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Figure 4.2. Fractionation between total overhead and highest sidestream product, catalytic cracking fractionators. (Used by permission of the American Petroleum Institute.)

Bottoms (Slurry Oil or Decant Oil)

The bottoms stream is actually nothing more than what is left over after the maximum distillate yield has been attained. This is a very foul stream and is usually blended into residual fuel. A rare but occasional practice is to design the base section of the tower as an HCO condensing section so that the bottoms streams will be HCO in addition to slurry oil. This operation requires that the bottoms slurry oil also be recycled to the reactor. This often results in low conversion to gasoline and more rapid coke formation.

Separation Criteria

As was the case in crude oil distillation, the sharpness of fractionation is described in terms of the ASTM (5 - 95) temperature differences. Packie's relationships, per se, do not apply to catalytic fractionation. However, Houghland, Lemieux, and Schreiner (5) have developed similar correlations for catalytic towers which are used in the same way as in which Packie is applied to atmospheric crude towers. Figures 4.2 and 4.3 have been drawn from this reference and are recommended for design and analysis of these units. The nomenclature for Figure 4.2 is as follows.

Ljs] = gallons per hour of equilibrium reflux from the top tray or gallons per hour equilibrium reflux from the bottom pumparound tray, measured as 60 degree F liquid.

D^ = gallons per hour total distillates to the top tray measured as 60 degree F liquid.

Dn„2 = first sidestream product below top tray, measured as 60 degree F liquid.

N-p = number of actual trays in section. Note that each tray in pumparound heat removal service counts as one third of an actual tray. The nomenclature for Figure 4.3 is as follows.

Ljsj = gallons per hour reflux from the upper draw tray or gallons per hour reflux from bottom pumparound tray, measured as 60 degree F liquid.

Pjvj = gallons per hour total product vapors to upper draw tray, measured as 60 degree F liquid.

Figure 4.3. Fractionation between sidestream products, catalytic cracking fractionators. (Used by permission of the American Petroleum Institute.)

Ny = number of actual trays in section. Note that each tray in pumparound heat removal service counts as one third of an actual tray.

Both correlations require a steam stripping rate to product strippers of at least 8.4 pounds per barrel of stripped product. Subsequent information indicates that the correlation is more realistic if the presence of n-butane and all lighter components is disregarded in defining the product vapors. Thus, the effective internal reflux ratio from draw trays is greater than the apparent ratio since n-butane and all lighter components is disregarded in defining the product vapors. Thus, the effective internal reflux ratio from draw trays is greater than the apparent ratio since n-butane and lighter do not count in defining the product vapor. However, the original definition of the correlation is recommended since it represents a more conservative view of the problem.

Estimate of Material Balance

In a typical design, the material balance and separation criteria may be defined by:

1. Gross overhead ASTM end point.

2. ASTM boiling ranges for LCO and HCO.

3. TBP cut point between slurry oil and HCO.

4. ASTM (5 - 95) temperature differences maybe specified or may be allowed to be the natural result of the above criteria.

These are translated into design information by the following stepwise procedure.

1. From the whole feed TBP curve, develop TBP cut points between fractions. Calculate and plot the ASTM curves of the naphtha and cycle oils. Figure 2.15 will be useful in converting between TBP and ASTM end and initial points.

2. Calculate and tabulate weight and molar yields of the fractions from which product gravities and molecular weights are calculated.

3. Calculate EFV curves for the overhead and for all side-streams. Plot the EFV curves, extrapolating those for the sidestreams, LCO and HCO, to minus 20 percent.

4. Using Figure 2.13, set a steam rate to the sidestream strippers and estimate the material balance around these towers. For design purposes, a steam rate of 10 pounds jjer barrel of stripped liquid is recommended. At the indicated vaporization, find the EFV temperatures of the sidestreams at the appropriate minus vaporization percentages. These are the 14.7 psia bubble points of the product liquids on the draw trays and will be used later in calculating draw tray temperatures.

5. Set the steam rate to the tower bottoms at 10 pounds per barrel of bottoms product, in the case of catalytic towers and unlike crude unit towers, it is not necessary to set up a material balance around the feed entry point and the bottoms stripping trays because the vapor-liquid traffic at this point does not influence the overall heat and material balance calculations.

6. Tabulate the properties of the combined product vapors in the various sections of the tower. This will be very useful in making the heat and material balance calculations.

Heat and Material Balance Calculations

The design of a catalytic fractionator is accomplished in essentially the same manner as an atmospheric crude tower. There is a slight difference in the techniques employed for analyzing fractionation capability, but, otherwise, the procedures are nearly identical.

