Reactors semibatch operation

A hst of polyol producers is shown in Table 6. Each producer has a varied line of PPO and EOPO copolymers for polyurethane use. Polyols are usually produced in a semibatch mode in stainless steel autoclaves using basic catalysis. Autoclaves in use range from one gallon (3.785 L) size in research faciUties to 20,000 gallon (75.7 m ) commercial vessels. In semibatch operation, starter and catalyst are charged to the reactor and the water formed is removed under vacuum. Sometimes an intermediate is made and stored because a 30—100 dilution of starter with PO would require an extraordinary reactor to provide adequate stirring. PO and/or EO are added continuously until the desired OH No. is reached the reaction is stopped and the catalyst is removed. A uniform addition rate and temperature profile is required to keep unsaturation the same from batch to batch. The KOH catalyst can be removed by absorbent treatment (140), extraction into water (141), neutralization and/or crystallization of the salt (142—147), and ion exchange (148—150). 

In a batch reactor, the reaclants are loaded at once the concentration then varies with time, but at any one time it is uniform throughout. Agitation seiwes to mix separate feeds initially and to enhance heat transfer. In a semibatch operation, some of the reactants are charged at once and the others are then charged gradually. 

Except in the laboratoiy, batch reactors are mostly liquid phase. In semibatch operation, a gas of limited solubility may be fed in gradually as it is used up. Batch reaclors are popular in practice because of their flexibility with respect to reaction time and to the lands and quantities of reactions that they can process. 

Unsteady material and energy balances are formulated with the conservation law, Eq. (7-68). The sink term of a material balance is and the accumulation term is the time derivative of the content of reactant in the vessel, or 3(V C )/3t, where both and depend on the time. An unsteady condition in the sense used in this section always has an accumulation term. This sense of unsteadiness excludes the batch reactor where conditions do change with time but are taken account of in the sink term. Startup and shutdown periods of batch reactors, however, are classified as unsteady their equations are developed in the Batch Reactors subsection. For a semibatch operation in which some of the reactants are preloaded and the others are fed in gradually, equations are developed in Example 11, following. 

With batch reactors, it may be possible to add all reactants in their proper quantities initially if the reaction rate can be controlled by injection of initiator or acqustment of temperature. In semibatch operation, one key ingredient is flow-controlled into the batch at a rate that sets the production. This ingredient should not be manipiilated for temperature control of an exothermic reactor, as the loop includes two dominant lags—concentration of the reactant and heat capacity of the reaction mass—and can easily go unstable. 

The reactor is operated in the semibatch mode with component A being sparged into the stirred tank. Unreacted A and the reaction products leave through the gas phase so that the mass of liquid remains constant. To the extent that these assumptions are true and the catalyst does not deactivate, a pseudo-steady-state can be achieved. Find (flg)o j. Assume that Henry s law is valid throughout the composition range and ignore any changes in the gas density. 

Semibatch reactors are operated in two different modes (1) Some components of the reaction mixture are loaded into the reactor. After the operating conditions have reached the required level, the other components are dosed continuously or portion-wise, whereby temperature and pressure are kept as close as possible to profiles determined as optimum ones. This mode of... 

Even if it is decided to use semibatch operation in the above example in which FEEDl is charged to the reactor initially and then FEEDl as the reaction progresses, it is not known how fast the feed should be added to obtain the optimum performance. Feed addition rate and product takeoff rate are degrees of freedom that need to be optimized. 

Just as a reactor can be operated in batch or semibatch mode, a crystallizer can also be operated in batch or semibatch mode. Table 14.3 contrasts the optimization variables for batch and semibatch operation of cooling crystallization. 

Batch operation requires a larger inventory than the corresponding continuous reactor. Thus, there may be a safety incentive to change from batch to continuous operation. Alternatively, the batch operation can be changed to semibatch in which one (or more) of the reactants is added over a period. The advantage of semibatch operation is that the feed can be switched off in the event of a temperature (or pressure) excursion. This minimizes the chemical energy stored up for a subsequent exotherm. 

Semibatch or Semiflow Reactors. Semibatch or semiflow operations usually take place in a single stirred tank using equipment extremely similar to that described for batch operations. Figure 8.1 indicates some of the many modes in which semibatch reactors may be operated. 

