INDUCED REFLUX ON TRAY (pl-l) R ¡01 — 1 «=LDif(htDt-l-htoci)>(HtD1_2-htot~l)J
irst sidestream product draw tray—Type R tower.
reboiling, temperatures at the next draw tray upward and all those higher will be increased. If this is by steam stripping, however, these temperatures will be lowered due to the reduction of hydrocarbon partial pressures.
Having estimated draw tray temperatures and having set the operating pressure profile in previous work, plot a temperature and pressure profile for the tower by assuming linear change between draw trays.
Estimating the temperature of the cooled pumpback reflux requires a good deal of experience with the crude oil upon which the design is being based, and, unfortunately, this is usually only available in an operations-analysis situation. In design work, this temperature must be estimated, and later, after the heat duties and temperature levels of all the reflux coolers have been determined, it must be verified that the crude oil in being preheated is capable of absorbing the heat from the pumpback reflux streams. Optimizing the crude preheat-tower cooling heat-exchange train is the heart of crude unit design, and each case must be studied on an individual basis in order to arrive at the most economical processing scheme. It should be remembered that most designs will be based on several different operations, and this will exert a far greater influence on the design of the heat exchange equipment than will the assumed temperature levels alone. Since a large portion of the reflux heat removal is accomplished by the latent heat of vaporization and a relatively small portion by sensible heat, some latitude is available in choosing the cool reflux temperatures. The key point is not to assume a temperature level which is unat-tainably low. High values can be adjusted downward, but low values will require redoing the work. This subject would be more fittingly treated in a work on process optimization and is mentioned here to alert the reader to possible difficulties which will result from assuming unrealistic cool reflux temperatures.
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 2.22. An expanded view of these is illustrated by Figure 2.24, which also gives the equations to be used in making the calculations. These equations are to be used in the following sequence. Note that most of these points have already been encountered in the discussion of the Type U tower. Thus, detailed discussion will be presented here only to explain new concepts.
2. Calculate the heat removal capability of the cooled pumpback reflux.
3. Calculate the amount of cool pumpback reflux which is required to absorb the reflux heat at Tray (Dl - 1). Calculate the heat content of this stream as it reenters the tower.
Balance above Tray D1—Envelope HI
1. Calculate the reflux heat at Tray Dl. The liquid leaving this draw tray is the sum of the product, Dl, and the pumpback reflux, Ljjj . The liquid which is revaporized in the product stripper falls to the draw tray as a small part of the reflux from Tray (Dl + J) and, thus, for the purpose of this calculation, is not considered part of the exit liquid. Also note that, if the process is lo be designed for stripping of the pumpback reflux, the stripout liquid falling to Tray Dl will be accordingly greater.
2. Calculate the heat absorbed by the stripout liquid in passing across the draw tray.
3. Calculate the heat removal capability of the internal reflux falling from Tray (Dl + 1).
4. Calculate the internal reflux from Tray (Dl + 1) which is required to absorb the excess heat at Tray Dl.
5. Calculate the mole fraction of product vapors in the total vapors leaving the draw tray, remembering to neglect the presence in the vapor of the product which is to be withdrawn on the next draw tray up in the tower.
6. Calculate the hydrocarbon partial pressure of product in the total vapor leaving the draw tray. Convert the atmospheric bubble point of the unstripped liquid on the draw tray to this partial pressure. If this temperature does not check the value assumed earlier, repeat the procedure for a new assumed temperature. If recalculation is required, it is usually not necessary to revise the assumed temperature of the cooled pumpback reflux unless, for some reason, a gross error was made in assuming the draw tray temperature.
7. Calculate the heat effect at the sidestream stripper.
8. Calculate the reflux induced on Tray (Dl - 1) as the amount of vapor from Tray (Dl - 2) which enters and is condensed on Tray (Dl — 1) for the purpose of converting the subcooled pumpback reflux liquid to its bubble point.
9. Calculate and tabulate the vapor and liquid quantities to the base of Tray (Dl + 1).
10. Calculate and tabulate the external heat quantities to the base of Tray (Dl + 1).
The remaining sidestream draw trays are calculated by the same procedure as that outlined in the previous step. Remember that, in making partial pressure calculations, the presence of the next higher product vapor in the total vapor leaving the draw tray must be neglected.
The heat and material balance relationships at the top tray are determined by making a balance around Envelope IV as shown in Figure 2.22. The detailed flows around this section of the tower and the equations to be used in the calculations are the same as those given in Figure 2.21.
Condenser Calculations and Overall Heat Balance
The condenser duty is calculated by making a heat balance around Envelope V on Figure 2.22. This must be checked against an overall system heat balance. The two values must check to within 2 percent of the absolute value of the condenser duty.
This subject has been discussed in detail earlier in this work. The only new point to be considered here is the definition of reflux from the draw tray. For purposes of Packie's analysis, reflux is defined as the volume of liquid falling from the tray below the draw tray. In terms of the first side stream draw tray, this is calculated by making a heat balance above Tray (D1 - 2) and then calculating the internal reflux from Tray (D1 — 1) which is required to absorb the excess heat.
Tabulate the flows of vapor and liquid at all key trays in the tower as moles per hour. Plot these values versus tray number as vapor from tray and liquid to tray. This plot is of great value in tower design.
Heat and Material Balance Calculations for Type A Towers
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.
As will be seen in the development of the design procedure, there is some flexibility in the choice of draw tray locations and in the design of the pumparound systems. As in the case of the Type R tower, the optimum design of a crude unit utilizes crude oil as the coolant for the various tower heat removals, thus affording feed preheat which is an obvious operating economy. The final task of the designer is to verify that the assumed thermal aspects of the design are practically attainable. Since, by definition, this work is more concerned with the principles of petroleum fractionation rather than those of process optimization, the following procedures are written accordingly. This method may not always result in an optimum design after completion of the first calculation. However, the optimum can be found rather easily in subsequent iterations.
This procedure is based on the assumption thai a complete heat and material balance has been calculated for the process as a Type U tower. This analysis contains the following things.
1. A complete heat and material balance.
2. Complete temperature and pressure profile.
3. Draw tray locations and total number of trays.
4. Reflux volumetric flow rates from al! draw trays and the top tray.
It is an inherent property of a Type U tower and a given material balance that, since internal reflux flows are at their maximum values, column temperatures will also be at their maximum levels. Any modification to the system which removes tower heat will lower tower temperatures.
Looking back to Packie's work, it is known that for a given separation requirement expressed as (5-95) Gap, a given material balance expressed in terms of ASTM 50 volume percent temperature difference and a given number of trays in the separation section, there is a value, called F, which is the product of the number of actual trays and the volumetric reflux ratio in the section. Thus, a minimum allowable reflux falling from draw trays can be calculated. This is not minimum reflux in the sense that infinite plates are required for the separation. It is minimum allowable operating reflux for the specified number of trays and the required separation.
1. Based on Packie's analysis, calculate the minimum reflux which must be available from the product draw trays to make the required separation. These calculations must be made for two cases:
a. Assume no pumparound heat removal in the section. Thus, the number of trays to be used in Packie's F-Factor is the actual number of trays in the section.
b. In each section of the tower, assume a three-trav heat removal system. For fractionation purposes, these three trays are defined as being equivalent to one actual tray. Thus, the number of trays to be used in Packie's F-Factor is two less than the actual number of trays.
2. For purposes of comparison, note the reflux available in the tower as a Type U system.
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