AS A CHECK, (QC4-Q'C4)=[(QVD4-1 + QVI D4* - QO'4-Q VOHIJ
Figure 3.13. Heat and material balance—top sidestream product draw.
tower overhead stream comprises the inerts. Convert the atmospheric dew point of D4 to this hydrocarbon partial pressure and check the assumed temperature.
2. The temperature of the liquid on Tray D4 is found by converting the atmospheric bubble point of the product liquid to the hydrocarbon partial pressure existing above Tray D4.
3. Calculate the duty of the cooler as Qc4 by making an overall system heat balance, keeping in mind that the outlet temperature from this exchanger has already been defined,
4. Calculate the portion of required to cool the liquid (D4 + Lj)4) from the draw tray temperature to the exit temperature. Designate this quantity as Q'c4-
The remaining portion of is that required to cool the pumparound reflux, Lp^.
5. Check the calculation of Lp^ by making a heat balance around Envelope VI,
6. Check the value of Q^4 by calculating the heat requirements for cooling (LpA + D4 + Lj^) over the required temperature range. This must check the value calculated in Step 4 above.
At all the trays which were calculated in the design, vapor and liquid flow rates were developed. In this section, these pieces of information will be assembled to develop an internal traffic di agram. At this point, the following items are known.
1. Vapor leaving the flash zone including total distillate products, overflash, steam, air and hydrocarbon decomposition gas.
2. Product streams leaving their respective draw trays.
3. Material balance around all product strippers.
4. Pumpback and pumparound reflux rates.
5. Internal liquid flow rates to draw trays.
Using this information, developing the diagram reduces to a matter of arithmetic. The only place where caution must be used is in handling induced reflux. Consider a section of the tower containing the tray below a draw down to the tray above the next lowest draw. The known liquid rates in the section are pumpback reflux to the upper tray, its induced reflux and reflux from the lower tray. There is almost always a difference between the sum of the first two quantities and the third. In the analysis, this difference is assumed to be split equally between the trays in the see* tion. This information is required for tray design or for analysis of the trays in existing towers. Discussion of tray design techniques is outside the scope of this work, and the interested reader is referred to the previously cited vendor literature.
There are no correlations known to the writer for analyzing fractionation in vacuum towers. The Packie analysis can be employed but only to show that a proper trays-reflux balance exists in the various sections. Estimates of gap-overlap may be made as a matter of interest, but it must be remembered that Packie's curves were intended to apply only to atmospheric crude towers.
Heat and Material Balance Calculations for Fuels-Type Towers
This section presents the remainder of the procedures required to calculate the heat and material balance around a fuels-type vacuum tower. Instructions for making the calculations at the flash zone and at the overflash liquid condensing section were presented in the previous section. As a quick refresher, the following items must be accomplished to this point.
1. Set the overall material balance, including air leakage and hydrocarbon decomposition gas.
2. Establish a pressure profile across the tower using previously recommended guidelines.
3. Make flash zone calculations of steam requirements and heat input to the tower in accordance with earlier sections.
Set the temperature of the vapor and liquid streams at key points in the tower. Unlike an atmospheric tower or a lube-type tower, these temperatures can be established analytically and do not require a triai-and-error approach. In this type of tower, the entire vapor charge to the tower is generated in the flash zone, and, with the exception of the overflash condensing section, there is no internal equilibrium reflux. All the heat is removed by pumparound reflux. Since the operation of the sidestream product condensing sections can be described closely as equilibrium condensations, the vapor temperatures can be estimated from the reduced crude EFV curves at the appropriate degree of vaporization and hydrocarbon partial pressure. Liquid temperatures are set by converting the atmospheric bubble points of the products to the appropriate partial pressures existing above the draw trays. This analysis also applies to the overflash liquid condensing section because the reflux required to condense the overflash is generally the same relative number of moles as the overflash.
The temperatures of the exit products and the pumparound reflux streams from their coolers are established taking into account the temperature-viscosity relationships of these very heavy oils. Product temperatures may also be set by heat balance considerations in downstream units.
This section of the tower is calculated in the same manner as the lube-type tower. The procedure was discussed earlier and illustrated by Figures 3.19 and 3.10.
Sidestream Products (DI and D2) Condensing Sections
The calculation of a fuels-type tower is much simpler than a lube-type. The material balance and heat balance relationships are analytical rather than trial-and-error in nature. Figure 3.14 shows the complete heat and material balance relationships for such a tower. The expressions and equations on this figure are self-explanatory. Note also that this sketch contains all the vapor-liquid internal traffic data.
1. D.t. Wheeler, "Design Criteria for Chimney Trays," Hydrocarbon Processing 47, no. 7 (July, 1968), pp. 119-20.
2. "Ballast Tray Design Manual," Bulletin No. 4900, Fritz W. Glitsch & Sons, Inc., Dallas, Texas.
3. Koch Flex it ray Design Manual, Koch Engineering Co. Inc., Wichita, Kansas.
4. W.L. Nelson, Petroleum Refinery Engineering. 4th ed. (New York: McGraw-Hill Book Company, Inc., 1958).
5. "Glitsch Grid-A New Design for Column Internals," Bulletin No. 7070, Fritz W. Glitsch & Sons, Inc., Dallas, Texas.
6. E.E. Ludwig, Applied 'Process Design for Chemical and Petrochemical Plants, Vol. I (Houston: Gulf Publishing Company, 1964).
7. J.W. Packie, "Distillation Equipment in the Oil Refining Industry," AJChE Transactions 37 (1941), pp. 51-78.
8. W.C. Edmister, Applied Hydrocarbon Thermodynamics (Houston: Gulf Publishing Company, 1964).
9. J.B. Maxwell, Data Book on Hydrocarbons (Princeton, N.J.: D. van Nostrand Co., 1965).
10. F.W. Winn, "Physical Properties by Nomogram," Petroleum Refiner 36, no. 2 (February, 1957), p, 157.
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