Recycle Loops

Xylene Isomerization. After separation of the preferred xylenes, ie, PX or OX, using the adsorption or crystallization processes discussed herein, the remaining raffinate stream, which tends to be rich in MX, is typically fed to a xylenes isomerization unit in order to further produce the preferred xylenes. Isomerization units are fixed-bed catalytic processes that are used to produce a close-to-equiUbrium mixture of the xylenes. To prevent the buildup of EB in the recycle loop, the catalysts are also designed to convert EB to either xylenes, benzene and lights, or benzene and diethylbenzene. 

The SRC-II process, shown in Figure 2, was developed in order to minimise the production of soHds from the SRC-I coal processing scheme. The principal variation of the SRC-II process relative to SRC-I was incorporation of a recycle loop for the heavy ends of the primary Hquefaction process. It was quickly realized that minerals which were concentrated in this recycle stream served as heterogeneous hydrogenation catalysts which aided in the distillate production reactions. In particular, pyrrhotites, non stoichiometric iron sulfides, produced by reduction of iron pyrite were identified as being... 

Time-Delay Compensation Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow distance lags, and dead time in composition measurements resulting from use of chromatographic analysis. The presence of a time delay in a process severely hmits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that which could be used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

While it is easy to add materials to a fermentation, removal is difficult. Membrane devices have been placed in the fermenter or in external recycle loops to dialyze away a soluble component. Cells release wastes or metabolites that can be inhibitory these are sometimes referred to as staling factors. Their removal bv dialysis has allowed cell concentrations to reach ten to one hundred times that of control cultures. 

If chemical reactions occur only over the catalyst and none on the walls or in the homogeneous fluid stream in the recycle loop, then conservation laws require that the two balances should be equal. 

The unit was built in a loop because the needed 85 standard m /hour gas exceeded the laboratory capabilities. In addition, by controlling the recycle loop-to-makeup ratio, various quantities of product could be fed for the experiments. The adiabatic reactor was a 1.8 m long, 7.5 cm diameter stainless steel pipe (3 sch. 40 pipe) with thermocouples at every 5 centimeter distance. After a SS was reached at the desired condition, the bypass valve around the preheater was suddenly closed, forcing all the gas through the preheater. This generated a step change increase in the feed temperature that started the runaway. The 20 thermocouples were displayed on an oscilloscope to see the transient changes. This was also recorded on a videotape to play back later for detailed observation. 

Reflux overhead vapor recompression, staged crude pre-heat, mechanical vacuum pumps Fluid coking to gasification, turbine power recovery train at the FCC, hydraulic turbine power recovery, membrane hydrogen purification, unit to hydrocracker recycle loop Improved catalysts (reforming), and hydraulic turbine power recovery Process management and integration... 

Refiners can address the sulfur issue in stages, but decisions should be made that will leave the door open for further reductions. If hydrotreating is selected, the design can include oversized reactors, connections for a spare compressor, or connections for adding amine scrubbing inside the recycle loop. Some process or catalyst changes can buy time, some can solve the problem. 

Sulfonation plants are normally equipped with neutralization units based on pressure recycle loops, simple or multistage, composed of in-line high-shear... 

A real continuous-flow stirred tank will approximate a perfectly mixed CSTR provided that tmix h/i and tmix i. Mixing time correlations are developed using batch vessels, but they can be applied to flow vessels provided the ratio of throughput to circulatory flow is small. This idea is explored in Section 4.5.3 where a recycle loop reactor is used as a model of an internally agitated vessel. 

Recycling of partially reacted feed streams is usually carried out after the product is separated and recovered. Unreacted feedstock can be separated and recycled to (ultimate) extinction. Figure 4.2 shows a different situation. It is a loop reactor where some of the reaction mass is returned to the inlet without separation. Internal recycle exists in every stirred tank reactor. An external recycle loop as shown in Figure 4.2 is less common, but is used, particularly in large plants where a conventional stirred tank would have heat transfer limitations. The net throughput for the system is Q = but an amount q is recycled back to the reactor inlet so that the flow through the reactor is Qin + q- Performance of this loop reactor system depends on the recycle ratio qlQin and on the type of reactor that is in the loop. Fast external recycle has... 

Example 4.13 treated the case of a piston flow reactor inside a recycle loop. Replace the PER with two equal-volume stirred tanks in series. The reaction remains first order, irreversible, and at constant density. 

Reaction occurs in the loop as well as in the stirred tank, and it is possible to eliminate the stirred tank so that the reactor volume consists of the heat exchanger and piping. This approach is used for very large reactors. In the limiting case where the loop becomes the CSTR without a separate agitated vessel, Equation (5.35) becomes q/Q > 10. This is similar to the rule-of-thumb discussed in Section 4.5.3 that a recycle loop reactor approximates a CSTR. The reader may wonder why the rule-of-thumb proposed a minimum recycle ratio of 8 in Chapter 4 but 10 here. Thumbs vary in size. More conservative designers have... 

Example 15.6 Determine the washout function if a diffusion-free, laminar flow reactor is put in a recycle loop. Assume that 75% of the reactor effluent is recycled per pass. 

Figure 6 shows typical results obtained with the plug-flow quartz reactor containing 0.5 g of Sr(lwt%)/La203 catalyst operated in the continuous flow recycle mode. The inlet CH partial pressure was 20 kPa (20% CH in He) at inlet flowrates of 7.1 and 14.3 cm STP/min. A 20% O2 in He mixture was supplied directly, at a flowrate Fog, in the recycle loop via a needle valve placed after the reactor (Fig. 1). The methane conversion was controlled by adjusting Fog, which was kept at appropriately low levels so that the oxygen conversion...

The model assumes a well-mixed gas phase composition in the recycle loop, a well justified assumption in view of the very high (10-200) recycle ratio values used in the present work. For the batch electrocatalytic version we also neglect volume changes and assume linear kinetics for steps 1,3 and 4 of the consecutive OCM network (1), i.e. ... 

Following the arguments given above, micro mixers are valuable tools for bench-scale continuous processing to fill the recycle loop. For first tests on feasibility, the total volume of the recycle loop has to be kept small, demanding also for small-scale processing units. Conventional devices would probably suffer here from too large internal volumes. 

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