Gas Dehydration 211 well as using stripping gas. Sometimes the addition of a vacuum will help extend the range of an existing glycol system. Figure 8-11 can be used to estimate the effect of vacuum on lean gly- col. concentration, Stripping Gas The lean glycol concentration leaving the reboiler can be lowered by contacting the glycol with stripping gas. Often, wet gas that is saturated with water vapor at ambient temperature and 25 to 100 psig is used. At 25 psig and 100°F this gas is saturated with 1,500 Ib/MMscf of water vapor. At atmospheric pressure and the temperatures in the reboiler the gas can absorb over 100,000 Ib/MMscf. In most situations the additional fuel gas required to heat the reboiler to increase lean glycol concentration is less than the stripping gas required for the same effect. Thus, it is normally desirable to use strip- ping gas only to increase lean glycol concentration above 98.5 to 98.9%, which can be reached with normal reboiler temperatures and normal back pressure on the still column. If the glycol circulation rate must be increased above design on an existing unit and the reboiler cannot reach desired temperature, it is often possible to use stripping gas to achieve the desired lean glycol concentration. Figure 8-12 shows the effects on the glycol purity of stripping gas flow rate for various reboiler temperatures, assuming the gas is injected directly into the reboiler. Greater purities are possible if stripping gas contacts the lean glycol in a column containing one or more stages of packing before entering the reboiler. Glycol Circulation Rate When the number of absorber trays and lean glycol concentration are fixed, the dew-point depression of a saturated gas is a function of the gly- col circulation rate. The more glycol that comes in contact with the gas, the more water vapor is stripped out of the gas. Whereas the glycol con- centration mainly affects the dew point of the dry gas, the glycol rate controls the total amount of water that can be removed. The minimum circulation rate to assure good glycol-gas contact is about two gallons of glycol for each pound of water to be removed. Seven gallons of glycol per pound of water removed is about the maximum rate. Most standard 212 Design of GAS-HANDLING Systems and Facilities Figure 8-12. Effect of stripping gas on glyco! concentration. dehydrators are designed for approximately three gallons of glycol per pound of water removed. An excessive circulation rate may overload the reboiler and prevent good glycol regeneration. The heat required by the reboiler is directly proportional to the circulation rate. Thus, an increase in circulation rate may decrease reboiler temperature, decreasing lean glycol concentration, and actually decrease the amount of water that is removed by the glycol from the gas. Only if the reboiler temperature remains constant will an increase in circulation rate lower the dew point of the gas. Stripping Still Temperature A higher temperature in the top of the still column can increase glycol losses due to excessive vaporization. The boiling point of water is 212°F and the boiling point of TEG is 546°R The recommended temperature in the top of the still column is approximately 225°F. When the temperature exceeds 250°F the glycol vaporization losses may become substantial. The still top temperature can be lowered by increasing the amount of gly- col flowing through the reflux coil. If the temperature in the top of the still column gets too low, too much water can be condensed and increase the reboiler heat load. Too much Gas Dehydration 213 cool glycoi circulation in the reflux coil can sometimes lower the still top temperature below 220°F. Thus, most reflux coils have a bypass to allow manual or automatic control of the stripping still temperature. Stripping gas will have the effect of requiring reduced top still temper- ature to produce the same reflux rate. System Sizing Glycoi system sizing involves specifying the correct contactor diame- ter and number of trays, which establishes its overall height; selecting a glycol circulation rate and lean glycoi concentration; and calculating the reboiler heat duty. As previously explained, the number of trays, glycoi circulation rate and lean glycol concentration are all interrelated. For example, the greater the number of trays the lower the circulation rate or lean glycol concentration required. Figures 8-13, 8-16, and 8-17 can be used to relate these three parameters. Figure 8-13. Glycoi concentration vs. glycoi circulation when n = 1 theoretical tray. 214 Design of GAS-HANDLING Systems and Facilities Figure 8-14. Structured (matrix) packing, (from Koch Industries.) Contactor Sizing Bubble cap contactors are the most common. The minimum diameter can be determined using the equation derived for gas separation in verti- cal separators (Volume 1, Chapter 4). This is: Gas Dehydration 215 Figure 8-15. Various types of packing. (Courtesy: McGraw-Hill Book Company.