Plants using Monoethanol Amine (MEA) are used throughout the industry to remove CO2 and H2S from Natural Gas Streams. The use of Deethanol Amine (DEA) and other amine based chemicals is increasing in an effort to reduce energy consumption and operating costs. Plants using MEA or DEA used basically the same process.
Amine Gas Sweetening Units / Natural Gas Sweetening Process
In the typical Amine plant, the sour gas enters the bottom of the tower and flows upward through contactor trays. The amine solution enters the top of the tower and flows downward over the trays extracting the H2S and CO2. Sweet gas leaves the top of the tower. The rich amine, saturated with acid gas, leaves the bottom of the contactor, passes through amine/amine heat exchanger and enters the top of the still column. The rich amine flows down through the trays while steam generated by the reboiler bubbles up from the bottom. The steam strips the acid gas from the amine. The steam and acid gas exit the top of the still column and enter the reflux condenser where the steam is condensed. The condensed steam or water goes to the surge tank and back into the system while the acid gas is now sent to sulfur recovery units instead of burned or flared in the past.
The amine solution, stripped of the acid gas, leaves the still column and enters the reboiler. The reboiler heats the amine solution to produce the stripping steam. A small amount of the amine is taken from the reboiler and purified in the reclaimer. The lean amine from the reboiler is pumped back through the amine/amine heat exchanger to the top of the contactor to repeat the cycle.
Energy Efficiency Optimization
The major energy consumer in a typical amine unit is the reboiler. As mentioned, the purpose of the reboiler is to heat the amine solution in order to produce steam. The steam passes up through the still column stripping the H2S and CO2 from the rich amine. The energy consumed by the reboiler can be reduced by either reducing the amount of amine solution the reboiler must heat or improving its thermal efficiency.
Most MEA designs specify a reboiler temperature of 240ºF. This temperature is below the boiling point of MEA which is 339ºF and above the boiling point of water. This will allow the reboiler to produce steam to strip the rich amine while not boiling off or thermally degrading the MEA.
Amine Circulation Rate
The amine circulation rate; gas volume; amine concentration; concentration of the contaminants along with other parameters, are considered during the design of the plant. Various charts, calculation methods, and analytical procedures have been developed to estimate the optimum circulation rate. However, the most effective method is to reduce the circulation while monitoring the amount of H2S and CO2 in the sales gas.
Determine the maximum acceptable percent of contaminants the purchaser will accept and treat to that limit.
Varying the circulation rate at existing plants may be a problem. Most use constant speed electric motors to drive multiple cylinders positive displacement pumps. Varying the speed of an electric motor is an expensive proposition and at current electrical costs an acceptable payout is unlikely.
Installing a bypass line around the pump may be the best way of controlling the circulation.
To further reduce the circulation rate the amine concentration can be increased. For MEA systems the amine solution is usually 10 – 20% MEA and the balance water. For DEA systems the concentration increases to 20 – 30%. The only reason for diluting the amine is to reduce the corrosion problem. The trend has been to operate at the lower percentage to minimize corrosion. Reducing the amine concentration reduces the ability of the solution to remove the contaminants and therefore increases the circulation rate. If an effective control program is initiated there is no reason to run less than 15% MEA or 25% DEA.
- Measure energy consumption of each individual component
- Optimize energy consumption of each component.
- Major energy consumer is the reboiler
- Improve thermal efficiency of reboiler
- Optimize amine circulation rate.
- Increase amine percentage to at least 15% for MEA or 25% for DEA.
Typical Amine Process