TABLE OF CONTENTS Section
Content
1.0
Executive Summary
2.0
Process Background
Page Number 2
2.1 Dehydrogenation of ethylbenzene
3
2.2 Oxidation Process
4
2.3 Advantages and Disadvantages
4
2.4 Process Flow Diagram
5
2.5 Selection of The Process
6
2.6 Input And Output Materials of The
7
Reaction
11 2.7 Dehydrogenation of Ethylbenzene Into
Styrene
11 12
3.0
2.8 Reactor Background
15
4.0
2.9 Operation and Uses
16
5.0
Mass Balance Analysis
18
6.0
Stoichiometric Table
19
7.0
Reaction Kinetics
21
8.0
Reactor Design and Sizing
22
9.0
Conclusion
23
References Appendices
1.0 EXECUTIVE SUMMARY Styrene monomer is a very important monomer, its number of applications and demand are still growing. Currently, styrene is produced by catalytic dehydrogenation, or by a peroxidation process.There are two different approaches to the reaction, which is the adiabatic process and the isothermal process. The isothermal process is chosen because the process takes place in the PFR. Since the reaction taking place is an endothermic reaction which requires 1.26-1.33 MJ/kg of ethyl benzene to be converted at 25 oC; the heat must be supplied. In the PFR, the reaction heat is provided 1
by indirect heat exchange between the process fluid and a suitable heat transfer medium. There are few assumptions made in order to calculate the mass balance of the reaction. The conversion selected is 65%, the mole fraction of ethyl benzene is equal to 1 and the reactor operates for 50 weeks a year, 6 days a week and 24 hours a day, making the production mass flow rate is 25.2kg/hr. The reaction is a reversible reaction, however to simplify the calculation to calculate the volume of the reactor, the reaction was assumed to be irreversible, making the reaction rate of the reaction as –r A =kCA; without considering the value of KC calculated in section 4 of reaction kinetics. The volume of the reactor calculated is 4.6536 L . The dehydrogenation reaction was conducted isobarically at atmospheric pressure, 1atm.
2.0 PROCESS BACKGROUND Styrene (St) is one of the most important monomers in modern petrochemical industry. The world production at present is approximately 20 million tons per year. Styrene is a colorless liquid with a distinctive, sweetish odor. Styrene is produced in industry mainly by two processes, which is Dehydrogenation of ethyl benzene and oxidation process.
2
2.1) DEHYDROGENATION OF ETHYL-BENZENE
C6H5CH2CH3 C6H5CH=CH2 + H2 Dehydrogenation of ethyl benzene is one of the most common process in production of styrene. Direct dehydrogenation of ethylbenzene to styrene accounts for 85 % of commercial production. The reaction is carried out in the vapor phase with steam over a catalyst consisting primarily of iron oxide. The reaction is endothermic, and can be accomplished either adiabatically or isothermally. Both methods are used in practice. The major reaction is the reversible, endothermic conversion of ethylbenzene to styrene and hydrogen:
C6H5CH2CH3 C6H5CH=CH2 + H2
ΔH (600 °C) = 124.9 kJ/mol
This reaction proceeds thermally with low yield and catalytically with high yield. As it is a reversible gas phase reaction producing 2 mol of product from 1 mol of starting material, low pressure favours the forward reaction. Competing thermal reactions degrade ethylbenzene to benzene, and also to carbon:
C6H5CH2CH3 C6H6 + C2H4 C6H5CH2CH3 8 C + 5 H2
ΔH = 101.8 kJ/mol ΔH = 1.72 kJ/mol
Dehydrogenation of ethylbenzene is carried out in the presence of steam, which has a threefold role, which is it lowers the partial pressure of ethylbenzene, shifting the equilibrium toward styrene and minimizing the loss to thermal cracking. Besides that, it cleans the catalyst by reacting with carbon to produce carbon dioxide and hydrogen and t supplies the necessary heat of reaction. (MANUFACTURING PROCESES AND SELECTION)
2.2) OXIDATION PROCESS C6H5CH2CH3 + O2 C6H5CH(OOH)CH3 One of the most notable oxidation processes is Halcon international’s process to produce styrene from propylene oxide. The reaction takes place in the liquid phase with air bubbling through the liquid, and no catalyst is required. However, since hydro peroxides are unstable compounds, exposure to high 3
