Continuous stirred tank separators

Figure 7-26 Typical flow pattern for continuous stirred tank separation of stereoisomers (series operation).

Open separators bulk flow parallel to force and continuous stirred tank separators... 

Section 6.4 covers continuous stirred tank separators. Section 6.4.1 studies equilibrium separation processes most of this section is devoted to crystallization, with additional coverage of liquid extraction. Membrane separation processes/devices are sometimes modeled as CSTRs. Section 6.4.2 touches upon a few of these examples, encountered, for example, in ultrafllUation and gas permeation. There are brief treatments of batch systems that are well-stirred in Sections 6.4.1 and 6.4.2 for both equilibrium based and membrane separation processes. 

In a continuous stirred tank separator (CSTS), fluid streams (single or multiphase, with or without solids) enter and leave, and the contents are kept well stirred or well mixed. There is no spatial dependence of species concentrations or any other quantity (such as temperature, pressure, etc.) inside the separator vessel. The species concentrations inside the vessel are at a steady state, sometimes they may change with time however, the outlet stream concentration is different from the inlet stream concentration, and is equal to that inside the CSTS (Figure 6.2.4). Meanwhile, whatever separation mechanism is employed is operative inside the vessel. (A batch stirred tank separator operates such that there is no spatial gradients of concentration, temperature or pressure inside the vessel. Except when the batch is introduced or withdrawn from the vessel, no fluid/solid streams enter or leave the vessel. The conditions inside the vessel may change with time. Generally there is vigorous bulk motion in the vessel.)... 

Figure 6.2.4. Continuous stirred tank separator (CSTS) with one incoming stream and one outgoing stream.

Continuous stirred tank separator in particulate systems... 

In a continuous stirred tank separator (CSTS) employed in particulate systems, there are no spatial gradients inside the separator therefore we need to integrate either equation (6.2.51b) or equation (6.2.51c) over the total volume V of the separator. To that end, we may employ the type of relations (6.2.4b-d) utilized in volume-averaging procedures. For example, the second term, V- Upn rp)j, on the left-hand side of (6.2.51b) will lead to... 

In this section, the flow vs. force configuration of a continuous stirred tank separator (CSTS) will be illustrated with a few examples. The examples cover crystallization, solvent extraction, ultrafiltration and gas permeation. 

If fiuid streams with or without solid particles are entering and other streams are exiting the well-stirred separator continuously, we have a continuous stirred tank separator (CSTS), provided that its properties are uniform throughout the separator. Figure 6.4.1(a) illustrates a CSTS which is a crystallizer. The conditions in such a separator are time- and space-invariant However, the intensity of mixing conditions in the separator is such that the fresh feed introduced into the separator is mixed in a time interval which is very short compared to the mean residence time of the fluid elements (and solid particles) in the separator. Figure 6.4.1(b) illustrates a continuous well-stirred extractor... 

If we assume that the processes of crystal breakage, attritional change and a omeration may be neglected, then B = 0 = De. The governing equation for such a continuous stirred tank separator is... 

Note Using equation (6.4.122a) in mass balance (6.4.120b) for spedes A, we obtain tbe equation corresponding to (6.2.44) for a continuous stirred tank separator having one membrane surface.)... 

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