Reactor heat capacity

Normalized activation energy (.E fR) Heat of reaction Mass of metal wall in reactor Heat capacity of metal wall Heat losses... 

The slow, stable response of the GT-MHR to internally or externally initiated transients, in combination with passive safety features, provides a strong defence against internal or external threats to the plant. These characteristics arise from the inert coolant, large reactor heat capacity, low power density, strong negative reactivity feedback coefficients and passive decay heat removal by conduction, radiation and convection. 

Reactor heat carrier. Also as pointed out in Sec. 2.6, if adiabatic operation is not possible and it is not possible to control temperature by direct heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flow rate (i.e., product of mass flow rate and specific heat capacity) and to reduce... 

Heat carriers. If adiabatic operation produces an unacceptable rise or fall in temperature, then the option discussed in Chap. 2 is to introduce a heat carrier. The operation is still adiabatic, but an inert material is introduced with the reactor feed as a heat carrier. The heat integration characteristics are as before. The reactor feed is a cold stream and the reactor efiluent a hot stream. The heat carrier serves to increase the heat capacity fiow rate of both streams. 

The preceding appropriate placement arguments assume that the process has the capacity to accept or give up the reactor heat duties at the given reactor temperature. A quantitative tool is needed to assess the capacity of the background process. For this purpose, the grand composite curve can be used and the reactor profile treated as if it was a utility, as explained in Chap. 6. 

Solution Polymerization. In this process an inert solvent is added to the reaction mass. The solvent adds its heat capacity and reduces the viscosity, faciUtating convective heat transfer. The solvent can also be refluxed to remove heat. On the other hand, the solvent wastes reactor space and reduces both rate and molecular weight as compared to bulk polymerisation. Additional technology is needed to separate the polymer product and to recover and store the solvent. Both batch and continuous processes are used. 

Suspension Polymerization. In this process the organic reaction mass is dispersed in the form of droplets 0.01—1 mm in diameter in a continuous aqueous phase. Each droplet is a tiny bulk reactor. Heat is readily transferred from the droplets to the water, which has a large heat capacity and a low viscosity, faciUtating heat removal through a cooling jacket. 

With batch reactors, it may be possible to add all reactants in their proper quantities initially if the reaction rate can be controlled by injection of initiator or acqustment of temperature. In semibatch operation, one key ingredient is flow-controlled into the batch at a rate that sets the production. This ingredient should not be manipiilated for temperature control of an exothermic reactor, as the loop includes two dominant lags—concentration of the reactant and heat capacity of the reaction mass—and can easily go unstable. 

A useful approximation to estimate the possibility of a particular reaction which depends on internal heat generation to produce the products in the proper state for separation is to ignore the heat losses from die reactor, and assumes an average heat capacity calculated from die Neumann-Kopp law... 

Similar approaches are applicable in the chemical industry. For example, maleic anhydride is manufactured by partial oxidation of benzene in a fixed catalyst bed tubular reactor. There is a potential for extremely high temperatures due to thermal runaway if feed ratios are not maintained within safe limits. Catalyst geometry, heat capacity, and partial catalyst deactivation have been used to create a self-regulatory mechanism to prevent excessive temperature (Raghaven, 1992). 

A stirred reactor contains a batch of 700 kg reactants of specific heat 3.8 kJ/kg K initially at 290 K, which is heated by dry saturated steam at 170 kN/m2 fed to a helical coil. During the heating period the steam supply rate is constant at 0.1 kg/s and condensate leaves at the temperature of the steam. If heat losses arc neglected, calculate the true temperature of the reactants when a thermometer immersed in the material reads 360 K. The bulb of the thermometer is approximately cylindrical and is 100 mm long by 10 mm diameter with a water equivalent of 15 g, and the overall heat transfer coefficient to the thermometer is 300 W/m2 K. What would a thermometer with a similar bulb of half the length and half the heat capacity indicate under these conditions ... 

Figure 1. Typical reactor temperature profile for continuous addition polymerization a plug-flow tubular reactor. Kinetic parameters for the initiator 1 = 10 ppm Ea = 32.921 kcal/mol In = 26.492 In sec f = 0.5. Reactor parameter [(4hT r)/ (DpCp)] = 5148.2. [(Cp) = heat capacity of the reaction mixture (p) = density of the reaction mixture (h) = overall heat-transfer coefficient (Tf) = reactor jacket temperature (r) = reactor residence time (D) = reactor diameter].

BMO = inlet monomer (ethylene) concentration, mole/l PP = reactor pressure, psia BM = monomer (ethylene) molecular weight CP = heat capacity at constant pressure of the reaction fluid, cal/mole- C... 

T = temperature p = density AH = heat of reaction h = heat-transfer coefficient D = reactor diameter Cp = heat capacity Rp = polymerization rate Tj = reactor jacket temperature P = pressure... 

The design equations for a nonisothermal batch reactor include A-fl DDEs, one for each component and one for energy. These DDEs are coupled by the temperature and compositional dependence of 91/. They may also be weakly coupled through the temperature and compositional dependence of physical properties such as density and heat capacity, but the strong coupling is through the reaction rate. 

Chapter 7 has two goals. The first is to show how reaction rate expressions, SI a, b,..., T), are obtained from experimental data. The second is to review the thermodynamic underpinnings for calculating reaction equilibria, heats of reactions and heat capacities needed for the rigorous design of chemical reactors. 

Thermod5mamics is a fundamental engineering science that has many applications to chemical reactor design. Here we give a summary of two important topics determination of heat capacities and heats of reaction for inclusion in energy balances, and determination of free energies of reaction to calculate equihbrium compositions and to aid in the determination of reverse reaction... 

These equations assume that the reactor is single phase and that the surroundings have negligible heat capacity. In principle, Equations (14.19) and (14.20) can be solved numerically using the simple methods of Chapters 8 and 9. The two-dimensional problem in r and is solved for a fixed value of t. A step forward in t is taken, the two-dimensional problem is resolved at the new t, and so on. 

In the model equations, A represents the cross sectional area of reactor, a is the mole fraction of combustor fuel gas, C is the molar concentration of component gas, Cp the heat capacity of insulation and F is the molar flow rate of feed. The AH denotes the heat of reaction, L is the reactor length, P is the reactor pressure, R is the gas constant, T represents the temperature of gas, U is the overall heat transfer coefficient, v represents velocity of gas, W is the reactor width, and z denotes the reactor distance from the inlet. The Greek letters, e is the void fraction of catalyst bed, p the molar density of gas, and rj is the stoichiometric coefficient of reaction. The subscript, c, cat, r, b and a represent the combustor, catalyst, reformer, the insulation, and ambient, respectively. The obtained PDE model is solved using Finite Difference Method (FDM). 

Fig. 3.2 shows the case of a jacketed, stirred-tank reactor, in which either heating by steam or cooling medium can be applied to the jacket. Here V is volume, Cp is specific heat capacity, p is density, Q is the rate of heat transfer, U is the overall heat transfer coefficient, A is the area for heat transfer, T is temperature, H is enthalpy of vapour, h is liquid enthalpy, F is volumetric flow... 

The heat transfer coefficient, U, to the reactor wall (cal/h cm K) and the coolant temperature is Tj. The nitrobenzene feed rate is Nafo (mol/h), with hydrogen in very large excess. The heat of reaction is AH (cal/mol). The heat capacity of hydrogen is Cp (cal/mol K). 

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