Hydrocarbon Compression

Amine systems are extremely corrosive due to the acid-gas concentrations 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 corrosion. 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

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.

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 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 CS2 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.

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.

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 exchanger and the reboiler. The reboiler duties recommended above assume a 30°F approach.

Amine-stripper overhead condensers are typically air-cooled, fin-fan exchangers. 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 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 for two phase separators.

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. 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 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.

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.

The reboiler provides the heat input to an amine stripper, which reverses the chemical reactions and drives off the acid gases. Amine reboilers may be either a kettle reboiler or an indirect fired heater.

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

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.

The rich amine 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.

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:

For design, the following solution strengths and loadings are recommended to provide an effective system without an excess of corrosion:

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.

Amine absorbers use counter-current flow through a trayed or packed tower to provide intimate mixing between the amine solution and the sour gas. Typically, small diameter towers use stainless steel packing, while larger towers use stainless steel trays. For systems using the recommended solution concentrations and loadings, a tower with 20 to 24 actual trays is normal. Variations in solution concentrations and loadings may require further investigation to determine the number of trays.

In a trayed absorber the amine falls from one tray to the one below in the same manner as the liquid in a condensate stabilizer. It flows across the tray and over a weir before flowing into the next downcomer. The gas bubbles up through the liquid and creates a froth that must be separated from the gas before it reaches the underside of the next tray. For preliminary design, a tray spacing of 24 in. and a minimum diameter capable of separating 150 to 200 micron droplets can be assumed. The size of packed towers must be obtained from manufacturer’s published literature.

Commonly, amine absorbers include an integral gas scrubber section in the bottom of the tower. This scrubber would be the same diameter as required for the tower. The gas entering the tower would have to pass through a mist eliminator and then a chimney tray. The purpose of this scrubber is to remove entrained water and hydrocarbon liquids from the gas to protect the amine solution from contamination.

Alternately, a separate scrubber vessel can be provided so that the tower height can be decreased.

For ME A systems with a large gas flow rate, a scrubber should be considered 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.