Semibatch reactors liquid phase

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. 

The kinetic models for the gas phase polymerization of propylene in semibatch and continuous backmix reactors are based on the respective proven models for hexane slurry polymerization ( ). They are also very similar to the models for bulk polymerization. The primary difference between them lies in the substitution of the appropriate gas phase correlations and parameters for those pertaining to the liquid phase. 

Single phase Gas-liquid or liquid-liquid (semibatch reactors) Catalytic (three-phase) (semibatch or continuous) ... 

Column reactors can contain a draft tube - possibly filled with a packing characterized by low pressure drop - or be coupled with a loop tube, to make the gas recirculating within the reaction zone (see Fig. 5.4-9). In recent years, the Buss loop reactor has found many applications in two- and three-phase processes About 200 Buss loop systems are now in operation worldwide, also in fine chemicals plants. This is due to the high mass-transfer rate between the gas and the liquid phase. The Buss loop reactor can be operated semibatch-wise or continuously. As a semibach reactor it is mostly used for catalytic hydrogenations. 

Price and Schiewetz Ind Eng. Chem. 49 (807), 1957] have studied the catalytic liquid phase hydrogenation of cyclohexene in a laboratory scale semibatch reactor. A supported platinum catalyst was suspended in a cyclohexene solution of the reactant by mechanical... 

In pulp and paper processing, anthraquinone (AQ) accelerates the delignification of wood and improves liquor selectivity. The kinetics of the liquid-phase oxidation of anthracene (AN) to AQ with NO2 in acetic acid as solvent has been studied by Rodriguez and Tijero (1989) in a semibatch reactor (batch with respect to the liquid phase), under conditions such that the kinetics of the overall gas-liquid process is controlled by the rate of the liquid-phase reaction. This reaction proceeds through the formation of the intermediate compound anthrone (ANT) ... 

Figure 12.3 illustrates some modes of operation of semibatch reactors. In Figure 12.3(a), depicting a homogeneous liquid-phase reaction of the type A + B - products, reactant A is initially charged to the vessel, and reactant B is added at a prescribed rate, as reaction proceeds. In Figure 12.3(b), depicting a liquid-phase reaction in which a... 

Gas-liquid bubble column This reactor is of tubular shape (Figure 3.5). The liquid phase is agitated by the bubble rise of the gas phase. The gas phase flows through the reactor upward at a constant rate. The liquid phase is continuous. This reactor could be of continuous type, if the liquid is flowing through the reactor continuously or semibatch, if the liquid is stationary in the reactor. 

Agitated slurry reactor (ASR) This is a mechanically agitated gas-liquid-solid reactor (Figure 3.13). The liquid is agitated by a mechanical apparatus (impeller). The fine solid particles are suspended in the liquid phase by means of agitation. Gas is sparged into the liquid phase, entering at the bottom of the tank, normally just under the impeller. This reactor can also be of continuous type or of semibatch type. This type is used only in catalysis. 

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. 

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. 

Finally, some remarks on the operation of mechanically agitated gas-liquid reactors are worth mentioning. The mode of operation (i.e., batch, semibatch, continuous, periodic, etc.) depends on the specific need of the system. For example, the level of liquid-phase backmixing can be controlled to any desired level by operating the gas-liquid reactor in a periodic or semibatch manner. This provides an alternative to the tanks in series or plug flow with recycle system and provides a potential method of increasing the yield of the desired intermediate in complex reaction schemes. In some cases of industrial importance, the mode of operation needs to be such that the concentration of the solute gas (such as Cl2, H2S, etc.) as the reactor outlet is kept at a specific value. As shown by Joshi et al. (1982), this can be achieved by a number of different operational and control strategies. 

The liquid-phase hydrogenation of cyclohexene to cyclohexane (in an inert solvent) is conducted over solid catalyst in a semibatch reactor (dihydrogen addition to keep the total pressure constant). 

A liquid-phase chemical reaction with stoichiometry A B takes place in a semibatch reactor. The rate of consumption of A per unit volume of the reactor contents is given by the first-order rate expression (see Problem 11.14)... 

Of the two types of semibatch reactors described earlier, we focus atten tion primarily on the one with constant molar feed. A schematic diagram of this semibatch reactor is shown in Figure 4-14. We shall consider the elemen- tary liquid-phase reaction ... 

This chapter focuses attention on reactors that are operated isotherraally. We begin by studying a liquid-phase batch reactor to determine the specific reaction rate constant needed for the design of a CSTR. After iilustrating the design of a CSTR from batch reaction rate data, we carry out the design of a tubular reactor for a gas-phase pyrolysis reaction. This is followed by a discussion of pressure drop in packed-bed reactors, equilibrium conversion, and finally, the principles of unsteady operation and semibatch reactors. 

Key Words Propylene, Propylene oxide. Liquid-phase epoxidation, Semibatch reactor. Supercritical CO2, Gas-phase epoxidation, Pd/TS-1 catalyst, Au/Ti02 catalyst, Au/Ti-Si02 catalyst, Ag catalyst, Mesoporous... 

