Heat balance reactors

In a case of where the radical steady-state assumption can be made, the reactor heat balance can be written in dimensionless form at constant pressure (l )... 

It must be emphasised that it is unnecessary to correct a heat of reaction to the reaction temperature for use in a reactor heat-balance calculation. To do so is to carry out two heat balances, whereas with a suitable choice of datum only one need be made. For a practical reactor, the heat added (or removed) Qp to maintain the design reactor temperature will be given by (from equation 3.10) ... 

Illustrates the manual calculation of a reactor heat balance. 

Consider the case of a proportional controller, which is required to maintain a desired reactor temperature, by regulating the flow of coolant. Neglecting dynamic jacket effects, the reactor heat balance can then be modified to include the effect of the varying coolant flow rate, hj, in the model equation as ... 

Conversion per pass is the principal correlating variable in the equations predicting recycle rate and composition. In the second loop, this variable is resolved by computing the catalyst circulation rate which simultaneously satisfies the reactor heat balance and supports the conversion per pass. 

A further specific aspect of the reactor heat balance is the multiplicity of solutions to the system of equations. This situation may arise with a CSTR in which an exothermal reaction is performed. The mass balance [Eq. (12)] is coupled with the heat balance [Eq. (13)], which gives a system of equations [Eq. (14)] that is represented graphically in Figure 11.4. 

On line instrumentation has been developed and used for the measurement of density (conversion), surface tension (25), turbidity spectra (21), reactor heat balances, and monomer composition (26). Such instrumentation will be essential to the development of functional control systems. Very little work, however, has been published on actual control studies. Papers by Hamielec and Mac Gregor (27), Gordon and Weidner (28) and Leffew and Deshpande (29) represent some of the most recent efforts in this area. 

Example 21-6. Reactor Heat Balance. The stock of Examples 21-3 and 21-1 will be cracked. The amount of recirculation was found to be 9.18 in Example 21-4 and this is different from the original assumption of 8 which was used in Example 21-1 in determining yields. A heat of reaction of 200 (Table 21-18) will be used. The size of the reactor will not be computed, and the heat loss through the walls (imder 0.5 per cent) will be neglected. 

Pure zirconium tetrachloride is obtained by the fractional distillation of the anhydrous tetrachlorides in a high pressure system (58). Commercial operation of the fractional distillation process in a batch mode was proposed by Ishizuka Research Institute (59). The mixed tetrachlorides are heated above 437°C, the triple point of zirconium tetrachloride. AH of the hafnium tetrachloride and some of the zirconium tetrachloride are distiUed, leaving pure zirconium tetrachloride. The innovative aspect of this operation is the use of a double-sheU reactor. The autogenous pressure of 3—4.5 MPa (30—45 atm) inside the heated reactor is balanced by the nitrogen pressure contained in the cold outer reactor (60). However, previous evaluation in the former USSR of the binary distiUation process (61) has cast doubt on the feasibHity of also producing zirconium-free hafnium tetrachloride by this method because of the limited range of operating temperature imposed by the smaH difference in temperature between the triple point, 433°C, and critical temperature, 453°C, a hafnium tetrachloride. 

Thus the ECCU always operates in complete heat balance at any desired hydrocarbon feed rate and reactor temperature this heat balance is achieved in units such as the one shown in Eigure 1 by varying the catalyst circulation rate. Catalyst flow is controlled by a sHde valve located in the catalyst transfer line from the regenerator to the reactor and in the catalyst return line from the reactor to the regenerator. In some older style units of the Exxon Model IV-type, where catalyst flow is controlled by pressure balance between the reactor and regenerator, the heat-balance control is more often achieved by changing the temperature of the hydrocarbon feed entering the riser. 

Fig. 2. Heat balance of the FCCU. Heat balance around the reactor A, heat balance around the regenerator B, and the overall heat balance around the entire...

Heat balances of several lands of reactors are summarized in Tables 7-5, 7-6, 7-7 and 7-10. 

In tubular reactors of only a few cm in diameter, the temperature is substantially uniform over the cross section so only an axial gradient occurs in the heat balance. 

