Heat Balance Equations

(2) PUMPAROUND CIRCULATION RATE LC3 = QC3/(c)(t(n-2)-tC32)

(3) CONDENSER DUTY-ENVELOPE ZT Qcoh= Qvn-(qnv+QNl + QH2o)

Figure 4.7. Heat and material balance equations-top tray and condenser.

presence in the vapor of the overhead distillate liquid and considering the overhead distillate vapor and steam as inerts.

g. Convert the bubble point of the unstripped hydrocarbon liquid on the draw tray to this partial pressure and check the assumed temperature.

h. Set the temperature drop across the sidestream stripper at 30 degrees F and calculate the heat content of the product leaving the system.

i. Calculate and tabulate the external heat quantities to the base of Tray (B + 1).

j. Calculate and tabulate the vapor and liquid quantities above Tray B.

Top Tray Calculations and Overhead Condenser Duty

The heat and material balance relationships at this section of the tower are determined by making balances around Envelopes V and VI as shown on Figure 4.4. An expanded view of this section together with the equations to be used in the computations is given by Figure 4.7.

1. Calculations of Top Pumparound Heat Removal-Envelope V

a. In Step D, the temperature of the overhead product vapor was calculated on the basis that there would be zero external reflux to the tower from the condenser. At this temperature, calculate the heat content of the vapor leaving the tower. Note tlVat this stream consists only of product materials and stripping steam.

b. The top pumparound heat removal is now calculated as that amount required to balance the toWer.

c. In practice it may not be feasible to set the temperature of the cool pumparound liquid 150 degrees F lower than the overhead vapor temperature, In this case, take a reasonable approach to the temperature of the available cooling medium and calculate the top pumparound circulation rate.

2. Calculation of Overhead Condenser Duty—Envelope VI The overhead condenser duty is calculated as the enthalpy difference between the overhead vapor and the products from the overhead accumulator.

Fractionation Calculations

1. Calculation of Internal Reflux to Key Trays

By making heat balances, calculate the internal reflux at the following points in the tower.

a. Liquid to the top tray in the bottom pumparound section, Tray (A — 1).

b. Liquid from the bottom tray in the midpumpa-round section, Tray (B - 3).

c. Liquid from the bottom tray in the top pumparound section, Tray (N — 2). Lg _ 3 and Ljsj _ 2 are the liquid rates which are used in the fractionation analysis.

2. Fractionation Calculations

Using Figures 4.2 and 4.3, determine the degree of separation possible for the system as calculated. If the fractionation criteria have not been satisifed, additional trays or reduced heat removal may be employed to achieve the desired separation.

Vapor-Liquid Traffic

Tabulate and plot the vapor and liquid traffic at key points in the tower. This plot will usually identify the points of maximum load in the tower and will be of great assistance in tower sizing and tray design calculations.

Process Design Considerations

Assuming that the heat and material balance calculations have been finished and that the design appears feasible, the remaining tasks are to ensure that the assumed pumparound configuration can indeed remove the required amount of heat from the tower.

Neeld and O'Bara (7) and Fair (8) have published procedures for calculating the heat transfer capabilities of "jet trays" and side-to-side trays, respectively. Jet trays are similar to conventional trays, e.g., bubble-cap or valve trays, which would normally be specified for this service, and, thus, Neeld and O'Bara's correlations may be used as checks.

Since tower diameters are used in the heat transfer calculations, tray sizing calculations may be made using procedures from Glitsch (9) or Koch (10) or any other method which the reader might prefer.

if an insufficient number of heat transfer trays were provided in the original design assumptions, they can be added as required with only minor modifications to the calculations.

References

1. "1970 Refining Processes Handbook," Hydrocarbon Processing (September, 1970), pp. 174-79.

2. W.C. Edmister, Applied Hydrocarbon Thermodynamics (Houston: Gulf Publishing Company, 1961).

3. J.B. Maxwell, Data Book on Hydrocarbons (Princeton. N.J.: D. van Nostrand Co., 1965).

4. Technical Data Book-Petroleum Refining (Washington, D.C.: American Pelroleum Institute, 1966).

5. G.S. Houghland, E.J. Lemieux, and W.C. Schreiner, L'The Performance of Catalytic Cracking Unit Fractionating Towers," 19th Midyear Meeting, API Division of Refining, (May 13, 1954).

6. J.W. Packie, "Distillation Equipment in the Oil Refining Industry," AIChE Transactions 37 (1941), pp. 51-78.

7. R.K. Neeld and J.T. O'Bara, "Jet Trays in Heat Transfer Service," CEP 66, no 7 (July, 1970) pp. 53-59. /

8. J.R. Fair, "Design of Direct-Contact Gas Coolers," PetroJChemical Engineer (August, 196 1).

9. "Ballast Tray Design Manual," Bulletin No. 4900, Fritz W. Glitsch & Sons Inc., Dallas, Texas.

10. "Koch Flexitray Design Manual," Koch Engineering Co., Inc., Wichita, Kansas.

fracti fracti

Glitsch Bulletin 4900

Up to this point, this work has considered gross or rough separations. In the case of the atmospheric crude tower, petroleum was separated into relatively narrow fractions while, in the case of the FCCU main fractionator, the tower produced fewer fractions having wider boiling ranges. In both cases, the lightest (overhead) fraction was a full-range naphtha, i.e., everything in the tower feed up to a certain predetermined end point. This section will concern itself with the distillation processes required to separate light hydrocarbon components from the heavier continu-ous-boiling fractions, to fractionate the discrete light ends components and to separate the heavier continuous boiling materials into two or more fractions.

In the refinery, the term "light ends" generally means any discrete component lighter than heptane which can be identified by a name. This includes everything from hydrogen through the hexanes. A more narrow definition might consider only C3 and C4 liquids as light ends since, in many refineries, ethane and lighter is used as fuel gas and the pentanes and hexanes are blended dircctly into gasoline. As will be seen in the development of this section, there are many reasons for recovering light ends, each being dictated by the process configuration and economics of the refinery in question.

For a moment, let us explore the historical development of light ends recovery plants within the context of the total refinery. In the early days of the industry, kerosene, stove oil and lubricants were the principal products. With the advent of the automobile, gasoline became valuable and was produced. In most cases, the raw naphtha distillate was stored in open tankage where the light components vaporized and the gasoline became self-stabilized. Besides being wasteful, this practice was dangerous.

As the automobile engine developed, gasoline specifications became more stringent, and a need developed for better control of vapor pressure which radically affects carburetion and ignition. The refiner accomplished this by installing a naphtha stabilizer with which he could closely control gasoline vapor pressure. A secondary benefit was that the vapor distillate from this tower—essentially butanes and lighter—could be used as fuel gas within the refinery. Figure 5.1 shows a typical whole naphtha stabilizer producing a vapor distillate to the refinery fuel gas system.

In time, some enterprising person invented a stove utilizing as fuel either propane or butane, which the refinery could easily produce. It required that the naphtha stabilizer be redesigned at a higher pressure level in order to yield a liquid distillate which, in turn, could be fractionated into propane and butane as liquefied petroleum gas (LPG). Figure 5.2 shows a typical scheme for processing the whole naphtha stabilizer liquid distillate into propane and butane while still yielding fuel gas consisting of ethane and lighter.

As time, passes and the petroleum industry and society develops, the demand for refinery products skyrockets, and product specifications continually tighten. New processes are developed to improve gasoline octane. Petrochemical

Was this article helpful?

0 0

Post a comment