell, which uses the reduced pressure and temperature of the mixture. The values obtained from the fugacity graphs in Maxwell provide a correction for pressure and temperature. They do not take into account interactions between components in the vapor or liquid phase; in other words, they assume ideal mixtures. The simple fugacity relations greatly extend the pressure range for which liquid-vapor equilibria for hydrocarbon systems may be predicted with confidence; they can be used up to equilibrium pressures of 20 to 25 atm with a fair degree of accuracy. Beyond these pressures and especially as the critical point of the mixture is approached, serious deviations from true equilibrium conditions are encountered. Under these circumstances, the assumptions of ideal mixtures no longer hold, and the fugacities of the individual compounds depend upon the compositions of the liquid and vapor phases as well as temperature and pressure. If other gases such as air, H2, and CO2, are present in the vapor phase, in addition to hydrocarbon vapors, an effective pressure should be used in determining the fugacities of individual hydrocarbons. The effective pressure is equal to the total pressure multiplied by the square root of the mole fraction of the entire hydrocarbon portion of the vapor, or Peff = PT yHC


35



Fugacity Function of Propane Figure 12


36



Equilibrium K-Values The definition of equilibrium K-value, also called K factor or distribution coefficient, of component i in a mixture is given in the following equation: Ki =

yi xi

The K-value is simply the ratio of the vapor to the liquid mole fraction of i. This ratio has no special thermodynamic significance, but has found extensive use in high-pressure VLE work. For ideal systems where Raoult's law applies, it can be expressed as: Ki =

yi VPi xi = PT

It can also be expressed in terms of fugacities as: Ki =

yi fVPi = xi fP T

Equilibrium K values can be obtained from graphs or nomographs like the De Priester nomograph, Figure 13. K values are a function of temperature and pressure. For nonideal mixtures, K values are also a function of composition. Relative Volatility Relative volatility is a relation widely used in distillation. It is defined by: αij =

yi /xi Ki = yj /xj Kj

Relative volatility is a measure of separability. The larger the value of αij, the easier the separation. For close boiling components, such as pentane and isopentane, the relative volatility approaches 1.0. Because the value of relatively volatility is not as sensitive to temperature as other measures of equilibrium, it is used in a number of shortcut distillation calculations. Relative volatility graphs are available in Maxwell, Pages 64-66. For ideal mixtures (Raoult's law applies), the relative volatility of two components is equal to the ratio of their vapor pressures. αij = VPi VPj


37



De Priester Nomograph Figure 13 "Light Hydrocarbon Vapor-Liquid Distribution Coefficients", C. L. De Priester, Chemical Engineering Symposium Series Vol. 49, No. 7, pp 1-43 (1953) Reproduced by permission of the American Institute of Chemical Engineers.

Nonideal Liquids Saudi Aramco DeskTop Standards

38



In liquids and liquid mixtures, the distances between molecules are much smaller than in gases, and the forces attracting molecules to each other are much greater. Nonideal behavior of liquids is indicated by heat of mixing and nonadditivity of volumes when two liquids are mixed. The deviation from ideality is greater for chemically dissimilar substances. The activity coefficient, γ, measures the deviation from ideal liquid solution behavior. Using the coefficient in Raoult's law results in: yi PT = PPi = γixi VPi Generally, γ is greater than 1.0. For very dissimilar systems, such as hydrocarbons and water, γ can be much greater than 1.0, in the order of 1000. There are cases where two components attract each other, leading to γ < 1.0 Activity coefficients are used in a number of VLE methods such as the Chao-Seader and the Grayson-Streed correlations. The Chao-Seader correlation requires relatively short computing times. It was used extensively in the '60s and '70s when computing was costly. Hydrocarbon VLE methods using activity coefficients have been replaced by the more rigorous equations of state. Equations Of State Equations of State (EOS) predict the PVT behavior of gases and liquids. The simplest equation of state is the one for ideal gases (per mole). P = RT/V In general, real fluids deviate from ideal fluids in two ways: there are variations in the sizes and shapes of the molecules, and specific interactions between molecules, such as polarity or hydrogen bonding, must be considered. The large variations in size and shape of molecules have a great effect on PVT behavior. The Soave-Redlich-Kwong (SRK) and the Peng-Robinson (PR) equations of state are among the best known.


39



SRK Equation of State - The Soave-Redlich-Kwong equation of state is a two-parameter equation of the following form: a P = RT V-b V(V+b) where: a = • • (1 - cij) ( ai aj ) (xi xj) i

j

b = • xi bi i

The parameters a and b must be specified for each component in a mixture and then combined as a function of composition. The a parameter is temperature-dependent. In addition, a binary interaction parameter cij is used to calculate the aij term for mixtures, to improve vapor-liquid equilibrium calculations. Peng-Robinson Equation of State - The Peng-Robinson equation is similar to the SRK equation of state, except that it has an expanded volume term: a P = RT V-b V(V+b) +b (V-b) The a parameter varies with temperature. Both constants use the same mixing equations as the SRK equation of state.


