Article pubs.acs.org/EF
Design of a Petroleum Preflash Column William L. Luyben* Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States ABSTRACT: Crude oil contains saturated hydrocarbon chemical components with boiling points that range from very low (methane) to very high. A small preflash column is frequently used upstream of the main pipestill to remove light ends and some of the naphtha to reduce the load on the pipestill furnace and column. The major design optimization variables in the preflash column include pressure, reflux-drum temperature, and furnace coil outlet temperature. This paper explores the design of a typical preflash column.
1. INTRODUCTION The first unit operation in a petroleum refinery is a pipestill (crude distillation unit) that separates the crude oil into various streams on the basis of boiling points. The amount of crude oil consumed worldwide1 is astronomical (84 × 106 barrels/day in 2009). Oil consumption in the United States is 18 × 106 barrels/day. There are 148 U.S. refineries. The largest of these is the ExxonMobil refinery in Baytown, TX, that handles 560 000 barrels/day. Typical crude distillation units contain three columns: a preflash column, an atmospheric pipestill, and a vacuum pipestill. Each of these columns has an upstream furnace to partially vaporize the feed. Each has an overhead condenser and reflux drum. There are no reboilers. The major source of vapor going up the column is the vapor in the partially vaporizer effluent stream from the furnace. A small amount of stripping steam is added at the base of the column to remove very light components and prevent them from going downstream to the atmospheric pipestill. The atmospheric and vacuum pipestills also have sidestream strippers, from which various products are produced, and pumparounds, which remove high-temperature heat at intermediate locations in the column. Two product streams leave the reflux drum of the preflash column. A vapor stream contains most of the very volatile components (methane, ethane, propane, and butane) that are dissolved in the crude oil. A liquid light-naphtha stream (100 °F ASTM D86 Engler 5% point and 375 °F 95% point) is sent on for further separation and processing. Typical product streams leaving the atmospheric pipestill are heavy naphtha, kerosene, diesel, and atmospheric gas oil. Additional gas oil is removed in the vacuum pipestill. The Aspen PetroFrac model is used to simulate all of the columns. Crude assay data are used to generate a petroleum pseudo-component using the BK10 method. All boiling-point data are reported as Engler ASTM D86 temperatures. The design of the preflash column involves finding the economic optimum combination of design optimization variables: pressure, reflux-drum temperature, and furnace outlet temperature. We explore the preflash design problem in this paper. It should be emphasized that the only effect on the downstream atmospheric pipestill of modifying the design of the preflash tower is to change the flow rate and low-boiling © 2011 American Chemical Society
portion of the heavy naphtha stream. There are essentially no effects on the other products from the pipestill (flow rates or boiling points).
2. BASE CASE 2.1. Preflash Column. Figure 1A shows the flowsheet of the preflash column presented as an example in the Aspen Technology manual2 Getting Started Modeling Petroleum Processes. This same example has been used for dynamic simulations and control studies.3 Crude oil feed to the unit is 100 000 barrels/day. After the crude oil feed is desalted and preheated with hot product streams and pumparounds, it enters the preflash furnace at 200 °F and 44.7 psia. The oil is partially vaporized and heated to 450 °F before entering the base of the 11-stage preflash column. Stripping steam is also added in the base. The furnace duty is 203.1 MM Btu/h. Overhead vapor is partially condensed at 170 °F and 39.7 psia in a condenser that removes 65.9 MM Btu/h. The reflux drum acts as a decanter with 201 lb-mol/h of water removed from the water boot, 1860 lb-mol/h of light naphtha removed as the liquid product, and 575.2 lb-mol/h of vapor leaving from the top of the reflux drum. In the Aspen Plus steady-state simulation, the flow rate of the light-naphtha stream (“naphtha”) is adjusted by a design spec function to give an ASTM D86 Engler 95% point of 375 °F. Note that the ASTM D86 Engler 5% point of the light naphtha is 102 °F under the operating conditions used in the base case: 170 °F reflux drum, 39.7 psia, and 450 °F furnace coil outlet temperature (COT). The reflux flow rate is 854.7 lb-mol/h. The vapor stream from the top of the reflux drum (“lights”) is 575.3 lb-mol/h, with composition of 6.9 mol % methane (C1), 8.6 mol % ethane (C2), 14.4 mol % propane (C3), 9.2 mol % isobutane (iC4), plus other heavier components. The methane and ethane can be used for fuel. The propane and heavier components are valuable and need to be recovered. There are several ways to recover these components. We assume in this work that the gas is compressed up to a pressure where 95% of the propane in the lights can be recovered in a liquid stream after condensation at a temperature (120 °F) that permits the use of cooling water. The total condensation of the gas would require high pressure or low temperature because of the light methane and ethane present. An Aspen Plus flowsheet design spec is used to achieve the 95% propane recovery in the liquid stream, leaving the partial condenser downstream of the compressor by varying the vapor/feed fraction specification. Thus, the condenser has two specifications: Received: November 12, 2011 Revised: November 30, 2011 Published: December 5, 2011 1268