Although the tower has the same process configuration as a Type A atmospheric crude tower, a different approach to setting tray and heat removal requirements has been taken here. Such an approach would easily lend itself to crude unit design and is of particular utility in revamp studies. In expansion studies, one is usually interested in maximizing throughput by increasing heat removal via pumparound. This procedure quickly shows how much heat is available for a given material balance.

Preliminary Steps

In getting the design underway, certain assumptions will be made and later checked against the results of the calculations.

Number of Trays

It is generally true that most towers in similar services will have about the same number of total trays and draw tray locations, regardless of where they are located geographically. Catalytic fractionators are no exception. The preliminary tray configuration of the tower is established based on the following general guidelines.

1. Trays are required between the feed entry point and the first product draw tray. These trays are for desu-perheating the feed vapor to its dew point and for quenching and condensing that portion of the total feed which is to be yielded as bottoms product (slurry or decant oil). Depending upon the value of this heat quantity which must be removed, between four and eight trays will suffice. Because of the high tendency toward coke formation in this section of the tower, conventional trays such as valves or bubble-caps are not used here. Rather, trays having a high percentage of open area such as disc-and-donut, side-to-side baffles or "shed" angle iron decks are the types most commonly employed.

Four steam stripping trays are provided below the feed point for stripping recoverable material from the slurry oil. These trays are also of the high open area type. In determining the design material balance and product properties, it is generally true that the separation requirements for the HCO-LCO separation are much less stringent than those for the LCO-naphtha separation. Accordingly, nine trays between the HCO and

7. At the conditions of temperature and pressure existing in the overhead accumulator, calculate the vapor-liquid separation which will occur in this drum, remembering that water will exert its pure-component vapor pressure in the vapor phase. This procedure is exactly the same -, as that presented in Chapter 2.

8. After the number of trays and the draw tray locations have been set, plot the API gravity and molecular 3. weight of liquid leaving trays versus tray number. Assume that liquids leaving the tray above and tray below draw trays have the same gravity and molecular weight as the product liquid.

LCO draw trays and 11 trays above the LCO draw tray are typical configurations. Each of these two sections will normally contain a three-tray pumparound heat removal section.

Definable Heat Quantities

At this early point in the design, the following heat quantities can be defined.

1. The feed temperature and pressure is given. Calculate the heat content of the feed. Note that the coke is treated as zero degree API' liquid, both in the feed and in the bottoms product stream.

2. It is common practice to limit the temperature of the slurry oil to a maximum of 700 degrees F to minimize thermal decomposition in the section of the tower below the first product draw tray.

3. Assume a value for the temperature of the stripping steam. Calculate the heat content of all stripping steam streams.

initial Premises for Process Design

It is necessary to make initial assumptions relative to proposed heat removal schemes prior to commencing design calculations. Since the separations made in catalytic frac-tionators are relatively easy in comparison with atmospheric crude tower separations and since the number of trays available is quite high by petroleum fractionation standards, it follows that internal reflux requirements are comparatively low. Thus, it is desirable to remove as much heat as possible from the system. Maximizing heat removal is usually accomplished by process-to-process exchange at various points in the unit, and this affords substantia] utility savings. A secondary benefit is that internal reflux is minimized which, in turn, minimizes the tower diameter.

In normal practice, one should design for a high level of heat removal by pumparound systems. This is accomplished by setting the internal reflux from sidestream draw trays at zero or, at most, at very low values and employing zero reflux from the overhead condenser.

Many designers employ zero internal reflux from the lowest sidestream draw tray and accomplish the total heat duty in the lower section of the tower by pumparound heat removal. It has been the author's experience that a small amount of internal reflux to the lower section of the tower will do a more effective job of washing back the pyrolysis solids than does total heat removal by pumparound, even at very high liquid wash rates across the trays. This indicates that these pyrolysis solids are fractionated back to the bottoms rather than washed back by the scrubbing action of the liquid.

For the purposes of the illustrative design procedure, the pumparound heat removals were assumed on the following bases.

1. For the section of the tower up through the tray below the lower sidestream product (HCO) draw tray, let the pumparound heat removal take less than 100 percent of the total reflux heat, the remainder being satisfied by internal reflux from Tray A. A suggested value for

. design is 75 percent.

2. Set zero internal reflux from the upper sidestream draw tray, Tray B. A three-tray pumparound heat removal system is required for Trays (B - 1) to (B - 3).

3. Provide a pumparound heat removal system utilizing the top three trays in the tower. This heat removal will be such that no external reflux will be required from the condenser to the top tray. Another way of considering this is that the upper pumparound heat removal balances the system so that only overhead product leaves the top tray.

