Design Guidelines for Distillation Columns in Ethyl-benzene and Styrene Monomer Service

Transcription

1 Design Guidelines for Distillation Columns in Ethyl-benzene and Styrene Monomer Service Peter W. Faessler Karl Kolmetz Wai Kiong Ng Krishnamoorthy Senthil Tau Yee Lim Sulzer Chemtech Pte. Ltd. Singapore Andrew W. Sloley Veco USA, Inc Bellingham, Washington Timothy M. Zygula Nova Chemicals Corp. Pasadena, Texas Prepared for, DISTILLATION Spring AIChE Meeting Atlanta, Georgia April 10-14, 2005 Abstract Styrene monomer is the fourth largest chemical produced on an industrial scale and most ethylbenzene is utilized in styrene monomer production. The largest chemical produced on an industrial scale is ammonia for fertilizer production, followed by crude oil refining, and then ethylene by furnace pyrolysis. Styrene monomer has been manufactured commercially for more than fifty years with advances in the key unit operation areas of reactor design and distillation. The original distillation internals were trays, which have faced many challenges in services that are prone to polymerization. The higher pressure drop and longer residence time of trays are disadvantages in this service. First generation structured packing was introduced in 1980 s. This advance greatly improved styrene distillation operations. The limit of first generation structured packing was capacity, where the stage efficiency began to decrease. The rapid progress in computational fluid dynamic modeling (CFD) over the last ten years has also led to further improvements in the optimization of column internal design. CFD modeling of the structured packing, vapor-liquid distributors, and feed inlets can improve the performance of a distillation column. Contemporary styrene unit designs include second generation structured packing and optimized liquid-vapor distributors. In the current designs, consideration of packing bed height and how the packing bed height affect stage efficiency due to mal-distribution should be reviewed. Designs are now limited by the maximum allowable pressure drop in order to avoid bottoms temperatures above 120 C (248 F) to 122 C (252 F). The authors will review the progression of Styrene Monomer Distillation advances and where the future research might be exploring.

2 Introduction The ethyl benzene and styrene monomer industry is a mature industry with basically two technology licensers; 1) ABB Lummus and 2) Shaw Stone & Webster Badger. The technology has the typical two unit operations, reaction followed by separation. Each operation can be optimized and there is on going research in each area to improve the conversion of the reactors and the efficiency of the separation. The advances in reactor technology in ethyl benzene conversion include converting from gas phase to liquid phase reactors utilizing zeolite catalyst with a greater the 95% yield. The advances in reactor technology in the styrene monomer conversion include continuous addition of active catalytic components and advances in the endothermic reheat technology, which utilizes deep vacuum, adiabatic dehydrogenation. The original distillation internals were trays, which have faced many challenges in services that are prone to polymerization. The higher pressure drop and longer residence time of trays are disadvantages in this service. First generation structured packing was introduced in 1980 s. This advance greatly improved styrene distillation operations. The limit of first generation structured packing was capacity, where the stage efficiency began to decrease. Ethyl Benzene Overview Reactor Overview In the early 1970's, aluminum chloride technology was the preferred means for the production of ethylbenzene (EB). However, environmental concerns and the high maintenance costs associated with this technology created a market for alternative processes. In response, technology licensers developed new processes. The process consists of two major subsystems: 1. An alkylation section, where ethylene is reacted with benzene 2. A distillation section, where un-reacted benzene, PEBs (Poly Ethyl Benzene), and heavier compounds are separated from the alkylation and secondary reactor effluents to produce EB of high purity. In the alkylation section, ethylene is reacted with benzene in a liquid-filled reactor that is equipped with multiple fixed-catalyst beds. A portion of the total ethylene feed is injected upstream of each bed. The benzene feed is heated to reaction temperature prior to being introduced to the first reactor bed. The heat of reaction is used to generate steam. (3) Distillation Overview The distillation section consists of three or four columns, each of which can generate steam in its overhead system. The benzene column separates untreated benzene from the reactor effluent. Distillate from the benzene column is recycled to the reactor system, and its bottoms feed the second distillation column, where the EB product is recovered as liquid distillate. The bottoms from the second column feed the PEB column, which recovers recyclable PEBs for conversion to EB in the transalkylation reactor. Bottoms from the PEB column, which consist primarily of diphenyl compounds, are withdrawn from the process as a residue stream. (3)

