Beneficiation feed, composition effects

If the process feed does not He in the Hquid—Hquid region it can be made to do so by dehberately feeding either pure or pure B to the decanter, as required. This may only be necessary during start-up or for control purposes because the recycled azeotrope has the beneficial effect of dragging the decanter composition further into the Hquid—Hquid region. 

Three issues are discussed under this heading (1) the effect that different feed materials might have on the propensity for PCDD/F formation in combustors (2) whether restrictions in feedstock composition are likely to have a beneficial effect in terms of reduced PCDD/F emissions (3) whether the manner in which the feedstock is presented to the combustor influences PCDD/F formation. Each issue is discussed below. 

Coal beneficiation involves a series of steps to separate the mineral matter from the combustible portion of the coal. Current coal characterization for beneficiation is usually limited to measurements of the particle specific gravity distribution (washability). It is further assumed that the properties of the coal feed stream and related mineral matter remain constant during the separation or cleaning process, but the compositions of the streams do change. These changes are important in understanding the lack of expected separations. The effects of specific mineral constituents on different unit operations are described. Better measurement and analytical systems will permit improved control of the processes and better separations. 

Part (or conceivably all, in the limit) of either or both the permeate and reject products may be recycled or refluxed. For instance, in the case of concurrent flow, part of the reject product L, can be recycled to the feed f, where the composition and rate of L, would be affected. In effect the reject is making another pass through the membrane cell. For that matter, part of the permeate product V, can be recycled to constitute V, The beneficial effects are dubious and can be ascertained only by a more complex mode of calculation. For the case of assuming perfect mixing, the benefits are nil, as already discussed in Section 3.5 of Chapter 3. 

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