dx.doi.org/10.1021/ef201763g | Energy Fuels 2012, 26, 1268−1274
Energy & Fuels
Article
Figure 1. (A) Preflash column, base case. (B) Deethanizer column, base case. vapor/feed fraction and 120 °F temperature. These two specifications and the composition of the lights stream fix the pressure in the partial condenser downstream of the compressor. This pressure is then used as the compressor discharge pressure to calculate the power requirement to compress the lights stream. Changes in design parameters change the compressor discharge pressure and the flow rate and composition of the lights stream, which determine compressor work. The compressor discharge pressure is 282 psia, and the power requirement is 497 hp. The partial condenser heat duty is 7.42 MM Btu/h. Gas and liquid streams leave the separator downstream from the partial condenser. The small gas steam (33.99 lb-mol/h) can be used for refinery fuel. It represents a small loss of propane and a little isobutane. The liquid stream (541.2 lb-mol/h) contains 3.8 mol % methane and 7.7 mol % ethane; therefore, it is sent to a distillation
column to remove these components. The design of this distillation column is discussed in section 2.3. 2.2. Atmospheric Pipestill. The bottoms from the preflash column are fed at 443 °F into the atmospheric pipestill furnace, which consumes 200.5 MM Btu/h to provide a 3% “overflash” (fraction of the vapor leaving the furnace that is returned as liquid to the section of the pipestill below the gas oil drawoff to prevent entrainment of crude oil into the gas oil). The COT of the atmospheric furnace to achieve the specified overflash is 683 °F. The atmospheric pipestill operates with a 15.7 psia reflux-drum pressure. The overhead vapor from the column goes to a total condenser that produces a liquid organic phase and an aqueous phase. The latter is withdrawn from a “boot” at the bottom of the reflux drum. The organic liquid phase is split between a reflux stream, which is pumped to the top of the column, and a product stream, which is a liquid heavy-naphtha stream (“HNAPH”) with an ASTM D86 Engler 1269
dx.doi.org/10.1021/ef201763g | Energy Fuels 2012, 26, 1268−1274
Energy & Fuels
Article
Figure 2. Effect of the reflux-drum temperature at 39.7 psia and 450 °F COT. 95% point of 375 °F. This specification is maintained by an Aspen Plus flowsheet design spec that varies the flow rate of the heavy naphtha. Under the base-case conditions, the heavy-naphtha flow rate is 649.4 lb-mol/h. The initial boiling point is 115 °F, and the 5% point is 195 °F. The temperature (178 °F) in the total condenser in the pipestill is established by the specified pressure and the composition of the heavy naphtha. Note that the conditions of the heavy-naphtha product are shown in the upper left corner of Figure 1A. The “IBP” is the initial boiling point temperature (the temperature at which the first bubble of vapor is formed). 2.3. Deethanizer Column. The liquid stream from the separator downstream of the compressor and partial condenser is pumped into a distillation column to remove the methane and ethane. Because the critical pressures of ethane and propane are 708 and 617 psia, respectively, the operating pressure of the column was set at 400 psia to avoid hydraulic problems. The separation in this “deethanizer” is between ethane and propane. The impurity of ethane in the bottoms should be small, so that the propane can be produced in a downstream “depropanizer” at high purity. The loss of propane in the distillate should be kept small because the value of propane as a refinery fuel gas is less than its value as a commercial product. The flowsheet of the deethanizer is shown in Figure 1B. The distillate product is removed as a vapor. The design specification for the distillate is 5 mol % propane impurity. A modest reflux ratio of 2 and a 20-stage column feeding on stage 10 are selected as reasonable design parameters for this fairly easy separation. With the vapor distillate composition of 31.3 mol % methane, 63.7 mol % ethane, and 5 mol % propane and a column pressure of 400 psia, the reflux-drum temperature is 53 °F. Therefore, refrigeration is required. The condenser heat duty is fairly small at 0.911 MM Btu/h. The bottoms stream at 461.2 lb-mol/h has an ethane impurity composition of only 52 ppm. This deethanizer bottoms stream along with the light-naphtha stream from the preflash column are then fed to a downstream refinery light-end process for the recovery of all of the valuable light hydrocarbons (propane, isobutane, etc). The base temperature in the deethanizer is 314 °F at the high pressure in the column; therefore, medium-pressure steam at 160 psia and 363 °F is required. The reboiler duty is 4.89 MM Btu/h. The column diameter is 3.81 ft. As we show in the next section, the flow rates, compositions, and boiling point temperatures of the light naphtha from the preflash column, the heavy naphtha from the pipestill, and the liquid stream fed to the deethanizer vary with the conditions in the preflash column.