3 ALKYLATION BENZENE RECOVERY EB RECOVERY INHIBITOR BE INHIBITOR EB PEB ETHYLENE LIGHTS ETHYLENE STEAM ETHYLENE BENZENE ETHYLENE PEB BE FLUX OIL Advances in Ethyl Benzene In recent years, interest has been growing in using dilute ethylene from fluidized catalytic cracking (FCC) off-gas to produce ethylbenzene (EB). Although efforts for economically converting the off-gas ethylene into a more valuable petrochemical product (instead of using it as a fuel in refinery operations) were initiated in 1950s, it was not until May 1991 that the first commercial EB unit using dilute ethylene as feedstock came on stream at Shell's Stanlow, United Kingdom plant. The plant, based on the vapor-phase benzene alkylation technology of Mobil/Badger was soon followed by Dow's unit at Ternuezen, the Netherlands. The Dow plant, built in late 1991, is also based on Mobil/Badger technology. Recently, Dow has patented a different approach for EB production. In its new process version, ethylene is generated by the catalytic dehydrogenation of ethane instead of using cat-cracker off-gas as its source or polymer-grade ethylene as the feedstock. Gallium- or zinc-promoted mordenite catalyzes ethane dehydrogenation to ethylene at a milder temperature of 700 C (1292 F) that the higher temperature range of C ( F) employed in steam cracking of ethane. Conversion of ethane is about 14 to 50 wt%, depending on the promoter used, and selectivity to ethylene is 85 wt%. Dow states that the products from the dehydrogenation reactor are free of acetylene and butadiene, and can be directly fed into the alkylation reactor for EB production without purification of the dilute ethylene stream. The benzene alkylation reaction is catalyzed by dealuminated mordenite, and the reaction products are separated by conventional means such as distillation to obtain EB. Unreacted ethane and benzene are recycled. Lower per-pass conversion rates in the alkylation phase depress formation of aromatic by-products, whose presence adversely affects product purity. The Dow process is probably appropriate only for a region where ethane is abundantly available, and where FCC off-gas or ethylene are not. Alternatively, the Dow process may be a useful, innovative technology for the large-scale catalytic production of ethylene because its process economics are comparable to those of conventional ethane cracking units. (6)

4 Ethyl Benzene Distillation Guidelines The EB distillation section consists of three or four columns, each of which can generate steam in its overhead system. Benzene Recycle Column The first column is typically the benzene recycle column. This column removes the un-reacted benzene and the reacted products are in the tower bottoms. In this column the overhead condenser can be utilized to generate steam. This requires the column to be at medium pressures, therefore trays are typically specified. Typically about 60 trays are utilized. This is a straight forward distillation column. Benzene Drying Column The second column is the benzene drying column and the overhead condenser can be utilized to generate steam. This requires the column to be at medium pressures, therefore trays are typically specified. Typically about 50 trays are utilized. This is also a straight forward distillation column. EB Recovery Column The third column is the EB Recovery Column. This column is designed at lower pressures (1.7 bar) to reduce the polymerization of the heavy reacted products. Earlier designs utilized trays in this application. Present designs utilize structured packing and trays to increase capacity. EB Recovery Column Guidelines Separation columns where relative volatility between the key components is higher than 2.0 and more than 40 theoretical stagers are required; column design has to consider at least two packed beds with a single bed height not exceeding 15 theoretical stages and trays where applicable. Looking into the styrene process we find similar conditions in the BE/TO separation column, EB recovery column and in the EB plant, where PEB has to be separated from the heavies. These columns are operating in the positive pressure range, as well as vacuum and have relative volatilities higher than 2.0. PEB Recovery Column The final column EB distillation section is the PEB Recovery Column. Many designs for this column are trays due to the high incidents of fouling. For revamp cases packing can be utilized with success, but care needs to be taken in the design to cater for the high polymerization potential and feed inlet devices.

