Chemical reactors external heat exchange reactor

FIG. 23-1 Heat transfer to stirred tank reactors, a) Jacket, (h) Internal coils, (c) Internal tubes, (d) External heat exchanger, (e) External reflux condenser. if) Fired heater. (Walas, Reaction Kinetics for Chemical Engineers, McGraw-Hill, 1959). 

There are many reactors in industry that use evaporative cooling. The liquid in the reactor boils to remove reaction heat. The vapor leaving the reactor is condensed in an external heat exchanger, and the liquid is returned to the reactor. Clearly the vapor phase is important in these autorefrigerated reactors. In addition to chemical kinetics, the vapor-liquid equilibrium properties influence the design of the reactor-condenser system. 

Hydrogenation reactions are frequently run in fed-batch reactors. The chemical component to be hydrogenated is charged to the reactor vessel. The hydrogen is then fed into the vessel on pressure control. The temperature of the reactor is controlled by manipulating the flowrate of coolant to the jacket, coil, or external heat exchanger. Thus this system has two manipulated variables (the flowrate of hydrogen and the flowrate of coolant) and two controlled variables (pressure and temperature). 

Jet-loop reactors tend to replace stirred-tank reactors in recently built equipment for fine-chemical hydrogenation. The external heat exchanger on the liquid circu-... 

Suppressing the influence of external disturbances on a process is the most common objective of a controller in a chemical plant. Such disturbances, which denote the effect that the surroundings (external world) have on a reactor, separator, heat exchanger, compressor, and so on, are usually out of the reach of the human operator. Consequently, we need to introduce a control mechanism that will make the proper changes on the process to cancel the negative impact that such disturbances may have on the desired operation of a chemical plant. 

Figure 6.12 An autothermal reactor system with details on external heat exchange from product to feed. [After J.M. Smith, Chemical Engineering Kinetics, 3rd ed., with permission of McGraw-Hill Book Co., New York, NY, (1981).]...

Formation of quasi-plug-flow mode in turbulent flows under fast chemical processes when reaction zone reaches heat exchanging reactor walls determines possibility of application of effective external heat removal that allows regulating of molecular characteristics of resulting polymer [62-65]. 

In many industrially important situations, it is impossible to maintain geometric, mechanical (kinematic/hydrodynamic and turbulence similarities), and thermal similarities simultaneously. Consider a stirred tank reactor with heat exchange only through a jacket on its external surface. The jacket heat transfer area to vessel volume ratio is proportional to (l/T). Evidently, with scale-up, this ratio decreases, and it is difficult to maintain the same heat transfer area per unit volume as in the small-scale unit. Additional heat transfer area is required to cater to the extra heat load resulting from increase in reactor volume. This area can be provided in the form of a coil inside the reactor or an external heat exchanger circuit. In both cases, the flow patterns are significantly different than the model contactor used in bench-scale studies and kinematic similarity is violated. This is the classic dilemma of a chemical engineer it is impossible to preserve the different types of similarities simultaneously. 

The obvious way to heat or cool a chemical reactor is by heat exchange through a wall, either by an external jacket or with a cooling or heating coil. As we discussed previously, the coolant energy balance must be solved along with the reactor energy balance to determine temperatures and heat loads. 

These are not state variables of the system. They are external to the system and affect the system, or in other words they work on the system . For example, the feed temperature and composition of the feed stream for a distillation column or a chemical reactor, or the feed temperature of a heat exchanger are input variables. 

A second example of interest in the present context refers to the scaling of thermal effects. Any object (a chemical reactor such as a living body) that produces heat at a rate proportional to its volume ( <2r a Vr) and exchanges heat with a cooling device or with the ambient at a rate proportional to its lateral surface Sl and to the temperature difference with respect to the external heat sink (i.e., Qe = USe(Tt - Ta)) can maintain the same temperature, independently of its dimensions, only if the ratio USe/Vx is kept constant. In general, this condition cannot be satisfied, since the ratio SeJ V) is inversely proportional to the characteristic linear dimension, and the... 

Monolithic Loop Reactor A novel MLR was developed af Air Products and Chemicals (Figure 17) (144). The reactor contains a monolithic catalyst operating under cocurrent downflow condifions. Because the residence time in the monolith is short and the heat of reaction has to be removed, the liquid is continually circulated via an external heat exchanger until the desired conversion is reached. The concept was patented for the hydrogenation of dinifrofoluene fo give toluenediamine (37). 

External sinface area of catalyst per unit bed volume (m /m ) Area of heat exchange per unit volume of reactor (m ) External surface area per volume of catalyst pellets (mVm ) Chemical species... 

Further discussion of nuclear reactor control problems resulting from water leakage is beyond the scope of the present paper, and the discussion presented hereafter is on the effects of the chemical reaction between sodium and water. As a corollary of this discussion, the necessity for the use of a double-barrier heat exchanger in systems where nuclear control problems are absent has been examined. The desirability of eliminating the double-barrier design where feasible is obvious. The double-barrier results in a more complex design with associated fabrication and operational problems, requires additional heat transfer area due to the increased thermal resistance of the double barrier, and requires external equipment to handle the third-fluid system. All these factors increase the size and cost of the heat exchanger. 

Unless required as part of the design of the separator or reactor, provide necessary heat exchange for heating or cooling process fluid streams, with or without utilities, in an external shell-and-tube heat exchanger using countercurrent flow. However, if a process stream requires heating above 750°F, use a furnace unless the process fluid is subject to chemical decomposition. 

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