temperature must be minimized to reduce the rate of decomposition. Fewer by-products are formed from the decomposition if the reaction temperature is gradually reduced during the course of reaction. The reaction is more selective to the production of by-product acids, when it is carried out at constant temperature than when the temperature is gradually reduced. In practice, the temperature is reduced by means of a series of reactors, each of which is maintained at a progressively lower temperature. The pressure required for the reaction is not critical; 800-1500 kPa is sufficient to maintain the reactants at liquid phase. Catalysts for this reaction are compounds of metals such as molybdenum, tungsten, vanadium. The oxidation reaction generally proceeds at 100- 130’C in the liquid phase under self-generated pressures. The conversion of ethyl benzene hydro peroxide is nearly complete, and selectivity of the reaction in producing propylene oxide is greater than 70%. The alcohol can be dehydrated to styrene or it is reduced to ethyl benzene for recycle if styrene is not desired. (MANUFACTURING PROCESES AND SELECTION) 2.3 ADVANTAGES AND DISADVANTAGES Presently used methods for dehydrogenation of ethyl benzene to styrene suffer the disadvantage of relatively low conversions of the feed hydrocarbon per pass. To obtain conversions of even as high as 40%, it is necessary to dilute the hydrocarbon stream by adding more than two pounds of steam for every pound of hydrocarbon fed. At the high temperatures required for dehydrogenation, the heating requirements for converting water to steam constitute a major part of the operating cost. Another major disadvantage of the low conversion is the high cost of separating styrene from ethyl benzene. The separation of styrene is expensive and a large amount of ethyl benzene must be recycled. Other disadvantages of oxidative dehydrogenation using oxygen are the necessity to operate outside the explosive limits of the gas mixture and the loss of part of the feed hydrocarbon by combustion. Also, the presence of elemental oxygen leads to formation of various oxygen containing by-products which must be separated out and disposed of. The aforementioned disadvantages of conventional oxidative dehydrogenation processes are reduced or eliminated by the present invention. The invention utilizes the conjoint presence of Fischer-Tropsch catalysts with carbon dioxide. The carbon dioxide not only acts as the diluting gas, but also reacts with hydrogen to produce water and carbon monoxide thus apparently shifting the equilibrium and eliminating the equilibrium conversion barrier encountered by conventional dehydrogenation processes. Oxidative dehydrogenation enjoys two advantages: it is not necessary to feed large quantities of steam or other diluent to dilute the hydrocarbon, higher conversions to styrene are possible, thus reducing 4
the cost of the separating equipment and the amount of ethyl benzene which must be recycled. (Olson, 1966) 2.4 PROCESS FLOW DIAGRAM
1) DEHYDROGENATION OF ETHYL BENZENE
Figure 1: PFD of dehydrogenation of ethyl benzene.
2) OXIDATION PROCESS
5
Figure 2: PFD of oxidation process of manufacturing styrene.
2.5 SELECTION OF THE PROCESS Dehydrogenation process is more stable than oxidation process. This is because oxidation is more selective to the production of by product acids, when it is carried out at constant temperature than when the temperature is gradually reduced. In practice, the temperature is reduced by means of a series of reactors, each of which is maintained at a progressively lower temperature. Several licensed process are available for the conversion of ethyl benzene to styrene. Although all such processes share the catalytic dehydrogenation of ethyl benzene to styrene in the presence of steam, there are two distinct approaches to the reaction section design which is the adiabatic process and the isothermal process. As the dehydrogenation of ethyl benzene reaction is endothermic, requiring 1.26-1.33 MJ/kg of ethyl benzene converted at 25’C, heat must be supplied. In the adiabatic process, steam superheated to 800950’ C is mixed with preheated ethyl benzene (EB) feed prior to exposure to the catalyst. The reactors are run at the lowest pressure that is safe and practicable. Some units operate under vacuum, while others operate at a low positive pressure. While, for the isothermal process, steam temperatures are lower than in the adiabatic process. (Behr) The main difference between the isothermal and adiabatic processes is in the way the endothermic reaction heat is supplied. In principle, the isothermal reactor is designed like a shell & tube Heat Exchanger is a fixed bed dehydrogenation catalyst and reactant gas is on the tube side, and a suitable heat transfer medium is on the shell side. (MANUFACTURING PROCESES AND SELECTION) 6