Example 2-8 Benzene is chlorinated in the liquid phase in a kettle-type reactor operated on a semibatch basis i.e., the reactor is initially charged with liquid benzene, and then chlorine gas is bubbled into the well-agitated solution. The reactor is equipped with a reflux condenser which will condense the benzene and chlorinated products but will not interfere with the removal of hydrogen,chloride. Assume that the chlorine is added sufiiciently slowly that (1) the chlorine and hydrogen chloride concentrations in the liquid phase are small and (2) all the chlorine reacts. 

Solution The process described is neither flow nor batch, but semibatch in nature. However, with assumptions which are reasonably valid, the problem can be reduced to that for a constant-density batch reactor. If the density of the solution remains constant and the hydrogen chloride vaporizes and leaves the solution, the volume of the liquid-phase reaction will be constant. Then the relationship between the composition of the substances in the liquid phase is governed by rate expressions of the type used in this chapter. Assume that the reactions are second order. Then the rate of disappearance of benzene, determined entirely by the first reaction, is... 

Example 5-5 Hexamethylenetetramine (HMT) is to be produced in a semibatch reactor by adding an aqueous ammonia solution (25 wt % NH3) at the rate of 2 gpm to an initial charge of 238 gal (at 25°C) of formalin solution containing 42% by weight formaldehyde. The original temperature of the formalin solution is raised to 50°C in order to start the reaction. The temperature of the NH4.OH solution is 25°C. The heat of reaction in the liquid phase may be, assumed independent of temperature and concentration and taken as —960 Btu/lbbf HMT. If the reactor can be operated at a temperature of 100°C, the rate of reaction is very fast in comparison with the rate of heat transfer with the surroundings. Temperatures higher than 100°C are not desirable because of vaporization and increase in pressure. 

Figure 9.1 Semibatch reactors (a) liquid phase (variable volume) and (b) gas phase (constant volume).

Consider first a semibatch reactor with liquid-phase reactions. To derive a relation for the change in the volume of the reactor, we write an overall mass balance over the reactor ... 

For liquid-phase semibatch reactors, the enthalpy of the injected stream is... 

Example 9,2 Valuable product V is produced in a semibatch reactor where the following simultaneous, liquid-phase chemical reactions take place. 

You are asked to design a semibatch reactor to be used in the production of specialized polymers (ethylene glycol-ethylene oxide co-polymers). The semi-batch operation is used to improve the molecular-weight distribution. Reactant B (EG) and a fixed amount of homogeneous catalyst are charged initially into the reactor (the proportion is 6.75 moles of catalyst per 1000 moles of Reactant B). Reactant A (EO) is injected at a constant rate during the operation. The polymerization reactions are represented by the following liquid-phase chemical reactions ... 

The key issue in effective catalytic oxidation of organics is finding a suitable catalyst. Oxidation of aqueous phenol solutions by using different transition metal oxides as heterogeneous catalysts is already known [4-6]. On the other hand, the potential of molecular sieves to catalyze oxidative phenol destruction has not been examined yet. The objective of this contribution is to provide kinetic and mechanistic data on the catalytic liquid-phase oxidation of aqueous phenol solutions obtained in the presence of various transition metal oxides and molecular sieves. The reaction was studied in a semibatch slurry as well as two-and three-phase continuous-flow reactors. Another matter of concern was the chemical stability of catalysts under the reaction conditions. 

The activity tests of liquid-phase oxidation of aqueous phenol solution were conducted in a semibatch slurry reactor at operating conditions given in the caption of Figure 1. The experimental apparatus, the procedure of these measurements and the analysis of the reaction samples are described in detail in a preceding paper [6]. Additional kinetic and mechanistic investigations were carried out in an isothermal, differentially operated "liquid-saturated" fixed-bed reactor [8, 9] which was packed with a pretreated EX-1144.3 catalyst (Sfld-Chemie... 

Figure 2. Percentage of CO2 formed during the course of catalytic liquid-phase phenol oxidation carried out in a semibatch slurry reactor.

In a kinetic investigation of the catalytic liquid-phase phenol oxidation carried out in a semibatch slurry reactor [6], it has been found that homogeneous stepwise polymerization reactions are enhanced in the bulk liquid-phase due to the high liquid-to-solid volumetric ratio. The rate of phenol disappearance has been expressed on the basis of power-law kinetics as a sum of heterogeneous and homogeneous (polymerization) contributions, thus... 

Figure 8. X-ray powder diffraction patterns of molecular sieves before and after liquid-phase phenol oxidation performed in a semibatch slurry reactor at 403 K.

X 10 N/m. Choose room-temperature operation, T = 30°C = 303.16 K. Initial gas volume = nRgT/P = (267.75 x 8314 x 303.16)/ .8 x 10 = 3749 m. Since this volume is very high compared to the reactor volume of 10 m, choose semibatch mode of operation with continuous supply of feed gas and removal of residual gas to maintain the reactor pressure. In view of this, a slurry of Ca(OH)2 in water is prepared and CO2 is bubbled through it for 4 h. The mother liquid phase will be initially saturated with Ca(OH)2. As the reaction proceeds, CaCOj precipitates out. The mother liquor can be separated from the solid CaC03 precipitate by filtration. For ease of operation and control, assume constant gas flow rate. 

A model has been developed for oxidation of calcium sulfite in a three-phase, semibatch reactor, The overall rate of conversion to sulfate depends on the rates of solid dissolution and liquid phase chemical reaction. In this first treatment of the problem, gas-liquid mass transfer resistance did not affect the overall rate of oxidation. 

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