186 Design of GAS-HANDLING Systems and Facilities Alternately, a separate scrubber vessel can be provided so that the tower height can be decreased. This vessel should be designed in accordance with the procedures in Volume 1 for design of two-phase separators, For ME A systems with a large gas flow rate, a scrubber should be con- sidered for the outlet sweet gas. The vapor pressure of MEA is such that the separator may be helpful in reducing MEA losses in the overhead sweet gas. DEA systems do not require this scrubber because the vapor pressure of DEA is very low. Amine Circulation Rates The circulation rates for amine systems can be determined from the acid gas flow rates by selecting a solution concentration and an acid gas loading. The following equations can be used: where L M EA = MEA circulation rate, gpm LDEA — DEA circulation rate, gpm Q g = gas flow rate, MMscfd ME = total acid-gas fraction in inlet gas, moles acid gas/mole inlet gas c = amine weight fraction, Ib amine/lb solution p = solution density, Ib/gal at 60°F A L = acid-gas loading, mole acid-gas/mole amine For design, the following solution strengths and loadings are recom- mended to provide an effective system without an excess of corrosion; MEA: c = 20 wt. % A L = 0.33 mole acid gas/mole MEA DEA: c = 35 wt. % A L = 0.5 mole acid gas/mole DEA Acid Gas Treating 187 For the recommended concentrations the densities at 60°F are: 20% MEA= 8.41 Ib/gal = 0.028 mole MEA/gal 35 % DBA = 8.71 Ib/gal = 0.029 mole DEA/gal Using these design limits, Equations 7-24 and 7-25 can be simplified to: The circulation rate determined with these equations should be increased by 10-15% to supply an excess of amine. Flash Drum The rich arnine solution from the absorber is flashed to a separator to remove any hydrocarbons. A small percentage of acid gases will also flash when the pressure is reduced. The dissolved hydrocarbons should flash to the vapor phase and be removed. However, a small amount of hydrocarbon liquid may begin to collect in this separator. Therefore, a provision should be made to remove these liquid hydrocarbons. Typically the flash tanks are designed for 2 to 3 minutes of retention time for the amine solution while operating half full. Amine Reboiler The reboiler provides the heat input to an amine stripper, which revers- es the chemical reactions and drives off the acid gases. Amine reboilers may be either a kettle reboiler (see Chapter 3) or an indirect fired heater (see Chapter5). The heat duty of amine reboilers varies with the system design. The higher the reboiler duty, the higher the overhead condenser duty, the higher the reflux ratio, and thus the lower the number of trays required. The lower the reboiler duty, the lower the reflux ratio will be and the more trays the tower must have. Typically for a stripper with 20 trays, the reboiler duties will be as follows: ME A system—1,000 to 1,200 Btu/gal lean solution DBA system—900 to 1,000 Btu/gal lean solution 188 Design of GAS-HANDLING Systems and Facilities For design, reboiler temperatures in a stripper operating at 10 psig can be assumed to be 245°F for 20% MEA and 250°F for 35% DBA. Amirie Stripper Amine strippers use heat and steam to reverse the chemical reactions with CO 2 and H 2 S, The steam acts as a stripping gas to remove the CO 2 and H 2 S from the liquid solution and to carry these gases to the overhead. To promote mixing of the solution and the steam, the stripper is a trayed or packed tower with packing normally used for small diameter columns. The typical stripper consists of a tower operating at 10-20 psig with 20 trays, a reboiler, and an overhead condenser. The rich amine feed is intro- duced on the third or fourth tray from the top. The lean amine is removed at the bottom of the stripper and acid gases are removed from the top, Liquid flow rates are greatest near the bottom tray of the tower where the liquid from the bottom tray must provide the lean-amine flow rate from the tower plus enough water to provide the steam generated by the reboiler. The lean-amine circulation rate is known, and from the reboiler duty, pressure, and temperature, the amount of steam generated and thus the amount of water can be calculated. The vapor flow rate within the tower must be studied at both ends of the stripper. The higher of these vapor