40



VLE Calculations Equilibrium Diagram Figure 14 depicts a simple flash separation. The feed consists of two components, propane and n-butane. The feed temperature and composition vary. The table in Figure 14 lists vapor and liquid concentrations of propane, distribution coefficients (K1 and K2) for propane and nbutane, and the relative volatility of the two components for five temperatures. Pressure is fixed at 100 psia. At 70°F, the mole fraction of propane in the liquid phase is 0.746. Its mole fraction in the vapor phase is higher, 0.907, since propane is the more volatile of the two components. The distribution coefficient K1 for propane is equal to the ratio y1/x1 = 0.907/0.746 = 1.22. The relative volatility α12 = K1/K2 = 3.31. As the temperature increases, the K values increase by a factor greater than two. From 70°F to 140°F, the value of the relative volatility, however, changes by only about 25%. The small effect of temperature on relative volatility is the reason for using relative volatility in shortcut distillation calculations. Relative volatility data for only two or three points in the column provide results of acceptable accuracy.

F

V , y1

Comp 1 = C 3 P = 100 psia Comp 2 = nC4

L,x 1 x 1

Temp, °F

y1

K1= y1/ x1

K2= y2/ x2

70

0.746

0.907

1.22

0.37

80

0.607

0.832

1.37

0.43

100

0.376

0.644

1.71

0.57

120

0.191

0.398

2.09

0.74

140

0.035

0.087

2.49

0.95

Vapor-Liquid Equilibrium Figure 14


41



Figure 15 is an equilibrium diagram for the propane/n-butane system using the data from Figure 14 at 100 psia. The horizontal axis indicates the mole fraction of the more volatile component, propane, in the liquid phase. The vertical axis indicates its mole fraction in the vapor phase. The equilibrium line connects all the (x1, y1) points. Given the mole fraction in the liquid phase, the equilibrium line can be used to obtain the mole fraction in the vapor phase. Given the mole fraction in the vapor phase, the mole fraction in the liquid phase can be found.

Equilibrium Diagram Figure 15 Figure 6 contains a second line, the reference line. It is simply the diagonal of the diagram: for all the points on the reference line, x = y. The reference line makes it easier to see the differences between the vapor and the liquid phase compositions. Since by convention the horizontal axis represents the composition of the more volatile component, y1 is larger than x1. Therefore, the equilibrium line is above the reference line. Large differences in y1, x1 mole fractions indicate large differences in the volatility of the two components. Accordingly, equilibrium lines bulging away from the reference line are indicative of mixtures that are easy to separate by successive vaporization and condensation steps, that is, by multistage distillation.


42



For some mixtures, there is a reversal in relative volatilities and the equilibrium line intersects the x = y reference line. Because the vapor and liquid fractions at that point are equal, these mixtures cannot be separated by distillation. Such mixtures are called azeotropes. Vapor-Liquid Phase Diagrams Phase diagrams are used to describe two-phase systems by plotting two of the three independent variables (composition, temperature, and pressure) at a constant value of the third variable. Figure 16 is a phase diagram at constant pressure for the binary mixture of propane and n-butane. The two lines indicate the temperatures at which a phase change takes place. The temperatures and concentrations (at the diagram pressure) below the two lines correspond to an all-liquid mixture. In the region between the two lines, the vapor and liquid phases are present. Above the lines there is only a vapor phase. The phase lines in Figure 16 were drawn from data in Figure 14. For example, at 120°F, the point on the liquid phase line corresponds to x1 = 0.191 and the point on the vapor line to y1 = 0.398 (see Figure 14, 120°F, x1, y1 data). The phase diagram can be used to determine the compositions of the vapor and liquid phases from the pressure and temperature at equilibrium.

Phase Diagram Figure 16


43



Bubble Point and Dew Point The phase diagram can also be used to determine the phase transition points. Figure 17 is an example for a mixture of 45% propane and 55% n-butane at 70°F. The phase diagram in Figure 8 indicates that the mixture is in the liquid phase. If the temperature is increased at constant pressure, 100 psia, the mixture will be liquid to 92°F, at which point vaporization begins. This is the bubble point of the mixture, the temperature and pressure at which a liquid is in equilibrium with an infinitesimal amount of vapor.

Bubble Point and Dew Point Figure 17 Between 92°F and 117°F the mixture is in two phases, vapor and liquid. At 117°F, all the liquid is vaporized. This is the dew point, the temperature and pressure at which vapor is in equilibrium with an infinitesimal amount of liquid. At temperatures above the dew point, this is only a vapor phase. The liquid phase line is the bubble point curve; the vapor phase line is the dew point curve.


