Recycle control

It should be recognized that implementing this control structure will require some piping changes, and pump heads may need to be modified. However, the improvement in dynamic performance may well justify these modifications. 

The dynamic responses of these three control structures are compared using a numerical example discussed in the next section. Dynamic simulations are performed using Aspen Dynamics. 

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Recycle control loops intended for accident prevention Emergency shutdown features... 

A fourth method for waste reduction in reactors focuses on spent catalyst wastes where recycling, controlling attrition, and limiting deactivation may reduce wastes. Consider the case of hydroprocessing catalyst wastes, which are extensively recycled to recover cobalt, molybdenum, nickel, vanadium, and alumina (Chemical and Engineering News, October 26, 1992, p. 20). More than a third of the domestic demand for vanadium is met via these recovery processes (Chemical and Engineering News, November 23, 1992,... 

Figure 13.20 CSTR-Separator-Recycle control structures...

Figure 15.10 gives a direct comparison of the performances of the three control structures for the same ramp disturbances. The dashed lines are for the two-temperature control structure. The dotted lines are for the VPC control structure. The solid lines are for the recycle control structure. 

Figure 15.13 gives results for the recycle control stmcture. The top left graph shows the series of step changes in the liquid feed to the upstream flash tank. The third left graph... 

Figure 15.14 provides a direct comparison among the three control structures. The superiority of the recycle control structure is clearly demonstrated. Notice that the other... 

Raw biomass is extremely persistent to enzymatic degradation. Therefore, methods of thermochemical pretreatment have been developed to improve the digestibility of biomass (18). For example, ammonia explosion, aqueous ammonia recycle, controlled pH, dilute acid, flow through, and hme treatment were applied to prepare com stover, i.e., the leaves and stalks of maize, for a subsequent conversion to sugars. 

The installation of pumps requires special care to provide enough suction head to avoid cavitation. The net suction head can disappear if a pump generates too much heat in the condensate. It is important to maintain enough flow through the pump at all times to prevent this. Minimum flow can be provided by recycle controlled by an automatic valve or by placing an orifice assembly in a recycle line. As a rule of thumb, a flow of 1 hr is necessary for each 12kW of input if the temperature rise is to be kept to 10° C. The recycle flow must be cooled or go into a heat sink such as a condensate hotwell or boiler feed deaerator. A simple recycle into the pump suction line is inadequate. 

Further, we will briefly comment other interesting works in this area. Pushpavanam Kienle [16] studied the reaction A- P in a non-isothermal CSTR / Separation / Recycle process. Assuming infinite activation energy and equal eoolant and reactor-inlet temperatures, they reported state multiplicity, isolated solution branches and instability, for both conventional and fixed-recycle control structures. In addition, the conventional structure showed regions of unfeasibility. The authors claimed the superiority of the fixed-recycle control structure over the fixed-fresh flow rate control. 

If one of the reactants is recycled, control of its flow is not critical at all, because it is always in excess. And since it is ordinarily separated entirely from the products, addition of fresh reactant can be manipulated by inventory control. Figure 10.8 shows how a liquid reactant is added to the recycle stream to make up what, is consumed in the reaction. 

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