PRODUCTION OF STYRENE BY ADIABATIC DEHYDROGENATION: A TWO-STAGE REACTOR WITH STEAM REHEAT. Aspen Model Documentation

Transcription

1 PRODUCTION OF STYRENE BY ADIABATIC DEYDROGENATION: A TWO-STAGE REACTOR WIT STEAM REEAT Aspen Model Documentation Index Process Summary About This Process Process Definition Process Conditions Physical Property Models and Data Chemistry/Kinetics Key Parameters Selected Simulation Results: Blocks Streams References PEP Process Module 1 //99

2 Process Summary This Aspen Plus simulation models the production of styrene from EB by adiabatic dehydrogenation: a process using a two-stage reactor with steam reheat. It is intended to resemble the CLASSIC technology of Lummus Crest/Monsanto/UOP. In the Aspen model, the plant (base case) is designed to produce 1 billion lb/yr (44,000 t/yr) of styrene. The process consists of three sections: dehydrogenation, vent gas and condensate treatment, and styrene recovery. The major differences between this process and the design that used two-reactor process with interstage reheat (resembling the Fina/Badger process) are in the dehydrogenation section. In the styrene recovery section, low-grade energy can be recovered by azeotropical vaporization of an EB/water mixture in the condenser of the EB/styrene splitter. Results from the Aspen simulation showed that the purity of styrene obtained is 99.9%. Other valuable by-products include benzene (purity 9.9%) and toluene (purity 94.8%) at a rate of approximately 17 and 717 lb/hr, respectively. PEP Process Module //99

3 About This Process Styrene is produced from EB primarily by adiabatic dehydrogenation. Styrene is also produced by oxidation and dehydration as a coproduct with propylene oxide (PO) from EB via -methyl benzyl alcohol. Other oxidative dehydrogenation processes that react EB with an oxygen-containing compound can also produce styrene. In addition, styrene can be prepared by the following processes: stilbene disproportionation with ethylene; toluene oxidative coupling with methane; toluene alkylation with acetone, formaldehyde, and methanol; benzene alkylation with ethylene oxide; aromatization of ethylene or n-octane; and thermolytic formation or selective adsorption of C 8 aromatics. Table 1 provides a summary of the commercial processes for styrene and their licensor. TABLE 1. COMMERCIAL PROCESSES FOR PRODUCING STYRENE Process Type / Technology EB Dehydrogenation Adiabatic, steam reheat Adiabatic, steam reheat Adiabatic, steam reheat (CLASSIC) Adiabatic, oxidative reheat (Smart ) Adiabatic, oxidative rehear (Styro-Plus ) Isothermal, molten salt bath heating Isothermal, hot flue gas heating Coproduction with propylene oxide EB oxidation, propylene epoxidation with EB hydroperoxide and α-methyl benzyl alcohol dehydration EB oxidation, propylene epoxidation with EB hydroperoxide and α-methyl benzyl alcohol dehydration Licensor / Developer Dow Fina/Badger Lummus Crest/Monsanto/UOP Lummus Crest/Monsanto/UOP UOP Montedison/DWE/Lurgi BASF Arco Shell This Aspen Plus simulation models the production of styrene from EB by adiabatic dehydrogenation: a process using a two-stage reactor with steam reheat. It is intended to resemble the CLASSIC technology of Lummus Crest/Monsanto/UOP. In the EB dehydrogenation process, the important variables are the reaction temperature, the reaction pressure, and the steam/eb ratio. Because the process is endothermic, the conversion of EB increases as the reaction temperature increases. For a given temperature and a fixed EB/steam ratio, the conversion of EB also increases as the reaction pressure decreases. Under constant temperature and pressure, a higher EB/steam ratio results in greater EB conversion because the dilute steam reduces the partial pressure of the reacting components and gives an effect similar to that achieved by reducing the reaction pressure. owever, steam quantities are limited by the pressure drop allowed and by the energy cost increase that would be required to improve performance. In addition to steam, nitrogen can also be used as diluent (170, 1700, 1714). PEP Process Module //99

