Semibatch mode

Table 3. Semibatch Mode Emulsion Recipe for SAN Copolymers...

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). 

Many industrial reactors operate in the fed-batch mode. It is also called the semibatch mode. In this mode of operation, reactants are charged to the system at various times, and products are removed at various times. Occasionally, a heel of material from a previous batch is retained to start the new batch. 

There are a variety of reasons for operating in a semibatch mode. Some typical ones are as follows ... 

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. 

Therefore, many traditional designs, such as stirred tank reactors, incorporate heat transfer in the process (jacket, external or internal coil, etc.). However, in these devices, there is a significant distance between the heat transfer site and the site of the chemical reaction where heat is released. As a consequence semibatch mode is implemented while batch mode and/or systems are diluted. 

The main limitation of HEX reactors is the short residence time, typically from a few seconds to a few minutes. Indeed, the apparatuses are smaller than the traditional ones and fast flow velocities are necessary in order to maintain good level of heat-transfer coefficients. However, as described in the previous paragraph, the highlighted transfer properties of HEX reactors allow us to operate in a few minutes, whereas it takes many hours in batch or semibatch mode. 

Clearly, the oxidation reaction could not have been implemented in a pure batch operating reactor. Indeed, heat removal capacity would not have been sufficient (100—1200 kW m removed versus 20 x 10 kW m generated). As a consequence, a semibatch mode is necessarily required. Besides, Table 12.10 shows that the feeding times are much higher than the residence time of the Shimtec reactor (around 15 s). 

As expected, heat exchanged per unit of volume in the Shimtec reactor is better than the one in batch reactors (15-200 times higher) and operation periods are much smaller than in a semibatch reactor. These characteristics allow the implementation of exo- or endothermic reactions at extreme operating temperatures or concentrations while reducing needs in purifying and separating processes and thus in raw materials. Indeed, since supply or removal of heat is enhanced, semibatch mode or dilutions become useless and therefore, there is an increase in selectivity and yield. 

The following reaction system of industrial importance was studied a liquid reactant A simultaneously reacts with a liquid B and a gas G to give products R and S and by-products lumped as L. For proprietary reasons, the names of the species are not specified. The reaction is carried out in semibatch mode. The amounts of A, B, G, R, and S at the end of experiments leading to identification of the kinetic model were determined analytically while the amount of L was evaluated from the material balance. 

The vast majority of chemical reactions are sufficiently slow not to observe a dramatic influence of mixing on yields and selectivities. Exceptions are polymerizations, interfacial polycondensations, precipitations, and some fast reactions - usually performed in semibatch mode - such as autocatalytic reactions, neutralizations, nitrations, diazo couplings, brominations, iodinations, and alkaline hydrolysis, which are often encountered in the manufacture of fine chemicals. 

Example 14.1 Consider again the chlorination reaction in Example 7.3. This was examined as a continuous process. Now assume it is carried out in batch or semibatch mode. The same reactor model will be used as in Example 7.3. The liquid feed of butanoic acid is 13.3 kmol. The butanoic acid and chlorine addition rates and the temperature profile need to be optimized simultaneously through the batch, and the batch time optimized. The reaction takes place isobarically at 10 bar. The upper and lower temperature bounds are 50°C and 150°C respectively. Assume the reactor vessel to be perfectly mixed and assume that the batch operation can be modeled as a series of mixed-flow reactors. The objective is to maximize the fractional yield of a-monochlorobutanoic acid with respect to butanoic acid. Specialized software is required to perform the calculations, in this case using simulated annealing3. 

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. 

It is possible to operate a fluidized bed in either batch or continuous mode. Strictly, most batch applications are in tact operated in semibatch mode where the solids are treated as a batch but the fluidizing medium enters and leaves the bed continuously. In the case ot gas-solid beds used in termentation (see Chapter 6), the fluidizing gas is recirculated although reactants and products flow continuously. In true continuous operation the solids may be ted into a fluidized bed via screw conveyors, weigh teeders or pneumatic conveying lines and can be withdrawn trom the bed via standpipes or by flowing over weirs. 

If reaction took place instantaneously as A was fed to the reactor, the heat release rate would be F [A]j(—Ai/) and eqn. (56) shows how the rate at which heat is released may be controlled by operating in the semibatch mode. 

If the operations run in the semibatch mode and the linear superficial slurry velocity wsL is zero, the above equation would become the mean bubble rise velocity in the swarm (Shah et al, 1982). 

A condensation reaction is to be performed in a stirred tank reactor in the semibatch mode. The solvent is acetone, the industrial charge (final reaction mass) is 2500 kg, and the reaction temperature is 40 °C. The second reactant is added in a stoichiometric amount at a constant rate over two hours. Under these conditions, the maximum accumulation is 30%. The reaction does not produce any gas and its heat release rate is 20 Wkg h The reactor is equipped with a condenser with a cooling power of 250kW and the vapor tube has a diameter of 250mm. The reactor can be considered open. 

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... 

Slurry Bubble Column Reactors As in the case of gas-liquid slurry agitated reactors, bubble column reactors may also be used when solids are present. Most issues associated with multiphase bubble columns are analogous to the gas-liquid bubble columns. In addition, the gas flow and/or the liquid flow have to be sufficient to maintain the solid phase suspended. In the case of a bubble column fermenter, the sparged oxygen is partly used to grow biomass that serves as the catalyst in the system. Many bubble columns operate in semibatch mode with gas sparged continuously and liquid and catalyst in batch mode. 

The virgin bitumen is vaporized only partially at normal cracking temperatures and obtaining reproducible catalyst-bitumen contact presents a significant problem. Two modes of operation were used—semibatch and batch. In the semibatch mode, the catalyst and feed were preheated separately before contact was made in a downflow fixed-bed reactor. Preheat of the feed was held to 350°C to minimize thermal-cracking reactions prior to contact. This reactor had contact between bitumen and catalyst at reaction temperature, but it did not achieve uniform contact and meaningful cat-to-oil ratios. The batch mode provided ultimate contact with the bitumen but it had longer heat-up times... 

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