Reactors, chemical tubular flow

Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

A useful classification of types of chemical reactors is in terms of their concentration patterns. Certain limiting or ideal types are represented by Figure 4.1 which illustrates batch reactors, continuous stirred tanks and tubular flow reactors. This chapter is concerned with the sizes, performances and heat effects of these ideal types. They afford standards of comparison and are often as close enough to the truth as available information allows. 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

All chemical reactions are accompanied by some heat effects so that the temperature will tend to change, a serious result in view of the sensitivity of most reaction rates to temperature. Factors of equipment size, controllability, and possibly unfavorable product distribution of complex reactions often necessitate provision of means of heat transfer to keep the temperature within bounds. In practical operation of nonflow or tubular flow reactors, truly isothermal conditions are not feasible even if they were desirable. Individual continuous stirred tanks, however, do maintain substantially uniform temperatures at steady state when the mixing is intense enough the level is determined by the heat of reaction as well as the rate of heat transfer provided. 

Figure 4-13. Liquid-liquid heterogeneous tubular flow reactor (e.g., alkylation of olefins and isobutane). (Source J. M. Smith, Chemical Engineering Kinetics, 3rd ed., McGraw-Hill, Inc., 1981.)...

Chemical vapor deposition is a key process for thin film formation in the development and manufacture of microelectronic devices. It shares many kinetic and transport phenomena with heterogeneous catalysis, but CVD reactor design has not yet reached the level of sophistication used in analyzing heterogeneous catalytic reactors. With the exception of the tubular LPCVD reactor, conventional CVD reactors may be viewed as variations on the original horizontal reactor. These reactors have complex flow fields and it is consequently difficult to control and predict the effect of operating conditions on the film thickness and composition. 

FIG. 19-14 Batch and continuous polymerizations, (a) Polyethylene in a tubular flow reactor, up to 2 km long by 6.4-cm ID. (b) Batch process for polystyrene, (c) Batch-continuous process for polystyrene, (d) Suspension (bead) process for polyvinylchloride, (e) Emulsion process for polyvinylchloride. Ray and Laurence, in Lapidus and Amundson (eds.). Chemical Reactor Theory Review, Frentice-Halt, 1977.)... 

Tubular flow reactors are usually operated under steady conditions so that, at any point, physical and chemical properties do not vary with time. Unlike the batch and tank flow reactors, there is no mechanical mixing. Thus, the state of the reacting fluid will vary from point to point in the system, and this variation may be in both the radial and axial direction. The describing equations are then differential, with position as the independent variable. 

Chakraborty, S. and V. Balakotaiah, Low-dimensional models for describing mixing effects in laminar flow tubular reactors. Chemical Engineering Science, 2002, 57 2545-2564. 

The OBR can be viewed as a series stirred cells or a tubular flow reactor. The tube can be jacketed to provide cooling or heating. Its potential has been demonstrated for processing in the pharmaceutical, fine-chemicals and biochemical industries. An account of these can be found elsewhere [26, 37],... 

Chemical reactors, particularly for continuous processes, are often custom designed to involve multiple phases (e.g., vapor, liquid, reacting solid, and solid catalyst), different geometries (e.g., stirred tanks, tubular flows, converging and diverging nozzles, spiral flows, and membrane transport), and various regimes of momentum, heat, and mass transfer (e.g., viscous flow, turbulent flow, conduction, radiation, di sion, and dispersion). There... 

Pancharatnam S. Homsy GM. An asymptotic solution for tubular flow reactor with catalytic wall at high Peclet numbers. Chemical Engineering Science 1972 27 1337-1340. 

Plug Flow Reactor (PFR) Tubular reactor that fluid moves through at a continuous, steady pace so that the conversion of chemicals are functions of the position within the reactor rather than time. Also called continuous tubular reactor (CTR). 

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