Transalkylation reactor

Fig. 3. Unocal—Lummus—UOP ethylbenzene process AR = alkylation reactor TR = transalkylation reactor BC = benzene column ...

The trialkylaluminum compounds leaving this transalkylation reactor now contain primarily C -C10 alkyl groups simply by the law of mass action. The low molecular weight olefins are separated from the trialkylaluminum compounds by distillation (19). This is a specially designed flash unit which contains provisions for scrubbing the overhead olefin vapors with partially (2/3) oxidized alkylaluminum dialkoxide to remove any trialkylaluminum carried overhead (20). 

A slip-stream (1/5 to 1/3) of the second growth product can be sent to the first transalkylation reactor rather than the second. This procedure is required in order to balance this growth scheme. 

To increase the overall yield, the DIPB is reconverted to cumene in a transalkylation reactor in the presence of a large excess of benzene. The same zeolite catalyst may be used. Practical data for the design are temperatures of140-150 °C, benzene/ DIPB ratios between 20-30 and a WHSV of 2 to 3. 

Description The technology encompasses three main processing areas splitter, reactor and stabilizer sections. The heavy-aromatics stream (C9+s feed) is fed to the splitter. The overhead C9 aromatic product is the feed to the transalkylation reactor section. The splitter bottoms is exchanged with other streams for heat recovery before leaving the system. 

Description The process includes a fixed-bed alkylation reactor, a fixed-bed transalkylation reactor and a distillation section. Liquid propylene and benzene are premixed and fed to the alkylation reactor (1) where propylene is completely reacted. Separately, recycled polyisopropylbenzene (PIPB) is premixed with benzene and fed to the transalkylation reactor (2) where PIPB reacts to form additional cumene. The transalkylation and alkylation effluents are fed to the distillation section. The distillation section consists of as many as four columns in series. The depropanizer (3) recovers propane overhead as LPG. The benzene column (4) recovers excess benzene for recycle to the reactors. The cumene column (5) recovers cumene product overhead. The PIPB column (6) recovers PIPB overhead for recycle to the transalkylation reactor. 

In the fractionation section, propane that accompanies the propylene feedstock is recovered as LPG product from the overhead of the depropanizer column (2), unreacted benzene is recovered from the overhead of the benzene column (4) and cumene product is taken as overhead from the cumene column (5). Di-isopropylbenzene (DIPB) is recovered in the overhead of the DIPB column (6) and recycled to the transalkylation reactor (3) where it is transalkylated with benzene over a second zeolite catalyst to produce additional cumene. A small quantity of heavy byproduct is recovered from the bottom of the DIPB column (6) and is typically blended to fuel oil. The cumene product has a high purity (99.96-99.97 wt%), and cumene yields of 99.7 wt% and higher are achieved. 

Description Ethylene reacts with benzene in either a totally liquid-filled or mixed-phase alkylation reactor (1) containing multiple fixed-beds of ExxonMobil s proprietary catalyst, forming EB and very small quantities of polyethylbenzenes (PEB). In the transalkylation reactor (2), PEB is converted to EB by reaction with benzene over ExxonMobil s transalkylation catalyst. PEB and benzene recovered from the crude EB enter the transalkylation reactor. 

Effluents from the alkylation and transalkylation reactors are fed to the benzene column (3), where unreacted benzene is recovered from crude EB. The fresh benzene feedstock and a small vent stream from the benzene column are fed to the lights column (4) to reject light impurities. The lights column bottoms is returned to the benzene column. The bottoms from the benzene column is fed to the EB column (5) to recover EB product. The bottoms from the EB column is fed to the PEB column (6) where recyclable alkylbenzenes are recovered as a distillate and diphenyl compounds are rejected in a bottoms stream that can be used as fuel. 

A small amount of polyethylbenzene (PEB) is recovered in the overhead of the PEB column (5) and recycled back to the transalkylation reactor (2) where it is combined with benzene over a second proprietary zeolite catalyst to produce additional EB product. A small amount of flux oil is recovered from the bottom of the PEB column (5) and is usually burned as fuel. 

Figure 4.10 illustrates the results of typical calculations of the reaction mixture composition evolution in the plug flow reactor the calculations are made using the preceding relationships, the relevant mass balance equa tions, and literature data on Kpi at 210° C. The evolution in time of the ini tial product concentrations including DEB is seen to lead eventually to the situation when the inlet and outlet DEB concentrations become equal. This means that the proper choice of the composition of the initial reaction mix ture makes the process 100% selective in respect to the conversion of the initial reactants, benzene and ethylene, to EB (see Figure 4.10) even though no transalkylation reactor is used. 

