Reflux internal

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... 

The slope L/V of the operating line is termea the internal-reflux ratio. This ratio in the operating-line equation for the top section of the column [see Eq. (13-21)] is related to the external-reflux ratio R = L + i/D by... 

As shown in Fig. 13-92, methods of providing column reflux include (a) conventional top-tray reflux, (b) pump-back reflux from side-cut strippers, and (c) pump-around reflux. The latter two methods essentially function as intercondenser schemes that reduce the top-tray-refliix requirement. As shown in Fig. 13-93 for the example being considered, the internal-reflux flow rate decreases rapidly from the top tray to the feed-flash zone for case a. The other two cases, particularly case c, result in better balancing of the column-refliix traffic. Because of this and the opportunity provided to recover energy at a moderate- to high-temperature level, pump-around reflirx is the most commonly used technique. However, not indicated in Fig. 13-93 is the fact that in cases h and c the smaller quantity of reflux present in the upper portion of the column increases the tray requirements. Furthermore, the pump-around circuits, which extend over three trays each, are believed to be equivalent for mass-transfer purposes to only one tray each. Bepresentative tray requirements for the three cases are included in Fig. 13-92. In case c heat-transfer rates associated with the two pump-around circuits account for approximately 40 percent of the total heat removed in the overhead condenser and from the two pump-around circuits combined. 

Tray requirements depend on internal-reflux ratios and ASTM 5-95 gaps or overlaps, and may be estimated by the correlation of Paclde (op. cit.) for crude units and the correlation of Houghland, Lemieux, and Schreiner (op. cit.) for main fractionators. 

FIG. 13-93 Comp arisen of internal-reflux rates for three methods of providing reflux. 

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,... 

The condition of minimum reflux for an equihbrium curve with an inflection point P is shown in Fig. 13-102. In this case the minimum internal reflux is... 

Internal reflux can he controlled without affecting product yield. The maximum internal liquid reflux is fixed by the thermodynamic state of the feed relative to the product stream. Excessive reflux will diminish product yield. 

A very low gas rate in a column several feet tall with internal reflux can sometimes be used to effect difficult multicomponent separations in batch operation [Lemlich, Principles of Foam Fractionation, in Periy (ed.). Progress in Separation and Purification, vol. 1, Inter-science, New York, 1968, chap. 1]. 

Internal reflux is induced by means of externally cooled liquid pumparounds. A pumparound simply removes hot liquid from the tower, pumps it through a heat exchanger and then introduces this cooled liquid into the tower a few trays above. Use of pumparounds allows a better distribution of tower loadings than if all the heat were removed from the VPS using an overhead condenser. Four to six trays between sidestreams and two pumparounds are normally specified for a lube VPS. The three liquid sidestream products to be used as lube plant feed stocks are steam stripped to remove lighter boiling components which condense with tire sidestreams. 

Internal reflux condensers and reboilers where practical (Dale, 1987)... 

The internal reflux ratio is L/V, and is the slope of the operating line. The external reflux is [133] ... 

Use the ethanol curve similar to Figure 8-37, or refer to the data of Reference 133 the point of tangency of the line from the distillate composition of the diagonal is xj) = 0.80 andyv = 0.80. Thus the minimum internal reflux is set by this tangent line ... 

For practical design, select L/V = (1.5) (0.525) = 0.7875. Select L/V internal reflux lines and add to the equilibrium plot, similar to that shown for a normal curve of Figure 8-35, but unlike the abnormal ethanol curve shown. 

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,... 

Liquid mass velocity, Ib/hr-ft" based on superficial cross section of column Internal reflux ratio, dimensionless Weir length, in. 

Heavy cycle oil, heavy naphtha, and other circulating side pumparound reflux streams are used to remove heat from the fractionator. They supply reboil heat to the gas plant and generate steam. The amount of heat removed at any pumparound point is set to distribute vapor and liquid loads evenly throughout the column and to provide the necessary internal reflux. 

Ambient conditions, which cause changes in internal reflux (see Chapter 11). 

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. 

In the present estimation, a continuous dehydrogenation reactor in which decalin is supplied to the catalyst at various feed rates without internal refluxing is assumed. Here all the condensable products and unconverted decalin were removed from the reactor to the condensation part (see Figure 13.22). Now, the stationary rates of hydrogen generation (VH), naphthalene formation (VN), and evaporation of unconverted decalin (VD) are defined as the magnitudes per area of the catalyst layer (mol/m2h). All these rates are expressed from mass balance as follows . 

NITECH A cryogenic process for removing nitrogen from natural gas, mainly methane. The high-pressure gas is liquified by expansion and then fractionated. The essential feature is the use of an internal reflux condenser within the fractionating column. Developed by BCCK Engineering and demonstrated on a full-scale plant in Oregon in 1994. 

The proof strength at which the volatility curve of a particular component intersects that of ethyl alcohol indicates that proof at which the minor constituent will be concentrated in a fractionating column at the limiting condition of total reflux. For practical conditions, the maximum concentration of a particular congener or minor component occurs at a proof in the column at which its volatility is approximately equal to the internal reflux ratio (L/V where L is the molal liquid or overflow rate and V the molal vapor rate). This can be established by the technique... 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

In any type of packed tower, the liquid or internal reflux drips through the packing, and forms a thin film of liquid on the surface of the packing. Vapor percolates up through the packing, and exchanges heat and molecules with the thin film of liquid on the surface of the packing. 

Unfortunately, packing does not redistribute liquid, or internal reflux. Unless the initial reflux distribution is good, the liquid flow distribution through the entire packed bed will be poor. Figure 7.1 shows a common orifice plate liquid distributor. Vapor flows up through the large chimneys, and liquid drains through the smaller distribution holes in the tray deck. 

Let s further assume that the orifice plate distributor is 1 in out-of-level. This could easily happen in a 14-ft 0-in-ID tower. Figure 7.2 shows the results. The flow of internal reflux or liquid through the higher portion of the tray deck falls to zero. Worse yet, vapor starts to... 

As the heavier components in the vapor condense into a liquid, they give off heat. This heat is called the latent heat of condensation. This latent heat is picked up by the liquid flowing across the tray. This liquid flow is called the internal reflux. This latent heat promotes extra vaporization of the internal reflux. Naturally, the lighter, lower-boiling-point components preferentially vaporize from the internal reflux. These lighter components have a relatively low molecular weight. 

The way we increase the fractionation efficiency of trays is to make the trays work harder. The correct engineering way to say this is To improve the separation efficiency between a light and heavy product, the vapor flow rate through the trays is increased, and the internal reflux flowing across the trays is increased. ... 

The distillation tray efficiency is reduced. The water may settle out on the hydrocarbon liquid on the tray. This reduces contact between the upflowing vapor and the downflowing, internal reflux. 

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