4 To improve dehydrogenation, catalysts and increased heat economy are being emphasized. Low-temperature heat from the reactor effluent (170) or the overhead of the EB column (1700) is recovered azeotropically by vaporizing the EB/water mixture; no compression equipment is required. Steam consumption can be reduced by periodically changing the steam/hydrocarbon ratio (17199). Unlike traditional steam-reheated processes in the Styrene Monomer Advanced Reheat Technology (SMART) process of Lummus Crest/Monsanto/UOP and UOP s Styro-Plus, reactants leaving the dehydrogenattion catalyst bed are reheated by selectively oxidizing part of the hydrogen coproduced (1708, 17, 17). The reaction is fed by air or by oxygen rich air and takes place over a proprietary UOP high selectivity catalyst (0184, 004). In a semi-commercial operation, selectivity is 9-9%, with conversion in the range of 77-9% (170). Advantages of the Styro-Plus process include high conversion, high styrene selectivity, reduced steam requirements, and less high-temperature heat transfer equipment. owever, some hydrocarbon coproduct has to be consumed, and an appreciable amount of phenylacetylene may form at high conversion levels. Other novel reaction systems have also been proposed. A transient reactor system is reported to provide better styrene yield than does a conventional steady state reactor system (179). The transient system consists of reactors, with one of them on stream while a favorable temperature profile is being regenerated in the other. Quick Contact (QC), a process developed by Stone & Webster, is a fluid-solid downflow adiabatic reaction system with a short residence time; QC is capable of plug flow operation in the millisecond contact time range (17). Although this technology was developed for ethylene production by pyrolysis in the 1970s, it is not process-specific and may be appropriate for EB dehydrogenation with a suitable abrasion-resistant catalyst. PEP Process Module 4 //99

5 Process Definition The Aspen Plus model is developed to simulate the production of styrene from EB by adiabatic dehydrogenation: a process using a two-stage reactor with steam reheat, resembling the CLASSIC technology of Lummus Crest/Monsanto/UOP. Figure 1 in the Aspen Plus model file shows the process flow diagram, which consists of three sections: dehydrogenation, vent gas and condensate treatment, and styrene recovery. In the Aspen model, 1,1,-triphenylethane is used to represent the heavies. Compound m-xylene is used to represent the xylenes, and 1,4-diethylbenzene is used to represent diethylbenzene. Aspen Plus Radfrac models are used to represent the distillation columns. Due to insufficient kinetics information, Aspen Plus RSTOIC reactor models is used to represent R-101. The reactor is considered to have valid phases; vapor and liquid phases. Fresh EB and recycled EB are combined with water to form a feed stream with a water/eb weight ratio of about This stream is a minimum-temperature azeotrope at 87 o C (189 o F) and 14 psia. By recovering lowtemperature heat, the feed stream is vaporized azeotropically in condensers E-01A&B of the EB/styrene splitter. The vaporized feed stream is then preheated to 8 o C (1000 o F) in feed/effluent exchangers E-10A through D and E-101A through D by exchanging heat with the reactor effluent. EB is dehydrogenated in two-stage reactor R-101, where one stage of the reaction takes place above the other. A static mixer heat exchanger is positioned on the reactor top, and a perforated inner core pipe runs through the center of both stages to supply steam. The feed stream from E-10A through D enters the bottom of R-101 and is heated to 9 o C by mixing with superheated low-pressure ( psig) steam from the inner core pipe. The mixture then passes radially through the catalyst bed of the first stage, and about % of the EB is converted. Because EB dehydrogenation is endothermic, a temperature drop occurs across the catalyst bed. From the annular space around the catalyst bed of the first stage, the effluent passes through apertures to the outer annular space of the upper second stage, rises to the reactor top, and passes through the tube side of a static mixer heat exchanger. In the exchanger, the stream is reheated by the superheated low-pressure steam from F-101 that passes through the shell. The reheated stream then becomes the feed to the second stage of R-101 and flows downward to the inner annular space of the catalyst bed of the second stage. Part of the steam from the exchanger diffuses through the inner core pipe to mix with the feed to the second stage; the remaining steam flows down to the first stage. The total steam/eb weight ratio is 1.8 (or a molar ratio of about 10.). Another % of the EB is converted as the reaction mixture passes radially through the catalyst bed of the second stage. The effluent flows downward through apertures to the outer annular space of the first stage and leaves the reactor at 77 o C (1071 o F). PEP Process Module //99

