Semi-batch

Using a batch reactor, a constant concentration of sulfuric acid can be maintained by adding concentrated sulfuric acid as the reaction progresses, i.e., semi-batch operation. Good temperature control of such systems can be maintained, as we shall discuss later. 

Stirred-tank reactors can be operated in batch, semi-batch, or... 

The Center for Chemical Process Safety (CCPS) has identified the need for a publication dealing with process safety issues unique to batch reaction systems. This book, Guidelines for Process Safety in Batch Reaction Systems, attempts to aid in the safe design, operation and maintenance of batch and semi-batch reaction systems. In this book the terms batch and semi-batch are used interchangeably for simplicity. The objectives of the book are to ... 

Semi-Batch Reactor In a semi-batch reactor, some reactants are added to the reactor at the start of the batch, while others are fed intermittently or continuously during the course of the reaction. 

Figure 4-4 shows a semi-batch reactor with outside circulation and the addition of one reactant through the pump. Semi-batch reactors have some reactants that are charged into the reactor at time zero, while other reactants are added during the reaction. The reactor has no outlet stream. Some reactions are unsuited to either batch or continuous operation in a stirred vessel because the heat liberated during the reaction may cause dangerous conditions. Under these... 

A semi-batch reactor has the same disadvantages as the batch reactor. However, it has the advantages of good temperature control and the capability of minimizing unwanted side reactions by maintaining a low concentration of one of the reactants. Semi-batch reactors are also of value when parallel reactions of different orders occur, where it may be more profitable to use semi-batch rather than batch operations. In many applications semi-batch reactors involve a substantial increase in the volume of reaction mixture during a processing cycle (i.e., emulsion polymerization). 

It is important to know how much heat of reaction can accumulate when assessing the hazards related to an exothermic reaction. Accumulation in a batch or semi-batch process can be the result of ... 

Thermal runaway reactions are the results of chemical reactions in batch or semi-batch reactors. A thermal runaway commences when the heat generated by a chemical reaction exceeds the heat that can be removed to the surroundings as shown in Figure 12-5. The surplus heat increases the temperature of the reaction mass, which causes the reaction rate to increase, and subsequently accelerates the rate of heat production. Thermal runaway occurs as follows as the temperature rises, the rate of heat loss to the surroundings increases approximately linearly with temperature. However, the rate of reaction, and thus the... 

Whether the reactor operates in batch or semi-batch mode... 

Replacement of batch reaction processes with semi-batch or continuous processes reducing the quantity of reactant present. 

Some batch reactions have the potential for very high energy levels. If all the reactants (and sometimes catalysts) are put into a kettle before the reaction is initiated, some exothermic reactions may result in a runaway. The use of continuous or semi-batch reactors to limit the energy present and to reduce the risk of a runaway should be considered. The term semi-batch refers to a system where one reactant and, if necessary, a catalyst is initially charged to a batch reactor. A second reactant is subsequently fed to the reactor under conditions such that an upset in reacting conditions can be detected and the flow of the reactant stopped, thus limiting the total amount of potential energy in the reactor. 

Singh, J. (1993). "Assessing Semi-Batch Reaction Hazards. The Chemical Engineer, 537 (February 5), 21, 23-25. 

The product crystals were agglomerates of needles or dendrites. Loose floes of dendroid strontium carbonate are compacted by agitation, which is an important factor in controlling the habit of product particles. Semi-batch operation produces larger particles compared to batch or continuous operation. 

Wachi and Jones (1991b) used a gas-liquid flat interface reactor as a semi-batch precipitation cell for the experimental measurement of calcium carbonate precipitation, as shown in Figure 8.15. 

Aslund, B.L. and Rasmuson, A.C., 1992. Semi batch reaction crystallization of benzoic acid. American Institution of Chemical Engineers Journal, 38, 328-342. 

Yagi, H., 1988. Semi-batch precipitation accompanying gas-liquid precipitation. Chemical Engineering Communications, 65, 109-119. 

Much has been written on RAFT polymerization under emulsion and miniemulsion conditions. Most work has focused on S polymerization,409-520 521 although polymerizations of BA,461 522 methacrylates382-409 and VAc471-472 have also been reported. The first communication on RAFT polymerization briefly mentioned the successful semi-batch emulsion polymerization of BMA with cumyl dithiobenzoate (175) to provide a polymer with a narrow molecular weight distribution.382 Additional examples and discussion of some of the important factors for successful use of RAFT polymerization in emulsion and miniemulsion were provided in a subsequent paper.409 Much research has shown that the success in RAFT emulsion polymerization depends strongly on the choice of RAFT agent and polymerization conditions.214-409-520027... 

With semi-continuous (more properly, semi-batch) reactors only part of the charge is added at the beginning of the cycle. Usually some reaction time is allowed to pass before the remaining part of the charge is added in a controlled manner. Sometimes... 

Presented in this paper is a specific example of a semi-batch, free radical, dispersion polymerization. In this example, SimuSolv is used to quantify a Icinetic model derived from free radical polymerization principles and then used to define a new finishing process to reduce residual monomer to an acceptable level. Finally, experimental results are compared with those predicted by the computer simulation. 

W° = total material in reactor at start of the semi-batch step. 

Table I. Lab Reactor Data Measured and Predicted Residual Monomer after the Semi-batch Step...

MODEL TO PREDICT RESIDUAL MONOMER FOR AN ISOTHERMAL SEMI-BATCH POLYMERIZATION AND AN ISOTHERMAL BATCH FINISHING STEP. 

TEMPERATURE FOR SEMI-BATCH AND FINISHING MONOMER FEED RATE DURING SEMI-BATCH STEP INITIATOR FEED RATE DURING SEMI-BATCH STEP TOTAL INITIAL LOADING TIME FOR SEMI-BATCH STEP MONOMER WT. % AT START OF FINISHING INITIATOR WT. PERCENT AT START OF FINISHING TIME FROM START OF... 

Thus, the initial value of the initiator concentrations, [Il]° and [I2]°, are calculated with Equation 15, for given values of the initial loading, feed rates, temperature, and time for the main semi-batch step, and [M]° is fixed according to experimental data from the base case semi-batch step. The nonlinear differential equation for [M] in terms of [II] and [I2] is given by Equation 16. Equation 10, with a redefinition of terms, is the differential equation mass balance for [II] and [12]. In the finishing step, only one of the initiators would be added for residual monomer reduction. Thus, Qm = 0,... 

---