Flash with Recycle Problem

To master the concepts of recycle analysis, it is recommended that the reader solve several of the exercises at the end of the chapter. Of these, the so-called flash with recycle problem (Exercise 4.1a) should be tackled first. Although it involves just one recycle loop, it demonstrates a very important principle. See if you can identify it  

Note that the flash with recycle process is a good representation of a quench vessel, in which hot gases, typically from an exothermic reactor, are quenched by a cold liquid recycle. Quenches are often needed to provide rapid cooling of a reactor effluent by direct-contact heat transfer. Cold liquid is showered over hot, rising gases. As some of the liquid vaporizes, the latent heat of vaporization is absorbed, and cooling occurs. Quenches are particularly 

Methane Ethane Propane n - Butane 1 - Butene 1,3 Butadiene 5,130 

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Next, it is recommended that the reader solve a variation on the flash with recycle problem. In this variation (Exercise 4.1b), case 3 is modified so as to determine the flash temperature to obtain 850 Ib/hr of overhead vapor. 

An excellent review or LLE applications in the food industry is given by Hamm,17 Since solvent toxicity is a major consideration, supercritical solvents, such as CO, which are nomoxic and LLE from an aqueous leachate are two popalar means of deriling with this problem. Figure 7.8-11 is a cooceptual supercritical extraction flowsheet. Because Tc is much lower than either 7 or rB, the solvent (e.g., C02) simply is flashed from the solute by throttling (an isenthalpic process). Then the vapors are compressed (an isentropic process) and cooled (an isobaric step) to complete the solveni recycle. Usually, costs are determined by the compressor requirearerUs. Several potential applications of supercritical or near-critical solvents are discussed in more datail else where,... 

It is impractical to analyze the multistage flash drum process with the graphical method we devised for a single flash drum. The problem is the recycle streams -it is awkward to determine the net composition of the flow into a particular drum. For example, how would you account for the liquid effluent from the second unit in Figure 4.62 being recycled to the first unit Yet multistage distillation should be no more difficult to analyze than multistage absorption. What is the problem ... 

A recycle single-entry separator may have part of its tails stream recycled to the feed (flash distillation. Figure 2.2.1(c)). Examples of radioactive rare gas separation with various recycle arrangements are given in Ohno et al. (1977, 1978). See Problem 2.2.7. One aspect to be noted while comparing separators with or without recycle is that since 0 is constant, but the actual feed flow rate to the separator changes with recycle from that without recycle, the dimensions of the two separators will be different See, however. Problem 2.2.8. 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

In the United States, automotive and other Industrial sources generate about 1.2 billion gallons of used oil each year. The most pertinent features of this oil with regard to recycle and disposal problems are flash point and water, sediments, ash, nitrogen, and oxygen content. Many of the impurities are dispersed in the oil due to the very effective dispersant characteristics of the additives in a modern-day lubricant system(1) ... 

The flowsheet in Fig 3.37 consisting of three nested loops of mixers and flashes, is a standard problem in flowsheeting known as the Cavett s problem (Rosen, 1980). Table 3.9 gives the cycle matrix, where 1 marks each stream included in a recycle loop. For example, the stream Z, appears both in the loops 1 and 3, while the stream Sj only in the loop 3. The last column shows the number of loops tom by each stream, and may be used to evaluate the tearing action. An obvious choice would be to consider three tear streams for three recycles, as for example R Rj, R3, which tom each loop only one. Another choice would be R, Sj, and R3, where Sj would replace Rj. However, it is possible to solve the flowsheet only with two tear streams, as Z, and Zj, now each breaking two loops. Zj may be also replaced by R3 or S3. Hence, several tear sets are... 

These facts have led to the review and finally the development of an alternative approach and in particular the possibilities of recovering vanadium for metallurgical use. Figure 100 illustrates the principles of the soot ash removal unit. The carbon slurry from the SGP unit is flashed to atmospheric pressure in the slurry tank (a). The slurry is then filtered on an automatic filter (b) to recover a filter cake with about 80% residual moisture and a clear water filtrate. The filter cake is subjected to a controlled oxidation process in a multiple-hearth furnace (c). This type of furnace, which is well established in many industries and specifically in the vanadium industry, allows combustion of the carbon to occur under conditions where the vanadium oxides neither melt nor corrode. This is not an easy task if one considers the problems of burning a high-vanadium fuel oil in a conventional boiler. The product is a vanadium concentrate, which contains about 75% V2O5. Compared to the old naphtha extraction-based recycle system, the new once-through process consists of only two proces-... 

Problem 8.2 Use the data for the system methanol-ethanol (from the previous problem) to design a separation process that takes a mixture with 20% methanol and produces a stream that contains 90% methanol by continuously flashing the vapor stream until the required purity is reached (the liquid streams are not recycled). The process is to be operated at 1 bar with a ratio V/L = 0.5 in all flash separators. 

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