Flash with recycle process

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... 

Flash with recycle a. Consider the flash separation process shown in Figure 4.15. If using ASPEN PLUS, solve all three cases using the MIXER, FLASH2, FSPLIT, and PUMP subroutines and the RK-SOAVE option set for thermophysical properties. Compare and discuss the flow rates and compositions for the overhead stream produced by each of the three cases. 

In the process (Figure 9-12), the feedstock (a vacuum residuum) is mixed with recycle vacuum residue from downstream fractionation, hydrogen-rich recycle gas, and fresh hydrogen. This combined stream is fed into the bottom of the reactor whereby the upward flow expands the catalyst bed. The mixed vapor liquid effluent from the reactor, either goes to flash drum for phase separation or the next reactor. A portion of the hydrogen rich gas is recycled to the reactor. The product oil is cooled and stabilized and the vacuum residue portion is recycled to increase conversion. 

Distillation, like flashing, is a process which is generally used to separate a mixture of two or more liquids based on their boiling points. However, what happens in a distillation column is essentially a series of flashes, which are connected with recycle loops. The liquid from each tray comes to equilibrium (ideally) with the vapor, and the vapor rises up to the next tray and the liquid falls to the tray beneath it. Each tray has a different temperature because a reboiler on the bottom and a condenser at the top maintain a temperature gradient across the column (in certain separation setups one of these components is omitted). 

For the reactor-flash-recycle process introduced in Example 20.4, Figure 20.15 shows a control system with the control objectives to ... 

The Fluor Solvent process uses flash regeneration as depicted in Figure 14-1(A). Commercial plants utilize several flashes of the rich solvent at decreasing pressure levels with recycling of the gas evolved in the high pressure flash. Lean solvent temperatures as low as... 

The liquid ammonia from the separator contains a small amount of dissolved gases. These are partly released by pressure reduction in a let-down vessel normally to about 20 bar. The gases released are normally used as fuel. Recycle to the synthesis gas compressor has been proposed [635]. Flashing at an intermediate pressure with recycle of the released gas to the process feed has been used, e.g. in revamp projects [452]. 

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. 

A flow diagram of the solvent-refined coal or SRC process is shown ia Figure 12. Coal is pulverized and mixed with a solvent to form a slurry containing 25—35 wt % coal. The slurry is pressurized to ca 7 MPa (1000 psig), mixed with hydrogen, and heated to ca 425°C. The solution reactions are completed ia ca 20 min and the reaction product flashed to separate gases. The Hquid is filtered to remove the mineral residue (ash and undissolved coal) and fractionated to recover the solvent, which is recycled. 

Meanwhile, the fatty acids are purified before they are reacted with caustics to produce soaps. The steps involve a flash evaporation to remove water, and a vacuum distillation that removes some more water, any gases, and a fatty residue, which is recycled through the splitter. The vacuum still also separates the acids into two different streams. One of these is used to make toilet soaps and the other, industrial soaps. The process for making the industrial soap is not shown, but it is similar to that shown for toilet soaps. The soap is made in the saponifier. A typical reaction is... 

In the slurry process, propylene monomer is dissolved in a hydrocarbon diluent in which the polymerization process occurs. The polymerization products are either soluble (the highly atactic components) or insoluble. Both the insoluble and soluble components are collected and form separate product streams. The insoluble species form a slurry in the solvent, from which they are removed by centrifugation. The soluble, atactic component is removed with the solvent as another product stream. To separate the atactic polymer from the solvent, the solution is heated allowing the solvent to flash off, leaving the atactic polymer behind. Any un reacted monomer is degassed from the solution and recycled to the start of the polymerization process. 

---