Cooling nonisothermal

Because the system likely is nonisothermal, the analysis of a closed-desiccant system requites knowledge of the temperature of the desiccant as well as the dew point (ice point) or water concentration (partial pressure) specification. Indeed, the whole system may undergo periodic temperature transients that may compHcate the analysis. Eor example, in dual-pane windows the desiccant temperature is approximately the average of the indoor and outdoor temperatures after a night of cooling. However, after a day in the sun, the desiccant temperature becomes much warmer than the outdoor temperature. When the sun sets, the outdoor pane cools quickly while the desiccant is still quite warm. The appropriate desiccant for such an appHcation must have sufficient water capacity and produce satisfactory dew points at the highest temperatures experienced by the desiccant. 

For nonisothermal flow of liquids across tube bundles, the fric tion factor is increased if the liquid is being cooled and decreased if the liquid is being heated. The factors previously given for nonisotherm flow of liquids in pipes ( Tncompressible Flow in Pipes and Channels ) should be used. 

External Exchanger with Liquid Continuously Added to Tank Nonisothermal Cooling Medium... 

A dynamic differential equation energy balance was written taking into account enthalpy accumulation, inflow, outflow, heats of reaction, and removal through the cooling jacket. This balance can be used to calculate the reactor temperature in a nonisothermal operation. 

Chen et al. [70] suggested that temperature gradients may have been responsible for the more than 90 % selectivity of the formation of acetylene from methane in a microwave heated activated carbon bed. The authors believed that the highly nonisothermal nature of the packed bed might allow reaction intermediates formed on the surface to desorb into a relatively cool gas stream where they are transformed via a different reaction pathway than in a conventional isothermal reactor. The results indicated that temperature gradients were approximately 20 K. The nonisothermal nature of this packed bed resulted in an apparent rate enhancement and altered the activation energy and pre-exponential factor [94]. Formation of hot spots was modeled by calculation and, in the case of solid materials, studied by several authors [105-108],... 

In adiabatic operation, there is no attempt to cool or heat the contents of the reactor (that is, there is no heat exchanger). As a result, T rises in an exothermic reaction and falls in an endothermic reaction. This case may be used as a limiting case for nonisothermal behavior, to determine if T changes sufficiently to require the additional expense of a heat exchanger and T controller. 

We stiU must use a PFTR in many chemical processes, and we must then determine how to program the cooling or heating to attain a temperature profile in the reactor close to that desired. The subject of this chapter is the proper temperature management to attain desired operation of a PFTR. In the next chapter the nonisothermal CSTR will be considered specifically. 

In this and the previous chapters we considered the effects of nonisothermal operation on reactor behavior. The effects of nonisothermal operation can be dramatic, especially for exothermic reactions, often leading to reactor volumes many times smaller than if isothermal and often leading to the possibility of multiple steady states. Further, in nonisothermal operation, the CSTR can require a smaller volume for a given conversion than a PFTR. In this section we summarize some of these characteristics and modes of operation. For endothermic reactions, nonisothermal operation cools the reactor, and this reduces the rate, so that these reactors are inherently stable. The modes of operation can be classified as follows ... 

The polarized state (nonequilibrium steady state) is created by applying a DC voltage at an elevated temperature and by subsequent cooling of the solid to a temperature that is sufficiently low that rapid relaxation is prevented. The next step of the experimental procedure is to remove the DC bias. The currents that can be measured during either isothermal or nonisothermal relaxation back to thermal equilibrium are used to monitor the relaxation processes involved. 

For most experiments on nonisothermal TSR, simple cooling of the sample to the desired initial temperature and a linear increase in T after excitation are sufficient to obtain TSC and TSL glow curves. Some techniques require more elaborate heating cycles, the details of which depend on the relaxation mechanism under study and on whether it is necessary to discriminate between simnltaneously occurring processes, e.g., thermally stimulated depolarization and thermally stimulated conductivity (see Chapter 2). 

Cool Flames. Cool flames are confined, roughly speaking, to the temperature regime which exhibits the negative temperature coefficient of the rate. The flames are clearly nonisothermal, and the light emission which is most intense at the end of the maximum rate period is probably caused by radical-radical reactions (27, 28) such as... 

A diffusion measurement at the temperature To is made by annealing a diffusion couple comprised of two semi-infinite bars. However, there is a complication after the completion of the isothermal anneal, carried out at To for the time to, the specimen must be cooled to room temperature at a finite rate. During this cooling period, a small amount of additional nonisothermal diffusion occurs. If an expression can be found for the amount of time, At, required to produce this same additional increment of diffusion at the constant temperature To, the specimen could be analyzed very simply at the end of the experiment by assuming that it was annealed at To for the time to + At. Assume that D = D0 exp —E/(kT)] and that the temperature during the cooling period is... 

In the case of the nonisothermal first-order exothermic reaction heat is auto catalytic, for it raises the temperature and provokes an increase of reaction rate, yet is itself a product of the reaction. In the Gray-Scott scheme, B is plainly autocatalytic and its degeneration by the second reaction plays the role of the direct cooling in the non-isothermal case. This reaction appears in the chemical engineering literature in 1983,16 and is the keynote reaction in Gray and Scott s 1990 monograph on Chemical Oscillations and Instabilities.17 A justification of the autocatalytic mechanism in terms of successive bimolecular reactions is the subject of Chapter 12. 

An industrial system consists of a nonisothermal CSTR (with a cooling jacket) and a tubular adiabatic reactor in series. The reaction is a first-order irreversible reaction ... 

Koelling et al. (70) conducted nonisothermal, pressurized gas-bubble Newtonian fluid-displacement experiments. The fluid used was PB H-300. It was injected into a capillary tube maintained at 60°C. The tube was then transferred in a different temperature bath at 0°C. The penetrating gas was then injected after different delay times, t. The longer the delay time, the deeper the cooling penetration thickness will be, since it is dependent on the Fourier number,... 

In general, influencing the reaction course via control of the cooling stream is more flexible than incorporating catalysts of different activities. Particularly with nonisothermal control of the heat transfer media, the reaction course can be influenced over a wide range by means of the inflow temperature of the heat transfer medium as well as by its volumetric flow rate [40],... 

BATCH COOLING INTERNAL COIL, NONISOTHERMAL COOLING MEDIUM 7.20... 

BATCH COOLING EXTERNAL HEAT EXCHANGER (COUNTERFLOW), NONISOTHERMAL COOLING MEDIUM 7.21... 

Select and apply the appropriate heat-transfer formula. When cooling a batch with an internal coil and a nonisothermal cooling medium, the following equation applies ... 

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