Stirred adiabatic operation

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. 

ILLUSTRATION 10.3 ADIABATIC OPERATION OF CONTINUOUS STIRRED TANK REACTORS OPERATING IN SERIES... 

Under conditions of adiabatic operation and negligible stirring work, both Q and Wr are zero, and the energy balance becomes... 

We have already obtained the basic design equations for the stirred tank [Eqs. (7.3.1) and (7.3.2)] and these will represent an adiabatic operation if we set Q = 0. Thus in the steady state... 

Even adiabatic operation results in the formation of considerable amounts of the undesirable dichloropropane. This occurs in the first part of the reactor, where the temperature of the flowing mixture is low. This is an illustration of the discussion at the beginning of the chapter with respect to Fig. 5-la and b. The conditions correspond to the low-conversion range of Fig. 5-lb before the maximum rate is reached. A tubular-flow reactor is less desirable for these conditions than a stirred-tank unit. The same reaction system is illustrated in Example 5-3 for a stirred-tank unit. 

A reactor for the production of drying oils by the decomposition of acetylated castor oil is to be designed for a conversion of 70%. The initial charge will be 500 lb and the initial temperature 340°C, as in Example 5-1. In fact, all the conditions of Example 5-1 apply, except instead of adiabatic operation, heat will be supplied electrically with a cal-rod unit in the form of a l-in.-OD coil inraiersed in the reaction mixture. The power input and the stirring in the reactor will be such that the surface temperature of the heater is maintained constant at 700°K. The heat-transfer coefficient may be taken equal to 60 Btu/ (hr)(ft )(°F). What length of heater will be required if the conversion of 70% is to be obtained in 20 min ... 

ILLUSTRATION 10.3 Adiabatic Operation of a Cascade of Continuous Flow Stirred-Tank Reactors... 

In lUusIration 10.2 we saw that when one nses a battery of stirred tanks for carrying out an exothermic reaction under isothermal conditions, there may be occasions when the heat requirements for the various tanks may be of opposite sign. Some tanks will require a net input of theamal energy, while others will need to be cooled. It is often useful in such situations to consider the possibility of adiabatic operation of one or more of the tanks in series, remembering the constraints that one desires to place on the temperatures of the process streams. Another means of achieving autothermal operation is to use a network consisting of a stirred-tank reactor followed by a tubular reactor. This case is considered in Illustration 10.6. 

Equation 4.77 for a nonisothermal CSTR establishes the temperature at which a stirred reactor operates for a given set of parameter values. This is also true for adiabatic operation. The only difference is that the heat exchange term UA T - T ) vanishes. In either case, the equation is transcendental and not amenable to extension to a CSTR sequence as a single generalized equation for N reactors. On the other hand, for a first-order reaction, a general recursion formula can be written for N reactors in series. This requires that the temperature of each stage is known to enable calculation of the rate constant. 

Keairns and Manning AIChE J., 15 (660), 1969] have used the reaction between sodium thiosulfate and hydrogen peroxide in a well-stirred flow reactor to check a computer simulation of adiabatic CSTR operation. Data on their experimental conditions and the reaction parameters are listed below. The reaction may be considered second-order in sodium thiosulfate. 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

Consider an exothermic irreversible reaction with first order kinetics in an adiabatic continuous flow stirred tank reactor. It is possible to determine the stable operating temperatures and conversions by combining both the mass and energy balance equations. For the mass balance equation at constant density and steady state condition,... 

We have a first-order homogeneous reaction, taking place in an ideal stirred tank reactor. The volume of the reactor is 20 X 10 3 m3. The reaction takes place in the liquid phase. The concentration of the reactant in the feed flow is 3.1 kmol/m3 and the volumetric flow rate of the feed is 58 X 10 m3/s. The density and specific heat of the reaction mixture are constant at 1000 kg/m3 and 4.184kJ/(kg K). The reactor operates at adiabatic conditions. If the feed flow is at 298 K, investigate the possibility of multiple solutions for conversion at various temperatures in the product stream. The heat of reaction and the rate of reaction are... 

Description Liquid or gaseous ethylene is fed, together with a solvent and required comonomer(s) into a stirred, liquid-filled, vessel-type reactor (1). The reactor is operated adiabatically thus, the feed is precooled. All heat of reaction is used to raise polymerization temperature up to approximately 200°C. Hydrogen is used to control polymer molecular weight. A high-activity, proprietaiy catalyst is prepared onsite from commercially available components. Ethylene... 

As we observed earlier, the adiabatic reactor is not so much a type, more a way of operation. We shall therefore refer to both stirred tank reactors and to tubular ones, and this chapter forms a suitable bridge between the two. We shall introduce the simplest model of the tubular reactor, but this is so elementary that the anticipation of the following chapter will cause no difficulty. 

Adiabatic or nonisothermal operation of a stirred tank reactor presents a different physical situation from that for plug flow, since spatial variations of concentration and temperature do not exist. Rather, reaction heat effects manifest themselves by establishing a temperature level within the CSTR that differs from that of the feed. Thus, when we use the terms adiabatic or nonisothermal in reference to CSTR systems, it will be understood to imply analysis where thermal effects are included in the conservation equations but not to imply the existence of thermal gradients. 

Thus, even in an adiabatic mode of tubular turbulent chlorination reactor operation (without heat removal), the temperature growth in the reaction zone in the case of BR chlorination (12-15% solution) with molecular chlorine in a tubular reactor, operating in the optimum plug-flow mode in turbulent flows, does not exceed 2 1 °C. The process can be thought to proceed under quasi-isothermal conditions and does not require external or internal heat removal, or special stirring devices for heat and mass exchange intensification. 

Two types of reactors are used for the production of LDPE either a stirred vessel (autoclave) or a tubular reactor. The autoclave reactor operates adiabatically. The tubular reactor is cooled with a jacket. The autoclave reactor has a length to diameter ratio (L/D) between 4 and 16. Tubular reactors have L/D ratios above 10000. The inner diameter of the high pressure tubes used for the tubular reactors range between 25 and 100 mm. The operating pressure ranges between 100 and 250 MPa (1000-2500 bars) for the autoclave reactor and between 200 - 350 MPa (2000 - 3500 bar) for the tubular reactor. A basic flow diagram for LDPE processes is shown in Figure 3.7. 

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