rates should be used to size the tower for vapor. At the bottom of the tower the vapor rate equals the amount of steam generated in the reboiler. Near the top of the tower the vapor rate equals the steam rate overhead plus the acid-gas rate. The steam overhead can be calculated from the steam generated in the reboil- er by subtracting the amount of steam condensed by raising the lean amine from its inlet temperature to the reboiler temperature and the amount of steam condensed by vaporizing the acid gases. For most field gas units it is not necessary to specify a stripper size. Vendors have standard design amine circulation packages for a given amine circulation rate, acid-gas loading, and reboiler. These concepts can be used in a preliminary check of the vendor's design. However, for detailed design and specification of large units, a process simulation computer model should be used. Overhead Condenser and Reflux Accumulator Amine-stripper overhead condensers are typically air-cooled, fin-fan exchangers. Their duty can be determined from the concepts in Chapter 3 as required to cool the overhead gases and condense the overhead steam Acid Gas Treating 189 to water. The inlet temperature to the cooler can be found using the par- tial pressure of the overhead steam to determine the temperature from steam tables. The cooler outlet temperature is typically 130 to 145°F depending on the ambient temperature. The reflux accumulator is a separator used to separate the acid gases from the condensed water. The water is accumulated and pumped back to the top of the stripper as reflux. With the vapor and liquid rates known, the accumulator can be sized using the procedures in Volume 1 for two- phase separators. Rich/lean Amine Exchanger Rich/lean amine exchangers are usually shell-and-tube exchangers with the corrosive rich amine flowing through the tubes. The purpose of these exchangers is to reduce the reboiler duty by recovering some of the sensible heat from the lean amine. The flow rates and inlet temperatures are typically known. Therefore, the outlet temperatures and duty can be determined by assuming an approach temperature for one outlet. The closer the approach temperature selected, the greater the duty and heat recovered, but the larger and more costly the exchanger. For design, an approach temperature of about 30°F provides an economic design balancing the cost of the rich/lean exchang- er and the reboiler. The reboiler duties recommended above assume a 30°F approach. Amine Cooler The amine cooler is typically an air-cooled, fin-fan cooler, which low- ers the lean amine temperature before it enters the absorber. The lean amine entering the absorber should be approximately 10°F warmer than the sour gas entering the absorber. Lower amine temperatures may cause the gas to cool in the absorber and thus condense hydrocarbon liquids. Higher temperatures would increase the amine vapor pressure and thus increase amine losses to the gas. The duty for the cooler can be calculat- ed from the lean-amine flow rate, the lean-amine temperature leaving the rich/lean exchanger and the sour-gas inlet temperature. Amine Solution Purification Due to side reactions and/or degradation, a variety of contaminants will begin to accumulate in an amine system. The method of removing these depends on the amine involved. 190 Design of GAS-HANDLING Systems and Facilities When MEA is used in the presence of COS and CS 2 , they react to form heat-stable salts. Therefore, MEA systems usually include a reclaimer, The reclaimer is a kettle-type reboiler operating on a small side stream of lean solution. The temperature in the reclaimer is maintained such that the water and MEA boil to the overhead and are piped back to the stripper, The heat-stable salts remain in the reclaimer until the reclaimer is full, Then the reclaimer is shut-in and dumped to a waste disposal. Thus, the impurities are removed but the MEA bonded to the salts is also lost. For DEA systems a reclaimer is not required because the reactions with COS and CS 2 are reversed in the stripper. The small amount of degradation products from COo can be removed by a carbon filter on a side stream of lean solution. Materials of Construction Amine systems are extremely corrosive due to the acid-gas concentra- tions and the high temperatures. It is