In using this scheme, check to see that the partial pressure of water in the overhead vapor is sufficiently low to preclude water condensation in the upper few trays of the tower. If water condensation does occur, it will be necessary to redesign the system to require some pumpback reflux from the condenser to the top tray. This will lower the partial pressure of water in the gross overhead vapor. Only enough pumpback reflux should be used to avoid the possibility of water condensation, the remainder of the heat removal being accomplished by the pumparound system.

Operating Pressure

The conditions of temperature and pressure of the feed at the tower inlet will almost invariably be set by the design of the reactor section. Assume a 5 psi pressure drop from the feed tray to the top of the toiyer and a 5 psi pressure drop from the top tray to the overhead product accumulator.

For later use in heat and material balance calculations, plot pressure versus tray number.

Vapor Temperatures to Draw Trays Assuming Zero Internal Reflux from Draw Trays

At this assumed operating condition, the vapor rising to draw trays will contain only products. Thus, the temperature of these vapors is the dew point of the particular product at the partial pressure at which it exists in the total vapor. As in the case of atmospheric and vacuum tower design, the presence of the product to be removed at the next draw tray up in the tower is neglected. All lighter product vapors and steam are considered as inert gases.

The temperature of vapor leaving the top tray is also calculated as the dew point of the overhead product—both vapor and liquid—at its partial pressure in this stream.

The flash zone temperature is determined from the phase diagram as the temperature of the total products leaving the feed section at the total hydrocarbon partial pressure existing there.

For later use in heat and material balance calculations, plot these temperature points versus tray number on the tray-pressure profile.

Process Flowsheet

Set up a process flowsheet such as Figure 4.4 which will be used in making the detailed heat and material balance calculations.

Heavy Cycle Oil (HCO) Draw Tray

The heat and material balance relationships at this section of the tower are determined by making balances around Envelopes I and II as shown on Figure 4.4. An expanded view of this section is given on Figure 4.5 together with the equations used in making the calculations. These equations are used in the following sequence.

1. Estimation of Lower Pumparound Heat Removal-

Envelope 1

a. In an earlier step, the temperature of the vapor leaving Tray (A - I) was calculated for the case of zero internal reflux from Tray A. At this temperature, calculate the heat content of the vapor leaving Tray (A - 1).

b. From this and the heat quantities at points lower in the tower, calculate the duty of the lower pumparound cooler, Qcis which is required to satisfy the stipulation of zero reflux heat and, thus, zero internal reflux at Tray (A - 1).

c. In accordance with the design premise, set Q^j equal to the predetermined percentage of the value calculated above.

d. Calculate and tabulate the external heat quantities above Tray (A - 1).

e. Set the temperature of the cooled pumparound liquid at least 150 degrees F lower than the Tray (A - 1) temperature and calculate the lower pumparound circulation rate.

2. Tray A Balance—Envelope II

a. Assume an operating temperature on Tray A. Calculate the heat content of the vapor and of the liquid product leaving Tray A at this temperature.

b. From these heat quantities and the external heat quantities below Tray A, i.e., the external heat quantities above Tray (A — 1), compute the reflux heat above Tray A as Qr.

c. The hydrocarbon which is to be revaporized in the product stripper falls to the draw tray as part of the internal reflux from Tray (A + 1). In passing across the tray, it absorbs a small amount of the reflux heat. This heat quantity is defined as

Qlsvhco d. Calculate the heat removal capability of the reflux available to the tray as q'j^.

e. Calculate the internal reflux required to absorb the excess heat at Tray A as L^ + j and convert it to moles per hour.

f. Calculate the hydrocarbon partial pressure of in the total vapor leaving Tray A. As in previous similar calculations, ignore the presence in the total vapor of the product to he removed on the next draw tray up in the column and consider all other lighter products and steam to be inerts.

g. Convert the atmospheric bubble point of the un-stripped hydrocarbon liquid on the draw tray to this partial pressure and check the assumed temperature. If there is a significant difference, assume a new temperature and repeat the procedure.

h. Set the temperature of the stripped HCO from the sidestream stripper at 30 degrees F lower than the draw tray temperature and calculate the heat content of this product stream leaving the system.

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

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

Light Cyde Oil (LCO) Draw Tray

The heat and material balance relationships at this section of the tower are determined by making balances around Envelopes III and IV as shown on Figure 4.4. An expanded view of this section together with the equations used in the computations is given by Figure 4.6. The following calculational sequence is recommended.

1. Calculation of Midpumparound Heat Removal-

Envelope III

a. In an earlier step, the vapor temperature leaving Tray (B - 1) was calculated for the condition of zero internal reflux from Tray B. At this temperature, calculate the heat content of the vapor leaving Tray (B — 1).

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Figure 4.4. Heat and material balance summary, catalytic cracking unit-.

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