44



Equilibrium Flash Separation The equilibrium flash separator is the simplest equilibrium process for engineers to consider. The process involves the separation of a two-phase feed into vapor and liquid in a vessel. The feed is at a desired temperature and pressure, achieved by heating, cooling, pumping, or letting down with a control valve. Calculations of the compositions and the relative amounts of the liquid and vapor phases at any given pressure and temperature involve a tedious trial-and-error solution. Since flash calculations can be performed easily by computer, manual methods for multicomponent flash calculations are not discussed here. Instead, phase and equilibrium diagrams will be used in a binary system to demonstrate and reinforce the concept of equilibrium. One-Stage Flash - Figure 18 shows an equilibrium separation. A propane and n-butane vapor mixture from a distillation column is cooled to 120°F at 100 psia. The vapor and liquid are separated and the vapor is condensed and collected in a second drum. Calculations are done to find the composition of the liquid in the two drums and the minimum cooling required to condense the vapor leaving the first drum. For simplicity, assume that the entire system is at 100 psia. The system is a one-stage flash. The second drum merely collects the condensed liquid. It is not an equilibrium stage.

One-Stage Flash Figure 18


45



The required compositions can be determined by using a phase diagram for the propane/nbutane system at 100 psia. The liquid in the flash drum is represented by a point (T, x1) = (120°F, 0.19) on the bubble point curve of the phase diagram at 120°F (Figure 19). Similarly, the vapor is represented by a point on the dew point curve at 120°F (T, y1) = (120°F, 0.4). Thus, the propane mole fractions in the liquid and vapor phases are 0.19 and 0.4, respectively.

One-Stage Flash on a Phase Diagram Figure 19 The minimum cooling required to condense the vapor leaving the first drum corresponds to its bubble point temperature. This is the maximum temperature at which all the vapor from the first drum can be condensed. Since the vapor from the first drum and the liquid from the second drum have the same composition, the bubble point can be located by drawing a vertical line between the dew point and the bubble point curves. The temperature obtained, 96°F, is the minimum temperature required to condense the vapor from the first drum.


46



The equilibrium in the flash drum is represented in the equilibrium diagram by a point, y1, x1 = 0.4, 0.19 (Figure 20). Condensation in the second drum is represented by a horizontal line from y1, x1, to the reference line, y1 x'1, where x'1 is the mole fraction of the liquid of the first drum, which is equal to y1. The equilibrium diagram does not provide temperature information; therefore, it cannot be used to determine the equilibrium concentrations or the temperature of the second drum. If the composition of one phase is known, however, it can be used to determine the composition of the other phase.

One-Stage Flash on An Equilibrium Diagram Figure 20 Two-Stage Flash - Figure 21 illustrates a two-stage flash. The vapor from the first drum is partially condensed in a second drum at 105°F. The vapor from the second drum is totally condensed, and the liquid is collected in a third drum. Pressure is constant at 100 psia.

Two-Stage Flash Figure 21


47



Representing the operation in a phase diagram (Figure 22) is similar to the one-stage flash. Compositions and temperatures can be determined from the diagram.

Two-Stage Flash on A Phase Diagram Figure 22 Figure 23 represents the two-phase flash on an equilibrium diagram. As with the diagram for one-stage flash, compositions of one of the phases in each equilibrium are required.

Two-Stage Flash On An Equilibrium Diagram Figure 23


48



Binary Flash Rates - The vapor and liquid compositions and rates for a binary system (see Figure 24) at a given pressure and temperature, can be determined from the following equations: x1 = 1 - K2 K1 - K2 y1 = K1 K2-K1 K2 - K1 z (K - K2) / (1 - K2) -1 V= 1 1 F K1 - 1 where K1, K2 are the distribution coefficients of the two components, V is the mole vapor rate, F is the mole feed rate, and z1 is the concentration of component 1 in the feed.

Flash Separation Figure 24


49



Bubble and Dew Point Calculations Bubble point and dew point calculations are performed when the liquid/vapor composition is known, the temperature or pressure is given, and the corresponding pressure or temperature needs to be determined. •

At the bubble point: xi = zi, therefore, Σ yi = ΣxiKi = ΣziKi = 1.0

The equation to be satisfied is: Σ zi Ki = 1.0 •

At the dew point: yi = zi, therefore, Σ xi = Σ yi /Ki = Σ zi /Ki = 1.0

The equation to be satisfied is: Σzi/Ki = 1.0 Finding the bubble point and the dew point for a multicomponent system involves tedious trail-and-error calculations. It is recommended that PRO/IITM or HYSIM be used for such calculations.