Plug Flow Reactor are wildly use in chemical industries. The advantages of PFR reactor is Plug flow reactors have a high volumetric unit conversion and run for long periods of time without maintenance. The control of Plug Flow Reactor process a problem frequently encountered in the chemical industries. The isothermal process takes place in PFR and reaction heat is provided by indirect heat exchange between the process fluid and a suitable heat transfer medium. (WAHAMID, 2010) In conclusion, for production of styrene process, dehydrogenating of ethylbenzene; isothermal in Plug Flow Reactor is the best modes to conduct the production process for styrene. 2.6 INPUT AND OUTPUT MATERIALS IN THE REACTION There are many ways on producing styrene (Styrene Production Methods, nd), but the most commonly used within industry is by dehydrogenation of ethylbenzene using selected catalysts. In this reaction, the only raw material used is ethylbenzene. From (ATSDR, 2005), Ethylbenzene is an aromatic hydrocarbon that occurs naturally in petroleum and is a component of aviation and automotive fuels. Commercial ethylbenzene (CAS # 100-41-4; (C6H5)CH2CH3) is more than 99.7% pure. It is manufactured in a closed continuous process by reacting ethylene and benzene with an aluminum chloride or zeolite catalyst. Ethylbenzene also occurs at 15 to 20% in the “mixed xylenes” stream isolated at some petroleum refineries for use as a solvent. It is used as a solvent and in the production of synthetic rubber and styrene. Ethylbenzene is a colorless liquid with an aromatic odor. Ethylbenzene is a flammable and combustible liquid. Vapors are heavier than air and may travel to a source of ignition and flash back. Liquid ethylbenzene floats on water and may travel to a source of ignition and spread fire. Combustion may produce irritants and toxic gases. Selected physical and chemical properties of ethylbenzene are shown in table 1 below:
7
Table 1: The chemical and physical properties of ethylbenzene (Reproduced from (Acute Exposure Guidelines Level for Ethylbenzene, 2009) With permission) In the reaction, there are two products formed which are styrene and also hydrogen. As described in (CEFIC, 2007) Styrene (C6H5-CH=CH2, CAS RN:100-42-5; EC No.: 202-851-5, EEC Annex I Index No.: 601-026-00-0), also known as ethenyl benzene, phenyl ethylene, phenyl ethene, vinyl benzene, cinnamene or styrene monomer, under ambient conditions is a colourless clear liquid with a distinctive sweetish aromatic odour. It is miscible with most organic solvents in any ratio and is a good solvent for synthetic rubber, polystyrene and other high molecular weight polymers. From the environmental impact point of view, it is only slightly soluble in water and consequently the acute hazard of spilled styrene will be very limited for most aquatic species. However styrene may cause unpleasant taste in food from aquatic organisms exposed to low environmental concentrations. Styrene is a monocyclic alkenyl aromatic compound with a molecular weight of 104. Being rather volatile and having a flash point of 32 oC, styrene is classified as a flammable substance, which in use may form flammable/explosive vapour-air mixtures. Despite its high boiling point, styrene will eventually end up in the air.The typical physical properties of styrene are the short-term exposure to styrene in humans’ results in respiratory effects, such as throat irritation and lung constriction, irritation to the eyes, and neurological effects such as very low concentrations of styrene vapour will react with bromine and with chlorine in sunlight to form an extremely potent lachrymatory agent. The 8
characteristic unpleasant odour and the low odour threshold (0.1 ppm; 0.43 mg/m3) allow styrene to be readily detected in the workplace at levels below the occupational exposure standards. Styrene will polymerise when contaminated by oxidising agents and most halides. The polymerisation reaction is exothermic and if contained may become violent. If the heat is not removed, the bulk styrene temperature may rise to a level at which polymerisation is self- sustaining and very rapid, evolving the release of large quantities of heat together with volumetric expansion. The most commonly used polymerization inhibitor is tertiary butyl catechol (4-tert- butylcatechol (TBC) or p-tert-butyl catechol; CAS RN 98-29-3; EC No.: 202-653-9.TBC may cause sensitization by skin contact, is toxic to aquatic organisms and may cause long-term adverse effects in the aquatic environment. Styrene can accumulate static charges; hence special attention should be paid to take precautionary measures against static discharge. Table 2 below summarizes the physical and chemical properties of styrene.
