Lecture 8 Natural Gas Sweetening Processes 2 2

Gas Sweetening Processes By Amines

Figure 15 Schematic of amine gas-sweetening process flow diagram.

4.4.3.3 Design Considerations This lecture describes some design aspects for the major equipment used in both MEA and DEA systems.

4.4.3.3.1 Amine Absorber Amine absorbers use countercurrent flow through a trayed or packed tower to provide intimate contact between the amine solvent and the sour gas so that the H2S and CO2 molecules can transfer from the gas phase to the solvent liquid phase. In tray columns, a liquid level is maintained on each tray by a weir usually 2 or 3 inches high. The gas passes up from underneath the trays through openings in the trays such as perforations, bubble caps, or valves and disperses into bubbles through the liquid, forming a froth. The gas disengages from the froth, travels through a vapor space, providing time for entrained amine solution to fall back down to the liquid on the tray, and passes through the next tray above.

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Schematic of Bubble Cap Trays

Schematic of Valve Trays

In packed columns the liquid solvent is dispersed in the gas stream by forming a film over the packing, providing a large surface area for CO2 and H2S transfer from the gas to the liquid solvent.

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Schematic of an Absorption/Desorption Cycle using Packed Column As Absorber

The degree of sweetening achieved is largely dependent on the number of trays or the height of packing available in the absorber. Twenty valve-type trays (spaced 24 inches apart) or the equivalent height in packing column are common and are often a standard design. Typically, small-diameter towers use packing, whereas larger towers use stainless steel trays. The cross-sectional area of the contactor (absorber) is sized for the gas and amine flow rates, where the maximum gas superficial velocity is obtained from the Souders and Brown (1932) equation as:

VSG = 0.25 [(ρ [( amine−ρgas)/ρgas]0.5 , ft/sec

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It may be necessary to reduce the gas velocity by 25 to 35% to avoid jet flooding and by 15% to allow for foaming. Use an amine velocity of 0.25 ft/sec in the downcomer.

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In most cases a mist eliminator pad is installed near the gas outlet of the absorber (the distance between the top tay and the mist pad is 3 to 4 feet) to trap entrained solvent, and an outlet knockout drum, similar to the inlet separator for the gas feed, is provided to collect solvent carryover. Some contactors have a water wash consisting of two to five trays at the top of the absorber to minimize vaporization losses of amine, which is often found in low-pressure MEA (monoethanolamine) systems. Absorbers will usually have multiple feed points, allowing the option of introducing the lean amine lower in the column or at multiple trays. If carbon dioxide absorption is desired, all of the lean amine should, in general, be fed on the top tray, thus utilizing all available stages.

4.4.3.3.2 Amine Pumps There are several different amine pumps in each of the processes. The amine booster and reflux pumps are centrifugal, preferably in-line or horizontal. Selection of the circulation pump depends on the contactor pressure and the amine circulation rate. Normally, reciprocating pumps are preferred only if the absorber pressure is very high. However, centrifugal pumps are used for low pressures (e.g., 100 psig) and multistage horizontal centrifugal pumps for high pressures (e.g., 700 psig) or high circulation rates (e.g., 300 gal/min). In sizing and rating pumps use a low positive suction pressure of 3 to 10 psig. The circulation flow rates for amine systems can be determined from the acid gas flow rates by selecting a solution concentration and an acid gas loading. For this purpose, the following equation can be used:

Q = [k(QG)(MF)] / [ρ(WF)(AG)]

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where Q is circulation rate for amine systems, gal/min; K is constant (112 for MEA system and 192 for DEA system); QG is gas flow rate, MMscfd; MF is total acid–gas fraction in inlet gas, moles acid gas/mole inlet gas; WF is amine weight fraction, lb amine/lb solution; ρ is solution density, lb/gal at 60◦F; and

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AG is acid gas loading, mole acid gas/mole amine. The rich solution acid gas loading depends on the acid gas partial pressure and corrosiveness of solution. The normal range of this parameter is 0.45 to 0.52 lbmole acid-gas/lbmole amine for MEA and 0.43 to 0.73 lbmole acid-gas/lbmole amine for DEA systems. For design, the following solution strengths and loading are recommended to provide an effective system without an excess of corrosion: MEA system: WF = 20 wt.% AG = 0.33 mole acid gas/mole MEA DEA system: WF = 35 wt.% AG = 0.5 mole acid gas/mole DEA For the recommended concentrations, the densities at 60◦F are 20% MEA = 8.41 lb/gal = 0.028 mole MEA/gal 35% DEA = 8.71 lb/gal = 0.029 mole DEA/gal Using these design limits, Equation (4-15) can be simplified to the following equation (Arnold and Stewart, 1999):

Q = k, (QG)(MF)

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where k, is constant (201 for MEA system and 126 for DEA system). The circulation rate determined with these equations should be increased by 10–15% to supply an excess of amine. Since the reboiler duty is almost always tied directly to the circulation rate, lower circulation rates reduce the overall energy requirements. Lower circulation rates also tend to increase the CO2 slip and can improve the quality of the feed to the sulfur recovery unit.

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4.4.3.3.3 Flash Tank The rich amine solution from the absorber enters a flah tank, allowing the lightest of the hydrocarbons to flash. A small percentage of acid gases will also flash when the pressure is reduced. The heavier hydrocarbons remain as a liquid, but separate from the aqueous amine, forming a separate liquid layer. Because the hydrocarbons have a lower density than the aqueous amine, they form the upper liquid layer and can be skimmed off the top. 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.