50



Flash Calculations: One Main Component Plus Gas In many cases, a gas stream is in equilibrium with liquid that contains only one component or a main component plus small amounts of heavier components. Such an equilibrium, for example, exists in the overhead condenser separator of a refluxed deethanizer, where ethane and other gases are in contact with the refluxed liquid. The liquid consists mainly of propane with very small amounts of heavier hydrocarbons. In this case, the amount of propane loss in the gas will need to be estimated. The calculation is simple and can be performed with a hand calculator. A similar type of equilibrium is found on the top tray of a gas scrubber, where water removes impurities from a gas mixture. An example is in Figure 25.

o

140 F Washed Gas 5 psig H2 0

V

H2 0 + Trace Impurities Water Wash of a Gas Figure 25


51



A gas with a dry (water not included) volumetric rate of 1000 moles/hr is contacted with water in a multistage tower. The pressure at the top of the tower is 5 psig and the temperature of the overhead gas is 140°F. The amount of water vapor in the overhead gas must be found. The first step is to examine the equilibrium at the top tray. A basic assumption for the solution of the problem is that the amount of gas in the liquid phase on the top tray is not significant (xw = 1.0). It is also safe to assume that the temperature of the top tray is equal to the temperature of the overhead gas, 140°F. From Raoult's law, the mole fraction xw for water is 1.0: PPw = xwVPw = 1.0 VPw = VPw

From steam tables:

PPw = VPw (140°F) = 2.9 psia

In the vapor phase:

PPw = ywPT yw = PPw/PT = 2.9/(14.7 + 5.0) = 0.147

The amount of water in the overhead gas is: Vw = yw V/(1 - yw) = 0.147 x 1000/(1 - 0.147) = 172 mole/hr


52



Physical Properties This section briefly covers the physical properties related to distillation calculations and distillation processes. Further information may be obtained from the course ChE 202, Physical Properties, or from the sources listed below. Physical Property Sources The following four sources are easily accessible to Saudi Aramco engineers and contain data suitable for distillation and VLE calculations. • • • •

Maxwell, Data Book on Hydrocarbons. Provided with this course. Data and format aimed for petroleum engineers. GPSA Engineering Data Book. Data for petroleum and gas-processing engineers. Perry's Chemical Engineers' Handbook. Source for a wide variety of properties. Not as simple to use for petroleum engineers as the previous two sources. PRO/IITM, HYSIM. The data libraries of these programs can be used directly for simulations and to develop properties for manual calculations or other computer programs.

Other sources of property data are: • •

•

W. C. Edmister and B. I. Lee; Applied Hydrocarbon Thermodynamics. Focuses on pure hydrocarbons and hydrocarbon mixtures. R. C. Reid, J. M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids. A classic reference for estimating properties. It contains extensive discussions on the many methods that have been proposed for predicting physical, thermodynamic, and transport properties. Reid's book has nothing on thermodynamics and focuses on defined compounds; there is no mention of petroleum fractions. American Petroleum Institute (API), Technical Data Book. Data and prediction methods for most properties related to petroleum processing.


53



Average Boiling Point Many physical properties of pure hydrocarbons can be correlated with specific gravity and normal boiling point as independent variables. However, for use in the petroleum industry, these correlations must also be applicable to petroleum fractions, which are mixtures of many components, that have a wide variation in boiling points. In all correlations involving the boiling points of petroleum fractions, the correct average boiling point should be used. For the following physical properties, these are: Average Boiling Point

Physical Property

Volume Average

Viscosity Liquid specific heat

Weight Average

True critical temperature

Molar Average

Pseudocritical temperature Thermal expansion of liquids

Mean Average

Molecular weight Characterization factor Specific Gravity Pseudocritical pressure Heat of combustion

Maxwell's Data Book on Hydrocarbons includes calculation and interconversion graphs for the different average boiling points.


54



Characterization Factor The characterization factor, also known as Watson K and UOP K, is an index of the chemical character of pure hydrocarbons and petroleum fractions. The characterization factor of a hydrocarbon is defined as the cube root of its absolute boiling point in °R divided by its specific gravity (60°F), or 3

Characterization Factor, Kw = TB / Sp Gr For hydrocarbon mixtures the mean average boiling point, MABP, is used in place of TB. Characterization factors are available in Maxwell, as a function of gravity in °API and boiling point in °F for hydrocarbons and petroleum fractions. The characterization factor is only an approximate index of the chemical nature of hydrocarbons, as indicated by its variation with boiling point for members of a homologous series and for fractions from the same crude. However, it has considerable value because it can be applied to the entire boiling range of a crude and has been generally accepted by the petroleum industry. In other words, we assume that the Kw of fractions is equal to the Kw of the entire crude. Crudes with high Kw are paraffinic, while crudes with low Kw are more aromatic. Below are average Kw for components with up to ten carbon atoms. Paraffins Naphthenes Aromatics