5 Styrene Monomer Overview Styrene monomer (SM) is an important petrochemical used in the production of polystyrene and other styrenic resins such as acrylonitrile butadiene styrene (ABS) and styrene acrylonitrile (SAN). Ethylbenzene (EB) is produced primarily by alkylation of benzene with ethylene. EB is then converted to SM by dehydrogenation. (4) Reactor Overview The feedstock, ethylbenzene, is catalytically dehydrogenated to styrene in the presence of steam in a fixed bed, radial flow reactor system. The dehydrogenation reaction is favored by low pressures and is generally conducted under deep vacuum. Toluene, benzene, and some light compounds are formed as by-products. The overall reaction is endothermic with heat supplied by steam in the adiabatic reactors. Reactor effluent waste heat is recovered through heat exchange with combined feed and by generating steam which is utilized in the process. The off gas stream is compressed, processed through the off gas recovery section, and used as fuel in the steam super heater. The condensates from the condenser and off gas recovery section flow into the separator where hydrocarbon and water phases separate. The dehydrogenated mixture is fractionated to recover the styrene monomer product and recycle ethylbenzene, as well as benzene and toluene by-products. Inhibitors are added to prevent styrene polymerization in the process equipment. (4)

6 Styrene Monomer Production The energy needed for the reaction is supplied by superheated steam (at about C) that is injected into a vertically mounted fixed bed catalytic reactor with vaporized ethylbenzene. The catalyst is iron oxide based and contains Cr 2 O 3 and a potassium compound (KOH or K 2 CO 3 ) which act as reaction promoters. Typically, kg steam is required for each kilogram of ethylbenzene to ensure sufficiently high temperatures throughout the reactor. The superheated steam supplies the necessary reaction temperature of C throughout the reactor. Ethylbenzene conversion is typically 60-65%. Styrene selectivity is greater than 90%. The three significant byproducts are toluene, benzene, and hydrogen. Styrene Distillation Overview After the reaction, the products are cooled rapidly (perhaps even quenched) to prevent polymerization. The product stream (containing styrene, toluene, benzene, and un-reacted ethylbenzene) is fractionally condensed after the hydrogen is flashed from the stream. The hydrogen from the reaction is used as fuel to heat the steam (boiler fuel). After adding a polymerization inhibitor (usually a phenol), the styrene is vacuum distilled in a series of four or five columns (often times packed columns) to reach the required 99.8% purity. The separation is difficult due to the similar boiling points of styrene and ethylbenzene. Typical capacity per plant ranges from 70,000 to 100,000 metric tones per year in each reactor and most plants contain multiple reactors or units. (5) Badger Process BENZENE EB BENZENE/TOLUENE COLUMN STEAM DEHYDROGENATION BENZENE/ TOLUENE RECYCLE COLUMN EB EB/SM COLUMN TOLUENE SM STYRENE STORAGE TANKS STRUCTURED PACKINGS TRAYS SM TAR RESIDUE FINISHING COLUMNS TAR CONCENTRATOR

7 Styrene Distillation Guidelines There are typically five columns in the Styrene Distillation Process, which can be ordered different by each licenser. Benzene Toluene Recycle Column This can be the first column after the reaction section. The purpose of this column is to remove the light reactant products, which are benzene and toluene, from the Styrene Product. This column can be trays, packing or a combination of the two. Case Example of a revamped Benzene Toluene Recycle Column Typical mass balance for this column Feed Distillate Bottom BENZENE (BE) TOLUENE (TO) ETHYLBENZENE (EB) AROMATICS STYRENE (SM) HEAVIES External reflux ratio : 13.0 Top pressure : 225 mbar Top temperature : 58 C Bottom pressure : 425 mbar Bottom temperature : 112 C Number of trays on top : 20 to 25 on bottom : 10 to 15 total : 30 to 40 Original Design 40 trays were replaced with three bed of structured packing T.L. expected NTS 8.4 m 250.Y/X 18 ID trays 22 m feed 2.5 m 250.X m 250.X 12

8 Results of the 1 st revamp Feed Distillate Bottom expected measured expected measured BE TO EB Arom SM Heavies External reflux ratio expected : 13.0 External reflux ratio measured : Data was gathered and then simulated. Simulating the measured results the following bed efficiencies emerged: expected guaranteed measured above feed 18 TS 14 TS 9-7 TS below feed TS 14 TS 3-5 TS Total 35 TS 28 TS 12 TS The result of the simulation and process study determined that there were insufficient stages in the rectifying section. The feed point was lowered to increase the rectifying section fractionation ability.

