Publisher Summary At the heart of the liquid-phase processes, gas scrubbing process, manufacturing of pure products, and biological systems, there exists the absorber or the reactor of a particular configuration best suited to the chemical absorption or reaction being carried out. Its selection, design, sizing, and performance depend on the hydrodynamics and axial dispersion, mass and heat transfer, and reaction kinetics. This chapter focuses on the subject of mass transfer with chemical reaction. It presents the techniques, results, and opinions on mass-transfer coefficients and interfacial areas in most types of absorbers and reactors. To study gas–liquid mass-transfer phenomena, it is convenient to consider steady-state situations in which the composition of the gas and the liquid are statistically constant when averaged over time in a specified region, such as a short, vertical slice of a tubular column or the entire volume of a single-compartment agitated vessel. Useful predictions have been developed for describing the behavior of complicated systems, using highly simplified models that simulate the situation for practical purposes without introducing a large number of parameters. The procedure differs depending on whether physical or chemical absorption is involved.

Mass-Transfer Rates in Gas-Liquid Absorbers and Reactors

Publisher Summary This chapter focuses on two idealized patterns (of all possible flow patterns), plug flow and back-mix flow. Plug flow assumes that fluid moves through the vessel “in single file” with no overtaking or mixing with earlier or later entering fluid. Back-mix flow assumes that the fluid in the equipment is perfectly mixed and uniform in composition throughout the vessel. All patterns of flow other than plug and back-mix flow may be called nonideal flow patterns because for these the design methods are not nearly as straightforward as those for the two ideal flow patterns. The treatment of nonideal flow patterns discussed in the chapter divides naturally into two parts: flow of single fluids and flow of two fluids. The main emphasis falls on the nonideal flow of single fluids through process equipment. Conversion in a reactor with nonideal flow can be determined either directly from tracer information or by use of flow models. The distribution of residence times gives information on how long various elements of fluid spend in the reactor but not on the detailed exchange of matter within and among the elements. The exact nature of the surrounding molecules is of no importance. Thus, the distribution of residence times yields sufficient information for the prediction of the average concentration in the reactor effluent. Because of a lack of applications, conversion expressions for two region models have not been developed for homogeneous systems. For heterogeneous systems, the appropriate expressions can be found in the works of the individual investigators.

Patterns of Flow in Chemical Process Vessels

In this literature survey flow patterns, mixing effects, heat transfer and power required for rotation in scraped surface heat exchangers (SSHE) are thoroughly discussed, with the emphasis on assumptions and results, while the principal design of different SSHEs are only briefly discussed.



The flow patterns control the desired radial mixing and the undesired axial mixing. the flow in a SSHE can be regarded as the sum of an axial flow and a rotational flow. the axial flow is laminar and the rotational flow is laminar or vortical.



With laminar flow the radial mixing is poor, which causes poor heat transfer and allows the axial flow profile to control the residence time distribution. the precise onset of vortical flow in a SSHE is hard to predict. the vortical flow makes the radial mixing very efficient, giving good heat transfer and perhaps plug flow behavior. However, vortical flow also causes axial mixing which reduces the apparent heat transfer coefficient and increases the residence time distribution.



The power required to rotate the shaft and blades is mainly determined by the design of the blades.

SCRAPED SURFACE HEAT EXCHANGERS.: A literature survey of flow patterns, mixing effects, residence time distribution, heat transfer and power requirements.

Publisher Summary It is primarily food aroma that distinguishes the flavor of one food from that of another, and aroma quality often determines the acceptability of a food. When water is removed from a food, the loss or retention of the food aroma is one of the major considerations in the design of the food processing operation. This chapter reviews the research work that examines the behavior of food aroma when water is removed by the various techniques used in food processing. Water is removed from foods to provide microbiological stability, and to reduce storage and transportation costs. This chapter distinguishes between concentration and dehydration processes; the former includes those processes that increase solids content to about 60 % and the latter those that reduce water content to less than 10%. This chapter discusses the various aroma recovery processes. It then takes a look at the concentration processes where aromas are retained in the concentrate and drying processes where aroma is retained. This chapter ends with a discussion of the economics of concentration and drying processes.

Aroma Recovery and Retention in Concentration and Drying of Foods

Flow regimes and wall shear rates determination within a scraped surface heat exchanger

The mathematical derivation of solutions for longitudinal dispersion in chemical-process equipment is extended to the case of cocurrent flow. The extent of completion of the process, in dimensionless form, is given as an analytical function of rates of dispersion in the two phases, over-all heat- or mass- transfer coefficient, partition coefficient, rates of fluid flow, and fractional height in the equipment. Numerical results for a large number of typical conditions are given in tubular form. (auth)

/pdf/longitudinal-dispersion-in-two-phase-continuous-flow-z1fdxmxamd.pdf

Theordore. Vermeulen

Papers

Longitudinal dispersion in two-phase continuous-flow operations: solution and tables for cocurrent flow