Minimum vapor flow

Many of the above factors that affect column operation are due to vapor flow conditions being either excessive or too low. Vapor flow velocity is dependent on column diameter. Weeping determines the minimum vapor flow required while flooding determines the maximum vapor flow allowed, hence column capacity. Thus, if the column diameter is not sized properly, the column will not perform well. Not only will operational problems occur, the desired separation duties may not be achieved. 

The transition split divides direct-type sphts from indirect-type splits as discussed by Doherty and Malone (Conceptual Desisn of Distillation Systems, 2001, chaps. 4 andS) also see Fidkowski, Doherty, and Malone [AlChE J., 39,1301(1993)]. The upper line in Fig. 13-70 is the minimum vapor flow leaving the reboiler of the main column, which also corresponds to the minimum vapor flow for the entire system since all the vapor for the total wstem is generated by this reboiler. For P = 0 the minimum vapor flow for the entire thermally coupled system (i.e., main column) becomes equal to the minimum vapor flow for the side rectifier system (i.e., main column of the side-rectifier system see Fig. 13-65b or c) (Vsr) for P = 1 it is equal to the minimum vapor flow of the entire side stripper system (Vss) (which is the sum of the vapor flows from both the reboilers in this system see Fig. 13-66h or c). Coincidentally, the values of these two minimum vapor flows are always the same (Vsr), = (Vss)mm- For P = Pr the main column is pinched at both feed locations i.e., the minimum vapor flows for separations A/B and B/C are equal. 

The minimum vapor flow for the entire thermally coupled system is flat over a wide range of P Pp < P < Pr. This is the reason why dividing wall columns usually work well without tight coutrol of the vapor or liquid spht betweeu both sides of the partitiou. The optimally designed fully thermally coupled system should operate with a fractional recovery of B in the top product of the prefractionator placed somewhere between points P and R. The transition spht P is located at one end of the optimal section PR, and it is not a recommended design point for normal operation because process disturbances may move the operating point outside the optimal section PR shown in Fig. 13-70. 

FIG. 13-70 Minimum vapor flows in the thermally coupled system bottom curve, minimum vapor flow in prefractionator top curve, minimum vapor flow from the reboiler of the main column (total minimum vapor flow for entire system) for = 6.25, = 2.5, ttc = 1.0 and feed mole fractions % = 0.33, Zb = 0.33, and Zc = 0.34. 

Applying Underwood s method gives us a minimum vapor flow of 72.5 kmol/h for a column accomplishing the separation of AB/CDE. Without species A, D, and E present, the minimum vapor flow is computed to be 44.5 kmol/h. The marginal vapor rate is therefore 38.0 kmol/h. 

Let us look more closely at the Underwood equations to see if we can quickly compute an approximate answer. The minimum vapor flow for the split AB/CDE is given by... 

This is a nonlinear equation with one unknown, which is a class of problems you know how to solve from Chapter 2. Then, you calculate the minimum vapor flow rate using Eq. (6.5) ... 

Glinos, K. and M.F. Malone, Minimum vapor flows in a distillation column with a sidestream stripper. Industrial Engineering Chemistry, Process Design and Develop menu 1985, 24 1087 1090. 

The objective in this chapter is to avoid minimum vapor flow rate limitations. If feed rates to a distillation column are reduced due to a reduction in plant throughput or a diversion of the feed stream to a more economical product (i.e., hydrogen gas to combustion turbine during peak power demand instead of methanol), the distillation control structure needs to be robust enough to handle these disturbances while maintaining product specifications. Three alternative control structures are explored to obtain this objective. 

Many distillation column use reboiler heat input as a primary manipulated variable, usually to control a temperature on an appropriate tray. This means that reboiler heat input is not constrained during normal operation with normal feed flow rates. However, as the feed flow rate to the column is decreased, less vapor boilup is required to achieve the same separation. If the feed drops to the point where the low vapor-boilup limit is encountered, the control structure must change. Three alternative control structures for achieving stable operation at minimum vapor flow rates are discussed in the following. 

Minimum reflux ratio. The minimum reflux ratio can be defined as the reflux ratio that will require an infinite number of trays for the given separation desired of Xj, and x,y. This corresponds to the minimum vapor flow in the tower, and hence the minimum reboiler and condenser sizes. This case is shown in Fig. 11.4-11. If R is decreased, the slope of the enriching operating line R/(R + 1) is decreased, and the intersection of this line and the stripping line with the q line moves farther from the 45° line and closer to the... 

Glinos, K., Malone, M. F. (1985). Minimum Vapor Flows in a Distillation Column with a Side Stream - Stripper. Ind. Eng. Chem. Process Des. Dev., 24, 1087-90. 

However, the largest operating cost for many process units is the energy supplied to the reboilers. We should therefore avoid high reflux rates, and try to achieve the best efficiency point for distillation tower trays at a minimum vapor flow. This is best done by designing and installing the tray decks and outlet weirs as level as possible. Damaged tray decks should not be reused unless they can be restored to their proper state of levelness, which is difficult, if not impossible. 

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