Downstream reactor , control

X 10 DOWNSTREAM REACTOR CONTROL-FREE SOAP AREA... 

The amount of combustion ait is tightly controlled to maximize sulfur recovery, ie, maintaining the appropriate reaction stoichiometry of 2 1 hydrogen sulfide to sulfur dioxide throughout downstream reactors. Typically, sulfur recoveries of up to 97% can be achieved (7). The recovery is heavily dependent on the concentration of hydrogen sulfide and contaminants, especially ammonia and heavy hydrocarbons, ia the feed to the Claus unit. 

When recycling material to the reactor for whatever reason, the pressure drop through the reactor, separator (if there is one), the heat transfer equipment upstream and downstream of the reactor, control valves, and so on must be overcome. This means the pressure of any material to be recycled must be increased. Again, for the case of a liquid recycle, the cost of this pressure increase is usually small. On the other hand, to increase the pressure of material in the gas phase for recycle requires a compressor and is expensive. 

Closed-loop response to process disturbances and step changes in setpoint is simulated with the model of Kiparissides extended to predict the behavior of downstream reactors. Additionally, a self-optimizing control loop is simulated for conversion control of downstream reactors when the first reactor of the train is operating under closed-loop control with dead-time compensation. 

These simulated results for the high emulsifier concentration operating condition demonstrate the utility of dead-time compensation to the control of conversion from the first reactor in a train. With implementation of this degree of control on the first reactor, control schemes for downstream reactors can be simplified as discussed in the next section. 

Figure 21. Block diagram of digital control loop for downstream reactors...

The empirical model of equation (11) predicted the response of the mechanistic model to a step change in initiator flow very closely (the average absolute deviation between the empirical model and mechanistic model was 0.8% of the response). Three algorithms have been considered for control of the downstream reactor modeled by equation (11). 

TABLE IV CONTROL SYSTEM PERFORMANCE - DOWNSTREAM REACTOR... 

Operotion/surface tension The reactor should be operated in such a manner that large transients in surface tension are avoided. Conversion and surface tension oscillations will tend to contribute to wall polymer formation. Start-up polides, system design, and control procedures should he selected to insure steady, free emulsifier levels in the particle formation reactor. In some cases it may also be desirable to add more emulsifier to downstream reactors. 

Reetz et al. [39] carried out the continuous kinetic resolution of chiral alcohols using IL/scCOj biphasic systems with high enantioselectivity. In this approach, the racemic alcohol and the acylating agent were transported into the reactor nsing scCOj as the mobile phase. The basis of the proposed approach is that one of the enantiomers is esterified selectively by the lipase in the ionic liqnid and the mixture of products is continuously extracted with the scCO stream. The ester and unreacted alcohol were then separated downstream by controlled density reduction via variation of temperature and/or pressure of CO. The authors found that vinyl laureate, which is a cheap acylation agent, renders an ester less soluble than the unreacted alcohol, which allows an efficient recovery of the former compound. 

The response of a CSTR system to inhibitors in the feed streams is, in some respects, similar to a semibatch system. Because inhibitor enters with the feed stream, the rate of initiation is reduced in proportion to the inhibitor flow. In extreme cases, the flow of inhibitor may be sufficient to prevent any initiation in the first reactor. When this happens the particle nucleation phenomena is shifted to the second tank, and serious control problems can be experienced. Increased initiator concentrations can be used to overcome high inhibitor concentrations, but such a course of action can produce initiation rates that are considerably higher in the downstream reactors. 

A second reason for intermediate monomer feed locations is to control copolymer composition and particle morphology. Unlike the batch reactor in which the more reactive monomer reacts first, the copolymer produced in a CSTR should have uniform composition. If all monomers are fed to the first reactor, however, the polymer formed in the downstream reactors will contain less of the most reactive monomer. Thus, the use of intermediate feed locations in a CSTR system is analogous to the programmed addition of reactive monomers in semibatch reactors. The location and rates of the various monomer streeims will influence copolymer composition and particle morphology. 

Note that the flow rate of the total methanol (D3 plus fresh methanol feed) is fixed by the two downstream flow controllers setting the flow rates to the reactor and to column Cl. This means there is an immediate effect of fresh feed flow rate on reflux drum level. The... 

The polymerization reactor is usually at the heart of the manufacturing process, impacting both downstream processing and final customer-related polymer properties. The following factors have contributed to the industrial significance of polymer reactor control ... 

The composition of copol3mier produced in a steady-state CSTR will not, except for small stochastic variations, change with time. However, the copolymer produced in different reactors of a CSTR train will usually be different. The polymer formed in the first reactor will contain a higher fraction of the more reactive monomer than that formed in downstream reactors. Copol mier composition can be controlled by feeding monomer at various points along the reactor system. The relative flow rates of these intermediate feed streams can be controlled to achieve a variety of composition profiles within the reactor train. 

The whole set-up for partial oxidation comprises a micro mixer for safe handling of explosive mixtures downstream (flame-arrestor effect), a micro heat exchanger for pre-heating reactant gases, the pressure vessel with the monolith reactor, a double-pipe heat exchanger for product gas cooling and a pneumatic pressure control valve to allow operation at elevated pressure [3]. 

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