Benzene, recovery

Eig. 4. Mobil—Badger process for ethylbenzene production H = heater Rx = reactor P = prefractionator BC = benzene recovery column ... 

Figure 3.9 The copolymerization process with a benzene recovery MEN.

Igure 3.1 la Representation of the two process MSAs for the benzene recovery example... 

Figure 3.1 lb Construction of the lean composite stream for the two process MSA s of the benzene recovery example. 

Figure 3.12 The pinch diagram for the benzene recovery example (x i = 2 = 0.001).

Optimizing the use of flie external MSA The pinch diagram (Fig. 3.12) demonstrates that below the pinch, the load of the waste stream has to be removed by the external MSA, S3. This renders the remainder of this example identical to Example 2.2. ThereftKc, the optimal flowrate of S3 is 0.0234 kg mol/s and the optimal outlet composition of S3 is 0.(X)85. Furthermore, the minimum total annualized cost of the benzene recovery system is 41,560/yr (see Fig. 2.13). 

Using an algebraic procedure, synthesize an optimal MEN for the benzene recovery example described in Section 3.7 (Example 3.1). 

Figure 6.2 CID for benzene recovery example with lean substrearnii.

In the previous problem, it is desired to compare the total annualized cost of the benzene-recovery system to the value of recovered benzene. The total annualized cost TAC for the network is defined as ... 

The first tower in Figure 11.44 gives the ternary azeotrope as an overhead vapour, and nearly pure ethanol as bottom product. The ternary azeotrope is condensed and splits into two liquid phases in the decanter. The benzene-rich phase from the decanter serves as reflux, while the water-ethanol-rich phase passes to two towers, one for benzene recovery and the other for water removal. The azeotropic overheads from these successive towers are returned to appropriate points in the primary tower. 

USEPA issued the NESHAP for benzene waste operations March 7, 1990, under the Clean Air Act. The compliance date was May 1992. It affects not only equipment leaks but also emissions of benzene in wastewater streams. Eacilities with greater than 10 tonnes/year benzene in wastewater streams are affected. They must identify wastewater streams containing greater than 10 mg/L benzene and divert them to units that will reduce benzene to acceptable levels, that is, below 10 mg/L or by 98%. This rule affected most major refineries and olefins plants. Mobil Corp. spent 10 million on a benzene recovery project at its Chalmette, LA, refinery. The refinery uses vacuum steam stripping to decrease benzene emissions by about 10 tonnes/year. One Gulf Coast petrochemical plant has also spent 10 million on a wastewater stripping facility, which reduced benzene levels from several thousand mg/L to less than 5 mg/L [70]. 

Still reetdue Receiver Benzene re-cowery (receiver), % r — Benzene recovery, lb Vapor required, Heat required, Btn... 

EBMax is a liquid phase ethylbenzene process that uses Mobil s proprietary MCM-22 zeolite as the catalyst. This process was first commercialized at the Chiba Styrene Monomer Co. in Chiba, Japan in 1995 (16-18). The MCM-22-based catalyst is very stable. Cycle lengths in excess of three years have been achieved commercially. The MCM-22 zeolite catalyst is more monoalkylate selective than large pore zeolites including zeolites beta and Y. This allows the process to use low feed ratios of benzene to ethylene. Typical benzene to ethylene ratios are in the range of 3 to 5. The lower benzene to ethylene ratios reduce the benzene circulation rate which, in turn, improves the efficiency and reduces the throughput of the benzene recovery column. Because the process operates with a reduced benzene circulation rate, plant capacity can be improved without adding distillation capacity. This is an important consideration, since distillation column capacity is a bottleneck in most ethylbenzene process units. The EBMax process operates at low temperatures, and therefore the level of xylenes in the ethylbenzene product is very low, typically less than 10 ppm. 

The monoalkylation selectivity of the alkylation step refers to the fraction of ethylene that reacts to form ethylbenzene, as opposed to forming polyethylated species. To suppress the formation of PEBs, benzene must be fed to the alkylation reactor in considerable excess (frequently five to seven times the stoichiometric requirement). Equipment in the alkylation reaction and benzene recovery systems must therefore be sized to accommodate the flow of excess benzene, and energy must be expended to recover the excess benzene from the reactor effluent. However, the superior monoalkylation selectivity and stability of MCM-22 permits operation with reduced benzene feed rates - in the range of two to four times the stoichiometric requirement - without excessive PEB formation (see Table 11.1). 

Benzene recovery from pyrolysis gasoline is usually above 99.5% at feed concentration above 80%. 

Part of the stream is washed countercurrently with a feed sidestream in the vent H2 absorber (9) for benzene recovery. The absorber overhead flows to the hydrogen purification unit (10) where hydrogen purity is increased to 90%+ so it can be recycled to the reactor. The stabilizer (11) removes light ends, mostly methane and ethane, from the flash drum liquid. The bottoms are sent to the benzene column (12) where high-purity benzene is produced overhead. The bottoms stream, containing unreacted toluene and heavier aromatics, is pumped to the recycle column (13). Toluene, C8 aromatics and diphenyl are distilled overhead and recycled to the reactor. A small purge stream prevents the heavy components from building up in the process. 

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 Shell benzene recovery process uses phenol or, in special cases, other solvents such as cresylic acids or sulfolanes, to separa te benzene from nonaromatics by extractive distillation. This process has been described by Dunn and Lieholm 22), and others 63). 

The benzene recovery section, between the condenser and the solvent feed stage, is represented on a Y-X pseudo-binary diagram where the acetone and chloroform are lumped as one component in solution with the benzene. 

An example of azeotropic distillation is the use of benzene to permit the complete separation of ethanol and water, which forms a minimum-boiling azeotrope with 95.6 weight percent alcohol. The alcohol-water mixture with about 95 percent alcohol is fed to the azeotropic distillation column with a benzene-rich stream added at the top. The bottom product is nearly pure alcohol, and the overhead vapor is a ternary azeotrope. The overhead vapor is condensed and separated into two phases. The organic layer is refluxed, and the water layer is sent to a benzene recovery column. All the benzene and some alcohol is taken overhead and sent back to the first column, and the bottoms stream is distilled in a third column to give pure water and some of the binary azeotrope. 

In the 1940s extractive distillation was used to increase aliphatic-benzene relative volatilities, thereby increasing benzene recovery and... 

Figure 3.15 Schematic diagram of the benzene-recovery process of Example 3.7.

Example 3.10 Benzene Recovery System Number of Ideal Stages... 

Benzene recovery and cnlorbenTene purification (Oistillation unit)... 

Description A typical SED unit mainly consists of an extractive distillation column and a solvent recovery column. The hydrocarbon feed is separated into non-aromatics and aromatics products through extractive distillation with the solvent. For the benzene-recovery case, benzene is directly produced from the SED unit. For the benzene and toluene recovery case, pure benzene and pure toluene are produced from the aromatics product of the SED unit through downstream fractionation. 

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