Cooling and recycling

Preliminary design work has been done on a process to recover a valuable product from an effluent gas stream. The gas will be scrubbed with a solvent in a packed column the recovered product and solvent separated by distillation and the solvent cooled and recycled. The major items of equipment that will be required are detailed below. 

Figure 8-3 Three basic types of cooling water systems. Top the once-through system where the cooling water is used once and then discharged. Middle the open recirculation system where the water is cooled and recycled through a system in which it comes in direct contact with air. Bottom a closed recirculation system where the water is cooled and recycled without coming in direct contact with the atmosphere.

In gas phase reactors, the monomer is introduced to the bottom of reactor where it percolates up through a fluidized bed of polymer granules and inert-media supported catalyst. A fraction of the monomer reacts to form more polymer granules, the remaining monomer being drawn from the top of the reactor, cooled, and recycled. Polymer granules are continuously wthdrawn from the bottom of the fluidized bed and the catalyst is replenished. 

In a packed bed, the condensing water joins the cool water stream. In a heat exchanger, the condensing water flows down the cool tube walls in a separate stream. In both cases the cooling water is air cooled and recycled to condensation. 

The heat of reaction is removed by excess ethylene leaving the reactor as an overhead stream. Ethylene is cooled and recycled to the reactor along with fresh ethylene feed. The reaction is conducted under an oxygen-free, anhydrous environment to prevent deactivation of the catalyst. The reaction is maintained at 80°C in order to minimize the potential isomerization of butene-1 to butene-2. Ethylene conversion is about 20 percent per pass and the assumed product yield from this process are 95 percent butene-1 and 5 percent butene-2. 

In a typical thermofor catalytic cracking unit, the preheated feedstock flows by gravity through the catalytic reactor bed. The vapors are separated from the catalyst and sent to a fractionating tower. The spent catalyst is regenerated, cooled, and recycled. The flue gas from regeneration is sent to a carbon monoxide boiler for heat recovery. 

Benzene vapor in the gaseous effluent of an industrial process is scrubbed with a wash oil in a countercurrent packed absorber. The resulting benzene-wash oil solution is then heated to 398 K and stripped in a tray tower, using steam as the stripping medium. The stripped wash oil is then cooled and recycled to the absorber. Some data relative to the operation follow ... 

Figure 24.20 A nuclear reactor produces heat that drives the formation of steam. The energy from the steam spins a turbine which produces electricity. The steam is eventually cooled and recycled. The water used to cool the steam enters the cooling tower where steam is released to the atmosphere. 

A azeotropic mixture of isopropyl alcohol and water is separated by addition of ethylene ,ycol. Extractive distillation leads to pure isopropyl alcohol as overhead product, and water and ethylene glycol as bottom product. This mixture is split into water and ethylene glycol under reduced pressure in a downstream distillation column. The ethylene glycol is then cooled and recycled to the extractive distillation (Figure 2.3.2-34). 

An iron automotive foimdry, using an automated moulding line with a production of 8000 t/yr castings produces 480 t/yr of dust. This sand is collected, cooled and recycled into the sand mixer. The collected dust contains 23 % active bentonite and 10 % carbon. 50 % of the dust can be re-circulated without any risk of loss of quality due to fine dust. 

A reflux ratio of 2.1 produces 316kmoI/h of high-purity methanol in the distillate (99.9 mol% MeOH) and 1026kmol/h of high-purity water in the bottoms (99.9 mol% H2O). The methanol is combined with 230kmol/h of fresh methanol feed, and the total is split between the methanol feed streams to the prereactor and to the reactive column. The water is combined with a small water makeup stream, cooled, and recycled back to the extractive column C2. 

Recirculated Caustic Feed to Electrolyzers. Most membrane cells operate at 30-35% caustic with a slightly lower feed caustic concentration. Thus, most of the caustic collected from the cells is diluted, cooled, and recycled to the cells. ITie circulating pumps must continue to run in case of a power failure and so should connect to a backup power supply. 

The upper section of the reactor is expanded to reduce velocity and minimize polymer entrainment. In the UNIPOL process, the gas and the small amount of entrained polymer are cooled and recycled back to the process. In the BP design, a eyelone is used first to recover the polymer before the gas is cooled and recycled. 

The absorption system consists of a packed absorber operating at the gas stream pressure and a solvent stripper. The lean solvent is fed to the absorber at as low a temperature as practical in order to minimize solute vapor pressure, as well as solvent losses. Due to the high boiling point of the solvent, regeneration usually is accomplished under vacuum. The rich solvent from the absorber is heated and then flashed under a reduced pressure to liberate the solute in vapor form. This solute vapor is condensed, and the lean solvent is cooled and recycled to the absorber. 

Coke-oven gas flora the exhauster, at a tenqterature of 45 -55 C (113 -131 F), is cooled in the secondary cooler to about 28 C (82°F)> then enters the absorption section of the vessel. A portion of the rich solution from the absorber is cooled and recycled over the bottom packed section to remove the heat of reaction and provide a high liquid flow rate in this zone. Cooled excess flushing liquor, which is a dilute solution of ammonia, is fed into the column in the lower third of the vessel. Water flora the free ammonia stripper is fed into the top of the absorber. This water still contains fixed ammonia, but since this form of ammonia has a negligible ammonia vapor pressure, it does not significantly affect the gas purity attainable. According to Svoboda and Diemer (1990), this type of washer typically reduces the ammonia content of coke-oven gas fiom 200-500 g/100 scf to 2-7 g/100 scf. 

Rich solution from the bottom of the wash tower is preheated by heat exchange with stripped water then fed to the top of the free ammonia stripper, which operates with direct steam injection at the bottom. Stripped water flnm this column is cooled and recycled to the top of the wash column. Excess stripped water, which stiU contains fixed ammonia, is processed in the fixed ammonia stripper before disposal. Caustic soda (or lime) is used to... 

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