IV Year B. Tech. Petrochemical Engineering II Sem. Multicomponent Distillation Azeotropes
Azeotropes
The term azeotrope means “nonboiling by any means” (Greek: a -non, zeo boil, tropos -way/mean), and denotes a mixture of two or more components where the equilibrium vapor and liquid compositions are equal at a given pressure and temperature . It was Wade and Merriman who had first introduce the term azeotrope in 1911 to designate mixtures that have a minimum or maximum boiling point at constant pressure or, equivalently, in the vapour pressure under isothermal conditions. They define the azeotropy state as a stationary point in the equilibrium T-x,y or P-x,y. The mixture whose composition corresponds to an extremal point is called an azeotrope. If at the equilibrium temperature the liquid mixture is homogeneous, the azeotrope is a homoazeotrope. If the vapor phase coexists with two liquid phases, it is a heteroazeotrope. Systems which do not form azeotropes are called zeotropic (Swietoslawski, 1963).
Figure 1: Graphical representations of the VLE for the most common types of binary mixtures at constant pressure: a) NonNon-az azeo eotr trop opic ic;; b) Minimu Minimum-b m-boil oiling ing homo homoaz azeot eotro rope; pe; c) Mi Min nimum imum--bo boiilin ling heteroazeotrope; d) Ma Maxim ximumum-bo boili iling ng azeot azeotro rope. pe. Lef Left: t: boiling temperature Tbp and condensation temperature Tdp and the equilibrium mapping vectors in T - x; y space. Right: x -y relationship relationship (equilibrium line)
The formation of azeotrope is due to the differences in the intermolecular forces of attraction among the mixture components. There are 3 groups that particularly deviate from the ideality that simply explains explains the binary mixtures. They are are : 1.
Posit ositiive devi devia ation tion from from Raoult’s law: The components “dislike” each other. other. The attraction between identical identica l molecules (A-Aand B-B) is stronger than between different molecules (A-B). This may cause the formation of a minimumboiling azeotrope and heterogeneity.
2.
Neg Negativ tive de devia viation tion from from Raoult’s law: The components “like” each other. The attraction between different molecules(A-B) is the strongest. strongest. This may may cause the formation of a maximumboiling azeotrope. azeotrope.
3.
Ideal mixture obeys Raoult’s law: The components component s have similar physiochemical properties. The intermolecular forces between identical and different molecules (A-A, B-B and A-B) are equal.
The tendency a mixture to form an azeotrope depends on 2 factors (Horsely, (Horsely, 1973; King, 1980): 1. 2.
The The diffe differe renc nces es in the the pure pure com compo pone nent nt boi boili ling ng poin pointt The The de degre gree of of non nonid idea eallity ity.
Due to azeotrope and resulting phase behaviour, there are profound effects on the feasibility and technology for distillationbased operation.
Azeotropic Mixtures Large deviations from ideal liquid solution behaviour relative to the difference between the pure component vapour pressures result in azeotrope formation. we are interested in: 1. Descri Describi bing ng az azeo eotro tropi picc mixtures both physically and in thermodynamic terms. 2. Detec Detecti ting ng azeotr azeotrop opic ic conditions and calculating their composition.
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Azeotropic Mixtures
Water / Hydrazine, P=1atm
Water / Pyridine, P=1atm 7
Azeotropes - Impact on Separation Processes Separation processes that exploit VLE behaviour (flash operations, distillation) are influenced greatly by azeotropic behavior.
An azeotropic mixture boils to evolve a vapour of the same composition and, conversely, condenses to generate a liquid of the same composition.
Ethanol(1)/Toluene(2) at P=1 atm
Lecture 24
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Predicting Whether an Azeotrope Exists
To determine whether an azeotrope will be encountered at a given pressure and temperature, we define the relative volatility. For a binary system, a12 is 12
y 1 x1 y2 x2
where xi and yi are the mole fractions of component i in the liquid and vapour fractions, respectively. 12 1
At an azeotrope, the composition of the vapour and liquid are identical. Since, y1=x1 and y2=x2 at this condition, To determine whether an azeotropic mixture exists, we need to determine whether at some composition, a 12 can 9 equal 1.
Predicting Whether an Azeotrope Exists We can derive an expression for α12 using modified Raoult’s Law Raoult’s Law as our phase equilibrium relationship,
y iP x i iPisat which when substituted into the relative volatility, yields sat
12
1P1
sat
2P2
α12 is therefore a function of T (Pisat, gi) and the composition of the liquid phase. Calculation of α12 therefore requires: Antoine’s equation Antoine’s equation an activity coefficient model (Margule’s (Margule’s,, Wilsons, …) a liquid composition Our goal is to determine whether an azeotrope exists. At some composition, can a12=1? 10 –
–
–
–
Predicting Whether an Azeotrope Exists One means of determining whether α12=1 is possible is to evaluate the function) over the entire composition range. This is plotted for the ethanol(1)/toluene(2) system using Wilson’s Wilson’s equation to describe liquid phase nonideality. According to this plot, a 12=1 at x1 = 0.82, meaning that an azeotrope exists at this composition.