6 The heat content in the reactor effluent is recovered sequentially to preheat the feed in feed/effluent exchangers E-101A through D, to generate medium-pressure (10 psig) steam in E-10A&B, and to preheat the feed in E-10A through D. The reactor effluent is further cooled and partially condensed in effluent air cooler E- 104 and condenser E-10; the effluent is subsequently separated into vapor, aqueous, and organic streams in V The vapor and the aqueous streams are sent to the vent gas and condensate treatment section, and the organic stream is routed to the styrene recovery section. The vent gas and condensate treatment section is similar to that for the oxidative reheat process; the vapor stream is cooled and flashed for aromatics recovery in two stages with vent gas compressor K-01 in between the stages. The noncondensable gases, mainly hydrogen, are used as fuel. The aqueous stream is combined with the water layer of the distillate from the EB/styrene splitter column in the styrene recovery section. The combined stream is then fed to condensate stripper C-01 and condensate treaters C- 0A&B, which contain diatomaceous earth to remove organic impurities. The treated condensate is used as boiler feedwater. In the styrene recovery section, a non-sulfur process polymerization inhibitor is added to the organic stream from V-101 then is fed to EB/styrene splitter column C-01. C-01 contains structured packing and operates under vacuum with a mixed benzene/toluene/ethylbenzene cut recovered overhead. The styrene recovery section differs only in the additional energy recovery scheme for azeotropical vaporization of EB/water feed. The equipment in the styrene recovery section is also larger because of conversion and selectivity that are lower than in the oxidative reheat process. Benzene recovered from the overhead of C-0 is credited as a by-product or, in the case of integrated EB and styrene production, recycled to the EB plant. Toluene is taken from the bottom of C- 0 and is credited as a by-product. The bottoms from C-0 are recycled to the reaction section. Styrene with 99.9% purity is recovered in the overhead of C-04. The bottom contains heavies, styrene, and the inhibitor. The product styrene is then inhibited with p-tert-butyl catechol (TBC), a product polymerization inhibitor, and cooled to 1 0 C (0 0 F) by brine before being sent to rundown tanks and product storage. Residue from the styrene column is used as fuel. Mediumpressure steam (10 psig) is used in the reboiler of the benzene column, whereas low-pressure steam ( psig) is used in other reboilers. PEP Process Module //99

7 Process Conditions Table provides the list of important blocks, design bases and assumptions for the process. TABLE. STYRENE FROM ETYLBENZENE BY ADIABATIC DEYDROGENATION: A TWO-STAGE REACTOR WIT STEAM REEAT, DESIGN BASES AND ASSUMPTIONS Capacity 1 billion lb/yr (44,000 t/yr), Styrene at 0.90 stream factor Process resemblance CLASSIC Lummus Crest/Monsanto/UOP References 09, 1700, 170 Reactor A two-stage reactor with steam reheat Catalysts Catalyst cycle life (years) Reactor temperature ( o C) 0-40 Reactor pressure (psia) 8-14 WSV (lb of EB/lb of catalyst/hr) 0.4 Steam/EB (wt ratio) 1.8 Conversion (%), 70.0 Selectivity (mol%) To styrene 9. To benzene.1 To toluene. To others 0.1 Plant yields (mol%) To styrene 9. To benzene 0. To toluene. To others.0 Net consumption/production (lb/lb of styrene) EB 1.07 Benzene Toluene Vent gas 0.01 Residue Columns Benzene/toluene column Design Spec G84C a or C0 b Valve trays Mass recovery of toluene in the distillate is 97.9% Benzene column Design Specs Valve trays Mass recovery of benzene in the distillate is 99% Mass recovery of toluene in the bottoms is 99% EB/styrene splitter Structured packing Styrene column Design Spec Sieve trays Mass recovery of styrene in the distillate is 99.87% Components Carbon monoxide Carbon dioxide ydrogen Methane Ethylene Benzene / Constituent of raw material Toluene Styrene Product 1,4 lb/hr Xylenes EB Raw material 1,147 lb/hr D-EB Constituent of raw material eavies Water Raw material Polymerization inhibitor Inhibitor p-tert-butyl catechol Inhibitor a By United Catalysts Inc. b by Criterion Catalyst Co. PEP Process Module 7 //99

8 Physical Property Methods and Data The physical property method used in the Aspen Plus simulation is UNIQUAC. The UNIQUAC model calculates liquid activity coefficients for these property methods: UNIQUAC, UNIQ-, UNIQ-OC, UNIQ-NT, and UNIQ-RK. It is recommended for highly non-ideal chemical systems, and can be used for VLE and LLE applications. This model can also be used in the advanced equations of state mixing rules, such as Wong-Sandler and MV. The UNIQUAC model can handle any combination of polar and non-polar compounds, up to very strong nonideality. Parameters should be fitted in the temperature, pressure, and composition range of operation. No component should be close to its critical temperature The UNIQUAC model can describe strongly nonideal liquid solutions and liquid-liquid equilibria. The model requires binary parameters. Many binary parameters for VLE and LLE, from literature and from regression of experimental data, are included in the ASPEN PLUS databanks. The property method has a vapor phase model that can be used up to moderate pressures, have the Poynting correction included in the liquid fugacity coefficient calculation. eats of mixing are calculated using the UNIQUAC model. PEP Process Module 8 //99

