Drawoff trays

Liquid sidestreams are used when there is less A in the feed than B or C. The vapor passing up through the sidestream drawoff tray contains all the A that is in the feed. The concentration of A in the liquid phase on this tray is smaller than its concentration in the vapor phase since A is the most volatile component. Therefore we withdraw a liquid sidestream. In the case where there is less C in the feed than A or B, we use a vapor sidestream below the feed. The concentration of the sidestream impurity C is less in the vapor phase than in the liquid phase since C is the heaviest component. 

Similar problems can occur with vapor sidestreams, but the solution is not as easy because we cannot provide vapor holdup in the system. One approach is to use an internal vapor controller. The flowrate of the vapor sidestream and the flowrate of the steam to the reboiler are measured. The net flowrate of vapor up the column above the vapor sidestream drawoff tray is calculated. This flow is then controlled by manipulating the vapor sidestream drawoff rate. 

Column pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude oil feed temperature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product drawoff tray and the flash zone. Calculations are made by using the crude oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed. 

Particular care should be taken to see that the nuts and bolts at the drawoff trays (particularly for total drawoffs) are properly tightened. At least one experience (85) has been reported where poor column performance resulted from failure to tighten the drawoff box nuts and bolts. 

The key separation in this liquid sidestream column is between DME and MeOH in the section above the feed tray. Because aU the DME in the feed must flow up the column past the sidestream drawoff tray, the concentration of DME in the vapor phase is significant. The liquid-phase concentration, however, is smaller if the relative volatility between DME and MeOH is large. The normal boiling points of these two components (DME = 248.4 K and MeOH = 337.7 K) are quite different. This gives a relative volatility at the sidestream drawoff tray of about 24. Thus, the vapor composition of 4.04 mol% DME has a liquid in equilibrium with it that is only 0.16mol% DME. The column diameter is 0.61 m. The reboiler heat input is 1.346 MW. 

All the water in the feed must flow down past the vapor sidestream drawoff tray. If the liquid composition on this tray is 5.4mol% water, the vapor composition is 3.2mol% water. Thus, the purity of the sidestream is low. The only way to reduce the impurity of water in the sidestream is to reduce the water concentration in the liquid by drastically increasing the internal flow rates of the vapor and liquid in the column, that is, increase the RR. This makes the sidestream configuration uneconomical for this chemical separation. 

In order to consider a reasonable system to illustrate a vapor sidestream column, we change the feed stream to contain -butanol (BuOH) instead of water. The normal boiling point of n-butanol is 390.8 K compared with 337.7 K for MeOH. This produces a relative volatility of about 4.4 thus, a vapor sidestream product with only 1 mol% BuOH can be produced with an RR of 1.07. The composition of the liquid on the sidestream drawoff tray is 4.3 mol% BuOH. 

Finally, the RR in the main column was reduced to see the effect on the DME impurity in the sidestream. Since DME is much more volatile than MeOH and is fed above the sidestream drawoff tray, the RR could be reduced to 0.5 without affecting sidestream composition. With an RR of 0.5, the liquid rate in the top section of the main column is quite small (17kmol/h) compared with the liquid rates lower in the column (180kmol/h). Therefore, RRs lower than 0.5 were not used. 

During a 20-minute period, the LCO color changed from a bright, clear yellow to a dirty blond and then to a translucent light brown. Coincident with this degradation of product color was a gradual increase in pressure drop between the LCO product drawoff tray and the reactor vapor inlet line. The Magnehelic AP cell leveled off at 79 in. of water-pressure drop. From these observations I could now draw two definite conclusions ... 

Colder cooling water lor ejector condensers Seal-weld HVGO and TOO drawoff trays Increase the number of bottom-stripping trays More superheat of exhaust stripping steam Add velocity steam... 

Leaking amine water cooler, 103 Leaking drawoff trays, 23-25 welded trapout pans, 23-24 Leaking feed-effluent exchanger (glycol), 444-445... 

A side-stream product drawn from a crude tower is only half fractionated. Refluxing below the drawoff tray controls the product end point. Front-end fractionation takes place in the side-stream stripper. 

Water entrained in the stripper feed is not properly removed at the water drawoff tray. (See Fig. 20—3 for details of a water drawoff tray.)... 

Inadequate flow of wash oil will allow entrained resid to reach the HVCO drawoff tray. Naturally, increasing the wash oil flow puts HVCO into black trim gas oil. 

To gain one more degree of fr dom for composition control, Doukas and Luyberr came up with the idea of changing the location of the sidestream drawoff tray. 

The concentration of the lightest component in the sidestream product is controlled by the location of the sidestream drawoff tray. As shown in Figure 7.6B, this easily can be implemented by conventional analog hardware. The fixed-gain relays are calibrated so that only two of the drawoff valves can be partially open simultaneously. Thus as the signal from the XSl composition controller increases, the valves for trays higher in the column open as valves for trays lower in the column close. 

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