Raffinate point

Fig. 9-20. Transient evolution of the concentration of both species in the raffinate. Points are experimental results lines are SMB model predictions (average concentrations over each cycle).

Zone III between the feed and the raffinate points where the more retained product (B) must be completely adsorbed. 

After determination of the difference point, it is possible to determine the necessary number of theoretical stages. Starting from the extract E the raffinate Rj for the first stage is found by using the tie line through E. A line through P and Rj intersects with the binodal curve and results in E2. This procedure is repeated until the final raffinate point R is reached. 

The principle of SMB operation can be easily understood by analogy with the equivalent True Moving Bed (TMB) process. The TMB unit (Fig. 3.4-8) is divided into four sections section 1, between the eluent and extract ports section 2, between the extract and feed ports section 3, between the feed and raffinate points and section 4, between the raffinate and the eluent inlet. In the ideal TMB operation, liquid and solid flow in opposite directions, and are continuously recycled the liquid flowing out of section 4 is recycled to section 1, while the solid coming out of section 1 is recycled to section 4. The feed is continuously injected between sections 2 and 3, and two product lines can be continuously collected the extract, rich in the more-retained species A, and the raffinate, rich in the less-retained species B, which move upwards with the liquid phase. Pure eluent or desorbent is injected at the beginning of section 1, with the liquid recycled from the end of section 4. 

Figure 13-15 shows the solution for the raffinate (stripping) section. Note that point W is outside the ternary diagram. This occurs because W + Pr = S. The procedure starts with the R point which is connected to by the appropriate tie line. Mass balance [equation (13-43)] is then used to locate the next raffinate point. The procedure is repeated until the point F is reached. 

Raffinate product, consisting of the less strongly adsorbed component B mixed with desorbent, is withdrawn from a position below the feed entry. Only a portion of the Hquid flowing in the bed is withdrawn at this point the remainder continues to flow into the next section of the bed. Extract product, consisting of the more strongly adsorbed component A mixed with desorbent, is withdrawn from the bed again, only a portion of the flowing Hquid in the bed is withdrawn, and the remainder continues to flow into the next bed section. 

An important use of the triangular equiHbrium diagram is the graphical solution of material balance problems, such as the calculation of the relative amounts of equiHbrium phases obtained from a given overall mixture composition. As an example, consider a mixture where the overall composition is represented by point M on Figure 2a. If the A-rich phase is denoted by point R (raffinate) and the B-rich phase is denoted by point E (extract), it can be shown that points R, M, and E are coUinear, and also... 

Xylene Isomeri tion. The objective of C-8-aromatics processing is the conversion of the usual four-component feedstream (ethylbenzene and the three xylenes) into an isomerically pure xylene. Although the bulk of current demand is for xylene isomer for polyester fiber manufacture, significant markets for the other isomers exist. The primary problem is separation of the 8—40% ethylbenzene that is present in the usual feedstocks, a task that is compHcated by the closeness of the boiling points of ethylbenzene and -xylene. In addition, the equiUbrium concentrations of the xylenes present in the isomer separation train raffinate have to be reestabUshed to maximize the yield of the desired isomer. 

In general, the sulfolane extraction unit consists of four basic parts extractor, extractive stripper, extract recovery column, and water—wash tower. The hydrocarbon feed is first contacted with sulfolane in the extractor, where the aromatics and some light nonaromatics dissolve in the sulfolane. The rich solvent then passes to the extractive stripper where the light nonaromatics are stripped. The bottom stream, which consists of sulfolane and aromatic components, and which at this point is essentiaHy free of nonaromatics, enters the recovery column where the aromatics are removed. The sulfolane is returned to the extractor. The non aromatic raffinate obtained initially from the extractor is contacted with water in the wash tower to remove dissolved sulfolane, which is subsequently recovered in the extract recovery column. Benzene and toluene recoveries in the process are routinely greater than 99%, and xylene recoveries exceed 95%. 