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Predicting Whether an Azeotrope Exists Because equation is continuous and monotonic, we do not need to evaluate a12 over the whole range of x1. It is sufficient to calculate a 12 at the endpoints, x1=0 and sat x1=1 1 P1 –
12
x1 0
At x1 = 0, we have 12
x1 1
P2sat P1sat
sat
2 P2
and at x1 = 1, we have
If one of these limits has a value greater than one, and the other less than one, at some intermediate composition we know a 12 =1. This is a simple means of determining whether an azeotrope exists. –
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Determining the Composition of an Azeotrope For an azeotropic mixture, the relative volatility equals one: sat
12
1P1
sat 2
2P
1
at an azeotrope. To find the azeotropic composition, two methods are available: trial and error (spreadsheet) analytical solution Rearranging as above yields: 1 P2sat 2 P1sat The azeotropic composition is that which satisfies this equation. Substitute an activity coefficient model for g 1, and g2. 13 Solve for x –
–
–
Example While azeotropes are undesirable from a processing point of view, we can still benefit from this phenomenon in obtaining parameters for GE models. For example, the binary mixture of 1,4-dioxane (1)/water (2) exhibits an azeotrope at x1=0.554 at T= 50 oC and P=0.223 bar. How can we use this information in the estimation of the Margules parameters A12 and A21 for the mixture? Given: P1sat(50 oC)=0.156 bar; P2sat(50 oC)=0.124 bar. ‘ Set all fugacity coefficients equal to 1.00.
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Azeotropic Distillation & Extractive Distillation Azeotropic distilla distillation tion usually refers to the specific technique of adding another component to generate a new, lower-boiling azeotrope that is heterogeneous (e.g. producing two immiscible liquid phases) to facilitate the separation of components in the original mixture into pure components or the desired compositions beyond the azeotropic azeotropic compositions.
A common historical example of azeotropic distillation is its use in dehydrating ethanol and water mixtures. For this, a near azeotropic mixture is sent to the final column where azeotropic distillation takes place. Several entrainers can be used for this specific process: benzene, pentane, cyclohexane, hexane, heptane, isooctane, acetone, trichloroethylene and diethyl ether are all options as the mixture. Of these benzene and cyclohexane have been used the most extensively.
Table: Some azeotropic mixtures
Mixture 1. 2. 3. 4.
Water-Ethanol Pyridine-Water Ethanol-Benzene Acetic acid-Toluene
% Composition of Boiling point azeotrope (pressure = 1 atm) 95.97 Ethanol 78.13oC 57.00 Pyridine 92.6oC 32.40 Ethanol 67.8oC 28.00 Acetic-acid 105.4oC
There are several liquid pairs which form maximum boiling point azeotrope. Some examples are tabulated below: Mixture 1. 2. 3. 4.
Nitric acid-Water Acetic acid-Pyridine Chloroform-Acetone Hydrogen chloride-Water
% composition of azeotrope 68% Nitric acid 65% Pyridine 80% Chloroform 79.8% Water
Boiling point (pressure = 1 atm) 125.5 C 139.0 C 65.0 C 108.6 C ° °
°
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Separation of Azeotropic Mixtures
Recall from previous lecture that an zoetrope is a special class of liquid mixture that boils at a constant temperature at a certain composition. It behaves as if it were one component with one constant boiling point. Such mixture cannot be separated using conventional conventional distillation methods. Distillation of a mixture that exhibits azeotropic behaviour begins similarly to conventional distillation. The difference is that, as the process continues, a temperature is reached at which the compositions of the vapour phase and liquid phase become the same. Once this happens an azeotrope has been formed, and the individual components can no longer be separated by conventional distillation. The compositions of the liquid and vapour remain the same until all of the liquid is eventually vapourised. vapourised.