important that all carbon steel exposed to the amine be stress-relieved after the completion of welding on the particular piece. A system fabricated from stress-relieved carbon steel for DEA solutions, as recommended, will not suffer excessive cor- rosion. For MEA systems, corrosion-resistant metals (304 SS) should be used in the following areas: 1. Absorber trays or packing 2. Stripper trays or packing 3. Rich/Lean amine exchanger tubes 4. Any part of the reboiler tube bundle that may be exposed to the vapor phase 5. Reclaimer tube bundle 6. Pressure-reduction valve and pipe leading to the flash tank 7. Pipe from the rich/lean exchange to the stripper inlet EXAMPLE PROBLEMS Example No. 7-1: Iron-Sponge Unit Acid Gas Treating 191 Problem: Design an Iron-Sponge Unit Solution: I. Minimum diameter for gas velocity: 2. Minimum diameter for deposition: 3. Minimum diameter to prevent channeling: Therefore, any diameter from 16.8 in. to 37.6 in. is acceptable. 4. Choose a cycle time for one month: Assume Fe = 9 Ib/Bu and rearrange: 192 Design of GAS-HANDLING Systems and Facilities ci H in. _ft_ 18 1.9.2 20 15,5 22 12.8 24 10.8 30 6.9 36 4.8 An acceptable choice is a 30-in. OD vessel. The wall thickness can be calculated from Chapter 12 and a value of bed height deter- mined. However, since t c and e are arbitrary, a 10-ft bed seems appropriate. 5. Calculate volume of iron sponge to purchase: Example No. 7-2: Specify Major Parameters for DEA Required: 1. Show that a DEA unit is an acceptable process selection. 2. Determine DEA circulation rate using 35 wt. % DEA and an acid- gas loading of 0.50 mole acid gas/mole DEA. 3. Determine preliminary diameter and height for DEA contact tower. 4. Calculate approximate reboiler duty with 250°F reboiler temperature. Acid Gas Treating 193 Solution; 1. Process selection Total acid gas inlet = 4.03 + 0.0019 = 4.032% Pacidin = 1.015 x (4.032/100) = 40.9 psia Total acid gas outlet = 2.0% Pa«d. out = 1,015 x (2.0/100) = 20.3 psia From Figure 7-12 for removing CO 2 and H 2 S, possible processes are amines, Sulfinol, or carbonates. The most common selection for this application is a DEA unit. 2. DEA circulation rate Note: In order to meet the H 2 S outlet, virtually all the CO 2 must be removed, as DEA is not selective for H 2 S. 3. Tower size From Volume 1: 194 Design of GAS-HANDLING Systems and Facilities Use 72-in. ID tower w/24 trays, 4. Determine reboiler duty: Using 1,000 Btu/gal lean solution q=l,000(508)(60 min/hr) q = 30.5 MMBtu/hr CHAPTER 8 Gas Dehydration * Gas dehydration is the process of removing water vapor from a gas stream to lower the temperature at which water will condense from the stream. This temperature is called the "dew point" of the gas. Most gas sales contracts specify a maximum value for the amount of water vapor allowable in the gas. Typical values are 7 Ib/MMscf in the Southern U.S., 4 Ib/MMscf in the Northern U.S. and 2 to 4 Ib/MMscf in Canada. These values correspond to dew points of approximately 32°F for 7 lb/ MMscf, 20°F for 4 lb MMscf, and 0°F for 2 Ib/MMscf in a 1,000 psi gas line. Dehydration to dew points below the temperature to which the gas will be subjected will prevent hydrate formation and corrosion from con- densed water. The latter consideration is especially important in gas streams containing CO 2 or H 2 S where the acid gas components will form an acid with the condensed water. The capacity of a gas stream for holding water vapor is reduced as the stream is compressed or cooled. Thus, water can be removed from the gas stream by compressing or cooling the stream. However, the gas stream is still saturated with water so that further reduction in tempera- ture or increase in pressure can result in water condensation. This chapter discusses the design of liquid glycol and solid bed dehy- dration systems that are the most common methods of dehydration used *Reviewed for the 1999 edition by Lindsey S. Stinson of Paragon Engineering Services, Inc. 195 [...]