Internal reflux calculation

Calculate reflux ratios. The minimum internal reflux ratio is a line from the intercept of the q line with the equiUbtium curve to the yP point on... 

As a first step in the calculation, the minimum-reflux ratio should be determined. In Fig. 13-100, point D, representing the distillate, is on the diagonal since a total condenser is assumed and Xo = yo- Point F represents the initial condition in the still pot with coordinates ip, y. Minimum internal reflux is represented by the slope of the line DF,... 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

In an operating column the effective reflux ratio will be increased by vapour condensed within the column due to heat leakage through the walls. With a well-lagged column the heat loss will be small and no allowance is normally made for this increased flow in design calculations. If a column is poorly insulated, changes in the internal reflux due to sudden changes in the external conditions, such as a sudden rain storm, can have a noticeable effect on the column operation and control. 

Stable column operation is guaranteed by keeping the internal reflux of the distillation tower constant. Consequently, internal reflux controls are designed to compensate for changes in the temperature of the external reflux caused by ambient conditions. Figure 2.90a is controlled by a typical internal reflux control system (top) and the equations that need to be solved in calculating the required external reflux rate are shown at the bottom. This control system corrects for either an increase in overhead vapor temperature or a decrease in external reflux liquid temperature. 

This equation can be rearranged to calculate the external reflux that maintains a specified internal reflux control (FJ ), i.e.. 

This approach, called internal reflux control, is shown schematically in Figure 15.65. Note that the composition controller outputs the internal reflux flow rate, and the internal reflux controller calculates the external reflux flow rate, which is used as the setpoint for the flow controller on the reflux. 

Example 12.4. Calculate the minimum internal reflux for the problem of Example 12.2 assuming a Class 1 separation. Check the validity of this assumption. 

Controlling the internal reflux to the section below the side draw Subtracting the measured side-product flow from the measured reflux flow (the latter may need correction for subcooling see Sec. 19.2) gives the internal reflux to the section below the side draw. An internal reflux controller (IRC) uses this computed internal reflux to manipulate side-product flow (Fig. 19.7a). A limitation of this technique is that the internal reflux is calculated as a small difference between two large numbers, and can therefore be in error. The error escalates as the internal reflux becomes a smaller fraction of the total liquid traffic above the side draw. 

Note that when f = 0, Eqs. 14-651 and 14-661 both say L1/V2 = L(/Vi- As the fraction condensed increases (reflux is subcooled more), the internal reflux ratio, L1/V2, becomes larger. Thus the net result of subcooled reflux is equivalent to increasing the reflux ratio. Numerical calculations (such as Problem 4.D51 show that a large amount of subcooling is required to have a significant effect on 17V. With highly subcooled reflux, an extra tray should be added for heating the reflux IKister. 19901. 

D5. A distillation column is operating with a subcooled reflux. The vapor streams have an enthalpy of Hi = H2 = 17,500 Btu/lbmol, while the saturated liquid hi = 3100 Btu/lbmol. Enthalpy of the reflux stream is hg = 1500 Btu/lbmol. The external reflux ratio is set at Lq/D = 1.1. Calculate the internal reflux ratio inside the column, L1/V2. 

D25. We are separating methanol and water. Calculate the internal reflux ratio inside the column,... 

Since the internal reflux ratio is not constant from stage to stage, the ratio Ro/Pb can be calculated by a series of material balances together with a consideration of the phase equilibria. Basis 100 lb. saturated feed. A complete plant material balance ... 

Flash-Type Calculations Based on Internal Reflux... 

On the other hand, when applied to multistage operations, the concept of F pertains to the introduction of both V and L (that is, to V + L) to the reject side of the membrane, where V and L originate from the adjacent membrane cells, that is, from the posterior and anterior cells. Thus, the so-called internal reflux ratio or recycle ratio L/V can be used to establish a value for V/F, from which a value for the permeate flux V" could in turn be calculated by the same methods of Chapter 3. The value of V" so determined is the uniform and constant permeate rate at each stage in the rectifying section. This would supersede the determination of the permeate flux V" based on the feedstream per se. 