9 Feed Point before - Stage 15 Feed point stripping section Feed point after - Stage 13 Feed point stripping section

10 Operation results with feed in new location Feed Distillate Bottom guaranteed measured guaranteed measured BE TO EB Arom SM Heavies External reflux ratio expected : External reflux ratio measured : Simulation with new feed location showed following efficiencies: expected guaranteed measured above feed below feed Total TS 12 TS 35 TS 18 TS 10 TS 28 TS = 22 TS 14 TS 36 TS

11 EB / SM Splitter Column The purpose of an ethyl benzene (EB) / styrene splitter is to separate ethyl benzene from styrene. The distillate EB is recycled to Styrene reactors and the bottom product Styrene Monomer (SM) is sent to the Styrene Finishing column for heavy key removal. The EB impurity in the SM should be in the range of 100 ~ 500 ppm. EB/SM Splitters are operated under vacuum due to the polymerization potential of styrene at elevated temperature. Polymers are undesirable in the monomer distillation column and can lead to plugging of distributors or packing and unit outages. The rate of polymerization is directly proportional to time and increases exponentially with temperature. Both residences time and temperate must be minimized to reduce polymerization deposits. Current guideline is to keep the tower bottoms temperature below 120 C Generally steam ejector systems are used to maintain vacuum at the top of the tower. The typical column top pressure is 100 to 400 mbar and the internals are carefully designed to reduce the tower overall pressure drop, minimize liquid hold up; reduce the bottom temperature and residence time. Some producers are increasing the tower pressure due to improvements in inhibitor formulations. This can increase capacity and improve heat recovery. Ethyl Benzene/Styrene Splitter Service Design Guidelines During the early days of styrene manufacturing, trays were used as column internals for the separation. The trayed columns have a high-pressure drop and high bottoms temperature. Currently the industry standard is to use high-capacity structured packing as column internals for EB/SM splitters. The EB/SM splitter may require 80 to 100 theoretical stages depending on the purity requirements. The columns may have 5 to 7 beds of structured packing and require very good quality liquid distributors and collectors.

12 Table of Number of Theoretical Stages Verses Reflux Ratio at Different Pressures NTS versus Reflux-Ratio NTS Reflux Ratio P=150mbar NTS Versus Reflux-Ratio NTS Reflux-Ratio P=400mbar CASE Top Pressure (mbar) NTS Reboiler (M.W) Condenser (M.W) Reflux-Ratio

13 A special design for overhead condenser and reboiler may be used to reduce the overall column pressure drop. Some of the advantages of using structured packing in these applications are as follows: 1. Minimum polymer formation because of reduced pressure drop leading to low tower bottoms temperatures. The pressure drop for high capacity structured packing may be in the range of 1-4 mmhg per meter. 2. Low inhibitor consumption because of low bottom temperature and minimum liquid hold-up, which results in significant savings in the costs of operation. 3. Smaller residence time distribution of the liquid phase is achieved with high capacity structured packing as compared to tray columns. It also reduces dead pockets and thermal degradation, which usually gives rise to further acceleration of polymer formation. 4. Increase in capacity, yield, and purity when compared to tray column of same size. 5. Energy saving by increasing the number of theoretical stages when the columns are revamped from trays or conventional packing to high capacity structured packing. Many trayed towers have been up graded to structured packing due to the polymer formation. Here is an example of polymer formation in a trayed styrene tower. Attached is a case example of one revamp.

14 Case example of trays verses packing with 200% increase Typical Mass Balance in wt% After Revamp with 200% increase Feed Distillate Bottom Before After Before After Benzene ~0 ~0 ~0 Toluene ~ Ethylbenzene ppm Styrene Heavies 4.75 ~ Design Case: 100% 200% Before After Reflux Ratio (External) : Pressure Drop Across Tower: 225 mm Hg 45 mm Hg Top Temperature: F Bottom Temperature: 235 F 185 F Number of Stages top : 15 to to 30 on bottom : 35 to to 60 total : 50 to to 90 The early design philosophy for EB/SM splitters relies mostly upon structured packing, notably 1 st generation 250.Y. Most of the columns featuring trays have been converted to packed columns due to process advantages generated by low pressure drop and liquid handling within the column and its ancillaries. The limit of 1 st generation structured packing was the total pressure drop with increased the tower bottoms temperature to the polymerization limit of 120 degrees C.