} 216 Design of GAS-HANDLING Systems and Facilities Figure 8-16. Glycol concentration vs. glycol circulation when n = 1.5 theoretical trays. where d = column inside diameter, in. d m = drop size, micron T = contactor operating temperature, °R Q g = design gas rate, MMscfd P = contactor operating presssure, psia C D = drag coefficent p g = gas density, lb/ft 3 p g = 2.7 SP/TZ (Volume 1, Chapter 3) p! = density of glycol, lb/ft 3 Z = compressibility factor (Volume 1, Chapter 3) S = specific gravity of gas relative to air Reasonable choices of contactor diameter are obtained when the con- tactor is sized to separate 120-150 micron droplets of glycol in the gas. The density of glycol can be estimated as 70 lb/ft 3 . The diameter of packed towers may differ depending upon parameters developed by the packing manufacturers and random packing. Conven- tional packing will require approximately the same diameter as bubble Gas Dehydration 217 Figure 8-17. Glycol concentration vs. glycol circulation when n = 2 theoretical trays. cap towers. Structured packing can handle higher gas flow rates than bubble cap trays in the same diameter contactor. (See Table 8-1.) Conventional and random packing will require approximately the same diameter as bubble caps. Structured packing can handle higher gas flow rates than bubble caps in the same diameter contactor while requiring half the height. The height per equivalent theoretical tray normally ranges from 8 ft for low dewpoints to 4 ft for moderate dewpoints. Adequate mist eliminator and glycol distribution is needed for high gas flow rates. Reboiler Heat Duty The reboiler heat duty can be calculated using the techniques in Chap- ter 2, by sizing the reflux coil and heat exchangers and calculating the temperature at which the wet glycol enters the still. The reboiler duty is then the sum of the sensible heat required to raise the wet glycol to reboil- er temperature, the heat required to vaporize the water in the glycol, the heat required for the reflux (which is estimated at 25 to 50% of the heat required to vaporize the water in the glycol) and losses to atmosphere. 218 Design of GAS-HANDLING Systems and Facilities Table 8-1 Example Contactor Sizes for Dehydrating 50 MMscfd at 1,000 psig and 100°F Tower Tower Diameter Internals (inch) A. Tray Bubble Cap B. Structural Packing (Figures 6-9, B 1-300 B 1-100 Flexipac#l Flexipac #2 C. Random Packing (Figure 8-15) 2" Pall Ring 48 8-14) 36 30 42 30 48 Troy/Pocking Height ffeet) 16 8 8 6 8 16 In sizing the various heat exchangers it is common to assume a 10°F loss of rich glycol temperature in the reflux coil, a desired temperature of 175°F to 200°F for the rich glycol after the preheater and a rich glycol temperature after the glycol/glycol heat exchanger of 275°F to 300°F It is necessary to make sure that the lean glycol temperature to the pumps does not exceed 200°F for glycol powered pumps and 250°F for plunger pumps. The temperature of lean glycol after the glycol/gas exhanger should be approximately 10°F above the temperature of the gas in the contactor. The water vapor boiled from the rich glycol plus the reflux water vapor must be cooled from approximately 320°F to 220°F by the reflux coil. Exchanger heat transfer factors, "U," can be approximated as 10 to 12 Btu/hr-ft 2 -°F for glycol/glycol exchangers, 45 Btu/hr-ft 2 -°F for the gas/ glycol exchanger, and 100 Btu/hr-ft 2 -°F for the reflux coil. The specific heat of triethylene glycol is given in Figure 8-15. Table 8-2 can be used for an initial approximation of reboiler duties. If the reboiler is heated with a fire tube, the fire tube should be sized for a maximum flux rate of 8,000 Btu/hr-ft 2 . Glycol Powered Pumps The process flow schematic in Figure 8-6 shows electric motor driven glycol pumps. On smaller systems it is common to use glycol powered pumps. These pumps use the energy contained in the rich (wet) glycol to Gas Dehydration 219 Figure 8-18. Specific heat of triethylene glycol. (Courtesy of Union Carbide, Gas Treating Chemicals.) Table 8-2 Approximate Reboiler Heat Duty Design Gallons of Glycol Circulated /Ib H 2 O Removed 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 Reboiler Heat Duty Btu/Gal of Glycol Circulated 1066 943 862 805 762 729 701 680 659 Size at 150% of above to allow for start-up, increased circulation, fouling. pump the lean (dry) glycol to the contactor. The action of this type pump is shown in Figures 8-19 and 8-20. With the main piston moving to the left (Figure 8-19), dry glycol is drawn into the left cylinder and dis- charged from the one at the right. Wet glycol is drawn into the right cylinder and discharged from the left cylinder. As the piston completes 220 Design of GAS-HANDLING Systems and Facilities Figure 8-19. Glycol-powered pump—piston moving to left. (Source: Kimroy, Inc.) its movement to the left, it moves the "D" slide to the position shown in Figure 8-19. This reverses the pilot slide position, which reverses the action of the piston. Even though the wet glycol drops in pressure from contactor pressure to condensate/separator pressure, it has enough energy to pump the dry glycol from atmospheric pressure to contactor pressure. This is because it contains more water and gas in solution, but also because gas from the contactor flows out with the wet glycol. There is no level control valve on the contactor when using a glycol powered pump. Sufficient contactor gas is automatically drawn into the wet glycol line to power the pump at the rate set by the speed control valves. This gas, as well as the approxi- mately 1 scf/gal gas in solution in the glycol, is separated in the conden- [...]