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ALKYLATION AND POLYMERIZATION P ● A ● R ● T ● 1 Source HANDBOOK OF PETROLEUM REFINING PROCESSES Downloaded from Digital Engineering Library McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 1.3 CHAPTER 1.1 NExOCTANE™ TECHNOLOGY FOR ISOOCTANE PRODUCTION Ronald Birkhoff Kellogg Brown it also provides a recycle isobutane stream. To meet overall gasoline pool RVP requirements, many of the recent alkylation designs require an alkylate RVP of 4 to 6 lb/in20.28 to 0.42 kg/cm2. To reduce the RVP of the alkylate, a large portion of the n-butane and isopentane must be removed. Low C5 con- tent of the n-butane product is difficult to meet with a vapor side draw on the DIB and STRATCO EFFLUENT REFRIGERATED H2SO4ALKYLATION PROCESS1.17 FIGURE 1.2.6Depropanizer feed treating. FIGURE 1.2.7Effluent treating section. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS Downloaded from Digital Engineering Library McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. requires the installation of a debutanizer tower Fig. 1.2.8. Typically, a debutanizer is required when the specified C5 content of the n-butane product must be less than 2 LV . A simpler system consisting of a deisobutanizer DIB with a side draw may suffice if a high-purity n-butane product is not required. The simplest fractionation system applies to a unit processing a high-purity olefin stream, such as an isobutane/isobutylene stream from a dehydrogenation unit. For these cases, a single isostripper can be used to produce a recycle isobutane stream, a low-RVP alkylate product, and a small isopentane product. An isostripper requires no reflux and many fewer trays than a DIB. Blowdown Section The acidic blowdown vapors from potential pressure relief valve releases are routed to the acid blowdown drum to knock out any entrained liquid sulfuric acid. Additionally, spent acid from the last Contactor reactor/acid settler systems in series is sent to the acid blowdown drum. This allows any residual hydrocarbon in the spent acid to flash. The acid blowdown drum also provides surge capacity for spent acid. The acidic vapor effluent from the acid blowdown drum is sent to the blowdown vapor scrubber. The acidic vapors are countercur- rently contacted with a circulating 12 wt caustic solution in a six-tray scrubber Fig. 1.2.9. TECHNOLOGY IMPROVEMENTS The following ination is provided to highlight important design ination about the STRATCO H2SO4effluent refrigerated alkylation process. STRATCO Contactor Reactor The alkylation reaction requires that the olefin be contacted with the acid catalyst concur- rently with a large excess of isobutane. If these conditions are not present, polymerization 1.18ALKYLATION AND POLYMERIZATION FIGURE 1.2.8Fractionation system. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS Downloaded from Digital Engineering Library McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. reactions will be promoted which result in a heavy, low-octane product and high acid con- sumption. Since the early days of alkylation, the Contactor reactor has been recognized as the superior alkylation reactor with higher product quality and lower acid consumption than those of competitive designs. However, STRATCO continues to modify and improve the Contactor reactor to further optimize the desirable alkylation reaction. Several of these improvements are listed next. The modern Contactor reactor has an eccentric shell as opposed to a concentric shell in older models. The eccentric shell design provides superior mixing of the acid and hydro- carbons and eliminates any localized “dead” zones where polymerization reactions can occur. The result is improved product quality and substantially lower acid consumption. The heat exchange bundle in the Contactor reactor has been modified to improve the flow path of the acid/hydrocarbon mixture around the tubes. Since this results in improved heat transfer, the temperature gradient across the reaction zone is quite small. This results in optimal reaction conditions. The heat exchange area per Contactor reactor has been increased by more than 15 per- cent compared to that in older models. This has resulted in an increased capacity per Contactor reactor and also contributes to continual optimization of the reaction conditions. The design of the internal feed distributor has been modified to ensure concurrent con- tact of the acid catalyst and olefin/isobutane mixture at the point of initial contact. The Contactor reactor hydraulic head has been modified to include a modern, cartridge- type mechanical seal system. This results in a reliable, easy-to-maintain, and long-lasting seal system. STRATCO offers two types of mechanical seals a single mechanical seal with a Teflon sleeve bearing and a double