There are a variety of limiting forms of equation 8.0.3 that are appropriate for use with different types of reactors and different modes of operation. For stirred tanks the reactor contents are uniform in temperature and composition throughout, and it is possible to write the energy balance over the entire reactor. In the case of a batch reactor, only the first two terms need be retained. For continuous flow systems operating at steady state, the accumulation term disappears. For adiabatic operation in the absence of shaft work effects the energy transfer term is omitted. For the case of semibatch operation it may be necessary to retain all four terms. For tubular flow reactors neither the composition nor the temperature need be independent of position, and the energy balance must be written on a differential element of reactor volume. The resultant differential equation must then be solved in conjunction with the differential equation describing the material balance on the differential element. 

For semibatch operation, the term fraction conversion is somewhat ambiguous for many of the cases of interest. If reactant is present initially in the reactor and is added or removed in feed and effluent streams, the question arises as to the proper basis for the definition of /. In such cases it is best to work either in terms of the weight fraction of a particular component present in the fluid of interest or in terms of concentrations when constant density systems are under consideration. In terms of the symbols shown in Figure 8.20 the fundamental material balance relation becomes ... 

A semibatch reactor is a variation of a batch reactor in which one reactant may be added intermittently or continuously to another contained as a batch in a vessel, or a product may be removed intermittently or continuously from the vessel as reaction proceeds. The reaction may be single-phase or multiphase. As in a batch reactor, the operation is inherently unsteady-state and usually characterized by a cycle of operation, although in a more complex manner. 

In comparison with a batch reactor, the gradual or intermittent addition of a reactant (say, B in A + B -> products, Figure 12.3(a)) in semibatch operation can result in improved control of T, particularly for a reaction with a large... 

Semibatch operation safety, 21 843 Semibatch polymer colloid process, 20 376 Semibatch polymerization of vinyl acetate, 25 608 Semibatch reactors, 21 332 Semibright nickel, 9 820 Semibulk containers, 18 5-6 Semibullvalene... 

Figure 26.1 Various contacting patterns in fluid-solid reactors a-d) countercurrent, crosscurrent, and cocurrent plug flow d) intermediate gas flow, mixed solid flow (e) semibatch operations.

In the common case, in slurry bubble column reactors, the catalyst phase remains in the reactor while the liquid phase could remain in the reactor with a continuous flow of gas (semibatch operation). Both gas and liquid could be in plug flow or could be well mixed. 

In a typical slurry bubble column operation, the liquid velocity is one order of magnitude lower than the one of gas, and in general, is very low. This mode of operation can be approximated by a semibatch operation. The semibatch operation is frequently used and is the case where the liquid and the catalyst comprise a stationary phase (sluny) in the reactor. In this case, the material balance, eq. (3.122) is used along with the overall rate based on the bulk gas-phase concentration (see Section 3.4.6). In the following, the semibatch operation is presented. 

Whereas much mechanistic information can be obtained by one of the above methods, any practical applications must be demonstrated under conditions similar to process operation, i.e., continuous flow. Small glass reactors which allow controlled addition of reagents by syringe pump and continuous removal and monitoring (IR spectroscopy) of product mixture have been developed. Much of the information obtained from semibatch operation has been reproduced under these continuous flow conditions. 

Comparative experiments are carried out between the SCISR and the STR for further verifying the good performance of the SCISR. Both the reactors are operated in semibatch mode and under the same optimized conditions as before. The structure and dimensions of the experimental reactors have been described in Section 13.2. The size distributions of the particles in the precipitates from the two reactors are illustrated in Fig. 13.2. Obviously, the product from the SCISR is finer with a narrower size distribution, i.e., more uniform in size. It should be noted that the effective volume of the experimental SCISR is six times that of the STR, suggesting the scales favor the... 

The semibatch reactor is a cross between an ordinary batch reactor and a continuous-stirred tank reactor. The reactor has continuous input of reactant through the course of the batch run with no output stream. Another possibility for semibatch operation is continuous withdrawal of product with no addition of reactant. Due to the crossover between the other ideal reactor types, the semibatch uses all of the terms in the general energy and material balances. This results in more complex mathematical expressions. Since the single continuous stream may be either an input or an output, the form of the equations depends upon the particular mode of operation. 

As long as the final temperature is less than some critical onset temperature where a secondary decomposition reaction occurs, then the process can safely handle a cooling system failure. If a batch reactor temperature cannot be assured to remain less than the onset temperature after a cooling system failure, then a semibatch operation should be used. As noted in Section 3.1.3.5, it is necessary to assure that reactant concentration is not increasing above an onset concentration where a similar decomposition could occur with a cooling system failure. 

Semibatch operations are commonly employed in copolymerizations to maintain a more uniform copolymer o>mposition. The monomers are fed at rates equsl to their consumption rates in the reactor. 

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