Kw ~ 12.7 Kw ~ 11.5 Kw ~ 10.5

55



Inspection Properties Inspection properties generally relate to the use of the products. For example, the cetane number is a measure of the ignition quality of a diesel fuel. It is expressed as the percentage of cetane (hexadecane) that must be mixed with liquid methylnaphthalene to produce the same performance as the diesel fuel being rated. The cetane number and other inspection properties are determined by standard tests. ASTM tests are generally accepted. Some of the inspection properties related to products of distillation units are as follows: • • • • • •

RVP (Reid Vapor Pressure) -- Gasolines and light distillates. Freezing Point -- Jet fuel. Flash Point -- Most products. Cetane Number -- Diesel fuel. Cloud Point -- Middle distillates, fuels. Pour Point -- Heavy fuels, heavy distillates, lubes.

The inspection properties measured in standardized tests are often correlated by predictive methods with other properties, such as average boiling point, Watson K, specific gravity, and distillation characteristics.


56



Petroleum Fraction Distillations A complete component-by-component analysis of crude oil or its fractions is not practical because of the large number of components. For this reason, the composition and the vaporization characteristics of petroleum fractions are represented by various distillation methods. Of these, the ASTM and 15/5 or TBP (true boiling point) are the most widely accepted and best standardized. The simple, inexpensive ASTM distillation is universally preferred for routine product testing and refinery operation control. Although seldom available, data on a 15/5 basis are required for refinery planning, engineering, designing fractionator towers, and evaluating major refining processes. 15/5 Distillation 15/5 is a standardized, accurate laboratory batch distillation that is used for crude assays and feed or product separations. The fractionator has 15 theoretical plates, calibrated under total reflux conditions, and is operated adiabatically with automated reflux of total condensate. The reflux ratio employed is 5:1 at atmospheric pressure and 2:1 at low pressures (2-10 mm Hg absolute). A maximum vapor temperature of 430°F is normal for atmospheric operations, while 700°F is the maximum atmospheric equivalent vapor temperature (°F AET) for operations at 10 mm pressure. The usual practical limit at 2 mm Hg absolute is 800°F AET. Generally, 15/5 vapor temperatures are approximations of true boiling points; they are not necessarily equivalent to those from an efficient analytical distillation such as GCD. The term true boiling point (TBP) is ambiguous. Theoretically, a TBP distillation utilizes a distillation system that is able to make very close separations; each compound present in the mixture will thus be separated at its own boiling point and in the quantity present in the original mixture. The concept is illustrated in Figure 26 for two components A and B boiling at TA and TB at the total pressure of the distillation. The stepwise plot (solid lines) represents an ideal TBP distillation. Component A, boiling at a lower temperature, is recovered first. Recovery of Component B starts after all of Component A is recovered. The distillation temperature then increases to TB.

Stepwise Plot of an Ideal TBP Distillation Figure 26 Saudi Aramco DeskTop Standards

57



The smooth curve in Figure 26 (broken line), represents an actual curve with imperfect fractionation, such as results from a 15/5 distillation. Recovery of Component B starts before the recovery of A is complete. As a result, the temperature of the distillation increases gradually, reflecting the increasing concentration of B in the distillate. Figure 27 shows similar curves for a mixture with seven components. If the mixture, like most petroleum fractions, contains many components, the TBP or 15/5 fractionation will produce a smooth curve (Figure 28).

TBP Curve for Seven Components Figure 27

TBP Curve of a Complex Mixture Figure 28


58



ASTM Distillations ASTM distillation procedures were developed by the American Society for Testing Materials. These methods are rapid batch distillations that employ no trays or reflux between the stillpot and the condenser. The only reflux is that generated by heat losses from the apparatus. ASTM test methods are used in control laboratories throughout the world. ASTM distillation data are considered to be roughly equivalent to those from a one-plate batch distillation. Figure 29 lists common ASTM distillations for petroleum products.

Pressure

Maximum Vapor Reproducibility Temp., °F , °F

ASTM

Range

D-86 Group 1&2

Naphtha and Kerosene

Atm

480

5 - 10

D-86 Group 3&4

Middle Distillates

Atm

760

5 - 10

D-158

Distillates and Gas Oil

Atm

760

Not Defined

D-1160

Heavy Distillates & Residua

Atm

620 under vacuum

15 - 20

D-216

Natural Gasoline

Atm ASTM Distillation Procedures Figure 29

In ASTM distillation, the thermometer reading when the first drop is recovered is the initial boiling point (IBP). The amount of distillate collected in the graduate may be recorded at specified temperature intervals, or the temperature may be recorded when the amount of distillate reaches specified levels. The maximum temperature, when the last vapor comes off, is recorded as the end point or final boiling point (FBP). The total amount of distillate collected is recorded as the recovery, and the volume of material (if any) remaining in the flask is recorded as the residue. The difference between the volume of the initial sample and the sum of the recovery and residue, is the distillation loss. Saudi Aramco DeskTop Standards

