Semibatch operations

The addition of an antisoivent can be earned out in different ways, as indicated in Fig. 9-1, where the concentration of product is shown on the ordinate and the amount of antisoivent added is shown on the abscissa. A typical equilibrium solubility curve is indicated as A-B-C. (This curve could be concave or linear but is shown as convex for clarity.) The metastable region is indicated as the area between B-C and E-D. From point A to point B, addition of antisoivent will proceed without crystallization because the solution concentration is below the equilibrium solubility. At point B, equilibrium solubility is reached. As the addition of antisoivent continues, supersaturation will develop. The amount of supersaturation that can be developed without nucleation is system specific and will depend on the addition rate, mixing, primaiy and/or secondary nucleation rate, and growth rate, as well as the amount and type of impurities present in solution. 

If growth-dominated crystallization is desired, with the presence of a sufficient quantity of seed and a sufficiently slow addition rate, the concentration in solution may remain completely in the metastable region as crystallization proceeds. The closer the solution concentration profile is to the equilibrium solubility curve (B-C), the higher the possibility of achieving an all-growth process. 

On the other hand, a system without seed or a high addition rate can develop a high degree of supersaturation, which can result in rapid precipitation or crash-out at point B , beyond the metastable zone. Crash-out could be followed by continued nucleation and some growth (B -C), and eventually to equilibrium at some time after aU the antisoivent 

Crystallization of Organic Compounds An Industrial Perspective. By H.-H. Tung, E. L. Paul, M. Midler, and J. A. McCauley 

A common practical approach to achieving growth while minimizing concern for seed dissolution is shown in curve B -F-C. The addition of antisolvent is stopped, and seed is added at point B, where the system is slightly supersaturated. As discussed in Chapter 2, in-line measurement by Fourier transform infrared (FTIR) or ultraviolet (U V) can be utilized to determine when the concentration reaches point B. Alternatively, the seed may be added in a sluiTy with the antisolvent starting before point B is reached, as discussed in Chapter 5 and below in Section 9.1.4. 

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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). 

Tank Cells. A direct extension of laboratory beaker cells is represented in the use of plate electrodes immersed into a lined, rectangular tank, which may be fitted with a cover for gas collection or vapor control. The tank cell, which is usually undivided, is used in batch or semibatch operations. The tank cell has the attraction of being both simple to design and usually inexpensive. However, it is not the most suitable for large-scale operation or where forced convection is needed. Rotating cylinders or rotating disks have been used to overcome mass-transfer problems in tank cells. An example for electroorganic synthesis is available (46). 

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. 

If there is no possibility to maintain a constant temperature by manipulating the temperature of the cooling medium the reaction can be slowed down by diluting the reaction mixture and/or the catalyst. After some components of the reaction mixture have been consumed to a sufficient extent and the reaction becomes too slow, more catalyst or reactants can be added to complete the reaction with the rate of heat generation not yet exceeding that of heat removal. This is the normally used semibatch operation. 

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. 

Should the process be continuous or intermittent Would batch or semibatch operation be advantageous ... 

Another mode of semibatch operation involves the use of a purge stream to remove continuously one or more of the products of a reversible reaction. For example, water may be removed in esterification reactions by the use of a purge stream or by distillation of the reacting mixture. Continuous removal of product(s) increases the net reaction rate by slowing down the reverse reaction. 

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. 

It may be desirable to operate in semibatch fashion in order to enhance reaction selectivity or to control the rate of energy release by reaction through manipulation of the rate of addition of one reactant. Other situations in which semibatch operation is employed include a variety of biological fermentations where various nutrients may be added at predetermined rates to achieve optimum production capacity and cases where one reactant is a gas of limited solubility that can be fed only as fast as it will dissolve. 

Semibatch operations usually employ a single well-stirred tank. In such cases it is possible to make the usual assumption that the composition... 

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 ... 

There are a number of specific cases of equation 8.6.1 that are of potential interest for commercial applications. We wish to consider one mode of semibatch operation using Illustration 8.11 in order to indicate the general principles involved in the analysis of these systems. 

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... 

For a semibatch operation in which some of the ingredients are preloaded and the others are fed in gradually, the equations are developed in problem P4.09.09. 

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... 

Semibatch Operations, The ion exchange column of Fig. 26.1(c) is an example of the batch treatment of solids in which the flow of fluid closely approximates... 

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. 

Gas-Liquid reaction and batch liquid (semibatch operation)... 

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