3. EFFECT OF DESIGN OPTIMIZATION VARIABLES The three main design optimization variables in the preflash column are pressure, reflux-drum temperature, and furnace COT. In theory, the number of stages is also a design optimization variable, but there are only 10 trays in this small column, which is about the minimum practical number. Increasing the number of stages was found to have little effect on the separation between the light naphtha in the preflash column and the heavy naphtha in the atmospheric pipestill (gap or overlap between the 95% boiling point of the light naphtha and 5% boiling point of the heavy naphtha). An iterative evolutionary procedure is used to find the optimum economic design. First, the effect of one design optimization variable is explored, with the other variables fixed at initial guesses. Using the optimum value of this variable, a second variable is explored. This procedure is repeated with all variables iterated until the global optimum is found. 3.1. Effect of the Reflux-Drum Temperature. With the pressure set at 39.7 psia and COT held at 450 °F, the temperature of the reflux drum is varied over a wide range from 130 to 180 °F. The base-case temperature is 170 °F, which permits the use of air coolers. Below about 150 °F, the use of cooling water is required. Figure 2 gives results. As expected, decreasing the refluxdrum temperature reduces the flow rates of the vapor streams (lights from the reflux drum and gas from the separator). The flow rate of the light naphtha increases quite significantly because the lower temperature in the preflash reflux drum condenses more of the vapor stream into the liquid phase. With the 95% point fixed, more of the total naphtha (light plus heavy) is withdrawn in the preflash column. Slightly less naphtha goes out the bottom of this column; thus, there is a small reduction of the flow rate of the heavy naphtha (“HNaph”) removed in the atmospheric pipestill as the preflash reflux-drum temperature is decreased. The composition of the gas streams contain more light components at lower temperatures; therefore, the compressor discharge pressure increases. However, the throughput through the compressor is smaller. The net effect is a reduction in 1270
dx.doi.org/10.1021/ef201763g | Energy Fuels 2012, 26, 1268−1274
Energy & Fuels
Article
Figure 3. Effect of the pressure at 130 °F reflux drum and 450 °F COT.
Because compressor power is lower at lower temperatures, we select 130 °F as a reasonable reflux-drum temperature. A water-cooled condenser will be required in this design, but cooling-tower water is relatively inexpensive. Air coolers conserve water but are expensive in terms of both capital investment and operating costs (fan motor power). 3.2. Effect of the Reflux-Drum Pressure. With the reflux-drum temperature at 130 °F and COT held at 450 °F, the pressure in the reflux drum is varied. The total pressure drop over the column is assumed to be fixed at 5 psi; therefore, the pressures in the column base and the furnace change. Figure 3 gives results over a range of pressures. A higher pressure reduces the flow rates of lights and gas. Raising the reflux-drum pressure increases the compressor suction pressure,
compressor work as the temperature is reduced (third left graph in Figure 2). Because the light naphtha stream contains more light components at lower temperatures, its 5% boiling point decreases as the temperature decreases. At the base-case 170 °F, the 5% point is 102 °F. At 130 °F, the 5% point is 59 °F. The 95% point is held constant for all cases at 375 °F. There is also an increase in the water phase withdrawn from the reflux drum because the lower temperature reduces the solubility of water in the organic phase. The energy consumptions in the preflash and atmospheric pipestill furnaces are essentially unaffected by the reflux-drum temperature, despite the fact that the flow rates of light naphtha and heavy naphtha are changing. 1271
dx.doi.org/10.1021/ef201763g | Energy Fuels 2012, 26, 1268−1274
Energy & Fuels
Article
Figure 4. Effect of the COT at 130 °F reflux drum and 42 psia.
Furnace duties in the two furnaces also vary with the preflash pressure. The preflash furnace duty decreases (less vaporization, leading to less light naphtha) and the pipestill furnace duty increases (more vaporization, leading to more heavy naphtha) as the preflash pressure is increased. The total furnace energy decreases with increasing pressure; therefore, we might conclude that high-pressure operation is best. However, the high compressor discharge pressure can cause problems. The component that we want to recover from the lights stream is propane, and the critical pressure of propane is 617 psia. To avoid phase separation problems, we want to keep the compressor discharge pressure well below 600 psia. Therefore, a preflash pressure of 42 psia is selected, which gives a compressor discharge pressure of 573 psia.
which tends to reduce compressor work. However, the compressor discharge pressure increases rapidly because the gas streams are richer in the very light components (methane and ethane). The net effect on compressor load of a smaller throughput, a higher suction pressure, and a higher discharge pressure is a reduction in work (HP) as the column pressure is increased. The higher pressure at the preflash furnace reduces the fraction of the feed that is vaporized at the fixed 450 °F COT. The flow rate of light naphtha decreases and the flow rate of heavy naphtha increases as the pressure is raised. The changes in the heavy naphtha flow rate are much larger for variations in the pressure than for variations in the reflux-drum temperature (compare Figures 2 and 3). 1272
dx.doi.org/10.1021/ef201763g | Energy Fuels 2012, 26, 1268−1274
Energy & Fuels
Article
Figure 5. (A) Preflash column, modified design. (B) Deethanizer column, modified design.