59



Gas Chromatographic Distillation (GCD) Gas chromatographic analytical techniques are used to obtain a breakdown of components in petroleum fractions. The results are automatically converted by a computer associated with the instrument into distillations that approximate 15/5 results. Equilibrium Flash Vaporization (EFV) A flash curve indicates the relative amounts of feed vaporized as a function of the flash temperature (Figure 30). Pressure is constant. The amount vaporized is usually expressed as a fraction or percentage of the feed on a mole, weight, or volume basis.

Equilibrium Flash Vaporization Figure 30 The separation between light and heavy components in a flash separation is relatively poor, because there is only one equilibrium stage. The vapor product is in equilibrium with the liquid product, and the flash curve is relatively flat when compared to the curves from multistage distillation processes. EFV curves are seldom run because of the time and expense involved. They are almost always limited to crude oil or to reduced crude samples (atmospheric tower bottoms liquid) that are being evaluated as vacuum tower charge stocks. The EFV initial boiling point is the bubble point of the fraction under study, and the EFV final boiling point is its dew point.


60



Distillation Curve Relationships Figure 31 illustrates the difference in the shapes of 15/5, ASTM, and EFV curves. The steepest curve is the 15/5 because it provides the best separation between the components. EFV is relatively flat, reflecting the poor separation obtained from one-stage flash. Techniques for converting the results of one method to another will not be covered here. Conversion techniques can be found in the API Technical Data Book.

Constant Pressure

0

10

20

30

40

50

60

70

80

90

100

LV % Distilled

Distillation Curves Figure 31


61



Crude Assays The complete and definitive analysis of a crude oil, usually called a crude assay, is considerably more detailed than a TBP curve and a whole crude API gravity. A complete crude assay will contain some or all of the following: • • • • • • • •

Properties such as whole crude gravity, viscosity, sulfur content, and pour point. Plots of properties such as TBP curve, mid-volume plot of gravity, viscosity, sulfur. Light-ends analysis through C8 and C9. Properties of fractions (naphthas, middle distillates, gas oils, and residua) -- yield as volume percent, gravity, sulfur, viscosity, octane number, diesel index, flash and fire point, freeze point, smoke point, pour point, vapor pressure, etc. Properties of lube distillates, if the crude is suitable for the manufacture of lube basestocks. Properties of asphalts, if the residua have suitable characteristics for preparation of asphalts. Detailed studies of fractions for various properties, such as octane number versus yield for naphthas or viscosity versus yield for lubestocks. EFV curve run at atmospheric pressure and/or phase diagram, although this is rarely done.

A Saudi Aramco assay of Abqaiq GOSP 283 is given in Addendum D. Curves that provide TBP, gravity, and sulfur content are reproduced in Figure 32.


62



Crude Assay Curves Figure 32


63



Computer Simulations Component Data - Computer simulations of distillation columns break the hydrocarbon streams fractionated into their constituents. Generally, hydrocarbons with up to five or six carbons are identified as individual components. Hydrocarbons with more than five or six carbons are represented by narrow fractions. The narrow fractions are defined by their volume average boiling point (VABP) and their average gravity. In other words, components boiling within certain ranges are represented in the simulation as one component. Such a component is called a pseudocomponent. Figure 33 illustrates the division of a wide petroleum fraction into 11 pseudocomponents. The fraction can be divided into pseudocomponents of equal volume or equal boiling range. Alternatively, there can be an increased number of components in the region where the distillation column will split the products.

Divide TBP Curve Into Pseudocomponents

10

TBP

11

1 0

2 5

3 10

4 20

5 30

6 40

50

7 60

70

8 80

9 90

95 98 100

Vol. %

Pseudocomponent Breakdown Figure 33 PRO/IITM and HYSIM offer a variety of options for representing petroleum fractions and determining their pseudocomponents.