Separation of Azeotropic Mixtures can be broadly classified into the following methods:
by changing system system pressure by addition of an entrainer (azeotropic distillation) systems) ems) by combination with other processes (hybrid syst Azeotropic Distillation - Separation of Binary Azeotropes by Addition of Entrainer
Basic Concepts
Azeotropic distillation refers to processes whereby a new component (called the entrainer) is added to the original feed mixture to form (or nearly form) an azeotrope with one (or more) of the feed components. The azeotrope is then removed as either the distillate or the bottoms. Azeotropic distillation also refers to those processes in which a new component is added to an original feed mixture to break an azeotrope that otherwise would be formed by the feed components. Thus, the purpose of deliberately adding the entrainer is either to separate one component of a closely boiling pair or to separate one component of an azeotrope. To illustrate the basic concepts consider the set-up in the Figure below for the separation of a mixture A-B that forms a minimum-boiling azeotrope. azeotrope. The entrainer E is a medium boiler (i.e. its boiling point in intermediate between components A and B), or is a low boiler that can form an intermediate boiling maximum azeotrope with A.
he feed (A and B) is mixed with the entrainer E before entering column C1. Component B (which is essentially free of the azeotrope A-E) is removed from the bottom of column C1, while the overhead vapour from C1 is fed to column C2. Component A is removed as overhead product and entrainer E as the bottoms product. The entrainer entrainer is recycled back to column C1. One example of such a separation is for the mixture acetone-heptane with benzene as the entrainer. The respective boiling points are: A - acetone (56.2 oC), B - heptane (98.4 oC), E - benzene (80.1 oC) and A-B Minimum-boiling azeotrope (55.6 oC). Alternatively, component A can be separated first as overhead from column C1. In such a process, the column C2 then splits the bottoms from C1 into the entrainer entrainer E (as overhead product) and component B (as bottoms product). For a maximum-boiling azeotrope, the entrainer should either be a medium boiler or a high boiler that forms an intermediate-boiling azeotrope with component B.
Separation by Distillation)
Changing
System
Pressure
(Pressure-Swing
Sometimes azeotropic distillation can be carried out without the use of entrainer. Instead the distillation columns are operated at different pressures, as it has been known that the azeotropic concentration can be shifted substantially by changing system pressure. This method of separation is especially feasible for systems with low-boiling azeotropes, as the azeotropic concentration depends significantly on temperature, which can be changed by changing the operating pressure. For example, as shown in the Figure, the system tetrahydrofuran (THF) and water forms a minimum-boiling azeotrope. In the first column, which operates at lower pressure (1 bar), the high boiling component (water) is removed as bottoms. The composition of the overhead product is as close as possible to that of the azeotrope at this pressure.
The pressure is increased to 8 bar in the second column. At this higher pressure, the azeotrope forms at a lower concentration of the low boiling component (THF), which can then be removed as bottoms. The overhead product of the second column is returned to the first column (in vapour state) after pressure reduction. In some systems, reducing the operation pressure can eliminate azeotropic behaviour. For example, in the ethanol-water system, azeotropism disappears at a pressure below 70mm Hg. Some other examples include:
PRESSURE SWING METHOD
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USES PRESSURE TO SHIFT AZEOTROPE CONCENTRATIONS
Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. The method is used for mixtures having a low value of relative volatility, nearing unity.
Close boiling mixtures cannot be separated by simple distillation, because the volatility of the two components in the mixture is nearly the same, causing them to vaporize at nearly the same temperature at a similar rate, making normal distillation impractical. This process is very similar to azeotropic distillation. Extractive distillation refers to those processes in which a high-boiling solvent is added alter the relative relative volatilities volatilities of components in the feed. The boiling point of the solvent is generally much higher than the boiling points of the feed mixture that formation of new azeotropes is impossible. The high boiling point will also ensure that the solvent is will not appreciably vapourise in the distillation process. As an illustration, consider the simplified system shown in the Figure below for separation of toluene and iso-octane using phenol as the solvent.
The separation of toluene (boiling point 110.8 oC) from iso-octane (boiling point 99.3 oC) is difficult using conventional distillation. Addition of phenol (boiling point 181.4 oC) results in the formation of phenol-toluene mixture that leaves the extractive distillation column as bottoms, while relatively iso-octane is recovered as overhead product. The phenol-toluene mixture is further separated in a second column (solvent recovery column) whereby toluene appears as distillate and the bottoms product, phenol, is recycled back to the first column. In the above example, when the solvent is added to the original feed mixture it forms a new mixture with one of the feed components by "absorbing" that component. This new mixture has a much higher boiling point than the other feed component that is not absorbed so that it leaves as bottoms product from the extractive distillation column. The unabsorbed feed component then leaves as the overhead product. The absence of azeotropes plus the fact that the solvent can be recovered by simple distillation makes extractive distillation a less complex and more widely useful process than azeotropic azeotropic distillation.
