Pumparound heat removal

Direct-contact condensers (Fig. 15.14f). These are used for minimizing pressure drop in vacuum condensation. To accomplish this, the direct-contact zone contains low-pressure-drop internals such as packings, or is a spray chamber. Another common application is intermediate heat removal ("pumparounds ) in refinery fractionators. Here the main purpose is to maximize heat recovery at the highest possible temperature levels. A third common application is intermediate heat removal from absorbers or reactive distillation columns in which an exothermic reaction takes places. In all these applications, condensation... 

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. 

Often, we remove heat from a tower, at an intermediate point, by use of apumparound or circulating reflux. Figure 7.11 is a sketch of such a pumparound. In many towers, the liquid flows in the pumparound section are greater than in the other sections, which are used for fractionation. That is why we are often short of capacity and initiate flooding in the pumparound or heat-removal section of a column. 

Let s calculate the heat removed in the pumparound circuit shown in Fig. 12.1. Assume that the specific heat of the pumparound liquid is 0.7 Btu/[(lb)(°F)] ... 

Let s say that the cooling-water outlet temperature from the condenser was 140°F. This is bad. The calcium carbonates in the cooling water will begin to deposit, as water-hardness deposits, inside the tubes. It is best to keep the cooling-water outlet temperature below 125°F, to retard such deposits. Increasing the pumparound heat removal will lower the cooling-water outlet temperature. 

We could reduce the amount of diesel product from the tower. That could wash the heavier gas oil out of the diesel. But it would also increase the amount of diesel in the gas oil. Increasing the heat removed in the pumparound would have a similar effect less gas oil in diesel, but more diesel in gas oil. 

How about decreasing the heat removed in the pumparound This would seem to allow the lighter diesel to more easily vaporize out of the gas-oil product. But will this action result in increasing the contamination of diesel, with heavier gas oil This answer is no—but why not ... 

Reducing the pumparound heat-removal duty increases the vapor flow from tray 8 in the column shown in Fig. 12.5. The extra pounds of... 

Reducing the pumparound duty increases the tray loadings on trays 1 through 7. But in so doing, the trays operate closer to their incipient flood point. This is fine. The incipient flood point corresponds to the optimum tray performance. But if we cross over the incipient flood point, and trays 5, 6, and 7 actually start to flood, their fractionation efficiency will be adversely affected. Then, as we decrease the pumparound heat-removal duty, the mutual contamination of diesel and gas oil will increase. 

The warm product streams and pumparound heat removal streams from the main column are used to... 

Rectification of reformer feed can be improved by providing more internal reflux for the upper trays of the crude column, perhaps by using less pumparound heat removal above the jet fuel draw point. 

The setup and results for the second pumparound P-2 are shown in Figure 11.37b. The stage from which the hot liquid is removed is Stage 14. The stage to which the cool liquid is returned is Stage 13. The flow rate is specified to be 11,000 B/D, and the heat removed is set at 15MMBtu/h. 

Pumparounds affect separation between cuts and furnace firing in opposite ways. Reducing a pumparound heat removal improves separation between cuts above the pumparound, but increases furnace energy consumption. 

By increasing the pumparound heat removal below the furnace oil draw (tray 2), less liquid is left to be condensed above the furnace oil drawoff. This, in turn, reduces the amount of liquid that spills over tray 2 and down to trays 3, 4, and 5. The decrease in liquid (i.e., internal reflux) flowing across these trays impairs the separation between furnace oil and FCCU feed. 

This calculation is approximate and applies when the heat removal from the tower pumparound sections cannot be increased because of equipment limitations. In order to calculate the heat savings quantitatively, a... 

Other causes of excessive coke drum back pressure are badly fouled combination tower overhead condensers, partially plugged trays, or insufficient tower pumparound heat removal. Fouled condensers and lack of pumparound heat removal overload the wet gas compressor. Plugged trays are best identified with a pressure drop survey. 