64



Thermodynamic Systems In PRO/IITM - Figure 34 is a table of the various thermodynamic methods for calculating VLE and physical properties in PRO/IITM. Figure 35 contains specific processing examples and the corresponding thermodynamic methods suggested by PRO/IITM. Figure 36 lists the main Saudi Aramco distillation processes and the recommended thermodynamic methods. Second-choice but acceptable options in Figure 36 are in parentheses. Applicability K Value Enthalpy

Method

Keyword

Correlation Type

Entropy

Braun K10

BK10

Empirical

Yes

-

-

B-W-R (Modified by Twu)

BWRST

Equation of state

Yes

Yes

Yes

Curl-Pitzer

CP

Corresponding state

-

Yes

Yes

Grayson-Streed

GS

Semi-empirical

Yes

-

-

Johnson-Grayson

JG

Empirical

-

Yes

-

K Delta

KDELTA

Corresponding state

Yes

-

-

Lee-Kesler

LK

Corresponding state

-

Yes

Yes

Lee-Kesler-Plocker

LKP

Corresponding state

Yes

Yes

Yes

Peng-Robinson

PR

Equation of state

Yes

Yes

Yes

Redlich-Kwong(1)

RK

Equation of state

-

-

Yes

Rice

RICE

Corresponding state

-

Yes

Yes

Soave Redlich-Kwong

SRK

Equation of state

Yes

Yes

Yes

SRK (Kabadi Danner)

SRK (KD)

Equation of state

Yes

Yes

Yes

(1)For Vapor Only

Thermodynamic Methods In Process Figure 34


65



Thermodynamic System

Comments

Process Demethanizers Deethanizers Depropanizers

PR or SRK PR or SRK PR or GS or SRK

Expander plant simulation. Chiller plant simulation.

Debutanizers or Deisobutanizers C2 or C3 Splitter B-T-X column

GS or SRK or PR PR or SRK BK10

Crude Units Bubble Towers

BK10 or GS BK10 or GS

FCC main fractionators Vacuum Columns

BK10 or GS

See above comment.

BK10

Resid representation is critical to accuracy.

Reformer systems Natural gas with high H2S or CO2 Nitrogen rejection

GS or LKP

High H2.

PR or SRK PR or SRK

Wellhead proc.

PR or SRK

Special data are built-in. Cryogenic natural gas towers. High pressure flashes.

Special data are built-in. Special data for aromatics at low pressures included. More vapor with GA. Component breakdown is very important to simulation.

Thermodynamic System Application Guidelines Figure 35


66



Distillation Process

Approx. Pressure, psia

Components

Crude Stabilization

30

BK10

Condensate Stripping

250-470

SRK (PR)

Crude Fractionation

Vacuum - 50

BK10

Demethanizer

160 (No Condenser)

SRK (PR)

Deethanizer

430 (50-210°F)

SRK (PR)

Depropanizer

330

SRK (PR)

Debutanizer

140

SRK (PR)

NGL Fractionation

Saudi Aramco Distillation Processes Suggested Thermodynamic Options Figure 36


67



Nomenclature α

Relative volatility

F

Feed rate, mole/hr

F

Fugacity Factor

f

Fugacity

MABP

Mean average boiling point

Sp Gr

Specific gravity

t

Temperature

VP

Vapor pressure

z

Mole fraction in the feed


68



ADDENDUM B Example 1 Sweet gas from a DGA contactor (Figure 37) is water washed in the top two trays of the column. a. Find the water weight rate in the gas leaving the column. b. What is the water dew point of the saturated gas at the tower pressure? c. The sweet gas is let down to 60 psig. Find the dew point at this pressure. Assume that there are no hydrocarbons in the liquid phase. Dry gas rate: Temperature: Pressure: Vol%:

300 MMSCFD (Million Standard Cubic Feet per Day) 120°F 150 psig C1 = 60 C2 = 20 C3 = 15 C4 = 4

Assume that the gas is an ideal gas. The ideal gas volume at standard conditions (60 °F, 1 atm) is 379.5 SCF/lb-mole), Water vapor pressures are available in Addendum E.

Figure 37


69



Answer a.

Since water is the only component in the liquid phase, its partial pressure in the vapor phase is its vapor pressure at the temperature of the top tray, 120°F. VPw = PPw = 1.7 psia (from steam tables) Water content in vapor

1.7 psia PPw = = 0.0103 PT (150 + 14.7) psia

Molar rate of dry gas: 300 MMSCFD/(379.5 SCF/lb-mole) = 790,500 lb-mole/day Water rate in gas: 790,500 lb-mole/day x 0.0103/(1 - 0.0103) = 8227 lb-mole/day Weight rate: (8227 x 18) lb/day = 148,000 lb/day = 6200 lb/hr b.

Since the gas is saturated with water, it is at its dew point, 120°F.

c.

Partial pressure of water at 60 psig = 0.0103 x (60 + 14.7) = 0.77. This is also the water vapor pressure at the dew point. From the steam tables, the temperature that corresponds to 0.77 psia water-vapor pressure is 93°F.


70



Example 2 Use the provided crude assay TBP curves (Figure 38) to obtain the gravity and sulfur content for 450°F and 600°F TBP components. Find the fraction of the crude in a 450-600°F cut.