... exchanger 226 Design of GAS-HANDLING Systems and Facilities Rich glycol heat duty Lean glycol flow rate (W]ean) Calculation of T4 Gas Dehydration * Glycol/glycol preheater—calculate lean side Temperature * Gas/glycol exchanger duty 227 228 Design of GAS-HANDLING Systems and Facilities It is possible to recover more heat from the lean glycol and reduce the lean glycol temperature to the pumps to 18 0°F to... compression and sales If it is not recovered in one of these ways and is just vented locally, the cost of using this type of pump can be very high Glyeol powered pumps are inexpensive and easy to repair in the field They have many moving parts and because of their slamming reciprocating motion require constant attention One spare pump should always be installed 222 Design of GAS-HANDLING Systems and Facilities. .. Pump Gas Consumption sc f/gal 300 400 500 600 700 800 900 10 00 11 00 12 00 13 00 14 00 15 00 1. 7 2.3 2.8 3.4 3.9 4.5 5.0 5.6 6 .1 6.7 7.2 7.9 8.3 Contactor Operating Pressure EXAMPLE 8 -1: GLYCOL DEHYDRATION Problem: 1 Calculate contactor diameter 2 Determine glycol circulation rate and estimate reboiler duty 3 Calculate duties for gas/glycol exchanger and glycol/glycol exchangers Gas Dehydration 223 Calculate... purpose of internal insulation is to reduce the total regeneration gas requirements and costs Internal insulation eliminates the need to heat and cool the steel walls of the adsorber vessel Normally, a castable re factory lining is used for internal insulation The refractory must be applied and properly cured to prevent liner cracks Liner cracks will per- 232 Design of GAS-HANDLING Systems and Facilities. .. 234 Design of GAS-HANDLING Systems and Facilities Figure 8-22 Maximum design velocity for solid bed adsorbers where d = vessel internal diameter, in Bed Height to Diameter Ratio In its simplest form, an adsorber is normally a cylindrical tower filled with a solid desiccant The depth of the desiccant may vary from a few feet to 30 ft or more The vessel diameter may be from a few inches to 10 or 15 ft... valves and controls to direct and control the flow of gases according to the process requirements In the drying cycle, the wet inlet gas first passes through an inlet separator where free liquids, entrained mist, and solid particles are removed, This is a very important part of the system because free liquids can damage or destroy the desiccant bed and solids may plug it If the adsorption 230 Design of GAS-HANDLING. .. Circulation Rate and Reboiler Duty Wj = 63 Ib/MMscf W0 = 7 Ib/MMscf AW = 63 - 7 = 56 Ib/MMscf AW/Wj = 56/63 = 889 Assume 8 actual trays or 2 theoretical trays From Figure 8 -17 the glycol circulation rate is 2.8 gal TEG/lb H20 Size for 3.0 gal/lb 224 Design of GAS-HANDLING Systems and Facilities Estimate reboiler duty: Use a 750 MBtu/hr reboiler to allow for startup heat loads Calculate Duties of Heat Exchangers... pressure drop, psi L = length of bed, ft jj, = gas viscosity, cp p = gas density, lb/ft3 Vm = gas superficial velocity, ft/min B and C are constants given by: Particle Type B C ^-in bead %-in extradate Mfi-in bead !4-in, extradate 0.0560 0.0722 0 .15 2 0.238 0.0000889 0.00 012 4 0.00 013 6 0.000 210 Pressure drops of greater than approximately 8 psi are not recommended Moisture Content of Inlet Gas An important... GAS-HANDLING Systems and Facilities Figure 8- 21 Simplified flow diagram of a solid bed dehydrator, unit is downstream from an amine unit, glycol unit or compressors, a filter separator is preferred In the adsorption cycle, the wet inlet gas flows downward through the tower The adsorbable components are adsorbed at rates dependent on their chemical nature, the size of their molecules, and the size of. .. temperature of the rich glycol flowing on the still to more than 300°F and would decrease the reboiler heat duty SOLID BED DEHYDRATION Solid bed dehydration systems work on the principle of adsorption Adsorption involves a form of adhesion between the surface of the solid desiccant and the water vapor in the gas The water forms an extremely thin film that is held to the desiccant surface by forces of attraction, . be installed. 222 Design of GAS-HANDLING Systems and Facilities Table 8-3 Gas Consumption by Glycol Powered Pump Contactor Operating Pressure psig 300 400 500 600 700 800 900 10 00 11 00 12 00 13 00 14 00 15 00 Pump Gas Consumption sc . Structural Packing (Figures 6-9, B 1- 300 B 1- 100 Flexipac#l Flexipac #2 C. Random Packing (Figure 8 -1 5) 2" Pall Ring 48 8 -1 4) 36 30 42 30 48 Troy/Pocking Height ffeet) 16 8 8 6 8 16 In sizing . atmosphere. 218 Design of GAS-HANDLING Systems and Facilities Table 8 -1 Example Contactor Sizes for Dehydrating 50 MMscfd at 1, 000 psig and 10 0°F Tower Tower Diameter Internals (inch) A.