Reactants makeup

The two fresh reactant makeup feed streams (one gas for hydrogen... 

Figure 2.11 Ternary process with complete one-pass conversion of reactant B. (a) Ratio control structure with fixed reactant feed (unworkable) (i>) reactant makeup control based on component inventory ( workable).

Figure 2.13 Ternary process flowsheet with incomplete conversion and two recycle streams (.heavv-out-first sequence . iai Control structure CS4 reactor composition and level control (workable. (61 control structure CS1 reactant makeup control based on component inventories t workable).

When the setpoint of a dominant variable is used to establish plant production rate, the control strategy must ensure that the right amounts of fresh reactants are brought into the process. This is often accomplished through fresh reactant makeup control based upon liquid levels or gas pressures that reflect component inventories. Wre must keep these ideas in mind when we reach Steps 6 and 7. 

However, design constraints may limit our ability to exercise this strategy concerning fresh reactant makeup, An upstream process may establish the reactant feed flow sent to the plant. A downstream process may require on-demand production, which fixes the product flowrate from the plant. In these cases, the development of the control strategy becomes more complex because we must somehow adjust the setpoint of the dominant variable on the basis of the production rate that has been specified externally. We must balance production rate with what has been specified externally. This cannot be done in an open-loop sense, Feedback of information about actual internal plant conditions is required to determine the accumulation or depletion of the reactant components. This concept was nicely illustrated by the control strategy in Fig. 2.16, In that scheme we fixed externally the flow of fresh reactant A feed. Also, we used reactor residence time (via the effluent flowrate)... 

Inventory may also be controlled with fresh reactant makeup streams as discussed in Step 4. Liquid fresh feed streams may be added to a location where level reflects the amount of that component in the pro-... 

If we select temperature, we would like the reactor flow and composition to be nearly constant and we are constrained by the upper reactor temperature limit of 1300°F. If we select toluene composition, we can control it either directly or indirectly. If directly, a reactor feed composition analyzer is needed and is used to adjust either the fresh toluene feed rate or the total reactor toluene feed rate. If indirectly, the separation section is used as an analyzer for toluene. This allows us to control the total flow of toluene to the reactor (recycle plus fresh). Fresh toluene feed flow is used to control toluene inventory reflected in the recycle column overhead receiver level as an indication of the need for reactant makeup. Controlling the total toluene flow sets the reactor composition indirectly and is advantageous because it is less complicated and does not require an on-line analyzer. 

The process considered in this chapter involved the production of vinyl acetate monomer. It features many unit operations, many components, nonideal phase equilibrium, unusual reaction kinetics, two recycle streams, and three fresh reactant makeup streams. 

The feed rate is manually set as required for either pH or reactant stoichiometry control. Soda ash is added to the fourth reactor as sodium makeup. Reactor effluent slurry flows by gravity to the thickener centerwell. Clarified liquor overflows from the thickener to the forward feed hold tank from which it is pumped to the tray tower. A horizontal belt filter is used for further dewatering of the thickener underflow solids. 

Estimating the inventory of reactants and anticipating their dynamic effects is fundamental in the design and control of chemical plants. The occurrence of nonlinear phenomena is often interrelated with the method of controlling the makeup of fresh reactants [8]. There are two methods for controlling the component inventory in a plant. By self-regulation the fresh reactant is set on flow control at a value given by the desired production rate. No attempt is made to measure or... 

When several reactants are involved, the two strategies may be combined. The suggested approach may be summarized as follows design the plant for high conversion of the reference reactant, set its feed on flow control, fix the recycle flows, and adjust the makeup of the other reactants to keep the reactor-inlet flows or concentrations at constant values. 

In the second control structure (Fig. 2.11b), which does work, the fresh feed makeup of the limiting reactant (.F0B) is flow-controlled. The other fresh feed makeup stream (FCvl) is brought into the system to control the liquid level in the reflux drum of the distillation column. The inventory in this drum reflects the amount of A inside the system. If more A is being consumed by reaction than is being fed into the process, the level in the reflux drum will go down. Thus this control structure employs knowledge about the amount of component A in the system to regulate this fresh reactant feed makeup to balance exactly the amount of B fed into the process. 

Increasing the reactor temperature setpoint increases the production rate of vinyl acetate, so there must ultimately be net increases in all three fresh reactant feed streams. Oxygen and ethylene flows respond fairly quickly within about 20 minutes. However, the acetic acid feed actually decreases for the first 60 minutes in response to an increase in column base level. These results demonstrate the slow dynamics of the liquid recycle loop and illustrate the need for controlling the total acetic acid flow to the reactor so that the separation section does not see these large swings in load ( snowball effect"). The variability is absorbed by the fresh feed makeup stream. 

In contrast to LC detectors, GC detectors often require a specific gas, either as a reactant gas or as fuel (such as hydrogen gas as fuel for flame ionization). Most GC detectors work best when the total gas flow rate through the detector is 20-40 mL/min. Because packed columns deliver 20-40 mL/min of carrier gas, this requirement is easily met. Capillary columns deliver 0.5-10 mL/min thus, the total flow rate of gas is too low for optimum detector performance. In order to overcome the problem when using capillary columns, an appropriate makeup gas should be supplied at the detector. Some detectors use the reactant gas as the makeup gas, thus eliminating the need for two gases. The type and flow rate of the detector gases are dependent on the detector and can be different even for the same type of detector from different manufacturers. It is often necessary to refer the specific instrument manuals for details to obtain the information on the proper selection of gases and flow rates. All detectors are heated, primarily to keep the... 

Silicone chemists create structures similar to those of the silicates by adding various reactants to create silicone chains, sheets, and frameworks. Chains are oily liquids used as lubricants and as components of car polish and makeup. Sheets are components of gaskets, space suits, and contact lenses. Frameworks find uses as laminates on circuit boards, in nonstick cookware, and in artificial skin and bone. 

In control scheme B, sketched in Fig. 6.8, the total recycle flow rate to the reactor (distillate plus makeup A) is flow controlled. The makeup of reactant A is used to hold the level in the reflux drum. This level indicates the inventory of component A in the system. 

Control structure B provides good control of the system. Figure 6.11 shows what happens using scheme B when the total recycle flow rate is reduced from 500 to 400 Ib-mol/hr. The system goes through a transient and ends up at the same fresh feed flow rate for reactant A. The reflux drum level controller adjusts the flow rate of Fq/[ to maintain the correct inventory of component A in the system. Note that the concentration of component A in the reactor, decreases when the recycle flow rate is decreased. This has no effect on the reaction rate because we have assumed the instantaneous reaction of component B. Figure 6.12 shows the response for a step increase in Fq/j. The control system automatically increases the makeup of component A to satisfy the stoichiometry of the reaction. 

Make sure that the overall component balances for all chemical components can be satisfied. Light, heavy, and intermediate inert components must have a way to exit the system. Reactant components must be consumed in the reaction section or leave the system as impurities in product streams. Therefore, either reaction rates (temperature, pressure, catalyst addition rate, etc.) must be changed or the flow rates of the fresh feed makeup streams must be manipulated somehow. Makeups can be used to control compositions in the reactor or in recycle streams, or to control inventories that reflect the amount of the specific components contained in the process. For example, bring in a gaseous fresh feed to hold the pressure somewhere in the system, or bring in a liquid fresh feed to hold the level in a reflux drum or column base where the component is in fairly high concentration (typically in a recycle stream). 

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