Reflux ratio definition

The key to doing process analysis is the identification of the equations that may be used to achieve zero degrees of freedom. These equations will come from a number of sources, including the balance equations themselves (Equations (1) and (19)), process specifications (such as the purity of output streams and the reflux ratio), physical relations (such as the definition of enthalpy for liquid and vapour streams) and other constraints imposed by the problem. Once a full set of equations has been developed, the equations can be solved, usually with little difficulty, and the desired results obtained. 

Since the aim of distillation is to remove a distillate, a definite reflux ratio must be employed. The reflux ratio is defined by the mass flow of reflux R and distillate D ... 

Finally, the distillate flowrate can be calculated from the definition of the reflux ratio recall (64) ... 

To compute the heat duties of the condenser and reboiler, we will need to know the flowrates in various parts of the column. We will start at the top of the column and work our way down. First we use the definition of reflux ratio ... 

The complete definition of column performance when the number of stages and feed location are fixed requires two specifications (Section 5.2.2). In studying the effect of reflux ratio on separation, other parameters, including product rates, are assumed constant. In Y-X graphical representation, a fixed product rate is implied by fixing the 7-line slope. This slope depends on the feed thermal conditions at feed tray temperature and pressure (Section 5.2.2). The column pressure is fixed, but its temperature depends on the product rates. Therefore, in order to maintain a fixed product rate, the (/-line slope must be held constant. The actual product rates corresponding to a given (/-line slope are calculated by material balance once the product compositions have been determined. 

The calculational procedures are presented first for conventional distillation columns and then for complex distillation columns. The conventional distillation column is completely determined by fixing the following variables (1) the complete definition of the feed (total flow rate, composition, and thermal condition), (2) the column pressure (or the pressure at one point in the column, say in the accumulator), (3) the type of condenser, (4) ku the number of plates above and including the feed plate, (5) /c2, the total number of plates, and (6) two other specifications which are usually taken to be the reflux ratio and the distillate rate LJD, D or two product specifications such as bjdh bh/d,, XDh xBh > Td, 7, or combinations of these. The subscript / is used to denote the light key and the subscript h is used to denote the heavy key. In all of the optimization problems considered herein, the variables listed in items (1), (2), and (3) are always fixed. For convenience these variables are referred to collectively as the usual specifications. The remaining four variables, ku /c2, and two other specifications such as LJD and D are called additional specifications. ... 

MINIMUM REFLUX. At any reflux less than total, the number of plates needed for a given separation is larger than at total reflux and increases continuously as the reflux ratio is decreased. As the ratio becomes smaller, the number of plates becomes very large, and at a definite minimum, called the minimum reflux ratio, the number of plates becomes infinite. All actual columns producing a finite amount of desired top and bottom products must operate at a reflux ratio between the minimum, at which the number of plates is infinity, and infinity, at which the number of plates is a minimum. If is the operating reflux ratio and (Lg/B) the minimum reflux ratio,... 

What is the most probable cause of this poor performance What further tests would you make to establish definitely the reason for the trouble Could some 90% product be obtained by increasing the reflux ratio still farther while keeping the vapor rate constant ... 

The automatic column heads are based on a time cycle by electronic or mechanical means (c/. section 8.4) the device is alternately switched for a definite period into each of two positions the periods in question determine the reflux ratio. This requires... 

Furthermore, using the definition of reflux ratio. Equation 3.5 can be written... 

Equation 3.21 is in fact equivalent to the standard definition of a reflux ration used throughout distillation literature. It is not usually common practice to refer to a reflux ratio for a stripping section, but with the general definition it can be defined by incorporating Equation 3.12... 

Using all the definitions given in Equations 5.15 5.18 and rewriting them in terms of a reflux ratio in the stripping section, yields Equation 5.19... 

Since the reflux ratios are, by definition, dependent on the net flow of a particular CS, the same arguments put forward in Sections 7.3.2 wiU hold for reflux ratios with regard to net FPs. For instance, choosing a FP that will lead to CS2 having a negative net flow will cause the value of R 2 to also be negative in this CS. However, because is defined as the ratio of liquid flowrate to the net flowrate in a particular CS, the actual value of in a particular CS depends on the quality of the feed, and whether the side-draw product is liquid or vapor. Just as we have conveniently represented net flow patterns in >-space, we can also then construct lines of constant reflux in the >-space, which will ultimately allow us to hone in on feasible designs, as will be illustrated in Section 7.4. In order to do this, it is necessary to write all our reflux equations in terms of v and as shown in the equations below... 

Typically, the colunm is completely specified first, including the definition of the number of equilibrium stages, the feed stage, the reflux ratio, and the heat supply. By solving the MESH equations, the internal temperature and concentration profiles and, in turn, the product concentrations are found. If the calculated product qualities do not meet the specifications, then a new column configuration has to be made and the calculation repeated. 

The combinatorial number of possible separation sequences for multicomponent mixtures is considerably large and calls for the definition of heuristics and thermodynamic guidelines based on ROMs to find convenient initial designs. Furthermore, operational issues should be addressed in terms of, for instance, predicting the controllability of the sequence and defining feasible operational parameters e.g. reflux ratios). 

This chapter presented a concept drawing of a distillation process that included flow rate sensors, liquid level sensors, temperature sensors, and process control valves. The process streams were identified and labeled. The standard definition of reflux ratio was presented, and the concepts of separation power and material balance split were introduced. 

Figure 12.69 shows the original version of the Ryskamp scheme. This maintains constant a different definition of reflux ratio, i.e. R/ R + D). This is chosen because it is the slope of the top operating line on the McCabe-Thiele diagram - see Equation (12.18). The output of the level controller isR + D. From this is subtracted R to generate the SP for the distillate flow controller. The reflux ratio target is multiplied by the level controller ouQjut to generate the SP for the reflux flow controller. 

Solution of Part 4. The determination of the minimum reflux ratio on the basis of the usual simplifying assumptions requires determining the difference Vm F . A given enthalpy for the feed does not determine this difference for the general case because the feed-plate location is still a variable however, in this case, a definite value is obtained at the minimum reflux ratio, because the pinched-in condition occurs at the feed plate. 

When the number of theoretical plates is plotted as a function of reflux ratio, the curve is hyperbolic in type with asymptotes at Nmin and (0/Z))min. Thcsc two limiting conditions as asymptotes are useful in drawing such a curve, but they would be more helpful as definite points on the diagram. By modifying the variables, they can be made definite points in fact they can be made the same points for all cases. There are many ways in which the variables can be modified, and one that has been useful (Ref. 6) is shown in Fig. 12-1. The ordinate is S — Sm)/ 8 + 1), where S is the total theoretical steps including any enrichment in the still and condenser, and Sm is the value of S... 

Since as is moved upward and downward the reflux ratio increases, the definition of minimum reflux ratio requires A and A, for the minimum reflux ratio to be located as shown, with A at the highest tie-line intersection and A, at the lowest tie-line intersection. In this case, it is the tie line which, when extended, passes through F, the feed, that determines both, and this is always the case when the xy equilibrium distribution curve is everywhere concave downward. 

At the minimum reflux ratio R , these coordinates are equilibrium values since they occur on the equilibrium curve. Substituting them into the definition of a, Eq. (9.2), gives... 

---