W

Figure 2.18. Draw tray temperatures—Type U tower.

seen later, it simplifies the calculations for the Type A system to a very great degree.

This section outlines procedures for calculating product draw tray temperatures at all points in the tower and for making an overall heat balance around the system. The method is based upon assuming a draw tray temperature and then calculating the internal reflux required by the system's heat balance. This internal reflux to the draw tray defines the hydrocarbon product partial pressure in the vapor above the tray. Converting the 14.7 psia bubble point of the unstripped liquid to this partial pressure gives a temperature which must check the assumed value. The top tray temperature is calculated by the same procedure, except that the reference temperature is the 14.7 psia dew point of the gross overhead product.

Preliminary Steps

At this point, it is assumed that all the calculations outlined in the previous sections of this chapter have been performed arid that the following have been fully defined.

1. Complete hydrocarbon material balance for feed and products.

2. Steam rates to stripping sections and steam distribution between overhead distillate vapor and liquid.

3. Hydrocarbon material balances around product strippers.

4. Atmospheric EFV temperatures for products corresponding to the estimated stripout for each product.

5. Draw tray locations, number of trays in each section and total number of trays in the tower.

6. Heat input to the base section of the tower from feed and bottoms-stripping steam, heat outflow in the bottoms liquid and external heat quantities at the flash zone. This bottoms—section heat balance is shown as Envelope 1 on Figure 2.17.

All these items must be completed before proceeding further with calculations.

Estimate of Tower Operating Conditions

Draw tray temperatures are estimated from the correlation of Figure 2.18. The following form the basis for this chart.

1. Flash-zone pressure = 24.7 psia.

2. Overflash = 2.0 volume percent of feed.

3. Reduced crude and lowest sidestream are steam stripped at H)^ pounds steam per barrel of product measured as 60 degree F liquid.

4. All other sidestreams are reboiled equivalent to the steam stripping at 10 pounds per barrel.

5. For sidestream products, use the estimated bubble point of the unstripped liquid from draw tray. For the total overhead product, use calculated dew point.

Note carefully the restrictions which apply and the indicated temperature variations which will occur as the process conditions differ from the stipulated bases.

1. An increase in flash-zone pressure will increase draw tray temperatures. 2y An increase in overflash will slightly decrease draw tray temperatures of the second sidestream product and all others above.

3. An increased stripping steam rate will decrease product draw tray temperatures due to the reduced hydrocarbon partial pressures.

4. Use of stripping steam in all product strippers rather than in only_ the first sidestream stripper will decrease draw tray temperatures of the second sidestream and all others above.

Having estimated draw tray temperatures and having set the operating pressures at key points in previous work, plot a temperature and pressure profile for the tower by assuming linear change between draw trays.

Some investigators have stated that 40 degree F temperature rises are experienced between the draw tray and the first and second trays below the draw and that a 15 degree F drop occurs between the draw tray and the one above. This does not seem logical in many cases, particularly in the lower sections of the tower. It seems particularly suspicious in the light of stating that heat removal is more effective on one of two adjacent trays when both trays are handling markedly similar fluids. Unless specific process information to the contrary is available, the assumption of linear temperature drops between draw trays is recom-mended since it results in greater reflux requirements and tower size. Thus, any error inherent in the assumption is on the safe side.

First Sidestream Product Draw Tray

The heat and material balance relationships at this section of the tower are determined by making a balance around Envelope II as shown on Figure 2.17. An expanded view of this is illustrated by Figure 2.19 which shows the equations used in making the calculations. These equations are to be used in the following sequence.

1. Calculate the reflux heat, Q^ at Tray Dl. Reflux heat is defined as the apparent heat imbalance between external heat quantities at the point in question in the tower. These external heat quantities are denoted as Q with appropriate subscripts to signify their location. External heat input quantities are defined as the heat contained in the feed plus all heat to the system at product strippers either directly as steam or indirectly through reboilers. External heat output quantities at a given tray are defined as the heat contained in liquid products leaving the system from points lower in the tower, the heat contained in the internal vapors of products plus steam and the heat contained by a product liquid flowing to the sidestream stripper. If the tray is not a sidestream draw tray, this latter quantity does not enter into the heat balance.

external heat quantities

Q'i di -Q'i fz + Q51 Q'ooi^Q'ofz +Qdi Sq'di (Q'i-Q'oIdi

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