15 Case Example of 1 st Generation Packing to 2 nd Generation Packing with 140% increase Converting 1 st generation structure packing to 2ndgeneration structure packing can lead to capacity increases of as much as 140%. Attached is a typical example. Typical Mass Balance in wt% After Revamp with 140% increase Feed Distillate Bottom Benzene Toluene Ethylbenzene <100 ppm Xylenes Styrene Heavies Design Case: 100 % 140% Before After Reflux Ratio (External) : Pressure Drop Across Tower: mm Hg mm Hg Top Temperature: F 112 F Bottom Temperature: 200 F 210 F Number of Stages top: 20 to to 30 on bottom: 50 to to 80 total: 70 to to 110 The new 2 nd generation high capacity packing offers even lower pressure drop and improved process conditions. As such, the packing is suitable for replacement of both Mellapak and trays in existing columns as well as for the applications in new columns. Two items are worth noting: first a much lower pressure drop and consequently lower bottom temperature, second a substantial reduction in product hold-up. Both items mean an enormous reduction of risk of polymer formation. To reduce the fear of fouling with polymer below the feed level we can supply our structured packings also with smooth surface, if preferred. Assuming that every reduction of 10 C in the bottom halves polymer formation and hold-up as well as mean residence time are proportional for polymer formation, the solution with structured packings compared to trays means lowering risk of polymer formation by a factor of more than 8. Furthermore anti polymer chemical consumption is also half because of half the tower inventory. The limit of 2 nd generation structured packing is the total number of stages per bed to reduce mal distribution. Mal distribution studies have limited current designs to 15 theoretical stages per bed. Current research is investigating how to increase the bed heights and reduce the number of redistributors, reducing total column height.

16 Typical EB/Styrene Splitter Conditions EB/SM Splitter Column Design Column configuration Number of beds: 5 7 Diameter for world scale plant* : 8 9 m Standard internals before 1980: Trays Standard internals : Mellapak 250Y Standard internals since 2000: MellapakPlus 252Y / 452Y Operational parameters Column top pressure mbar Energy / product (MW / t SM)** Distillate specification 1-2% SM Bottoms specification ppm EB

17 Styrene Finishing Column The last column can be one or two column depending on the technology licenser. The columns can be trays, packing or a combination of both. Care needs to be taken in the design of this column due to the high polymerization potential of this column. SM Finishing Column Design Column configuration Number of beds: 2 Diameter for world scale plant* : 4 5 m Standard internals before 1980: Trays Standard internals : Trays / Mellapak Standard internals since 2000: MellapakPlus 252Y Operational parameters Column top pressure mbar Energy / product (MW / t SM)** Distillate specification wt% SM Bottoms specification wt% SM Conclusions Contemporary styrene unit designs include second generation structured packing and optimized liquid-vapor distributors. In the current designs, consideration of packing bed height and how the packing bed height affect stage efficiency due to mal-distribution should be reviewed. Designs are now limited by the maximum allowable pressure drop in order to avoid bottoms temperatures above 120 C (248 F) to 122 C (252 F). Present ethyl benzene and styrene monomer reactor and distillation system are being improved with time. Current research is on going to improve each section. Present limits for the distillation section are the bed heights due to mal distribution. Investigations are to improve distributor and packings to increase this limit.

18 References 1. Karl Kolmetz, Dr. Wai Kiong Ng, Siang Hua Lee, Tau Yee Lim, Daniel R. Summers, Cyron Anthony Soyza, Optimize Distillation Column Design for Improved Reliability in Operation and Maintenance, 2 nd Best Practices in Process Plant Management, Nikko Hotel, Kuala Lumpur, Malaysia, March 14-15, Mauro Damiani, Advantages of the New Generation of Structured Packing for EB/SM Recovery Column Applications, Sulzer Chemtech Ltd., Winterthur, Switzerland 3 Exxon Mobil website, /Tech_Technologies_Aro_EBMax.asp 4. UOP website, 5. Cheresources website, 6. PEP website, 7. Christina J. Campbell, Industrial Experiences of MellapakPlus in the Styrene Distillation Field, Sulzer Chemtech USA, Inc. Tulsa, Oklahoma 8. Karl Kolmetz, Andrew W. Sloley, Timothy M. Zygula, Peter W. Faessler, Dr. Wai Kiong Ng, K. Senthil, Tau Yee Lim, Designing Distillation Columns for Vacuum Service, The 11 th India Oil and Gas Symposium and International Exhibition, 6-7 September 2004, Grand Hyatt, Mumbai, India