9 Chemistry/Kinetics Reactors This Aspen simulation models the production of styrene from EB by adiabatic dehydrogenation: a process using a two-stage reactor with steam reheat, which resembled the CLASSIC technology of Lummus Crest/Monsanto/UOP. The main reaction in the adiabatic EB dehydrogenation process is: C = + In the presence of steam, the equilibrium constant can be expressed as: K Px P = (1 x)(1 + x + S/EB) Where P is the total pressure, x the equilibrium conversion, and S/EB the steam EB molar ratio. The side reactions are thermal cracking, hydrocracking, and steam cracking: C 4 + O CO + 4 And the water-gas shift reaction is: CO + O CO + At high steam concentrations, the following reactions may occur: follows: C C C C 4 C + O + O CO + C O C C C + CO C CO + C + O C CO + The reactions (with their conversions) used to model RSTOIC reactor R-101 in the Aspen Plus model are as R -101: (1) EB Styrene + () EB Benzene + Ethylene () EB + O CO + Toluene + (4) EB + O Benzene () + O CO + 4 () CO + 4 Ethylene + O (7) + CO + CO O CO (8) Benzene + EB eavies + Ethylene + + (Fractional conversion (Fractional conversion (Fractional conversion (Fractional conversion (Molar extent 4.01lbmol/hr) (Fractional conversion =.8%EB) = 0.109%EB) = 1.09% EB) = 1.7% EB) (Fractional conversion = 0.019% O) (Molar extent 1.07lbmol/hr) = 0.0% EB) PEP Process Module 9 //99

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11 References -- Wang, S.-., Styrene, Supplement C, Process Economics Program Report No. C (March 199) 170 Zietz, A., et al. (to Standard Oil, Illinois), Doped Aluminum Borate, US 4,4,7 (Feb 4, 1987) 1700 Satek, L. C. (to Amoco), Dehydrogenation of Alkylaromatics, US 4,90,4 (May 0, 198) 1714 Ueno, A., et al. (to Nissan Girdler Catalyst), Manufacture of Nonferrous Metal Oxide Catalysts for EB Dehydrogenation, Japan Kokai -784 (March 19, 1990) 170 Sundaram, K.M., et.al., Styrene Plant Simulation and Optimization, ydrocarbon Processing, 70, 1 (January 1991), Sardina,., et al., New Styrene Plant Operation Optimizer, Paper No. B presented at the 1988 AlChE Spring National Meeting, New Orleans, Louisiana, March -10, Sardina,. (to Lummus Crest), Dehydrogenation Process for Production of Styrene from EB Comprising Low Temperature eat Recovery and Modification of the EB-Steam Feed Therewith, US 4,8,1 (Dec 9, 198) 170 Whittle, L. F. (to Badger), Method of Recovering eat from Low Temperature Effluent, US 4,9,4 (Sept, 1987) Choi, J., et al., Saving of Steam by Periodic Catalyst Reactivation in Styrene Synthesis, Chem.-Ing.- Tech., 1, 8, Imai, T., et al., The Principle of Styro-Plus, Paper No. 4A presented at the 1988 AlChE Spring National Meeting, New Orleans, Lousiana, March -10, Romatier, J., et al., The Smart SM Process. The Lowest Investment Route to a Styrene Unit Revamp, paper presented at 199 DeWitt Petrochemical Review, ouston, Texas, March -7, Egawa, K., et al., New Sytrene Monomer Process, Aromatics, 4, & (1991), Imai, T. (to UOP), Dehydrogenation of Dehydrogenatable ydrocarbons, US 4,4,07 (March, 1984) 004 erber, R., et al. (to UOP), Dehydrogenation of Dehydrogenatable ydrocarbons, US 4,87,0 (May, 1989) 170 Ward, D.J., et al., ow New Styrene Unit is Working, ydrocarbon Processing,, (March 1987), eggs, P.J., et al., Experimental Investigation of a Transient Catalytic Reactor for the Dehydrogenation of EB to Styrene, Ind.Eng.Chem.Res., 7, 1 (1988), QC Reaction System, Licensing Information (nonconfidential disclosure), Stone & Webster (1990) Report by: Noni Lim June, 1999 PEP Process Module //99