The sidedraw from the DIPB column containing mainly DIPB combines with a portion of the recycle benzene and is charged downflow into the transalkylation reactor. In the transalkylation reactor, DIPB and benzene are converted to additional cumene. The effluent from the transalkylation reactor is then sent to the benzene column. 

With both alkylation and transalkylation reactors working together to take full advantage of the QZ-2001/QZ-2000 catalyst system, the overall yield of cumene based on benzene and propylene feed in... 

Commercially it is found to be economically attractive to transalkylate all the PEB formed as a result of successive alkylation reactions with benzene in a separate transalkylation reactor to produce additional EB. 

The reaction temperature is the key variable for controlling the operation and performance of the transalkylation reactor. Transalkylation reactors are designed to operate across a relatively wide temperature range. During initial operation when catalyst activity is high, relatively low reaction temperatures are sufficient to obtain the desired conversion of polyalkylated compounds. As the catalyst ages and loses activity, the temperature is increased to maintain PEB conversion at or near the desired level. Liquid phase transalkylation reactors typically operate between 170°C and 270°C. 

To obtain an economically viable PEB conversion in the transalkylation reactor, a molar excess of benzene relative to PEB is needed. A high benzene-to-PEB ratio (Bz/PEB) results in high-equilibrium PEB conversion, but at the expense of increased capital cost and... 

The Mobil/Badger vapor phase process includes four distillation columns. The first major separation is in a benzene recovery column where unconverted benzene is recovered as an overhead product for recycle to the alkylation and transalkylation reactors. The bottom stream is fed to an EB recovery column where EB product is separated from cumene, the PEB, and other heavy components. The cumene, PEB, and other heavy by-products are further separated in the PEB recovery column. The heavy residue is typically used as fuel in the reactor feed heater. The PEB fraction is recovered in the overhead stream and recycled to the transalkylation reactor where it reacts to form additional EB. A fourth column is used as a stabilizer column to vent any light components and to remove water from the system. 

The transalkylation reactor is also maintained in the liquid phase but uses EBZ-100 catalyst, which is made using zeolite Y. Transalkylation reaction is nearly thermo-neutral, so it operates essentially isother-mally. The reactor temperature is generally adjusted to provide the desired level of PEB conversion. While a high temperature results in high PEB conversion that closely approaches equilibrium composition, these conditions can result in undesired side reactions. 

Deactivation of EBZ-100 catalyst is rare, usually only occurring because of unusual upsets or operation of the transalkylation reactor. Plants have operated for approximately lOyr without regenerating the transalkylation catalyst. If EBZ-100 catalyst requires regeneration, an inexpensive, mild carbon burn procedure is used. 

The alkylation and transalkylation reactor effluent streams are sent to the distillation section, which consists primarily of three fractionation columns. The first column is the benzene column. It separates unconverted benzene into the overhead stream for recycle to the reactors. The benzene column bottom stream is the feed to the EB column. The EB column recovers the EB product in an overhead stream at purities as high... 

The transalkylation reactor in an EBMax plant can be either vapor phase or liquid phase. More recently, the transalkylation reactor has been designed as liquid phase because of its improved energy efficiency. The transalkylation reaction is conducted in the liquid phase using Mobil TRANS-4 catalyst. 

Figure 2.29a shows atypical EBMax plant flow diagram [226]. The alkylation reactor is maintained in the liquid phase and uses multiple catalyst beds and ethylene injections. The ethylene conversion is essentially 100% in the alkylation reactors, and the reactors operate nearly adiabatically. The exothermic heat of reaction is recovered and used to generate steam, heat reactor feed streams, or as heat duty in the distillation columns. The transalkylation reactor can be in either the vapor or liquid phase, but the latter shows improved energy efficiency. 

Modem alkylation processes make use of solid catalysts based on zeolites. According to different technologies, the reaction can be performed in vapour or liquid phase. The selection of a suitable chemical reactor for ethylbenzene is discussed in the Example 8.3. A conceptual flowsheet is depicted in Fig. 7.31 for a vapour-phase process (Mobil-Badger), one of the most widely used. The reactor works at 390-440 C and 0.6-3 MPa. Besides the main product ethylbenzene (EB), polyethylbenzenes (PEB) are formed, their amount depending on the reaction conditions. Large excess of benzene, over 6 1, is needed to shift the equilibrium to the desired product. The reaction mixture is sent to the separation section. Final yield can increase over 99% by converting PEB s to EB in a separate transalkylation reactor. 

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