Example of Process with Recycle: TOLUENE HYDRODEALKYLATION Ref: Seider et al, Product and process design principles, 2nd ed., Chapter 4, Wiley, 2004.

TOLUENE HYDRODEALKYLATION Process Synthesis This process was used actively following World War II, when it became favorable to convert large quantities of toluene, which was no longer needed to make the explosive TNT, to benzene for use in the manufacture of cyclohexane, a precursor of nylon. The principal reaction path is C7H8 + H2  C6H6 + CH4 which is accompanied by the side reaction 2C6H6  C12H10 + H2 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.

TOLUENE HYDRODEALKYLATION Process Synthesis 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. The plant capacity is based on the conversion of 274.2 lbmol/hr of toluene, or approximately 200 MMlb/yr, assuming operation 330 days per year.

TOLUENE HYDRODEALKYLATION Process Synthesis One distribution of chemicals involves a large excess of hydrogen gas to prevent carbon deposition and absorb much of the heat of the exothermic hydrodealkylation reaction (the suggested mole ratio of H2 to C7H8 in the reactor inlet is 4). Furthermore, to avoid an expensive separation of the product methane from the hydrogen gas, a purge stream is utilized in which methane leaves the process, unavoidably with a comparable amount of hydrogen. Now the sources and sinks of the chemicals can be connected and an estimate for the toluene recycle prepared based on the assumption of 75 mol% conversion and complete recovery of toluene from the effluent stream. Recycled Toluene = (25,265/0.75) 0.25 = 8,421 lb/hr A typical operating condition for reactor is 1,268 oF and 494 psia

TOLUENE HYDRODEALKYLATION Process Synthesis In addition, an estimate for the gas recycle prepared based on the initial assumptions of a) 0.25 for purge/recycle ratio, b) 4.0 for H2/C7H8 mole ratio in the reactor inlet stream and c) complete recovery of H2 and CH4 from the effluent stream.

TOLUENE HYDRODEALKYLATION Process Synthesis 600 lb/hr 1,801 H2 + 13,197 CH4 lb/hr + 600 lb/hr H2 in reactor inlet = 4.0 (25,265+8,421) 2.016/92.14 = 2,948 lb/hr H2 in reactor outlet = 2,948 – 547 = 2,401 lb/hr H2 in purge = 2,401*0.25 = 600 lb/hr H2 in recycle = 1,801 lb/hr, CH4 in recycle = 13,197 lb/hr

TOLUENE HYDRODEALKYLATION Process Synthesis One selection of separation operations involves a flash separator at 100°F and a slightly reduced pressure, to account for anticipated pressure drops, at 484 psia. The liquid product is sent to a distillation train in which H2 and CH4 are recovered first, followed by C6H6 and then C7H8. Because the performance of the flash separator is unknown, the amount of hydrogen and methane in the purge stream and in the vent of first distillation column are uncertain at this point. Note that the pressures of the distillation columns have not yet been entered. These are computed to permit the usage of cooling water in the condensers; that is, the pressures are adjusted to set the bubble- or dew-point temperatures of the vapor streams to be condensed at 120°F or greater.

TOLUENE HYDRODEALKYLATION Process Synthesis

TOLUENE HYDRODEALKYLATION Process Synthesis The next synthesis step involves positioning operations to change the temperatures, pressures, and phases where differences exist between the reaction and separation operations, as well as the sources of the raw materials and sinks for the product chemicals. For this process, the toluene and hydrogen feed streams are assumed to be available at elevated pressure, above that required in the hydrodealkylation reactions. When this is not the case, the appropriate operations to increase the pressure must be inserted. One arrangement of the temperature-, pressure-, and phase-change operations is shown in the next Figure for the reaction section only.

TOLUENE HYDRODEALKYLATION Process Synthesis Clearly, large quantities of heat are needed to raise the temperature of the feed chemicals to 1,200°F, and similarly large quantities of heat must be removed to partially condense the reactor effluent.

TOLUENE HYDRODEALKYLATION Process Synthesis The next synthesis step involves task integration, that is, the combination of operations into process units. In one task integration (next Figure), reactor effluent is quenched rapidly to 1,150°F, primarily to avoid the need for a costly high-temperature heat exchanger, and is sent to a feed/product heat exchanger. There, it is cooled as it heats the mixture of feed and recycle chemicals to 1000°F. The stream is cooled further to 100°F, the temperature of the flash separator. The complete simulation of this process using ASPEN PLUS with RK-SOAVE equation of state is an exercise in homework 4.

TOLUENE HYDRODEALKYLATION Process Synthesis