Toluene hydrodealkylation plant

Toluene hydrodealkylation plant described in Chapter 1 (see Figures 1.3 and 15 and Tables 1.5 and LZ). 

Toluene is recovered as a high purity product by fractionating the mixed aromatics obtained from the extraction of catalytic reformate or powerformate. About 70 fractionation trays are required to produce toluene having a purity of 99.7 percent. Toluene is consumed principally as a feedstock for hydrodealkylation plants. Toluene is used in a number of solvent applications. 

Since toluene is nothing more than benzene with a methyl group attached, creating one from another is relatively easy. Benzene, toluene, and for that matter, xylenes too, are coproduced in the processes just described—coke making, cat reforming, and olefin plants operations. The ratio of benzene to the other aromatics production is rarely equal to the chemical feedstock requirements.. fo.r the three. One method for balancing supply and demand is toluene hydrodealkylation (HDA). This process accounts for 10—15% of the supply of benzene in the United States and is a good example of what can be done when one or more coproducts are produced in proportions out of balance with the marketplace. 

Figure 4.9 Toluene hydrodealkylation reactor/separation/recycle structure of the plant.

Make an order-of-magnitude estimate of the total capital investment, as of the year 2001 (MS = 1,110), to produce benzene according to the toluene hydrodealkylation process shown in Figure 5.13. Assume an overall conversion of toluene to benzene of 95% and 330 days of operation per year. Also, assume the makeup gas enters at the desired pressure and a clay adsorption treater must be added to the flow sheet after the stabilizer. The treater removes contaminants that would prevent the benzene product from meeting specifications. In addition, in order for the reactor to handle the high temperature, it must have a brick lining on the inside, so take a material factor of Fm = 15. Otherwise, aU major equipment is constructed of carbon steel. The plant will be constructed outdoors with major additions to existing facilities. 

The PFD for the toluene hydrodealkylation process (Figure 1.51 represents the battery-limits plant. The equipment necessary to produce the various service or utility streams, which are used in the process and are necessary for the plant to operate, are not shown on the PFD. However, the utility streams such as cooling water and steam for heating are shown on the PFD. These streams, termed utilities, are necessary for the control of stream temperatures as required by the process. These utilities can be supplied in a number of ways. 

The benzene—toluene fraction is further fractionated in a small column, not shown in Figure 5, to recover benzene for recycle to the alkylation unit and toluene for sale. This toluene can be converted to benzene by hydrodealkylation but the high selectivity catalyst has reduced the formation of toluene in the dehydrogenation reactor to the point where the cost of installing a hydrodealkylation unit is difficult to justify even in a large styrene plant. 

These same notions can be extended to an entire plant in which several unit operations are connected together. The HDA process for hydrodealkylation of toluene to form benzene is a good example of where an eigenstructure can be found that provides a more easily and simply controlled plant. See Fig. 8.15. Assuming that the toluene feed rate to the unit is fixed, this plant has 22 valves that must be set. There are 11 inventory loops (levels and pressures), so they require 11 valves. One possible conventional control structure is shown in Fig. 8.15. 

Hydrodealkylation of Toluene Plant to Produce Benzene A Case Study... 

The plant we will use for our example produces benzene from the hydrodealkylation of toluene and is adapted from a similar case study by Douglas [Ref. 21]. The basic reactions are ... 

HSS (see High selector switch) Hydraulics, of distillation trays, 72 Hydrodealkylation of toluene plant control system design, 519-29 description, 516-19... 

Laboratory data indicate that the reactions proceed irreversibly without a catalyst at temperatures in the range of 1,200-1,270°F with approximately 75 mol% of the toluene converted to benzene and approximately 2 mol% of the benzene produced in the hydrodealkylation reaction converted to biphenyl. Since the reactions occur in series in a single processing unit, just a single reaction operation is positioned in the flowsheet, as shown in Figure 4.16. The plant capacity is based on the conversion of 274.2 Ibmol/hr of toluene, or approximately 200 MMlb/yr, assuming operation 330 days per year. 

When steam cracking or naphtha reforming produce an aromatics mixture short in benzene or o- and p-xylene, some interconversion is practiced. Toluene can be hydrodealkylated to benzene. Xylene can be isomerized to increase yields of o- and p-xylene. The analysis for aromatics thus falls into two general types to meet two different needs. Analysis for process optimization assists in obtaining the maximum product at the minimum unit cost. This involves analysis of feeds, products, and raffinate (purge) streams. These analyses must be tailored to the process and the plant streams involved. Generally, it is desirable to have one analytical procedure to apply to a variety of sample types. The final product specification analysis can also be used for process control. The ASTM standard... 

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