... natural gas with a molecular weight of 26 that is in equilibrium with a 3% brine at a pressure of 3,000 psia and a temperature of 15 0°R From Figure 8 -1 at a temperature of 15 0°F and pressure of 3,000 psia there is 10 4 Ib of water per MMscf of wet gas The correction for salinity is 0 .93 and for molecular weight is 0 .98 Therefore, the total water content is 10 4 x 0 .93 x 0 .98 = 94 .8 Ib/MMscf A correction for... circulation rate of 6.2 gaVlb at 99 .95 %, 8.2 gal/lb at 99 .5% or in excess of 12 gal/lb at 99 % is required The lean glycol concentration is determined by the temperature of the reboiler the gas stripping rate, and the pressure of the reboiler Glycol concentrations between 98 and 99 % are common for most field gas units Glycol Reboiler Temperature The reboiler temperature controls the concentration of the water... 8 -11 Reboiler temperatures for triethylene glycol are limited to 400°F, which limits the maximum lean glycol concentration without stripping gas It is good practice to limit reboiler temperatures to between 370°F and 390 °F to minimize degradation of the glycol This effectively limits the lean glycol concentration to between 98 .5% and 98 .9%  210 Design of GAS-HANDLING Systems and Facilities Figure 8 -11 ... solids and a charcoal filter to absorb small amounts of hydrocarbons that may build up in the circulating glycol Sock filters are normally designed for the removal of 5-micron solids On units larger than 10 gpm 202 Design of GAS-HAN DUNG Systems and Facilities Figure 8-5 The bottom of the contactor is often used as a vertical inlet scrubber it is common to route only a sidestream of 10 to 50% of total... contactor inlet stream and injected into the reboiler The "leaness" of the gas depends on the purity of the wet glycol and the number of stages below the reconcentrator The stripping gas is saturated with water at the inlet temperature and pressure conditions, but adsorbs water at the reboiler conditions of atmospheric pres- 204 Design of GAS-HANDLING Systems and Facilities sure and high temperatures... concentration of the lean glycol the greater the dewpoint depression for a given glycol circulation rate and number of trays, Figure 8 -9 shows the equilibrium water dew point at different temperatures for gases in contact with various concentrations of glycoJ At 10 0°F contact temperature there is an equilibrium water dew point of 25°F for 98 % glycol and 10 °F for 99 % glycol Actual dew points of gas leaving... out of the glycol by the addition of heat This step is called "regeneration" or "reconcentration" and enables the glycol to be recovered for reuse in absorbing additional water with minimal loss of glycol Gas Dehydration 19 7 Water Content of Hydrocarbon Gas Figure 8 -1 McKetta-Wehe pressure-temperature correlation (From Gas Processors Suppliers Association, Engineering Data Book, 10 th Edition.) 19 8 Design. .. to tray it becomes richer and richer in water As the gas rises it becomes leaner and leaner in water vapor Glycol contactors will typically have between 6 and 12 trays, depending upon the water dew point required To obtain a 7 Ib/MMscf specification, 6 to 8 trays are common 200 Design of GAS-HANDLING Systems and Facilities Figure 8-4 Typical glycol contactor in which gas and liquid are in counter-current... the circulation rate 208 Design of GAS-HANDLING Systems and Facilities tor may occur when the lean glycol temperature gets too hot On the other hand, the lean glycol temperature should be kept slightly above the contactor gas temperature to prevent hydrocarbon condensation in the contactor and subsequent foaming of the glycol Most designs call for a lean glycol temperature 10 °F hotter than the gas... expansion of the glycol and to allow for reasonable time between additions of glycol A well designed and operated unit will have glycol losses to the dry gas from the contactor and the water vapor from the still of between 0. 01 and 0,05 gal/MMscf of gas processed The lean glycol from the atmospheric surge tank is then pumped back to the contactor to complete the cycle Depending upon the pump design, . system. The method of removing these depends on the amine involved. 19 0 Design of GAS-HANDLING Systems and Facilities When MEA is used in the presence of COS and CS 2 , they. Processors Suppliers Association, Engineering Data Book, 10 th Edition .) 19 8 Design of GAS-HANDLING Systems and Facilities Figure 8*2. Effective water content of CO 2 . (From Gas Processors Suppliers Association, . _ft_ 18 1. 9. 2 20 15 ,5 22 12 .8 24 10 .8 30 6 .9 36 4.8 An acceptable choice is a 30-in. OD vessel. The wall thickness can be calculated from Chapter 12 and a value of bed height