The single-stage flash-type calculation will establish an absolute value for V in consistent units. Knowing V, then L can be calculated from the internal reflux ratio L/V, and D can, in turn, be calculated from the initially assigned value for the external reflux ratio L/D. 

As in two-product columns, total reflux in multiple product columns is a limiting condition where the colnmn internal liqnid and vapor flows are very large compared to each of the products and feed(s). Multiproduct columns are considered to consist of column sections defined by the product locations. Each section is bonnded by two products, one at its top and the other at its bottom. Thus, a column with. y sections has h- 1 products. As in two-product columns, multiproduct columns operating at total reflux achieve the maximum separation possible with a given number of stages in each section and for a given set of product rates. Conversely, if the separation between the different products is specified, the minimum trays required in each section are evaluated by total reflux calculations. 

For columns with a sidestream liquid drawoff, it is necessary to maintain a minimum liquid flow down the column below the drawoff point. As shown by Figure 9.24, this is accomplished by subtracting the sidestream drawoff flow from the estimated internal reflux to calculate the net liquid flow down the column. If this flow is insufficient, the override controller pinches back on the drawoff valve tmtil the downflow becomes adequate. 

In the design of distillation columns, engineers use internal reflux in their calculations rather than external reflux. Important indices for colunrn separation ability are internal reflux/distillate and boilup/bottom-product ratios. 

Calculation of Distillation-Column Internal Reflux Temperature and Pressure Compensation of Gas Flow Meters... 

This section outlines procedures for calculating product draw tray temperatures at all points in the tower and for making an overall heat balance around the system. The method is based upon assuming a draw tray temperature and then calculating the internal reflux required by the... 

The hydrocarbon which is to be revaporized in the product stripper falls to the draw tray as part of the internal reflux from Tray (D1 + 1) rather than rising to the tray as part of the product vapors. In passing across the draw tray, this liquid absorbs a small amount of the reflux heat. This heat absorption is calculated as LSVr example calculations that this... 

Calculate the heat removal capability of the internal reflux falling from Tray (Di + 1). 

Calculate the internal reflux from Tray (Dl + 1) which is required to absorb the excess heat at Tray Dl. 

This subject has been discussed in detail earlier in this work. The only new point to be considered here is the definition of reflux from the draw tray. For purposes of Packie s analysis, reflux is defined as the volume of liquid falling from the tray below the draw tray. In terms of the first side stream draw tray, this is calculated by making a heat balance above Tray (Dl — 2) and then calculating the internal reflux from Tray (Dl - 1) which is required to absorb the excess heat. 

From Step 3, calculate the internal reflux falling to the draw trays, taking into account the location of pumparound systems. 

Estimating draw tray temperatures for vacuum operations is much more difficult than in atmospheric towers because of the greater relative effect of calculated internal reflux on hydrocarbon partial pressure. As good a rule as any is to assume a hydrocarbon partial pressure equal to 30 to 50 percent of the total pressure at any tray. Plot the assumed profile for the trayed section of the tower. 

Convert the internal reflux from pounds per hour to moles per hour. Calculate the mole fraction of hydrocarbon product vapor in the total vapor leaving the draw tray but neglect the presence of the product to be removed on the next draw tray up in the tower. From this calculate the hydrocarbon partial pressure in this vapor and convert the atmospheric bubble point of the unstripped liquid product on the tray to this partial pressure. If this temperature does not check the value assumed earlier, repeat the procedure for a new assumed temperature. 

Calculate the internal reflux to Tray D2 by making a balance above this tray as shown by Envelope IV. 

It is necessary to make initial assumptions relative to proposed heat removal schemes prior to commencing design calculations. Since the separations made in catalytic fractionators are relatively easy in comparison with atmospheric crude tower separations and since the number of trays available is quite high by petroleum fractionation standards, it follows that internal reflux requirements are comparatively low. Thus, it is desirable to remove as much heat as possible from the system. Maximizing heat removal is usually accomplished by process-to-process exchange at various points in the unit, and this affords substantial utility savings. A secondary benefit is that internal reflux is minimized which, in turn, minimizes the tower diameter. 

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