Recycling extent

Figure 27.2 summarizes the effect of recycle extent on first catalyst bed exit gas temperature for a feed gas containing 13.0% SO2 and 14.3% O2 with the balance being N2. For simplicity, it assumes that feed gas and recycle gas temperatures are the same, 660 K (Fig. 27.1). 

Figure 27.2 Effect of recycling cooled first catalyst exit gas on first catalyst bed exit gas temperature. As expected, exit gas temperature decreases with increasing recycle extent. First catalyst bed exit gas temperature falls below 900 K with 8% recycle.

Figure 27.5 Effect of recycle extent on the percentage of feed gas SO2 that is oxidized to SO3 in the first catalyst bed. The increase with increasing recycle is notable.

The three recycle structures shown in Fig. 4.2 also can be used with this case. Because the BYPRODUCT is now being formed by a secondary reaction which is reversible, its formation can be inhibited by recycling BYPRODUCT as shown in Fig. 4.3a. In Fig. 4.3a, the BYPRODUCT formation is inhibited to the extent that it is effectively stopped. In Fig. 4.36 it is only reduced and the net BYPRODUCT formation removed. Again, the separation configuration will change between different processes as the order of volatility between the components changes. 

Feed purification. Impurities that enter with the feed inevitably cause waste. If feed impurities undergo reaction, then this causes waste from the reactor, as already discussed. If the feed impurity does not undergo reaction, then it can be separated out from the process in a number of ways, as discussed in Sec. 4.1. The greatest source of waste occurs when we choose to use a purge. Impurity builds up in the recycle, and we would like it to build up to a high concentration to minimize waste of feed materials and product in the purge. However, two factors limit the extent to which the feed impurity can be allowed to build up ... 

Liquid Effluents. Recycling of acid, soda, and zinc have long been necessary economically, and the acid—soda reaction product, sodium sulfate, is extracted and sold into other sectors of the chemical industry. Acid recovery usually involves the degassing, filtering, and evaporative concentration of the spent acid leaving the spinning machines. Excess sodium sulfate is removed by crystallization and then dehydrated before sale. Traces of zinc that escape recovery are removable from the main Hquid effluent stream to the extent that practically all the zinc can now be retained in the process. 

Total consumption of lead in the United States in 1993 reached 1,318,800 t. Of this, 766,000 t (58%) is allocated to battery use suppHed as either a mixed oxide or as metal. Approximately 95% of batteries are recycled and the lead recovered. In 1993, 908,000 t of lead came from secondary smelters and refiners compared to 350,000 t originating in primary mines and smelters (39). Approximately 51,000 t of lead was consumed in U.S. production of all oxides and chemicals appHcable to all industries other than batteries. Estimates include 8000 t for plastics, 6000 t for gasoline additives, 2000 t for mbber, and 30,000 t for ceramics, glass, and electronics. Lead is not used to any extent in dispersive appHcations such as coatings. 

Solvent Process. In the solvent process, or solvent cook, water formed from the reaction is removed from the reactor as an a2eotropic mixture with an added solvent, typically xylene. Usually between 3 to 10 wt % of the solvent, based on the total charge, is added at the beginning of the esterification step. The mixed vapor passes through a condenser. The condensed water and solvent have low solubiUty in each other and phase separation is allowed to occur in an automatic decanter. The water is removed, usually to a measuring vessel. The amount of water collected can be monitored as one of the indicators of the extent of the reaction. The solvent is continuously returned to the reactor to be recycled. Typical equipment for this process is shown in Figure 2. The reactor temperature is modulated by the amount and type of refluxing solvent. Typical conditions are ... 

Either product can be favored over the other by proper selection of catalyst and reaction conditions. However, the principal source of DIPE is as a by-product from isopropyl alcohol production. Typically, excess DIPE is recycled over acidic catalysts ia the alcohol process where it is hydrated to isopropyl alcohol. DIPE is used to a minor extent ia iadustrial extraction and as a solvent. 

In cases where a large reactor operates similarly to a CSTR, fluid dynamics sometimes can be estabflshed in a smaller reactor by external recycle of product. For example, the extent of soflds back-mixing and Hquid recirculation increases with reactor diameter in a gas—Hquid—soflds reactor. Consequently, if gas and Hquid velocities are maintained constant when scaling and the same space velocities are used, then the smaller pilot unit should be of the same overall height. The net result is that the large-diameter reactor is well mixed and no temperature gradients occur even with a highly exothermic reaction. 

The various designs differ in the extent to which heat exchange is used, in the plan of the pipe-stiU furnace, in the distillation pressure, and in whether recycle of pitch or base tar is involved. 

Some automobile manufactuieis have found the in-shop recycling equipment to be ill-advised one service bulletin warns of the deterioration of engine-coolant ethylene glycol to the extent that no additive can restore it to an acceptable state (21). 

Recycling and Reuse 9. Recycle cooling water and treated wastewater to the extent feasible. 10. Recover and reuse spent solvents and other chemicals to the extent feasible. 

Recover zinc from EAF dust containing more than 15% total zinc recycle EAF dust to the extent feasible. 

The extent to which each of the above reactions occur is strongly influenced by feed quality and the levels selected for the major process variables pressure, temperature, recycle rate, and frequency of regeneration. From a process viewpoint, these variables affect catalyst requirement, gasoline yield, and coke make. 

The source-sink mapping diagram can also be used to determine the extent of interception needed. If a source lies to the right of a sink, it can be intercepted to render it within the band of acceptable recycle. The problem of simultaneously... 

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