Semi-batch operation mode

The testing materials used in the investigation are commercial sodium silicate with the modulus of 3.3 and sulfuric acid. The following substances are studied (1) experiments for optimizing the operating conditions for the SCISR in semi-batch operation mode ... 

The comparative experiments are carried out only in semi-batch operation mode the experimental conditions and the operation procedure are identical for both the SCISR and the STR, and the specific effective power inputs for the two reactors are controlled rigorously to be identical. 

In a common semi-batch operation mode (sometimes called semi-continuous mode), some fraction of reactants, i.e. the initial charge, are initially charged into the reactor, and the rest of the formulation is continuously provided to feed over some period of time. In a fully semi-batch emulsion process, the polymerization starts with an aqueous solution only containing a surfactant and an initiator (micellar solution), and then the monomer feed is provided in very small drops with a given rate. 

In this chapter batch and semi-batch operation modes to produce copolymers in emulsion polymerisation will be discussed. 

Considering the normal mode of semi-batch operation, when A is fed steadily to the vessel and nothing is removed during reaction, the material balance becomes... 

Bacterial inactivation is achieved by CO2 absorption in the liquid phase, even though the reason why it happens is still not clear. In this respect, batch- and semi-continuous operating modes are substantially different. In the batch system the residence time, i.e., the time of contact between gas- and liquid phase, must be sufficient to allow the diffusion of CO2 in the liquid, and is therefore a fundamental parameter to assure a desired efficiency. In the semi-continuous system the contact between the phases is localized in the surface of the moving micro-bubbles. In this second case, the efficiency of the process is influenced by temperature, pressure, gas flux, bubble diameter, and other parameters that modify the value of the mass-transfer coefficient. Therefore, it is not correct to use the residence time as a key parameter in the semi-continuous process. In fact, a remarkable microbial inactivation is reached even with an exposure time of 0 min (i.e., pressurizing and immediately depressurizing the system) these two steps are sufficient to allow CO2 to diffuse through the liquid phase. 

To handle the volume of solution (about 30,000 L) necessary in the plant operation, a semi-batch denitration was necessary. Slow evaporation during product accumulation reduced the volume to <12,000 L, but increased the nitric acid concentration to about 11M. Experiments indicated that for a semi-batch denitration mode, a projected nitric acid concentration of 2M was an excellent stopping point, because no residual formic acid remains through the reflux and evaporation steps. Additional high nitric acid solution can then be added to the evaporated-denitrated solution without auto-initiation of a formic acid-nitric acid reaction. After all the Am-bearing solution had been transferred to the denitration evaporator and denitrated to <2M, the solution could be evaporated to 2500 L and denitrated to a residual free-acid concentration of 0.5 to 0.8M. In actual practice, the final 2500 L of solution was denitrated to 0.25M HNO3. 

Lastly, the semi-automated batch operating mode of the ultraCLAVE facilitates high throughput reaction screening and full system automation is feasible. 

Rational process design of liquid-liquid extractors (or reactors) is in many ways similar to those of equipment used in gas-liquid operations. The latter is discussed in some detail by Alper in this Proceedings (15). The important design parameter is the time of reaction for a specified level of conversion in the case of batch and semi-batch operations and volume (or height) for the continuous mode of operation. The required data include firstly hold-ups, interfacial... 

The most commonly used mode of operation for precipitations in the life science and specialty chemicals field is the semi-batch operation, the fed-batch ( ometry D in Figure 12.1). One of the two solutions is present in a stirred tank vessel and the other solution is added. The goal of these precipitations is often not the (fine) particle size but rather the fact that no alternative and more gentle crystallization technique is available. 

The semi-batch operation appears somewhat complicated if one wants to calculate the variation of the concentrations with time. However, it is a very practical operation mode if one wants to restrict the reaction rate in the first part of the process, and acquire a complete conversion of one of the reactants at the end. 

There are many variants on the batch-recycle mode. A practical example is shown in Fig, 19. Semi-batch operation allows for continuous or intermittent addition of reactant. 

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

Mode of operation (Semi)batch or continuous concurrent upward motion of phases Continuous concurrent upward motion of phases Continuous concurrent upwani or downward in different parts of the reaction zone Continuous concurrent upward motion of phases Continuous concurrent (1) or countercurrent (2) upward (a) or downw-ard (b) irickle-bcd (1 +b) Continuous countwcurrcni gas upward liquid downward Cominumis concurrent downward... 

Three modes of reactor operation may be distinguished, batch, semi-batch and continuous. In a batch system all reactants are added to the tank at the given starting time. During the course of reaction, the reactant concentrations decrease continuously with time, and products are formed. On completion of the reaction, the reactor is emptied, cleaned and is made ready for another batch. 