mechanical seal with ball bearings that operates with a barri- er fluid. The STRATCO Contactor reactors can be taken off-line individually if any main- tenance is required. If seal replacement is required during normal operation, the Contactor reactor can be isolated, repaired, and back in service in less than 24 h. STRATCO EFFLUENT REFRIGERATED H2SO4ALKYLATION PROCESS1.19 FIGURE 1.2.9Blowdown system. STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS Downloaded from Digital Engineering Library McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Process Improvements Several process modifications have been made to provide better alkylation reaction condi- tions and improve overall unit operations. Some of these modifications are as follows Acid retention time in the acid settler has been reduced by employing coalescing media in the acid settler. The reduced retention time minimizes the potential for undesirable poly- merization reactions in the acid settler. Two stages of coalescing are employed to separate the hydrocarbon product from the acid phase. The first stage results in a 90 vol H2SO4 stream that is recycled to the Contactor reactor. The second stage reduces the acid carry- over rate to only 10 to 15 vol ppm. This is at least a threefold decrease in comparison to simple gravity settling with a typical 50 to 100 vol ppm in the hydrocarbon stream. Fresh H2SO4is continuously added to the unit, and spent H2SO4is continuously with- drawn. In multiple-Contactor reactor units, the H2SO4flows in series between the Contactor reactors. Thus, the acid strength across the unit is held at its most effective value, and the acid strength at any one location in the unit does not vary with time. This of han- dling H2SO4provides a very stable operation and continual acid strength optimization. To ensure complete and intimate mixing of the olefin and isobutane feeds before con- tacting with the acid catalyst, these hydrocarbon feeds are premixed outside the Contactor reactor and introduced as one homogeneous stream. Alkyl sulfates are removed in a fresh acid wash coalescer/warm alkaline water wash. Afterward, the net effluent stream is washed with fresh process water to remove traces of caustic, then is run through a coalescer to remove free water before being fed to the DIB tower. This system is superior to the caustic wash/water wash system which was imple- mented in older designs. The fractionation system can be designed to accommodate makeup isobutane of any purity, eliminating the need for upstream fractionation of the makeup isobutane. The alkylation unit is designed to take maximum advantage of the refinery’s steam and utility economics. Depending upon these economics, the refrigeration compressor and/or Contactor reactors can be driven with steam turbines condensing or noncondensing or electric motors, to minimize unit operating costs. STRATCO now employs a cascading caustic system in order to minimize the volume and strength of the waste caustic NaOH stream from the alkylation unit. In this system, fresh caustic is added to the blowdown vapor scrubber, from which it is cascaded to the depropanizer feed caustic wash and then to the alkaline water wash. The only waste stream from the alkylation unit containing caustic is the spent alkaline water stream. The spent alkaline water stream has a very low concentration of NaOH 0.05 wt and is com- pletely neutralized in the neutralization system before being released to the refinery waste- water treatment facility. Since the cascading system maintains a continuous caustic makeup flow, it has the additional advantages of reduced monitoring requirements and reduced chance of poor treating due to inadequate caustic strength. H2SO4ALKYLATION PROCESS COMPARISON The most important variables that affect product quality in a sulfuric acid alkylation unit are temperature, mixing, space velocity, acid strength, and concentration of isobutane feed in the reactors. It is usually possible to trade one operating variable for another, so there is often more than one way to design a new plant to meet octane requirements with a giv- en olefin feed. Going beyond the customary alkylation process variables, STRATCO has developed unique and patented expertise in separate processing of different olefin feeds. This tech- 1.20ALKYLATION AND POLYMERIZATION STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS Downloaded from Digital Engineering Library McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. nology can improve product quality compared to alkylation of the same olefins mixed together. The two major H2SO4alkylation processes are the STRATCO effluent refrigerated process and the autorefrigerated process by design; these two processes take different approaches to achieve product quality requirements. These