Recycle configurations

Sometimes it is extremely difficult to avoid vapor recycles without using very high pressures or very low levels of refrigeration, in which case we must accept the expense of a recycle compressor. However, when synthesizing the separation and recycle configuration, vapor recycles should be avoided, if possible, and liquid recycles used instead. 

The delay in the development of a general synthesis strategy that will set the basis for a rigorous and systematic search for the optimal reactor/separator/recycle configuration is mainly due to the difficulties arising from the large number of structural alternatives and the nonlinear design... 

Curcio S, Calabro V, Iorio G (2006) A theoretical and experimental analysis of a membrane bioreactor performance in recycle configuration. J Membr Sci 273 129-142... 

Exxon and Phillips manufacture polypropylene in tubular reactors where the monomer is in the liquid form (see Section 6.8.2). One of the manufacturing processes for polyethylene involves the use of a loop reactor that has a recycle configuration. Here, under elevated pressure and temperature, a mixture of the catalyst, comonomer, hydrogen, and a solvent are introduced from one end of the reactor. The product and the unreacted starting materials are collected at the other end, and recycled back into the reactor. 

The patent literature also teaches the importance of countercurrent contacting [13] and recycle configurations to improve process performance [14], Auvil et al. [14] describe two configurations in which the wet permeate is recycled to eliminate feed air losses. If the feed is at ambient pressure, the permeate is sent to the inlet of the feed compressor. If the feed is already at pressure, a recycle compressor is used to inCTease the permeate pressure to the feed pressure. In both configurations, water is removed from the process in the chiller/condenser that follows the compressor. 

Theoretical model solutions for single pass and recycle configurations are reported in Calabrb et al. (2002) and Curdo et al. (2006a), respectively. 

Curcio, S., V. Calabro, and G. lorio. 2006. A Theoretical and Experimental Analysis of a Membrane Bioreactor Performance in Recycle Configuration. Journal of Membrane Science 273 (1-2) 129-142. 

A direct recycle configuration was then added (Figure 2.4). Depending on the type of pretreatment, the recycle stream vapor can contain water or a mixture of water and chemicals. From the convergence methods available in Aspen, the Broyden method was selected to solve the direct recycle flowsheet. Several simulations were run in order to analyze the operation cost (energy cost + chemical cost) as a function of the vapor fraction sent to the recycle the vapor fraction with the minimum cost was then identified. 

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. 

Reactor diluents and solvents. As pointed out in Sec. 2.5, an inert diluent such as steam is sometimes needed in the reactor to lower the partial pressure of reactants in the vapor phase. Diluents are normally recycled. An example is shown in Fig. 4.5. The actual configuration used depends on the order of volatilities. 

The environmental appHcations of infrared spectrometry are many and varied. Many appHcations at industrial sites are analogous to those for on-line process analysis waste streams and recycling processes can be monitored in the same way. Commercial infrared stack-gas monitors are based on either an extractive probe attached to a long-path gas ceU or an open-path (across stack) configuration (69). Stack plume and flare monitoring can be done externally... 

Methanol Synthesis. AH commercial methanol processes employ a synthesis loop, and Figure 6 shows a typical example as part of the overall process flow sheet. This configuration overcomes equiUbtium conversion limitations at typical catalyst operating conditions as shown in Figure 1. A recycle system that gives high overall conversions is feasible because product methanol and water can be removed from the loop by condensation. 

Ammonia Synthesis and Recovery. The purified synthesis gas consists of hydrogen and nitrogen in about 3 1 molar ratio, having residual inerts (CH Ar, sometimes He). The fresh make-up gas is mixed with the loop recycle and compressed to synthesis pressures. AH modern synthesis loops recycle the unreacted gases because of equiUbrium limitations to attain high overall conversions. The loop configurations differ in terms of the pressure used and the point at which ammonia is recovered. 

In comparison, units that are designed with turbulent beds have a lower superficial velocity limit because of soflds entrainment and are unable to independently control the entrained soflds recycle. The soflds loading in the turbulent-bed regenerator configuration are equal to the reactor—regenerator circulation plus the entrained soflds via the cyclone diplegs. 

Another synthetic sequence leading to penicillin derivatives is illustrated in Scheme 61 (77MI51102). Note that the cycloaddition of azidoacetyl chloride to the thiazoline affords an azidopenam with the 6-epi configuration. Equilibration leads to a mixture of (81) and (82) in a ratio of 4 1, but repeated recycling allowed the isolation of (82) in 40% yield. A related synthesis was shown in Scheme 26. 

Figure 6-51. Recycle trip configured to minimize process disturbances.

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