Hot vapors, flowing up the fractionator from the flash zone, are partially condensed by contact with cooler pumparound liquid. The heat absorbed by the pumparound stream is used to preheat crude. As the pumparound circulation rale is increased, both heat removal from the fractionator and from crude preheat increase. This saves furnace fuel. 

The values of Xb,Xd and Xf are 0.05, 0.95, and 0.50, respectively. Relative volatility (binary system) is 2.0. Reflux at pumparound stage is 0.7, and heat removed at the pumparound is 1.7 times that removed from the condenser. 

Heat is recovered from each pumparound liquid stream to preheat the crude feed to the atmospheric still. Therefore, the temperature of the liquid return to the vacuum tower is fixed. As a result, the amount of heat removed in a pumparound bed is a function of the liquid circulation rate and the temperature of the liquid draw. This temperature is determined by the heat transfer coefficient developed by the tower packing used and depth of packing installed. The heat load for the packed bed is the difference in the total heat contents of the inlet and outlet vapor streams... 

The outlet vapor stream has much less mass than the inlet stream, due to a large percentage of vapor condensation in the section. The heat load on the packed bed is greater than the heat removed by external pumparound coolers because the vacuum gas oil condensate normally is discharged from the system at the same temperature it is withdrawn from the column. The logarithmic mean temperature difference driving force for each packed bed is determined from Equation 6-24. 

It is best to keep the cooling-water outlet temperature below 125°F to retard such deposits. Increasing the pumparound heat removal will lower the cooling-water outlet temperature. 

Reducing the pumparound heat-removal duty increases the vapor flow from tray 8 in the column shown in Fig. 17.5. The extra pounds of vapor flow up the tower and raise the tower-top temperature. The reflux control valve opens to cool the tower-top temperature back to its temperature set point. Then the liquid flow rates from trays 1, 2, and 3 onto tray 4 all increase. If the diesel draw-off rate is maintained constant, the liquid overflow rate onto trays 5, 6, and 7 will increase. This liquid flow is called the internal reflux. Trays 5, 6, and 7 are the... 

The operators planned to bypass boilers A and B in turn, as shown in Fig. 21.5, to determine which boiler had a tube leak. Boiler A was arbitrarily selected. The hot gas-oil pumparound from the fractionator was closed. With the reduction in the gas-oil pumparound heat removal, the generation of 150-psig steam dropped in half, and the water intrusion into the fractionator stopped. The operators concluded that with waste-heat boiler A out of service, and the water leak into the fractionator apparently eliminated, the leaking boiler was A. 

Nj = number of actual trays in the section, i.e., trays M til rough N inclusive = N - (M - 1) = N - M + 1. Note that each tray in pumparound heat removal service counts as one-third of an actual tray. 

He then defined the separation capability of the system as the product of the refiux-to-feed ratio at the upper draw tray as calculated on a volumetric basis and the number of actual trays in the section. This product is designated as the F-factor. In sections where pumparound heat removal systems are used, trays in this service are considered to be only one-third of an actual fractionating tray. 

A complete Type A Tower is shown in Figure 2.25. This drawing illustrates the basic process and its essential auxiliaries as well as the external heat and material balance quantities. The two pumparound heat removal systems which are shown are for a typical installation and would not necessarily be located in these particular sections of the tower, nor would every tower employ two systems. 

Since there is no pumparound heat removal system in the section between the flash zone and the first sidestream product draw tray, the results of the calculations for this tray will be the same as for a Type U tower. This is shown as Envelope I in Figure 2.25. 

Figure 3.1 shows utilization of cooled pumpback reflux from the draw tray to the tray below at all trays except the top sidestream. This liquid is condensed by pumparound heat removal using a grid type material for vapor-liquid contacting. Cooled reflux is pumped back to the tray below the draw tray to provide fractionation between the two light vacuum distillates. 

Figure 3.2 shows utilization of cooled pumparound reflux in two sections of the tower. In terms of heat removal and equipment requirements, the two methods are relatively equivalent. Both the external heat recovery and the internal vapor and liquid traffic quantities are essentially equal. The pumpback reflux system has the advantage... 

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