Pumparounds heat removal

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. 

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. 

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. 

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. 

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 3.14. Heat and material balance summary-fuels type vacuum tower with pumparound heat removal.

N-p = number of actual trays in section. Note that each tray in pumparound heat removal service counts as one third of an actual tray. 

LCO draw trays and 11 trays above the LCO draw tray are typical configurations. Each of these two sections will normally contain a three-tray pumparound heat removal section. 

Many designers employ zero internal reflux from the lowest sidestream draw tray and accomplish the total heat duty in the lower section of the tower by pumparound heat removal. It has been the author s experience that a small amount of internal reflux to the lower section of the tower will do a more effective job of washing back the pyrolysis solids than does total heat removal by pumparound, even at very high liquid wash rates across the trays. This indicates that these pyrolysis solids are fractionated back to the bottoms rather than washed back by the scrubbing action of the liquid. 

For the purposes of the illustrative design procedure, the pumparound heat removals were assumed on the following bases. 

For the section of the tower up through the tray below the lower sidestream product (HCO) draw tray, let the pumparound heat removal take less than 100 percent of the total reflux heat, the remainder being satisfied by internal reflux from Tray A. A suggested value for... 

Set zero internal reflux from the upper sidestream draw tray. Tray B. A three-tray pumparound heat removal system is required for Trays (B - 1) to (B - 3). 

Provide a pumparound heat removal system utilizing the top three trays in the tower. This heat removal will be such that no external reflux will be required from the condenser to the top tray. Another way of considering this is that the upper pumparound heat removal balances the system so that only overhead product leaves the top tray. 

The top pumparound heat removal is now calculated as that amount required to balance the toWer. 

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