I have a question regarding distillation
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If the heat condition parameter q of the binary feed entering the continuous distillation column remains unchanged, as do the composition xF and the feed rate F, then will there be a parameter (xq, yq) that changes due to variations in the distillation operation line? That is, if the operation line is given by yn+1 = xn*R/(R+1) + xD/(R+1), and the reflux ratio R changes, will the point (xq, yq) through which this line passes change as well? Please give me some advice. Thank you
The original question is as follows: In an operating distillation column, with F, q, xF, and V remaining constant, if the bottom product flow W increases, how does xD change? Does it increase, decrease, or remain unchanged? ），xW (how does it change? ), L/V (how does it change? ) The answer is that all three increase. I understand why the last L/V value increases, but I’m still not clear on why xD and xW increase. Please advise those who know the answer; thank you
It’s just a guess; it’s not necessarily correct. 1. As W (the output at the bottom of the tower) increases, D (D = F – W) decreases, while L (L = V – D) increases. Therefore, L/V (with V remaining constant) increases； 2. At the same time, the L/D also increases, indicating an increase in the reflux ratio; more heavy components go to the bottom of the tower, so Xw increases ； 3. Since there are fewer heavy components at the top of the tower and relatively more light components, XD increases.
Thank you, vortu, for the explanation. Could you provide a more specific answer? I’ve tried to find a solution by examining changes in the operating line and the slope and intersection points of the q-line, but I wasn’t successful; I’m quite stuck.
The point (xq, yq) is the intersection of the q-line equation and the distillation section equation. With the feed conditions remaining unchanged and the q-line equation staying the same, any change in the distillation section equation will cause the q-point to change as well. Therefore, when the reflux ratio changes, the distillation section equation changes, and consequently the q-point also changes!
It’s not easy to calculate using graphical methods; all three values are variable, so there’s no clear starting point
I suddenly thought of something: since F = D + W, then F*xF = D*xD + W*xW, so D/W = (xF - xW) / (xD - xF). Here, since xF remains constant and D/W is decreasing, if xD remains constant then Xw increases, or if xW remains constant then xD increases. So it should be either an increase in xD or an increase in xW.
The analysis on the 3rd floor is basically correct. In fact, it’s easier to understand this issue through qualitative analysis. Before the change, the entire column was in equilibrium, with constant concentrations of various components on each tray; that is, the concentration of the light component was highest on the top tray and lowest at the bottom of the column. If W increases, the upper liquid phase fails to reach its original equilibrium state and falls into the reactor more quickly, so xW increases. As W increases, the amount of liquid flowing downward increases, while the amount of vapor rising upward, V, remains unchanged; therefore, L/V increases. An increase in the reflux ratio certainly benefits the increase of the concentration of the light components at the top of the tower; thus, xD increases. This also avoids the incorrect conclusion stated on page 7, which was that either xD or xW must increase.