Table 2: Properties of styrene (reproduced from (CEFIC, 2007) with permission). Hydrogen, which is the by-product of this reaction is one of the most abundant elements in the world. According to (SBioInformatics, 2014) Hydrogen is lightest and simplest of all elements. It is fairly but not exceptionally reactive gas. It enters into chemical combination with most of the elements and hydrogen forms more compounds than any other element. Hydrogen gas is colourless, non-poisonous odourless and tasteless. Contrary to most other gases, the inversion temperature of hydrogen lies below ambient temperature. Liquid hydrogen is colourless as well, very mobile liquid with low viscosity and surface tension. Solid hydrogen is a colourless, and crystallizes in the hexagonal closest
9
packed structure. At higher several phase transaction occurs under extremely high pressure a metallic, electrically conductive hydrogen phase with a density of >1,000kg/m 3 occurs. Hydrogen is only slightly soluble in liquids. In contrast to the highly soluble gases however the solubility generally increases with increasing temperature. The relative solubility differences in between hydrogen compound is few percent and diminish with increasing temperature. Hydrogen is the element with highest diffusion capacity diffusion coefficient of hydrogen in gases and liquids are given. About the chemical properties, the first Electro shell can be filled with maximum of two electrons. Therefore the chemistry of hydrogen depends mainly on three processes, which are the loss of the valence electron to yield the hydrogen ion H+, then its gain of an electron to form the hydride ion H- and finally, the formation of an electron pair bond. Hydrogen is not exceptionally reactive, although hydrogen atoms with all other elements with the exception of noble gases. Hydrogen oxidizer leas electronegative elements and reduces more electronegative ones. The strength of the H – X bond in covalent hydrides depends on the electronegativity and size of the element X. The strength decreases in a group with increasing atomic number and generally increases across any period. The most stable covalent bond are those formed between two hydrogen atoms, or with hydrogen, oxygen carbon and nitrogen. On a laboratory scale, hydrogen can be made from the action of an aqueous acid on a metal or from the reaction of an alkali metal in water. Hydrogen gas can also be produced on laboratory scale by the electrolysis of an acidic solution, by steam reforming using hydrocarbons such as natural gas, petroleum, and coal. Hydrogen is all-important raw material for the chemical industry. It takes part in reaction either by addition or by means of its reduction potential. Most of hydrogen is used for these hydrogenation and reduction process. In utilization sector, hydrogen production is stagnating. A part from ammonia synthesis, synthesis with hydrogen-carbon monoxide gas mixture to produce methanol, hydrocarbons and Oxon synthesis products are of particular importance large quantities of hydrogen are needed in coal processing is just beginning. Hydrogen finds use in diverse applications covering many industries, including: Food to hydrogenate liquid oils (such as soybean, fish, cottonseed and corn), converting them to semisolid materials such as shortenings, margarine and peanut butter. Chemical processing primarily to manufacture ammonia and methanol, but also to hydrogenate non-edible oils for soaps, insulation, plastics, ointments and other specialty chemicals. Metal production and fabrication to serve as a protective atmosphere in high-temperature operations such as stainless steel manufacturing; commonly mixed with argon for welding austenitic stainless. Also used to support plasma welding and cutting operations.
10
2.7 DEHYDROGENATION OF ETHYLBENZENE INTO STYRENE The balanced chemical equation of dehydrogenation of the ethyl benzene into styrene in benzene in plug flow reactor (PFR) is
C6H5CH2CH3 C6H5CH=CH3 + H2 There are two different approaches to the reaction; the adiabatic process and the isothermal process. The isothermal process is chosen because the process takes place in the PFR. Since the reaction taking place is an endothermic reaction which requires 1.26-1.33 MJ/kg of ethyl benzene to be converted at 25oC; the heat must be supplied. In the PFR, the reaction heat is provided by indirect heat exchange between the process fluid and a suitable heat transfer medium (flue gas). The catalyst life, the molar conversion of ethyl benzene and also the molar selectivity of the reactant are affected by the operating pressure, reactor operating temperature and the reactor liquid hourly space velocity (LHSV). In isothermal process, the pressure is maintained at atmospheric pressure which is 1atm or 101.325kPa.Due to the endothermic reaction, the conversion increases as the temperature increases, the temperature is maintained at 580-610 oC across the catalyst bed in the isothermal process. The LHSV of the isothermal process is 4-6 m 3/hr ethyl benzene per cubic meter of catalyst. Using this, 60-70% molar conversion of ethyl benzene can be achieved with commercially available dehydrogenation. 2.8 REACTOR BACKGROUND The plug flow reactor model (PFR, sometimes called continuous tubular reactor, CTR, or piston flow reactors) is a model used to describe chemical reactions in continuous, flowing systems of cylindrical geometry. The PFR model is used to predict the behaviour of chemical reactors of such design, so that key reactor variables, such as the dimensions of the reactor, can be estimated. Fluid going through a PFR may be modelled as flowing through the reactor as a series of infinitely thin coherent "plugs", each with a uniform composition, traveling in the axial direction of the reactor, with each plug having a different composition from the ones before and after it. The key assumption is that as a plug flows through a PFR, the fluid is perfectly mixed in the radial direction but not in the axial direction (forwards or backwards). Each plug of differential volume is considered as a separate entity, effectively an infinitesimally small continuous stirred tank reactor, limiting to zero volume. As it flows down the tubular PFR, the residence time of the plug is a function of its position in the reactor. In the ideal PFR, the residence time distribution is therefore a Dirac delta function with a value equal to 11