Selection of A Solvent
The choice of solvent determines which of the two components in the original feed is removed predominantly in the distillation. For example, if the fresh feed to the distillation is a mixture of 83 mole% ethanol and 17 mole% water, and ethylene glycol (boiling point 197.35 oC) is the solvent; the volatility of ethanol is increased more than that of water. Therefore, ethanol is removed as the distillate from the extractive distillation column, and water is separated in the solvent recovery column. On the other hand, if a high-boiling hydrocarbon such as iso-octane (boiling point 99.3 oC, vs. n-octane with boiling point 125.6 oC) is used as solvent, the volatility of water is enhanced relative to that of ethanol, and water now becomes the distillate in the extractive distillation column. The number of possible solvents available for separation by extractive extractive distillation is usually much larger than for an azeotropic distillation because of less severe volatility restrictions. A general approach is to chose a compound that is more similar to the higher-boiling component in the original feed, and then go up the homologous series for that compound until a homolog is found that boils high
Comparison between Azeotropic and Extractive Distillation
Many of the entrainers used in azeotropic distillation are either proven or suspected carcinogens or otherwise classified as hazardous pollutants. Using the example of ethanol-water from previous section, noted that benzene is the entrainer. Ethanol is removed as the bottoms product from the column. Benzene is too hazardous for various reasons ranging from workplace to product to environmental safety. An alternative to recover ethanol is to use extractive distillation. The solvent used is Propylene Glycol. Recall also that ethanol forms a minimum-boiling azeotrope with water at approximately 89.4 mole% (96 wt%) ethanol.
A process schematic for the process is shown in the Figure below:
For this separation, propylene glycol meets all the requirements of an ideal extractive solvent:
.
It is miscible with water at all concentrations It has a higher boiling point than water water (187 0C at 1 atm) It does not form an azeotrope with water It has a molecular affinity for water (the hydroxyl -OH group forms a weak bond with water molecule) It is a relatively safe workplace material
In the above system, the first column is the ordinary azeotropic distillation that produces an ethanol-water azeotrope as the distillate and nearly pure water as the bottoms. The distillate is fed to the second column for extractive distillation, where propylene glycol is added. Ethanol is produced as the distillate, leaving the top of the column. This column can be conceptually divided into 3 sections. The middle section is the rectifying section where ethanol is purified by the removal of water. Bonding of the water molecules with glycol raises ethanol's relative volatility with respect to water, thus facilitating separation. The following table shows the change in relative volatility in the columns
The top section reduces the concentration of propylene glycol in the ethanol distillate to negligible level. The bottom section strips ethanol from water. The bottoms from the second column is sent to the third column, a glycol stripper, where the glycol is recovered. The propylene glycol leaves the stripper as a bottoms product and is recycled back to the extraction column as the source of solvent. The overhead from the glycol stripper (containing mainly water and some ethanol) is sent back to the first column where it combined with fresh feed.
We distinguish between three different conventional entrainer-addition based distillation methods depending on the properties and role of the entrainer and the organization (scheme) of the process: 1.
Homoge Homo gene neou ouss az azeo eotr trop opic ic di dist stil illa lati tion on (ordinary distillation of homoazeotropic mixtures): The entrainer is completely miscible with the components of the original mixture. It may form form homoazeotropes homoazeotropes with the original mixture mixture components. components. The distillation is carried out in a conventional conventional single-feed single-feed column.
2.
Heter Het eroa oazzeo eotr trop opic ic di dist stil illa lati tion on (decanter-distillation (decanter-distillation hybrids that involve heteroazeotropes): heteroazeotropes ): The entrainer forms a heteroazeotrope with at least one of the original mixture components. The distillation is carried out in a combined column and decanter system.
3.
Ext xtrrac acti tiv ve di dissti till lla ati tion on:: The entrainer has a boiling-point that is substantially higher than the original mixture components and is selective to one of the components. The distillation is carried out in a two-feed two-feed column where the entrainer is introduced above the original mixture feed point. point. The main main part of the entrainer entrainer is removed removed as bottom product.
4.
Reac acttive di distillati tio on: The entrainer reacts preferentially and reversibly with one of the original mixture components. The reaction product is distilled out from the non-reacting component and the reaction is reversed to recover the initial component. The distillation and reaction is usually carried out in one column (catalytic distillation).
5.
Chemic Chem ical al dryi drying ng (che (chemi mica call actio action n and and dist distil illa lati tion on): ): The volatility of one of the original mixture components is reduced reduced by by chemical chemical means. An example is dehydration by hydrate formation. Solid sodium hydroxide may be used as an entrainer to remove water from tetra hydro furan (THF). The entrainer and water forms a 35-50 % sodium hydroxide solution containing very little THF.
6.
Disti Dis till llat atio ion n in in the the pre prese senc nce e of of salt salts: s: The The entrainer (salt) dissociates in the mixture and alters the relative volatilities sufficiently so that theseparation becomes possible. A salt added to an azeotropic liquid mixture will reduce the vapor pressure of the component in which it is more soluble. Thus extractive distillation can be applied using a salt solution as the entrainer. An example is the dehydration of ethanol using potassium acetate solution (Furter, (Furter, 1968)