Figure 38


71



Answer 450°F TBP fraction Volume = 38% API gravity = 44 Sulfur = 0.2 wt% 600°F TBP fraction Volume = 54% API gravity = 34 Sulfur = 1.3 wt% The 450-600°F volume is: 54 - 38 = 16% of the crude volume


72



Example 3 Use the ideal gas law to solve the following problems. The material balance of a DGA unit indicates the following conditions for the contactor feed: Pressure: Temperature: Rate:

160 psig 115°F 53,963 lb-mole/hr

Find the following: a.

The actual volumetric rate of the gas in MMACFD.

b.

The actual volumetric rate of the gas at 200 psig and 115°F.

c.

The actual volumetric rate of the gas at 200 psig and 250°F.

d.

The standard volumetric rate of the gas (60°F, 1 atm) using the ideal gas molecular volume.

Fundamental constants are in Addendum C. Answer a.

Ideal gas law: PV = nRT n P T R V

= = = = =

53,963 lb-mole/hr 160 + 14.7 = 174.7 psia 115°F = (115 + 459.7)°R = 574.7°R 10.732 (psia-ft3)/(lb-mole °R) nRT/P = [53,963 lb-mole/hr x 10.732 (psia-ft3)/(lb-mole °R) x 574.7°R]/174.7 psia

= 1.905 x 106 ft3/hr = 45.72 MMACFD


73



b.

P1V1 = nRT1 P2V2 nRT1 P1V1 = P2V2 and V = P1 T2 2 V2 T2 P2 T1 Since T1 = T2: (160 + 14.7) psia V2 = V1 P1 = 45.72 MMACFD = 37.20 MMACFD P2 (200 + 14.7) psia

c.

174.7 psia (250 + 459.7)°R V2 = V1 P1 T2 = 45.72 MMACFD P2 T1 14.7 psia (115 + 459.7)°R

d.

Normal volume (1 atm, 0°C) of ideal gases: 359.039 ft3/lb-mole Standard volume (1 atm 60°F): 359.039

(459.67 + 60)°R (459.67 + 32)°R

= 379.5 SCF/lb-mole

Rate = 53,963 lb-mole/hr x 379.5 SCF/lb-mole x 24 hr/day = 491.50 MMSCFD


74



Example 4 Represent the illustrated condenser sequence (Figure 39) in the provided propane/n-butane phase and equilibrium diagrams (Figures 40 and 41). Assume that the pressure is 100 psia for all drums.

F, z 1

V, y 1 T = 110ÞF

V', y'1 T' = 100ÞF

L, x 1

L', x'1

T" = ? L", x"1

Figure 39


75



Propane - n-Butane 150 140 P = 100 psia 130 120

Vapor 110 100

x1

90

x1'

80

Liquid

y1

70

y1'=x1"

60 50

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Mole Fraction Propane, x ,1y

0.8

0.9

0.8

0.9

1.0

1

V-L Phase Diagram Figure 40 Propane - n-Butane 1.0 C 3- nC 4 at 100 psia

0.9 0.8 0.7

(x1',y1')

(y 1',x 1")

0.6 (x1,y1) 0.5 0.4 0.3 0.2 0.1 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1.0

Mole Fraction Propane in Liquid, x 1

Equilibrium Line Figure 41


76



ADDENDUM C Pg. 1 of 14

Figure 42 Saudi Aramco DeskTop Standards

77



Pg. 2 of 14


78



Pg. 3 of 14

Figure 44


79



Pg. 4 of 14

Figure 45


80



Pg. 5 of 14

Figure 46


81



Pg. 6 of 14

Figure 47


82



Pg. 7 of 14

Figure 48


83



Pg. 8 of 14

Figure 49


84



Pg. 9 of 14

Figure 50


85



Pg. 10 of 14

Figure 51


86



Pg. 11 of 14

Figure 52


87



Pg. 12 of 14

Figure 53


88



Pg. 13 of 14

Figure 54


89



Pg. 14 of 14

Figure 55


90



ADDENDUM D Pg. 1 of 9


91



Pg. 2 of 9


92



Pg. 3 of 9

Figure 58


93



Pg. 4 of 9

Figure 59


94



Pg. 5 of 9

Figure 60


95



Pg. 6 of 9


96



Pg. 7 of 9

Figure 62


97



Pg. 8 of 9


98



Pg. 9 of 9


99



ADDENDUM E Pg 1 of 2

v = specific volume h - enthalpy, BTU per lb Source: Steam Tables. Reprinted with permission of Combustion Engineering, Inc.

s = entropy, Btu per R per lb


100



Pg 2 of 2

v = specific volume h - enthalpy, BTU per lb s = entropy, Btu per R per lb Source: Steam Tables. Reprinted with permission of Combustion Engineering, Inc.

Figure 66


101

Distillation Process

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