Benzene Recovery Section

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. 

Bubble point temperature calculations are made, similar to the lower column sections, to determine vapor compositions at equilibrium with the liquid compositions. If a benzene mole fraction of 0.01 is allowed in the distillate, and with a benzene mole fraction of 0.8 on the solvent feed tray, the separation requirement is defined for this section of the column. 

Assuming the relative amount of acetone to chloroform in this section remains essentially unchanged, the following liquid compositions are used for the equilibrium calculations. Also listed are the corresponding bubble point temperatures and vapor compositions. 

The equilibrium curve used for estimating the number of stages in this section is a plot of 12 vs. 12 where and Xi2 = Xi -I- X.  

The total number of stages in the column is 38. With the total condenser designated as 1, the solvent feed stage is 20, the main feed stage is 28, and the reboiler is 38. 

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

Etliylene production involves liigh temperatures (1500°F) in tlie pyrolysis section and cryogenic temperatures in tlie purification section. The feedstocks, products, and by-products of pyrolysis are flaimnable and pose severe fire liazards. Benzene, wliich is produced in small amounts as a byproduct, is a known carcinogen. Table 21.7.1 summarizes some of the properties of etliane (feedstock) and tlie product gases. Figure 21.7.1 shows a simplified schematic diagram of the pyrolysis and waste heat recovery section on an etliylene plant. 

The process consists of a reactor section, continuous catalyst regeneration unit (CCR), and product recovery section. Stacked radial-flow reactors are used to minimize pressure drop and to facilitate catalyst recirculation to and from the CCR. The reactor feed consists solely of LPG plus the recycle of unconverted feed components no hydrogen is recycled. The liquid product contains about 92 wt% benzene, toluene, and xylenes (BTX) (Figure 6-7), with a balance of Cg aromatics and a low nonaromatic content. Therefore, the product could be used directly for the recovery of benzene by fractional distillation (without the extraction step needed in catalytic reforming). 

The UOP Paeol process for selective long-chain paraffin dehydrogenation to produee linear mono-olefins is shown in Fig. 15 in combination with the UOP detergent alkylation process. The Pacol process consists of a radial-flow reactor and a product recovery section. Worldwide, more than 2 million metric tons per year of linear alkyl benzene is produced employing this process. 

Column B can be put on top of column C, as depicted in Figure 7. The benzene-heptane raffinate of the purification section is merely additional feed for the recovery section. 

The liquid stream from the separator is pumped to the deheptanizer to remove light hydrocarbons. The liquid stream from the deheptanizer overhead contains benzene and toluene, and is sent to distillation section to produce high-purity benzene and toluene products. The liquid stream from the deheptanizer bottoms contains mixed xylenes and a small amount of Cg+ aromatics. This liquid stream is sent to the paraxy-lene (PX) recovery section. The mixed xylenes stream is very low in EB due to high EB conversion in the DX reactor, which debottlenecks the PX recovery unit. 

The loss of interest in the production of MAN ex benzene in the Western world has prevented any significant development of the technology after the 1980s. Only China is expressing a real interest in cost optimization for the existing technology. Domestic study and investment were allocated in an optimized semi-continuous water distillation unit, while producers are wishing for a solvent recovery section coupled with a total continuous distillation unit. 

A comparison of active (using pumps) and passive (relying on diffusion) sampling techniques for the determination of nitrobenzene, benzene and aniline in air was mentioned in Section IV.A77. Several LLE methods for nitroaromatic compounds dissolved in water were evaluated. High recoveries were achieved with discontinuous or continuous extraction with dichloromethane, adsorption on a 1 1 1 mixture of Amberlite XAD-2, -4 and -8 resins and elution with dichloromethane445. 

The distillation train first separates the benzene/toluene byproduct from main crude styrene stream (8). Unconverted EB is separated from styrene (9) and recycled to the reaction section. Various heat recovery schemes are used to conserve energy from the EB/SM column system. In the final purification step (10), trace C9 components and heavies are separated from the finished SM. To minimize polymerization in distillation equipment, a dinitrophenolic type inhibitor is co-fed with the crude feed from the reaction section. Typical SM purity ranges between 99.90% and 99.95%. 

In the fractionation section the SM unit, unconverted EB is separated from SM product in the EB/SM splitter (7). SM product is recovered as overhead from the SM column (8). Typical SM product purity is in the range of 99.9 to 99.95 wt-%. Recycle EB is taken from the bottom of the EB recovery column (9). A benzene-toluene splitter (10) is often used to recycle benzene to the EBOne unit and export toluene as a minor co-product. 

The overhead product from the EB/SM splitter is fed to an EB recovery column. The EB recovery column net bottoms stream is recycled to the dehydrogenation section. Benzene and toluene by-products in the recovery column overhead stream are separated in a benzene/toluene splitter. Oftentimes, the benzene recovered in this scheme is recycled as feed to the upstream EB plant. 

The reaction mixture, coming from the reaction section, is sent to the separation section for the recovery of benzene, water and phenol, by consecutive distillation. 

Toluene. The sources of toluene lie primarily in the catalytic reforming of selected petroleum fractions rich in naphthenes or in the recovery of toluene contained in aromatic concentrate (pyrolysis gasoline) produced as a byproduct of ethylene manufacture—mostly from naphtha/gas oil cracking. U.S. production and pricing for benzene and the aromatics discussed in Sections... 

Benzene. Benzene is derived from several sources. A small amount is still recovered from coke oven by-product streams. However, the primary sources are from the catalytic refonning of petroleum fractions rich in naphthenes (see Section 6.2.1.7, above), recovery from aromatic concentrate (pyrolysis gasoline) produced as a by-product of ethylene manufacture, hydrodealkylation of... 

Adjust the benzene product flow rate. This is the overhead product of the benzene distillation column and depends on the value of the feed flow rate. In the next section we will see how to adjust the overhead flow rate in order to maintain the desired benzene purity and fractional recovery by a material balance controller which manipulates the distillate to feed ratio (D/F control). 

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