Mass-transfer devices

An industrial chemical reacdor is a complex device in which heat transfer, mass transfer, diffusion, and friction may occur along with chemical reaction, and it must be safe and controllable. In large vessels, questions of mixing of reactants, flow distribution, residence time distribution, and efficient utilization of the surface of porous catalysts also arise. A particular process can be dominated by one of these factors or by several of them for example, a reactor may on occasion be predominantly a heat exchanger or a mass-transfer device. A successful commercial unit is an economic balance of all these factors. 

A mass transfer device, such as a packed or trayed contact section, should be considered (see Fig. 26-21). 

It should be noted here that the bioartificial fiver device is not only a bioreactor but also a mass transfer device. The mass transfer of various nutrients from the blood into the liver cells, and also the transfer of many products of biochemical reactions from the cells into the bloodstream, should be efficient processes. In human fiver, the oxygen-rich blood is delivered via the hepatic artery, and bioartificial devices should be so designed that the oxygen can be easily delivered to the cells. 

Process intensification refers to a chemical process using significantly smaller equipment. Examples include novel reactors, intense mixing devices, heat and mass transfer devices that provide high surface area per unit of volume, devices... 

The performance of OCFS as mass transfer devices is heavily dependent on the quality of the distribution of the gas and liquid phases across the column upon entry to the packed section—irrespective of whether its function is purely rectification/stripping or chemical conversion also. Optimal liquid distribution is, however, of additional importance in catalytic distillation, in ensuring contacting of reactants with the catalyst. 

Miniaturization of Absorption Heat and Mass Transfer Devices - Volatile Absorbents... 

Figure 2 MicroChannel Heat and Mass Transfer Device Concept...

The membrane in a contactor acts as a passive barrier and as a means of bringing two immiscible fluid phases (such as gas and hquid, or an aqueous hquid and an organic hquid, etc.) in contact with each other without dispersion. The phase interface is immobilized at the membrane pore surface, with the pore volume occupied by one of the two fluid phases that are in contact. Since it enables the phases to come in direct contact, the membrane contactor functions as a continuous-contact mass transfer device, such as a packed tower. However, there is no need to physically disperse one phase into the other, or to separate the phases after separation is completed. Several conventional chemical engineering separation processes that are based on mass exchange between phases (e.g., gas absorption, gas stripping, hquid-hquid extraction, etc.) can therefore be carried out in membrane contactors. 

Chapter 2 Application of Membrane Contactors as Mass Transfer Devices.7... 

Capacity and pressure drop characteristics superior to the best commercially available mass transfer devices. 

In mass transfer apparatus one of two processes can take place. Multicomponent mixtures can either be separated into their individual substances or in reverse can be produced from these individual components. This happens in mass transfer apparatus by bringing the components into contact with each other and using the different solubilities of the individual components in the phases to separate or bind them together. An example, which we have already discussed, was the transfer of a component from a liquid mixture into a gas by evaporation. In the following section we will limit ourselves to mass transfer devices in which physical processes take place. Apparatus where a chemical reaction also influences the mass transfer will be discussed in section 2.5. Mass will be transferred between two phases which are in direct contact with each other and are not separated by a membrane which is only permeable for certain components. The individual phases will mostly flow countercurrent to each other, in order to get the best mass transfer. The separation processes most frequently implemented are absorption, extraction and rectification. 

This type of calculation does not have to be carried out for a plate column because the two phases are well mixed on each plate. This means that on each individual plate a state of equilibrium can be presumed. Therefore a volume element is identical to an equilibrium stage, and the height of the column can be obtained from the number of equilibrium stages required for a particular separation. This is a thermodynamic rather then mass transfer problem. This explains why a mass transfer device, such as a distillation column can be sized without any knowledge of the laws of mass transfer. 

The mass-transfer devices may be sieves (holes), fixed valves, moveable valves, or bubble caps. Fig. 2 shows a selection of mass-transfer devices. The purpose of the device is intimate mixing of the vapor and liquid on the tray deck. An ideal device has high capacity, high flexibility, low leakage, low pressure drop, and low cost. 

Vapor mixing into the liquid through the mass-transfer device creates the tray s active area. The active area may have either liquid as the continuous phase (froth regime) or vapor as the continuous phase (spray regime). 

Fig. 2 Mass-transfer devices bubble caps and valve sections. (From Refs. . )...

At the bottom of the mass-transfer device, or plane zv the flow rates and concentrations are defined as follows ... 

Consider a countercurrent mass-transfer device for which the equilibrium-distribution relation consists of a set of discrete values XiD,YiD instead of a continuous model such as Henry s law. An analysis to determine minimum flow rates similar to that presented in Problem 3.14 is still possible using the cubic spline interpolation capabilities of Mathcad. Cubic spline interpolation passes a smooth curve through a set of points in such a way that the first and second derivatives are continuous across each point. Once the cubic spline describing a data set is assembled, it can be used as if it were a continuous model relating the two variables to predict accurately interpolated... 

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