design differences and their impacts on operability and reliability are discussed below. Cooling and Temperature Control The STRATCO effluent refrigerated process utilizes a liquid-full reactor/acid settler sys- tem. The heat of reaction is removed by an internal tube bundle. In the autorefrigerated process, the heat of reaction is removed by operating the reactor at a pressure where the acid/hydrocarbon mixture boils. The autorefrigerated reactor and acid settler therefore contain a vapor phase above the two mixed liquid phases. Both systems can be operated in the same temperature range. However, the STRATCO system is much easier to operate. Temperature control in the STRATCO effluent refrigerated process is simpler than that in the autorefrigerated process. The pressure of the refrigerant flash drum is used to con- trol the operating temperature of all the Contactor reactors in the reaction zone. The autorefrigerated process requires two or more pressure zones per reactor to control tem- perature and to maintain liquid flow between the reactor zones. Good control of the acid/hydrocarbon ratio in a sulfuric acid alkylation reactor is crit- ical to reactor perance. This is the area in which the STRATCO system has its largest operability advantage. Since the Contactor reactor system operates liquid-full, gravity flow is used between the Contactor reactor and acid settler. The Contactor/settler system is hydraulically designed to maintain the optimum acid-to-hydrocarbon ratio in the reactor as long as the acid level in the acid settler is controlled in the correct range. The acid/hydro- carbon ratio in the Contactor reactor can be easily verified by direct measurement. In con- trast, the autorefrigerated process requires manipulation of an external acid recycle stream in order to control the acid/hydrocarbon ratio in the reactor. As a result, the acid/hydro- carbon ratio in the different mixing zones varies and cannot be readily measured. The Contactor reactor/settler system is also designed to minimize acid inventory in the acid settler. Minimizing the unmixed acid inventory suppresses undesirable side reactions which degrade product quality and increase acid consumption. Quick, clean separation of the acid and hydrocarbon phases is much more difficult in the boiling autorefrigerated process. When operated at the same temperature, the effluent refrigerated system requires some- what greater refrigeration compressor horsepower than the autorefrigerated process because of resistance to heat transfer across the tube bundle. Mixing The topic of mixing in a sulfuric acid alkylation unit encompasses 1 the mixing of the isobutane and olefin feeds outside the reactor, 2 the of feed injection, and 3 the mixing intensity inside the reactor. The best-quality alkylate is produced with the lowest acid consumption when ●The “local” isobutene/olefin ratio in the mixing zone is maximized by premixing the olefin and isobutane feeds. ●The feed is rapidly dispersed into the acid/hydrocarbon emulsion. ●Intense mixing gives the emulsion a high interfacial area. STRATCO EFFLUENT REFRIGERATED H2SO4ALKYLATION PROCESS1.21 STRATCO EFFLUENT REFRIGERATED H2SO4 ALKYLATION PROCESS Downloaded from Digital Engineering Library McGraw-Hill Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. In STRATCO’s effluent refrigerated process, all the isobutane sent to the reactors is pre- mixed with olefin feed, maximizing the “local” isobutane concentration at the feed point. The feed mixture is rapidly dispersed into the acid catalyst via a special injection nozzle. Mixing occurs as the acid/hydrocarbon emulsion passes through the hydraulic head impeller and as it circulates through the tube bundle. The tube bundle in the Contactor reactor is an integral part of the mixing system. The superior mixing in the Contactor reactor produces an emulsion with a high interfacial area, even heat dissipation, and uni distribution of the hydrocarbons in the acid. Intense mix- ing reduces the temperature gradient within the Contactor’s 11,500-gal volume to less than 1F. The result is suppression of olefin polymerization reactions in favor of the alkylation reaction. Good mixing is particularly important when the olefin feed contains propylene. In the autorefrigerated process, only a portion of the isobutane is premixed with the olefin feed. The “local” concentration of isobutane is therefore lower when the feeds first make contact with acid catalyst. The less intensive mixing in the autorefrigerated process can result in nonuni distribution of the hydrocarbons in the acid. The desired finely dispersed hydrocarbon in acid emulsion cannot be easily controlled throughout the different reaction zones. As a consequence, the autorefrigerated al
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