2.9 OPERATION AND USES PFRs are used to model the chemical transformation of compounds as they are transported in systems resembling "pipes". The "pipe" can represent a variety of engineered or natural conduits through which liquids or gases flow. (E.g. rivers, pipelines, regions between two mountains, etc.) An ideal plug flow reactor has a fixed residence time: Any fluid (plug) that enters the reactor at time the reactor at time
, where
will exit
is the residence time of the reactor. The residence time
distribution function is therefore a dirac delta function at
. A real plug flow reactor has a residence
time distribution that is a narrow pulse around the mean residence time distribution. A typical plug flow reactor could be a tube packed with some solid material (frequently a catalyst). Typically these types of reactors are called packed bed reactors or PBR's. Sometimes the tube will be a tube in a shell and tube heat exchanger. Applications Plug flow reactors are used for some of the following applications:
Large-scale reactions Fast reactions Homogeneous or heterogeneous reactions Continuous production High-temperature reactions
Advantages and disadvantages CSTRs (Continuous Stirred Tank Reactor) and PFRs have fundamentally different equations, so the kinetics of the reaction being undertaken will to some extent determine which system should be used. However there are a few general comments that can be made with regards to PFRs compared to other reactor types.Plug flow reactors have a high volumetric unit conversion, run for long periods of time without maintenance, and the heat transfer rate can be optimized by using more, thinner tubes or fewer, thicker tubes in parallel. Disadvantages of plug flow reactors are that temperatures are hard to control and can result in undesirable temperature gradients. PFR maintenance is also more expensive than CSTR maintenance. Through a recycle loop a PFR is able to approximate a CSTR in operation. This occurs due to a decrease in the concentration change due to the smaller fraction of the flow determined by the feed; in the limiting case of total recycling, infinite recycle ratio, the PFR perfectly mimics a CSTR.
12
PFR Modelling The PFR model works well for many fluids: liquids, gases, and slurries. Although turbulent flow and axial diffusion cause a degree of mixing in the axial direction in real reactors, the PFR model is appropriate when these effects are sufficiently small that they can be ignored.In the simplest case of a PFR model, several key assumptions must be made in order to simplify the problem. Note that not all of these assumptions are necessary, however the removal of these assumptions does increase the complexity of the problem. The PFR model can be used to model multiple reactions as well as reactions involving changing temperatures, pressures and densities of the flow. Although these complications are ignored in what follows, they are often relevant to industrial processes. Plug Flow Reactor (PFR) Model: In a PFR, one or more fluid reagents are pumped through a pipe or tube. It is also referred to as Tubular Flow Reactors (TFRs).
PTRs may have several pipes or tubes in parallel,. The reactants are charged continuously at one end and products are removed at the other end.
The chemical reaction proceeds as the reagents travel through the PFR.
In this type of reactor, the reaction rate is gradient, that is, at the inlet to the PFR the rate is very high, but as the concentrations of the reagents decrease and the concentration of the product(s) increases the reaction rate slows. Normally a steady state is attained.
Both horizontal and vertical operations are common.
When heat transfer is needed, individual tubes are jacketed or shell and tube construction is used. In the latter case, the reactants may be on either the shell or the tube side.
The reactant side may be filled with solid particles, either catalytic (if required) or inert, to improve interphase contact in heterogeneous reactions.
Large diameter vessels with packing or trays may approach plug flow behaviour and are widely employed.
Some of the configurations in use are axial flow, radial flow, multiple shell with built in heat exchangers, horizontal, vertical and so on. Some important aspects of the PFR are :
All calculations performed with PFRs assume no upstream or downstream mixing, as implied by the term “plug flow”.
Reagents may be introduced into the PFR at locations in the reactor other than the inlet. In this way a higher efficiency may be obtained, or the size and cost of the PFR may be reduced.
13
A PFR typically has a higher efficiency than a CSTR of the same volume. That is, given the same space-time, a reaction will proceed to a higher percentage completion in a PFR than in a CSTR.
