of the solvent dissolves in the raffinate. Therefore, a third still is needed to recover this solvent. Bases for Economic Design
The most important single factor affecting process costs is selection of the solvent. Factors governing the selection of a solvent have been adequately discussed by Treybal (3). In this article it will be assumed that the process has already been piloted and the solvent fixed. These costs are under control of the process designer: 1. Value of lost product in the raffinate stream 2. Solvent in the waste stream 3. Steam and cooling water charges to operate the extract and solvent recovery stills 4. Annual charges related to capital investment The problem of optimizing a threetower process can be simplified somewhat. The solvent recovery still can be treated completely independent of the other two towers. This can be optimized by means well known to distillation processes, balancing the cost of lost solvent vs. a steam and cooling water cost plus the capital charge of the still. The designer can set two of the following three variables for the extract tower: 1. Solvent-to-feed ratio (extract strength) 2. Number of stages 3. Raffinite composition There is also a choice of two out of three variables for the extract still:
A COMPLETE INTEGRATED SERVICE
now offered by . . .
LIQUID SOLIDS s-e-p-a-r-a-t-i-o-n
SPECIALISTS
1. Reflux ratio 2. Number of plates 3. Amount of product in the returned solvent
Tolhursf
Concentration of solvent and/or diluent in the product is usually fixed by-product specifications. Hence, this is eliminated as a process variable. The phase diagram (7) for the process of Figure 1 is shown in Figure 2. Feed is shown at point F, raffinate at R, extract at E, and solvent at S. The dotted line is the locus of compositions which will produce a water-solvent azeotope. This brings up the point as to whether the solvent-to-feed ratio
Niagara
{Continued on page 50 A)
CENTRIFUGALS
PRESSURE
FILTERS
FEINC
CONTINUOUS
FILTERS
FEON D I V I S I O N S
OF
FILTER
MEDIA
American Machine and Metals, Inc. EAST MOLINE,
ILLINOIS
VOL. 52, NO. 4 ·
APRIL 1960
49 A
