Feed hydrodealkylation

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

A typical catalytic hydrodealkylation scheme is shown ia Figure 3 (49). The most common feedstock is toluene, but xylenes can also be used. Recent studies have demonstrated that and heavier monoaromatics produce benzene ia a conventional hydrodealkylation unit ia yields comparable to that of toluene (51). The use of feeds containing up to 100% of C —aromatics iacreases the flexibiUty of the hydrodealkylation procedure which is sensitive to the price differential of benzene and toluene. When toluene is ia demand, benzene suppHes can be maintained from dealkylation of heavy feedstocks. 

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. 

The feedstock in hydrodealkylation units is heated to 1,200° F (650°C) in a preheat furnace before entering the reactor. Above 1,100°F (590°C), metal dusting or catastrophic carburization occurs on ail alloys that are otherwise suitable for the temperature conditions. The attack is very rapid and takes the form of round bottom pits. The surface of the remaining metal is heavily carburized. A small quantity of sulfur (0.05 to 0.5 wt%) in the form of hydrogen sulfide or mercaptan added to the feed will prevent attack. Aluminizing has also been used to prevent attack. 

Mitsubishi Petrochemical Co. Benzene Alkyl-containing hydrocarbons Easy and stable operation without catalyst. Hydrodealkylation of feeds 7 1992... 

In Figure PI. 3 the distillation configuration for the separation of benzene from tuluene is given. The feed to the distillation comes from the reactor, where toluene has been hydrodealkylated to produce benzene ... 

The hydrodealkylation of toluene is to be carried out in a packed-bed reactor. The molar feed of toluene to the reactor is 50 mol/min and the reactor is operated at 40 bar and 640 °C. The feed consists of 30% toluene, 45% hydrogen, and 25% inerts. Hydrogen is used in excess to help prevent coking. 

Return to the design of the toluene hydrodealkylation process, as it is presented in Section 4.3. In the reactor section, after heuristics are utilized to set (1) the large excess of H2 in the hydrodealkylation reactor, (2) the temperature level of the quenched gases that enter the feed-product heat exchanger, and (3) the temperature in the flash vessel, the simulator is used to complete the material and energy balances and to examine the effects of these heuristics on the performance of the reactor section. In the distillation section, after heuristics are used to set (1) the quahty of the feed, (2) the use of partial or total condensers, (3) the use of cool-... 

Toluene is converted to benzene by hydrodealkylation. Typically, a 75% conversion is used in the reactor, which necessitates the recovery and recycle of unreacted toluene. In addition, a side reaction occurs that produces a small amount of a biphenyl byproduct, which is separated from the toluene. A hydrodealkylation process is being designed that includes a distillation column for separating toluene from biphenyl. The feed to the column is 3.4 Ibmoiyhr of benzene, 84.6 IbmoVhr of toluene, and 5.1 Ibmol/hr of biphenyl at 264°F and 37.1 psia. The distillate is to contain 99.5% of the toluene and 2% of the biphenyl. If the column operates at a bottoms pressure of 38.2 psia, determine the bottoms temperature and select a suitable heat source for the reboiler. Steam is available at pressures of 60, 160, and 445 psig. The barometer reads 14 psia. 

In a toluene hydrodealkylation process, 25,000 Ib/hr of toluene feed is pumped from 75°F and 30psia to 570 psia. Use a process simulator to compute the capacity in gpm, the pump head in feet of tolueM, the exit temperature, and brake horsepower (BHp) for ... 

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... 

The overall conversion tells us what fraction of the toluene in the feed to the process (Stream 1) is converted to products. For the hydrodealkylation process, it is seen that this fraction is high (99.3%). This high overall conversion is typical for chemical processes and shows that unreacted raw materials are not being lost from the process. 

As noted in Section 13.5. the results provided for the toluene hydrodealkylation process are based on the SRK model for both enthalpy and phase equilibria. Simulate the benzene column (T-101) with the 20. PR model instead, using the shortcut simulation module and the specifications given in Table 13.1 and the conditions of feed stream (10) given in Exanple 13.2. Determine the BIPs for the PR model used by the simulator. Rerun the simulation with all the BIPs set to zero. Conpare the results. 

Table 24.5 HAZOP for the Feed Heater of the Hydrodealkylation (HDA) Process...

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