4.4.3.3.4 Amine Reboiler The amine reboiler provides the heat input to an amine stripper, which reverses the chemical reactions and drives off the acid gases. Two different reboiler designs are used in amine plants: thermosiphon and kettle. Thermosiphon reboilers return the heated amine solution and steam to the regenerator tower by the same pipe.

Schematic of a Thermosyphon Reboiler

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Kettle reboilers return the heated amine solution and steam to the regenerator tower in different pipes.

Schematic of a Kettle Reboiler The reboiler heat duty includes: (1) sensible heat required to raise the temperatures of the rich amine feed, the reflux, and the makeup water to the temperature of the reboiler, (2) heat of reaction to break chemical bonds between the acid gas molecules and the amine, (3) heat of vaporization of water to produce a stripping vapor of steam. The heat duty and transfer area of the amine reboiler can be determined as follows (Jones and Perry, 1973):

HR = 432000 × Q A = 11.30 × Q

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where Q is amine circulation flow rate, gal/min; HR is heat duty of amine reboiler, Btu/min; and A is heat transfer area of reboiler, ft2. The reboiler duty should be maintained as low as possible, but must be adequate to regenerate the amine solution sufficiently to meet the sweet gas requirements and to ensure that the CO2 loadings in the reboiler do not cause excessive corrosion. Higher reboiler duties do not reduce circulation rates to any degree and just consume energy.

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Reboiler temperature is dependent on solution concentration, flare/vent line back pressure, and/or residual CO2 content required. It is good practice to operate the reboiler at as low a temperature as possible. The normal operating range for reboiler temperature is 225 to 260◦F for MEA and 230 to 250◦F for DEA systems.

4.4.3.3.5 Amine Stripper Amine strippers use heat and steam to reverse the chemical reactions with CO2 and H2S. The steam acts as a stripping gas to remove the CO2 and H2S from the liquid solution and to carry these gases to the overhead. Like the absorber, the stripper is either a tray or a packed column with approximately 20 trays or the equivalent height in packing. To minimize amine vaporization loss, there may be a water wash section at the top of the column with an additional four to six trays. The rich amine feed is introduced 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 reboiler 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.

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4.4.3.3.6 Amine Condensers Amine-stripper condensers are typically overhead air-cooled and fin-fan exchangers.

Fin-Fan Condenser (Top picture: Tubes, Bottom picture: Tubes assembled)

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Air-Cooled Condenser (picture from ground floor)

Air-Cooled Condenser (picture from platform)

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The amine reflux condenser is required to cool the overhead gases and condense the overhead steam to water. The inlet temperature to the cooler can be found using the partial 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 heat duty and transfer area of an amine reflux condenser can be determined as follows (Jones and Perry, 1973):

HC = 18 × 105 × Q A = 5.20 × Q

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where HC is heat duty of amine reflux condenser, Btu/min; and Q is amine circulation flow rate, gal/min. The reflux accumulator is a two-phase 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.

4.4.3.3.7 Lean/Rich Amine Exchanger The lean/rich amine heat exchanger: preheats the rich amine solution and reduces the duty of the reboiler. It also cools the lean amine and reduces the duty of the aerial cooler. Both shell and tube and plate and frame exchangers are used. The rich solution is passed through the tubes, which are usually made of stainless steel, with a low inlet velocity (2 to 3.5 ft/sec) to minimize corrosion. Typically (∆T) temperature change for both streams is 70/20 to 100/37.5 ◦F/C and (∆P) pressure drops 2 to 5 psi. The heat duty and transfer area of a shell and tube type amine exchanger can be determined as follows (Jones and Perry, 1973):

HE = 27 × 105 × Q A = 11.25 × Q

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where

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is HE is heat duty of amine heat exchanger, Btu/min; A is Heat transfer area, ft2; and Q is amine circulation flow rate, gal/min.

4.4.3.3.8 Amine Cooler The amine cooler is typically an air-cooled, fin-fan cooler, which lowers 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 calculated from the lean amine flow rate, the lean amine temperature leaving the rich/lean exchanger and the sour gas inlet temperature.

4.4.3.3..9 Amine Reclaimer Due to side reactions and/or degradation, a variety of contaminants will begin to accumulate in an amine system. The method of removing these using an amine reclaimer depends on the amine involved. When MEA is used in the presence of COS and CS2, 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 amine 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. Diethanolamine (DEA) has a higher boiling temperature than monoethanolamines, requiring other methods of reclaiming such as vacuum distillation in order to prevent thermal degradation of the amine. Moreover, DEA diethanolamine has a slow degradation rate. Consequently, in most cases it is not practical, economical, or necessary to reclaim DEA solutions. Solution purification is maintained by mechanical and carbon filtration and by caustic or soda ash addition to the system to neutralize the heat-stable amine salts.

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Moisture may be removed from hydrocarbon gases at the same time as hydrogen sulfide is removed. Moisture removal is necessary to prevent harm to anhydrous catalysts and to prevent the formation of hydrocarbon hydrates (e.g., C3H8 18H2O) at low temperatures. A widely used dehydration and desulfhurization process is the glycol/amine process, in which the treatment solution is a mixture of ethanolamine and a large amount of glycol. The mixture is circulated through an absorber and a reactivator in the same way as ethanolamine is circulated in the Girbotol process. The glycol absorbs moisture from the hydrocarbon gas passing up the absorber; the ethanolamine absorbs hydrogen sulfide and carbon dioxide. The treated gas leaves the top of the absorber; the spent ethanolamine glycol mixture enters the reactivator tower, where heat drives off the absorbed acid gases and water.

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