Pumparounds draw tray

Figure 17.1 shows an alternate method, called circulating reflux or pumparound, to remove heat from a tower. Hot liquid, at 500°F, is drawn from tray 10, which is called the pumparound draw tray. The liquid pumparound is cooled to 400°F. The cooled liquid is returned to the tower at a higher elevation onto tray 9. It appears from Fig. 17.1 that the cold 400°F pumparound return liquid is entering the downcomer from tray 8. This is often good design practice. Tray 9 is called the pumparound return tray. 

Pumparound draw tray The tray from which liquid is drawn off to the pumparound pump. 

Either the entire amount of the liquid on a tray or part of it may be taken out and returned to the tray directly below it. The combined liquid flow (the pumparound and the liquid flowing directly to the tray below) would be the same as the liquid flow down to the tray below had there been no pumparound. From the standpoint of equilibrium stages, the column would perform exactly as though the pumparound did not exist although the tray efficiency may be affected due to the different flow pattern. If the liquid is returned several trays below the draw tray, the trays between the draw tray and the return tray are bypassed by the pumparound liquid. The amount of fractionation in that column section is therefore reduced. This pumparound thus tends to lower the overall number of effective trays in the column. 

In another type of pumparound, the liquid drawn from one tray is returned a number of trays above the draw tray. This process also tends to lessen the effective number of trays and fractionation in that column section because of back-mixing higher-boiling liquid from the lower tray with lower-boiling liquid in the upper tray. 

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. 

After the retrofit, opening the drawoff line below tray 10 (refer to Fig. 3-6) increased the pumparound draw temperature from 575°F to 620 F without any noticeable effect on duty. 

Table 8-4 summarizes the product distillations for the FCCU fractionator. These distillations are consistent with the vapor and liquid flows presented in Table 8-1. The degree of separation between the tower overhead and LCO products is represented by the ASTM 5% to 95% gap of —2°F. Based on the Packie method, calculations indicate that there are 10 effective trays above the LCO draw tray. This is equivalent to roughly six theoretical separation stages. Since each pumparound section is usually represented as a single stage, we can see by referring to Figure 8-2 that the naphtha wash section is equivalent to four theoretical separation stages.

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. 

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

By definition, the reflux from the upper draw tray in a section employing a pumparound system will be the minimum value. Keeping this in mind, estimate the draw tray temperafures and plot this profile on the one developed in the previous section, Estimate of Draw Tray Temperatures at Minimum Reflux Conditions. ... 

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. 

The following discussion assumes a pumparound system between the first and second sidestream product draw trays. The heat and material balance relationships at this section of the tower are determined by making two balances which are shown as Envelopes II and III on Figure 2.25. An expanded view of this section is illustrated by Figure 2.26 which also gives the equations which are to be used in making the calculations. The sequence of calculations is discussed as follows. 

In setting up a design, the location of pumparound systems will usually be predetermined. Thus, a section containing no pumparound is calculated as a conventional Type U draw tray which has been covered earlier. 

Normally, there will be no pumparound system in the section between the highest sidestream product draw tray and the top tray. Thus, it is calculated in the normal manner for a top tray as illustrated by Figure 2.21. 

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. 

The manufacture of distillates either directly for fuels or for feed to downstream processing units ordinarily does not require any particular degree of fractionation between cuts. Also, wide cuts are usually acceptable. For these reasons, the distillates can be condensed by cooled pumparound reflux, grid type contacting sections and chimney draw trays. For all practical purposes, the operation of the main condensing sections can be described as a single-stage equilibrium condensation. 

The analysis of this section of the tower involves calculating the required pumpback reflux to the tray below the draw tray together with the pumparound reflux and external heat removal requirements for condensing the side-... 

Product streams leaving their respective draw trays. Material balance around all product strippers. Pumpback and pumparound reflux rates. 

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. 

In normal practice, one should design for a high level of heat removal by pumparound systems. This is accomplished by setting the internal reflux from sidestream draw trays at zero or, at most, at very low values and employing zero reflux from the overhead condenser. 

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 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). 

From an operating standpoint, we can see when this flooding starts. As we decrease the pumparound duty, the temperature difference between the diesel- and gas-oil product draws should increase. When these two temperatures start to come together, we may assume that we have exceeded the incipient flood point, and that trays 5, 6, and 7 are beginning to flood. 

As will be shown later, the trays in the main column are mostly rectifying sections for the side products some of them are pumparound contact condensers. There is a wide variation in the industry of tray count between side draws, from 4 to 15, depending on the degree of rectification desired. 

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. 

Partial drawoffs (commonly utilizing downcomer trapouts) are used when a side draw does not share a pumparound drawoff. Figure 19.116 shows the preferred controls (234). Operator action is required to ensiure that the correct distillate quantity is drawn and to prevent drying of trays below the drawoff. The dryout problem can often be mitigated by drawing the side product from the bottom seal pan (i.e., just above a chimney tray). In both arrangements (Fig. 19.11a and b), note the seal loop in the line from the main fractionator to the stripper. This loop prevents vapor backflow at low liquid rates (Sec. 5.1). 

A portion of the valves on three pumparound trays was removed and their opening blanked. Ettds of distribution pans and draw pans were seal-welded. Leakage was reduced, sqrara-tion and heat recovery woe improved. 

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