3.3. Effect of the COT. The final design optimization variable studied is the furnace COT. The pressure is set at 42 psia, and the reflux-drum temperature is set at 130 °F. Overflash in the atmospheric pipestill furnace is fixed at 3%. Figure 4 gives results for COT ranging from 360 to 460 °F. As expected, increasing the COT increases the light-naphtha flow rate and preflash furnace duty. The effect on heavy naphtha and the pipestill furnace duty is the reverse. Total furnace energy consumption reaches a minimum at 380 °F preflash furnace COT. A preflash furnace COT of 400 °F is selected because the increase in the total energy in going from 380 to 400 °F is quite small, while there is a reasonably large reduction in compressor
work (from 276 to 257 hp). Note that the compressor discharge pressure has decreased to 476 psia.
4. MODIFIED FLOWSHEET Figure 5 gives the modified flowsheet with the optimum design optimization variables. The preflash reflux-drum temperature is 130 °F, and the pressure is 42 psia. The furnace COT is 400 °F. A comparison to the original flowsheet shown in Figure 1 shows the following improvements: (1) Total furnace energy is reduced from 403.6 to 397.7 MM Btu/h. (2) Compressor work is reduced from 497 to 257 hp. (3) Gas stream sent to fuel is reduced from 33.99 to 31.27 lb-mol/h. (4) Propane losses in the gas stream are reduced from (33.99 lb-mol/h)(0.122) = 4.15 lb-mol/h propane to (31.27 lb-mol/h)(0.084) = 2.63 lb1273
dx.doi.org/10.1021/ef201763g | Energy Fuels 2012, 26, 1268−1274
Energy & Fuels
Article
mol/h propane. (5) Preflash condenser heat removal is reduced from 65.9 to 49.3 MM Btu/h. (6) Compressor aftercooler heat removal is reduced from 7.42 to 2.45 MM Btu/h. (7) Deethanizer condenser refrigeration duty is reduced from 0.911 to 0.764 MM Btu/h. (8) Deethanizer reboiler duty is reduced from 4.89 to 2.13 MM Btu/h. (9) Propane losses in the deethanizer distillate stream are reduced from (65.75 lbmol/h)(0.05) = 3.29 lb-mol/h propane to (55.68 lb-mol/h) (0.05) = 2.78 lb-mol/h propane. (10) Water removed from the preflash reflux drum (decanter) is increased from 210 to 268 lbmol/h, which means that the water content in the light naphtha is lower. (11) Atmospheric pipestill furnace COT is reduced from 683 to 675 °F, which will reduce coking in the tubes. Notice that the light naphtha is lighter (5% point 50 versus 102 °F) and its flow rate is less. Light-naphtha composition changes from 0.07 to 0.18 mol % methane, from 0.36 to 0.95 mol % ethane, and from 1.79 to 3.87 mol % propane (compare Figures 1B and 5B). Likewise, the heavy naphtha is lighter (5% point 179 versus 195 °F). However, its flow rate is higher (1110 versus 649 lbmol/h). The concentration of ethane in the heavy naphtha is very small in both cases; thus, it does not need to be fed to the deethanizer. The preflash furnace duty is smaller (156.6 versus 203.1 MM Btu/h), but the pipestill furnace duty is larger (241.1 versus 200.5 MM Btu/h).
5. CONCLUSION An improved flowsheet of a petroleum preflash column has been developed. Energy costs are reduced in terms of furnace duties, compressor work, deethanizer refrigeration, and deethanizer reboiler duty. Losses of propane in the gas stream and the deethanizer distillate are reduced by 28%.
■
AUTHOR INFORMATION Corresponding Author *Telephone: 610-758-4256. Fax: 610-758-5057. E-mail: wll0@ lehigh.edu.
■
REFERENCES
(1) www.eia.doe.gov/energyexplained. (2) Aspen Technology, Inc. Getting Started Modeling Petroleum Processes; Aspen Technology, Inc.: Burlington, MA, 2004. (3) Luyben, W. L. Distillation Design and Control Using Aspen Simulation; Wiley: New York, 2006; p 291.
1274
dx.doi.org/10.1021/ef201763g | Energy Fuels 2012, 26, 1268−1274
