Rate of heat removal

Figure 13.1a shows two possible thermal profiles for exothermic plug-fiow reactors. If the rate of heat removal is low and/or the heat of reaction is high, then the temperature of the reacting stream will increase along the length of the reactor. If the rate of heat removal is high and/or the heat of reaction is low, then the temperature will fall. Under conditions between the two profiles shown in Fig. 13.1a, a maximum can occur in the temperature at an intermediate point between the reactor inlet and exit. 

Fig. 15. Temperature vs heat generation or removal in estabHshing stationary states. The heavy line (—) shows the effect of reaction temperature on heat-generation rates for an exothermic first-order reaction. Curve A represents a high rate of heat removal resulting in the reactor operating at a low temperature with low conversion, ie, stationary state at a B represents a low rate of heat removal and consequently both a high temperature and high conversion at its stationary state, b and at intermediate heat removal rates, ie, C, multiple stationary states are attainable, c and The stationary state at c ...

Let us consider a cooler in the equipment flow sheet. The required rate of heat removal is known from the balances, and the rate of cooling water can be calculated once the inlet and outlet temperatures of the water have been specified. The calculation of the consumption of other utilities is also straightforward. Allowances should be made for wastage. 

Under steady-state conditions the temperature of the evaporating surface increases until the rate of sensible heat transfer to the surface equals the rate of heat removed by evaporation from the surface. To calculate this temperature, it is convenient to modify Eq. (12-26) in terms of humidity rather than partial-pressure difference, as follows ... 

Assuming that the CESTR is operating at steady state, the rate of heat generation must equal the rate of heat removal from the reaetor. Plotting Qg as a funetion of T for fixed values of the other variables, an S-shaped eurve is obtained as shown in Eigure 6-19. 

If an external jaeket or internal eoil is used through whieh eoolant flows at temperature to remove the heat, then the rate of heat removal is represented by... 

The rate of heat removal also depends on four main faetors ... 

Heat transfer can, of course, be increased by increasing the agitator speed. An increase in speed by 10 will increase the relative heat transfer by 10. The relative power input, however, will increase by 10In viscous systems, therefore, one rapidly reaches the speed of maximum net heat removal beyond which the power input into the batch increases faster than the rate of heat removal out of the batch. In polymerization systems, the practical optimum will be significantly below this speed. The relative decrease in heat transfer coefficient for anchor and turbine agitated systems is shown in Fig. 9 as a function of conversion in polystyrene this was calculated from the previous viscosity relationships. Note that the relative heat transfer coefficient falls off less rapidly with the anchor than with the turbine. The relative heat transfer coefficient falls off very little for the anchor at low Reynolds numbers however, this means a relatively small decrease in ah already low heat transfer coefficient in the laminar region. In the regions where a turbine is effective,... 

As shown in Figure 5.6(b), the rate of heat removal is a linear function of Tout when physical properties are constant ... 

Rate of generation of heat / Rate of = heat removal... 

Determine Q and Tc (as functions of time, f) required to maintain isothermal conditions in the reactor in Example 12-1, if AHRA = -47,500 J mol-1, and UAC = 25.0 W K-1. Does Q represent a rate of heat removal or heat addition ... 

Also, the temperature of no return can be calculated from the data obtained in an ARC experiment [123]. In a given reaction vessel with given heat transfer characteristics, the temperature of no return (Tnr) is a metastable temperature such that below Tnr, the reaction temperature will not increase since the released heat from the reaction does not exceed the rate of heat removal from the system, and above Tnr the reaction temperature will in-... 

The reaction, A + B C, involves a high rate of heat generation and the rate of heat removal is limited. Accordingly, B is charged to the vessel at reaction temperature, and A is added gradually also at reaction temperature.The initial charge is 1.0 lbmol of B. The specific volumes of both A and B are 1 cuft/lbmol. The reaction is first order with respect to B with k = 3/hr, assumed independent of temperature ( ). Heat of reaction is 150,000 Btu/lbmol A reacted and the maximum rate of heat removal is 50,000 Btu/hr. What is the maximum possible rate of addition of A ... 

Quenching the vapour with cold air in the chamber may increase the rate of heat removal although excessive nucleation is likely and the product crystals will be very small. Condenser walls may be kept free of solid by using internal scrapers, brushes, and other devices, and all vapour lines in sublimation units should be of large diameter, be adequately insulated, and if necessary, be provided with supplementary heating to minimise blockage due to the buildup of sublimate. One of the main hazards of air-entrainment sublimation is the risk of explosion since many solids that are considered safe in their normal state can form explosive mixtures with air. All electrical equipment should therefore be flame-proof, and all parts of the plant should be efficiently earthed to avoid build-up of static electricity. 

The rate of heat removal from the reaction mass to the cooling coil is -Q (energy per time). The temperature of the feed stream is Tq and the temperature in the reactor is T (°R or K). Writing Eq. (2.18) for this system,... 

The first term is a sensible-heat term. If the feed temperature 7 is lower than the reactor temperature T, the feed stream tends to cool the reactor. The second term is the rate of heat removal by heat transfer to the cooling jacket. 

If the pressure is throttled down to 400psia at the inlet of the turbine, find the pump power required, turbine power produced, rate of heat added to the boiler, rate of heat removed in the condenser, and the cycle efficiency. Draw the T s diagram. 

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