For most chemical reactions, it is impossible for the reaction to proceed to 100% completion. The rate of reaction decreases as the percent completion increases until the point where the system reaches dynamic equilibrium (no net reaction, or change in chemical species occur). The equilibrium point for most systems is less than 100% complete. For this reason a separation process such as distillation often follows a chemical reactor in order to separate any remaining reagents or by products from the desired product. These reagents may sometimes be reused at the beginning of the process, such as in the Haber process. The PFR model is used to estimate the key unit operation variables when using a continuous tubular reactor to reach a specified output. The mathematical model works for all fluids : 20liquids, gases and slurries. In a PFR the fluid passes through a coherent manner, so that the residence time ‘τ’, is the same for all fluid elements. The coherent fluid passing through the ideal reactor is known as a plug. As a plug flows through a PFR, the fluid is perfectly mixed in the radial direction but not in the axial direction (forwards or backwards). Each plug of differential volume is considered as a separate entity (practically a batch reactor) as it flows down the tubular PFR. Application of PFRs are used to model the chemical transformation of compounds as they are transported in systems resembling pipes. There are many scenarios that must be considered when deciding on which type of reactor to use for a certain process. A plug flow reactor is one of many types of reactors. It is most useful when the reaction is not allowed to reach equilibrium, and the reaction is kinetically limited by the reaction rate. There are exceptions to the fact that a PFR is always better than a CSTR when the reaction is kinetically limited. However, most of the time a PFR does have a higher rate of product production than a CSTR if the reaction is kinetically limited. It is still necessary to analyze the details of the process before deciding that a plug flow reactor is the correct choice for the process. Plug flow reactors have a high volumetric unit conversion run for long periods of time without labor, and have excellent heat transfer. The limitations encountered with plug flow reactors are that temperatures are difficult to control and can result in undesirable temperature gradients. It is more expensive.
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3. 0 MASS BALANCE ANALYSIS Mass Balance Given the production rate of 400,000 lbm / year, the calculations were done to find out how much ethylbenzene is needed to produce this amount of styrene. Firstly, the production rate has to be converted into kilogram per hour by following steps: Conversion to kg/year: 400000 lbm / year x 1 kg / 2.20462 lbm = 181,437.2 kg/year Assuming production carried out 24 hour a day, 6 days a week, 50 week a year: = 24 x 6 x 50 = 7200 hours per year. Therefore, production per hour = 181,437.2/7200 = 25.2 kg/hour. Below is the balanced stoichiometric equation for the reaction:
C6H5CH2CH3 C6H5CH=CH2 + H2 In order to carry out the mass balance on the production plant, few assumptions have to be made which are as following (Styrene Material Balance, nd): a) The conversion in the isothermal reactor is 60 – 70 %, hence for the calculation, it is assumed as 65%. b). Feed of ethylbenzene given to the system is of 100% purity. The calculations for the mass balance around the reactor were carried out and can be found in the Appendix 1 of the report. The inlet and outlet values were summarized in table 1 below:
Stream flowrate (kg/hr) Component Inlet Outlet Ethylbenzene, A 39.5 13.8 Styrene, B 0 25.2 Hydrogen, C 0 0.5 Table 1: The mass balance around the reactor for production of styrene.
15
The values obtained then were converted back into their mass flowrates respectively: FAO = 0.3728 kmol/hr x 106 kg/kmol = 39.5 kg/hr FB = 25.2 kg/hr Fc = 0.2423 kmol/hr x 2 kg/kmol = 0.5 kg/hr FA = 0.1305 kmol/hr x 106 kg/kmol = 13.8 kg/hr In order to verify the calculation, both inlet and outlet values were compared: FAO = FB + FC + FA 39.5 = 25.2 + 0.5 + 13.8 39.5 = 39.5
Hence, the values are tally for both streams and hence the calculation is correct. 4.0 STOICHIOMETRIC TABLE Material Ethyl benzene Styrene
Symbol A B
Hydrogen
C
Initial FAo 0
Change -FAoX +
b a
FAoX
+
c a
FAoX
0
Remaining FA = FAo (1-X) FB =
b a
FAoX
FC =
c a
