Recycle to extinction

Heuristic 6 Byproducts that are produced in reversible reactions, in small quantities, are usually not recovered in separators or purged. Instead, they are usually recycled to extinction. 

When the reaction proceeds reversibly, however, it becomes possible to achieve an equilibrium conversion at steady state by recycling product species without removing them from the process. In so doing, it is often said that undesired byproducts are recycled to extinction. It is important to recognize this when distributing chemicals in a potential flowsheet so as to avoid the loss of chemicals through purge streams or the insertion of expensive separation operations. Recycle to extinction, which is considered in more detail in Section 8.5, is most effective when the equilibrium conversion of the side reaction is limited by a small chemical equilibrium constant at the temperature and pressure conditions in the reactor. 

Note that this eliminates one of the two waste streams from the process. The other, which loses large quantities of Hj, is the gas purge stream. To avoid this loss, the use of membrane or adsorption separators should be considered.  

Heuristic 7 For competing reactions, both in series and parallel, adjust the 

When selectivity is the key to the success of a process design, it is not uncommon to carry out an extensive analysis of the reactor alone, before distributing the chemicals, and proceeding with the synthesis of the flowsheet. In other cases, using simulation models, the distribution of chemicals is carried out as the process is optimized to achieve an economic objective. 

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The water—carbon slurry formed in the quench vessel is separated from the gas stream and flows to the carbon recovery system needed for environmental reasons and for better thermal efficiency. The recovered carbon is recycled to the reactor dispersed in the feedstock. If the fresh feed does not have too high an ash content, 100% of the carbon formed can be recycled to extinction. 

The thermodynamic equilibria are illustrated in Figures 1 and 2. Figure 1 shows the resulting composition after pure pseudocumene or a recycle mixture of C PMBs is disproportionated with a strong Friedel-Crafts catalyst. At 127°C (400 K), the reactor effluent contains approximately 3% toluene, 21% xylenes, 44% C PMBs, 29% C q PMBs, and 3% pentamethylbenzene. The equihbrium composition of the 44% C PMB isomers is shown in Figure 2. Based on the values at 127°C, the distribution is 29.5% mesitylene, 66.0% pseudocumene, and 4.5% hemimellitene (Fig. 2). After separating mesitylene and hemimellitene by fractionation, toluene, xylenes, pseudocumene (recycle plus fresh), C q PMBs, and pentamethylbenzene are recycled to extinction. 

Superffex C t lytic Crocking. A new process called Superflex is being commercialized to produce predorninantiy propylene and butylenes from low valued hydrocarbon streams from an olefins complex (74). In this process, raffinates (from the aromatics recovery unit and the B—B stream after the recovery of isobutylene) and pyrolysis gasoline (after the removal of the C —Cg aromatics fraction) are catalyticaHy cracked to produce propylene, isobutylene, and a cmde C —Cg aromatics fraction. AH other by-products are recycled to extinction. 

Figure 3-13. The influence of conversion severity on the theoretical product yield for the cracking of propane. Acetylene, methyl acetylene, and propadiene are hydrogenated and both ethane and propane are recycled to extinction (wt%)." ...

HCO is the sidecut stream from the main column that boils between LCO and decanted oil (DO). HCO is often used as a pumparound stream to transfer heat to the fresh feed and/or to the debutanizer reboiier. HCO is recycled to extinction, withdrawn as a product and processed in a hydrocracker, or blended with the decant oil. 

DO is the heaviest product from a cat cracker. DO is also called slurry oil, clarified oil, bottoms, and FCC residue. Depending on the refinery location and market availability, DO is typically blended into No. 6 fuel, sold as a carbon black feedstock (CBFS), or even recycled to extinction. 

Equipment. All of the catalysts were tested in continuous flow, fixed-bed pilot plants equipped for both liquid and gas recycle operation and continuous distillation of products. Hydrocarbons boiling above the desired product end point were recycled to extinction, that is, to 100 conversion of fresh feed. The product was cooled and passed into a high pressure phase separator. Here, hydrogen-rich recycle gas was flashed from the hydrocarbon product and recycled back to the reactor inlet. Electrolytic hydrogen make-up was added on demand to maintain constant system pressure. 

A possible solution is to gasify the more dilute vacuum tower bottoms product in an oxygen blown gasifier and to convert the excess synthesis gas to methanol. In those cases where a Flexicoker is used the heavy scrubber liquids could be recycled to extinction. Therefore, the plant products are SNG, naphtha, 300-800°F distillate and methanol. All of these products are of high quality or can be hydrotreated to achieve high quality. As a result, they could be easily integrated into the utility fuel mix with a minimum amount of disruption or special product handling facilities. 

Can any impurity be recycled to extinction This is possible if the impurities are formed by a reversible reaction while the main reaction is irreversible. 

Early attempts to convert methanol into olefins were based on the zeolite ZSM-5. The Mobil MTO process was based on the fluidised bed version of the MTG technology. Conversion took place at about 500°C allegedly producing almost complete methanol conversion. However, careful reading of the patent Uterature indicates that complete methanol conversion may not have been achieved by this means. Because of incomplete conversion, there would be a necessity to strip methanol and dimethyl ether from water and hydrocarbon products in order to recycle unconverted methanol. In this variant, the total olefin yield is less than 20% of the products of which ethylene is a minor but not insignificant product. The major product is gasoUne. Ethylene is difficult to process and has to be treated specially. Claims that it is possible that ethylene can be recycled to extinction conflict with the known behaviour of ethylene in zeolite catalyst systems and have to be viewed with some suspicion. 

The process has been demonstrated on a pilot scale by Lurgi and Statoil. Sufficient propylene has been produced to make polypropylene resin product by Borealis. This process appears to use an oxide doped ZSM-5 zeolite catalyst in fixed bed reactors. The oxide doping promotes the methanol conversion to olefins. All olefins, other than propylene, are recycled to extinction or purged as fuel gas or produced as naphtha. The flow sheet is illustrated in the Figure 11.8. 

Figure 5. Reactor temperature and specific gravity of the recycle feed as a function of time-on-stream for recycle-to-extinction and single-pass modes of hydrocracking using NiW/RE-X catalyst. (Reproduced from reference 45, Copyright 1983 American Chemical Society.)...

Recycling to extinction is uneconomic because polycyclic aromatics concentrate in the cycle gas oil and these compounds are not only hard to crack but are converted largely to coke and gaseous products. On the other hand, partial recycling is commonly practiced. In the United States, cycle gas oil constitutes, on the average, about 20% of the total feed to catalytic cracking (16,257). The material recycled may be a portion of... 

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