The mix point, = 0.0673, falls on a straight line connecting x and The extract composition is then determined hy drawing a straight line from x,-throiigh Zm until the line intersects the extract line at the final extract composition, i/e = 0.084. The delta point is then found at the intersection of two lines. One line connects the feed and extract compositions x and y. The other line connects the raffinate and solvent compositions x,- and y. ... 

On an XY diagram for case C the operating line will go through points Xr, Ys and Xf, with a slope of R /S similar to Fig. 15-13. When using the Kremser equation for case C, one uses the pseudo feed concentration X from Eq. (15-21) and the stripping factor from Eq. (15-22). One uses the raffinate concentration X and inlet solvent concentration Y, without modification. 

Nevertheless, near the feed point, there is a difference between TMB and all SMB cases due to the fact that the internal flow rates in the TMB are smaller than in the SMB, leading to a small dilution of the feed stream. As a consequence, near the feed inlet, TMB concentrations will be higher than in the SMB operation. The raffinate and extract purities in SMB units with four (95.2 % and 89.5 %), eight (98.7 % and 95.9%) and 12 columns (99.1 % and 96.8%) are increasing towards the one obtained in the equivalent TMB unit (99.3 % and 97.7 %). The optimum degree of subdivision of the SMB unit will depend of the difficulty of the separation and the product purity requirements. Typically, systems for the pharmaceutical industry have six to 16 columns. 

Zone IV between the raffinate and the eluent make-up points where the less-retained product (A) must be completely adsorbed. 

The simulation starts with the extractor operating at steady-state conditions, but with a relatively high outlet raffinate concentration. Control is implemented in order to reduce the raffinate concentration in accordance with a lower controller set point. 

INTEGRAL TIME CONSTANT BASE SOLVENT FLOW RATE RAFFINATE SET POINT... 

Find the composition of the raffinate leaving the first stage, n by judging the position of the tie-line from e. Draw a line from the pole point, P, through r to cut the curve at e2, the extract leaving stage 2. 

Mark required final raffinate composition, rm, on the equilibrium curve, at 10 per cent. Draw line from this point through point o to find final extract composition, e. ... 

Using the tie-lines plotted on the figure, judge the position that a tie-line would have from < i and mark it in, to find the point on the curve giving the composition of the raffinate leaving the first stage, n. 

The data are replotted in Figure 13b and the point F, representing the feed, is drawn in at 50 per cent acetaldehyde, 50 per cent water. Similarly, Rn, the raffinate from stage n located on the curve corresponding to 5 per cent acetaldehyde. (This solution then contains 2 per cent S and 93 per cent water.) FS is joined and point M located such that FM = MS, since the ratio of feed solution to solvent is unity. R M is projected to meet the equilibrium curve at Ei and FEi and RnS are projected to meet at P. The tie-line Ei Ri is drawn in and the line RiP then meets the curve at E2. The working is continued in this way and it is found that R4 is below Rn and hence four theoretical stages are required. 

The simulation starts from initially zero concentration conditions along the cascade. Control is implemented from time t=0, in order to obtain an outlet raffinate concentration equal to the controller set point The program is essentially the same as that used in EQBACK, but with added controller equations. 

In an ideal stage, the extract Ex leaves in equilibrium with the raffinate Rx, so that the point Rx is at the end of the tie line through Ex. To determine the extract E2, PRi is drawn to cut the binodal curve at E2. The points R2, E3, R3, E4, and so on, may be found in the same way. If the final tie line, say ER4, does not pass through R , then the amount of solvent added is incorrect for the desired change in composition. In general, this does not invalidate the method, since it gives the required number of ideal stages with sufficient accuracy. 

As Sherwood and Pigford(3) point out, the use of spray towers, packed towers or mechanical columns enables continuous countercurrent extraction to be obtained in a similar manner to that in gas absorption or distillation. Applying the two-film theory of mass transfer, explained in detail in Volume 1, Chapter 10, the concentration gradients for transfer to a desired solute from a raffinate to an extract phase are as shown in Figure 13.19, which is similar to Figure 12.1 for gas absorption. 

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