FAoX
FTo = FAo FT = FAo (1-X) +
c a
F A 0=28.32
b a
FAoX +
FAoX
kg 1 kmol 1000 mol x x hr 106.17 kmol 1 kmol
¿ 266.742
mol hr
16
C Ao =
¿
yA0P R To
(1)101.325 kPa 8.314 ×873.15 K
¿ 0.01396
υ o=
¿
mol L
F Ao C Ao
266.742 0.01396
¿ 19107.593
Material Ethyl benzene
L hr
Symbol A
Initial FAo = 266.742
Change -FAoX
Remaining FA = FAo (1-X) = 266.742 (1-0.65) = 93.3597
Styrene
B
0 +
b a
FAoX
FB =
b a
FAoX
= 266.742(0.65) = 173.3823 Hydrogen
C
0 +
c a
FAoX
FC =
c a
FAoX
= 266.742(0.65) = 173.3823 FTo = FAo =266.742mol/hr
FT = FAo (1-X) +
b a
FAoX +
= 0.2667kmol/hr
c a
FAoX
= 93.3597 + 173.3823 + 17
173.3823 = 440.1243 mol/hr = 0.4401kmol/hr
5.0 REACTION KINETICS The general chemical equation for dehydrogenation of ethylbenzene into styrene is given as follows:
C6H5CH2CH3 C6H5CH=CH2 + H2 Ethylbenzene is converted into styrene in reversible gas phase reaction (Rapra Review Reports, 2000). The production of styrene from ethylbenzene is a reversible law as mentioned in (Reaction Kinetics and the Development of Catalytic Processes, 1999). The rate law of the reaction equation is expressed as follows:
-rethylbenzene = k [
C Ethylbenzene -
C Styrene C Hydrogen Kc
]
Where Kc is the equilibrium constant for the reaction. As the reaction obeys the reversible rate law, the order of the reaction also follows the reversible rate law which is first order reversible reaction. Since the value for Kc was nowhere to be found in the literature, hence for the calculation of the rate of disappearance of ethylbenzene, the Kc value was assumed as 1 mol/L. In order to calculate the complete rate law for the reaction, -r A the value of specific reaction constant, k need to be known. As there were also no definite k value found from the literature, hence it must be calculated first using known pre-exponential factor (A) which is 8.32 x 10 3 hr-1 with activation energy (Ea) which is 0.909 x 10 -5 J/ mol (Seyed Mahdi Mousavi, 2012) and the temperature of the reaction which is 610 ˚C (Production of Styrene, 2003) k = A exp (-Ea/RT) k = (8320 hr-1) exp (-0.909 x 10-5 J mol-1 / (8.314 J mol-1 K-1) (610 + 273 K) k = 8318.9 hr-1 18
Therefore, the rate of reaction of dehydrogenation of ethylbenzene can be deduced as follows, using the concentrations of the reactant and products from Section 3 of this report:
-rethylbenzene = k [
C Ethylbenzene -
C Styrene C Hydrogen Kc
-rethylbenzene = (8318.9 hr ) [ . 002927 -1
]
(0.005436)2 1
] mol L-1
-rethylbenzene = (8318.9 hr-1) (0.0028975 mol L-1) -rethylbenzene = 24.1 mol L-1 hr-1 6.0 REACTOR DESIGN AND SIZING Volume of the PFR
C6H5CH2CH3 C6H5CH=CH2 + H2 Based on the previous calculations in section 3 and section 4 (Reaction Kinetics): CAO =
0.01396 mol/L
FAO = 0.3728 kmol/hr = 372.8 mol/hr X= 0.65 PO=P = 101.325 kPa TO=T=610oC = 883.15K K= 8318.9 hr-1 Assuming that the reaction is an irreversible reaction to calculate the volume. Therefore: -rA = kCA Using the formula to calculate the volume of the reactor x
V PFR =F AO∫ 0
dx −r A
19
CA=
F A F AO (1−x ) C AO (1−x ) = = ν ν O (1+ εx) 1+ εx
ε = y AO δ c b δ = + −a a a δ=1+1−1
δ=1 ε =1 x
V PFR =F AO∫ 0
dx kC A
F Ao x dx ¿ ∫ k 0 CA
¿
F Ao x ∫ k 0
dx ( 1−x) C Ao 1+εx
F Ao x 1+εx ¿ dx ∫ kC Ao 0 1−x
¿
F Ao 1 ( 1+ ε ) ln −εx kC Ao 1−x
¿
372.8 1 ( 1+1 ) ln −( 1 ) (0.65) 8318.9 × 0.01396 1−0.65
[
[
] ]
¿ 4.6536 L
20
PFR Schematic Diagram
7.0 CONCLUSION In order to produce 400 000 lbm/year of styrene using plug flow reactor (PFR), we use the method of dehydrogenation of ethyl benzene. The chemical equation for the reaction is
C6H5CH2CH3 C6H5CH=CH3 + H2 The reaction was set to a temperature of 610 oC (883.15 K), since the reaction is isothermal, the temperature outlet remained the same as the inlet temperature. The dehydrogenation reaction was conducted isobarically at atmospheric pressure, 1atm. There are few assumptions made in order to calculate the mass balance of the reaction. The conversion selected is 65%, the mole fraction of ethyl benzene is equal to 1 and the reactor operates for 50 weeks a year, 6 days a week and 24 hours a day, making the production mass flow rate is 25.2kg/hr. The reaction is a reversible reaction, however to simplify the calculation to calculate the volume of the reactor, the reaction was assumed to be irreversible, making the reaction rate of the 21
reaction as –rA =kCA; without considering the value of kC calculated in section 4 of reaction kinetics. The volume of the reactor calculated is 4.6536 L .