The stripped catalyst is picked up by a stream of air and carried into the regenerator where the carbon is burned at temperamres about 1100-1300°F. Entrained catalyst is again removed by cyclones and the flue gas goes out the stack. The hot, regenerated catalyst leaves the regenerator and takes with it much of the heat of combustion. This is carried over to the reactor to vaporize the feed and to balance the endothermic heat of cracking. Thus, the process is heat balanced. 

Non-lsothermal Reactors Table 6-8 Fractional conversions (mass and heat balances) at effluent temperatures 513... 

A cat cracker continually adjusts itself to stay in heat balance. This means that the reactor and regenerator heat flows must be equal (Figure 5-4). Simply stated, the unit produces and bums enough coke to provide energy to ... 

A heat balance can be performed around the reactor, around the stripper-regenerator, and as an overall heat balance around the reactor-regenerator. The stripper-regenerator heat balance can be used to calculate the catalyst circulation rate and the catalysi-to-oil ratio. 

The calculation of heat balance around the reactor is illustrated in Example 5-6. As shown, the unknown is the heat of reaction. It is calculated as the net heat from the heat balance divided by the feed flow in weight units. This approach to determining the heat of reaction is acceptable for unit monitoring. However, in designing a new cat cracker, a correlation is needed to calculate the heat of reaction. The heat of reaction is needed to specify other operating parameters, such... 

The coke yield of a given cat cracker is essentially constant. The FCC produces enough coke to satisfy the heat balance. However, a more important term is delta coke. Delta coke is the difference between the coke on the spent catalyst and the coke on the regenerated catalyst. At a given reactor temperature and constant CO2/CO ratio, delta coke controls the regenerator temperature. 

Steady state models of the automobile catalytic converter have been reported in the literature 138), but only a dynamic model can do justice to the demands of an urban car. The central importance of the transient thermal behavior of the reactor was pointed out by Vardi and Biller, who made a model of the pellet bed without chemical reactions as a onedimensional continuum 139). The gas and the solid are assumed to have different temperatures, with heat transfer between the phases. The equations of heat balance are ... 

In chemical processing the most fundamental constraint is that of the thermodynamics of the system. This constraint defines both the heat balance of the process and whether or not the processes in the reactor will be equilibrium limited. These constraints will limit the range of chemical engineering solutions to the problems of designing an economically viable process that can be found. 

A steady-state heat balance for a plug flow reactor with no radial temperature gradients is given by ... 

Use Scalable Heat Transfer. The feed flow rate scales as S and a cold feed stream removes heat from the reaction in direct proportion to the flow rate. If the energy needed to heat the feed from to Tout can absorb the reaction exotherm, the heat balance for the reactor can be scaled indefinitely. Cooling costs may be an issue, but there are large-volume industrial processes that have Tin —40°C and Tout 200°C. Obviously, cold feed to a PFR will not work since the reaction will not start at low temperatures. Injection of cold reactants at intermediate points along the reactor is a possibility. In the limiting case of many injections, this will degrade reactor performance toward that of a CSTR. See Section 3.3 on transpired-wall reactors. 

A relatively simple example of a confounded reactor is a nonisothermal batch reactor where the assumption of perfect mixing is reasonable but the temperature varies with time or axial position. The experimental data are fit to a model using Equation (7.8), but the model now requires a heat balance to be solved simultaneously with the component balances. For a batch reactor. 

This result is perfectly general for a constant-volume reactor. It continues to apply when p, Cp, and H are expressed in mass units, as is normally the case for liquid systems. The current example has a high level of inerts so that the molar density shows little variation. The approximate heat balance... 

The pilot plant stage Is vital in the scale-up of any new resin process, and in this paper we discuss the design philosophy of pilot plants and then describe two fully Instrumented and computer data logged reactors. Some indication is given of the use of the extracted data for both modelling and scale up. Both controlled and data logged parameters are tabulated and an example of data extraction for heat balance is illustrated. 

Reactor electrical heat input(l) Coil/jacket heat balance Condenser heat extract rate (1) Reactor heat loss... 

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