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

The next two steps after the development of a mathematical process model and before its implementation to "real life" applications, are to handle the numerical solution of the model s ode s and to estimate some unknown parameters. The computer program which handles the numerical solution of the present model has been written in a very general way. After inputing concentrations, flowrate data and reaction operating conditions, the user has the options to select from a variety of different modes of reactor operation (batch, semi-batch, single continuous, continuous train, CSTR-tube) or reactor startup conditions (seeded, unseeded, full or half-full of water or emulsion recipe and empty). Then, IMSL subroutine DCEAR handles the numerical integration of the ode s. Parameter estimation of the only two unknown parameters e and Dw has been described and is further discussed in (32). 

Equations (1-1) and (1-2) are true in the general case and can be used to study several modes of reactor operation (e.g. batch, semi-batch, continuous, start-up procedures, etc.). If the assumption is made that the reactor is a vessel continuously operating full, i.e. overflow CSTR, then the right hand side (RHS) of equation (1-2) is zero and (1-1) is considerably simplified to yield ... 

The catalytic ethylene oligomerization was performed in a 0.3 L well-mixed three-phase reactor operating in semi-batch mode, at constant temperature (70 or 150 °C) and pressure (4 MPa of ethylene) in 68 g of n-heptane (solvent). Prior to each experiment, the catalyst was successively pretreated, firstly in a tubular electrical furnace (550 °C, 8 h) and then in the oligomerization autoclave (200 °C, 3 h), under nitrogen flow at atmospheric pressure. After 30 min of reaction, the autoclave was cooled at -20 °C and the products were collected, weighted and analyzed by GC (FID, DB-1 60 m capillary column). 

During the development of a chemical process, a choice must be made regarding the type of reactor to be used on a plant scale. Some theoretical considerations and their practical impact on reactor issues are presented here. Choosing the right type of reactor can indeed improve the safety of the process. The considerations are reflected as well in the mode of operation. Reactors are characterized by type of operation (i.e., batch, semi-batch, and continuous). 

Generally, although not exclusively, a scrubber with a recycle loop of the caustic scrubbing liquor is used cases of once-through scrubbing liquor operation do exist. The scrubber may be operated in batch, semi-batch or continuous mode with respect to the liquid. Process hazards exist in batch and continuous mode, the most significant of which is over-chlorination. Batch-wise operations leads to periodic high loads on the hypochlorite destruction unit. In order to even out these loads, and improve the process safety, a study of alternative treatment options has been undertaken. 

Consider a chlorine scrubbing unit operated semi-batch-wise. That is, a system similar to that shown in Fig. 26.7 is operated batch-wise but, periodically, a portion of the contents of the sump are sent to treatment and the sump refilled with fresh caustic. This mode of operation allows continuous treatment of the chlorine-laden vent stream while allowing batch operation without the need for a dual scrubbing-liquor reservoir. 

The bioreactor operation mode is normally defined at the outset of process configuration. Insect cells have been cultured in almost all known cultivation modes batch [10], repeated-batch [70], perfusion [71-74], fed-batch [75, 76], semi-continuous [77,78] and continuous [79]. In spite of this multitude of different strategies, the batch or, eventually, fed-batch mode is normally preferred due to the lytic infection cycle of the baculovirus. 

It may be possible to minimise the time spent on relief system assessment and design if the basis of safety for a multi-purpose reactor can be changed to prevention rather than emergency pressure relief. For example, if it can be arranged for all the reactions to operate in semi-batch mode with no significant reactant accumulation, then the use of a trip system of sufficient integrity may provide a suitable basis of safety. (This may not always be possible.)... 

Biochemical reactors can be operated either batchwise or continuously, as noted in Section 1.5. Figure 7.1 shows, in schematic form, four modes of operation with two types of reactors for chemical and/or biochemical reactions in Uquid phases, with or without suspended solid particles, such as catalyst particles or microbial cells. The modes of operation include stirred batch stirred semi-batch continuous stirred and continuous plug flow reactors (PFRs). In the first three types, the contents of the tanks arc completely stirred and uniform in composition. 

Figure 7.1 Modes of reactor operation (a) batch reactor, (b) semi-batch reactor, (c) continuous stirred-tank reactor, and (d) continuous plug flow reactor.

Figure 1-2 Operating parameters necessary for ozone mass balance(s) on a continuous-flow stirred tank reactor (for operation in semi-batch mode Ol - 0).

Using it in the semi-batch mode of operation this reactor type has been used in the determination of the reaction rate constants of fast direct reactions of ozone with certain waste water pollutants, e. g. phenol or azo-dyes (Beltran and Alvarez, 1996). Beltran and coworkers have also successfully studied the reaction kinetics of various fast reacting substances using semi-batch mode STRs (see further references of Beltran, Benitez or Sotelo et al. in Chapters B 3 and B 4). 

Reactors can be operated either in a batch or continuous-flow mode. The combination, batch with respect to the liquid and continuous-flow with respect to the gas, is called semibatch. Often this fine distinction is ignored and it is commonly referred to as batch. The majority of ozonation experiments reported in the literature have been performed in one-stage semi-batch heterogeneous systems, with liquid phase reactor volumes in the range VL = 1-10 L. Most full-scale applications are operated in continuous-flow for both phases. 

---