8.0 REFERENCES Acute Exposure Guidelines Level for Ethylbenzene. (2009) EPA USA [Online]. [Accessed 1 October 2014]. Available at World Wide Web: http://www.epa.gov/oppt/aegl/pubs/ethylbenzene_interim_sep_09.v1.pdf. Anon., Rapra Review Reports, Rapra Technology Limited, Shrewsbury, United Kingdom, 2000, p.9. Anon., Ullmann’s Encyclopedia of Chemical Technology, Wiley Interscience, 2003, Production of Styrene, p.386. Behr, P. D. (n.d.). STYRENE. Faculty of Biochemical and Chemical engineering. G. Froment, & K. Waugh., Reaction Kinetics and the Development of Catalytic Processes, Elsevier, Amsterdam, 1999, p.230. Hydrogen Properties and Uses. (nd) SBioInformatics. [Online]. [Accessed 1 October 2014]. Available at World Wide Web: http://www.sbioinformatics.com/design_thesis/Hydrogen/Hydrogen_Properties&us es..pdf
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Mousavi, S.M et al, Modelling and Simulation of Styrene Monomer Reactor: Mathematical and Artificial Neural Network Model, 2012, International Journal of Scientific & Engineering Research, p.2. Olson, D. H. ( 1966, Aug 1). Process for the dehydrogenation of ethyl benzene. Grant, p. 52. Styrene Material Balance.(nd). SBioInformatics. [Online]. [Accessed 29 October 2014]. Available from World Wide Web: http://www.sbioinformatics.com/design_thesis/Styrene/Styrene_Material2520Balance.pdf Styrene Monomer: Environmental, Health, Safety,Transport and Storage Guidelines. (2007) CEFIC, S. P. A. [Online]. [Accessed 1 October 2014]. Available at World Wide Web: http://www.cefic.org/Documents/IndustrySupport/Transport-and-Logistics/Best %20Practice%20Guidelines%20-%20Product%20Specific%20Guidelines/StyreneMonomer-Environmental-health-Safety-and-Distribution-Guidelines.pdf Styrene Production Methods.(nd) SBioInformatics [Online]. [Accessed 1 October 2014]. Available at World Wide Web: http://www.sbioinformatics.com/design_thesis/Styrene/Styrene_Methods-2520of2520Production.pdf Toxicity Profiles of Ethylbenzene.(nd). ATSDR [Online]. [Accessed 1 October 2014]. Available at World Wide Web: http://www.atsdr.cdc.gov/toxprofiles/tp110-c4.pdf WAHAMID, A. A. (2010). PLUG FLOW REACTOR. DEVELOPMENT OF NONLINEAR MODEL FOR PLUG FLOW REACTOR, 1.
9.0 APPENDICES Mass Balance Calculations The balanced equation of the reaction is given by: C6H5CH2CH3 C6H5CH=CH2 + H2 Where each compound are denoted as follows: a) Ethylbenzene = A b) Styrene = B
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c) Hydrogen = C Production per hour of styrene is given by, FB = 25.2 kg/hr. In order to keep the accuracy of the values during calculation, the flowrates were converted from mass flowrate to the molar flowrate. Molecular weight of styrene is 104 kg/kmol. Converting flowrate of styrene in the outlet stream (FB), 25.2 kg/hr x (1/104) kmol/kg = 0.2423 kmol/hr Roughly, the process that went through the reactor is illustrated in figure 1 below:
X = 0.65 FAO
F B = 0.2423 kmol / hr Fc FA Figure 1: The mass balance diagram on the reactor for the production of styrene.
The value of FAO, FC, and FA can be deduced by using atomic mass balance. Balance on Carbon, C. Amount in the inlet stream = Amount in the outlet stream + Amount in unreacted ethylbenzene 8 FAO = 8 FB + 8 FA 8 FAO = 8 (0.2423) + 8 FA 8 FAO = 1.9384 + 8 FA
(Equation 1)
Balance on Hydrogen, H. Amount in the inlet stream = Amount in the outlet stream + Amount in unreacted ethylbenzene 10 FAO = 8 FB + 2 Fc + 10 FA 10 FAO = 8 (0.2423) + 2 FC + 10 FA 10 FAO = 1.9384 + 2 Fc + 10 FA (equation 2)
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Equation of the conversion, X 0.65 = (FAO - FA)/ FA FAO - FA = 0.65 FAO FA = 0.35 FAO (equation 3)
Replacing equation 3 into equation 1, 8 FAO = 1.9384 + 8 (0.35 FAO) 8 FAO = 1.9384 + 2.8 FAO 5.2 FAO = 1.9384 FAO = 0.3728 kmol/hr
In order to find FA, FA = 0.35 FAO FA = 0.35 (0.3728) FA = 0.1305 kmol/hr Putting these values into equation 2, 10 (0.3728) = 1.9384 + 2 FC + 10 (0.1305) 3.728 = 3.2434 + 2 Fc Fc = 0.2423 kmol/hr
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