Amine Contactor A/B(PV-1101A/B)

Raw Gas Flow to Amine Contactors

After the raw gas flow leaves the Inlet Gas Filter (FI-1105), the raw gas flow branches to feed the two, parallel Amine (PV-1101A/B) contactors (see Figure 2). The equipment numbering described below is for contactor A but contactor B is exactly the same.

The flow to the Amine contactor (PV-1101A) passes through a meter run that monitors and controls the raw gas flow to each Contactor. The meter run consists of the following equipment to monitor and control the raw gas flow to each contactor.

• Ultrasonic flow transmitter: this flow transmitter calculates the basic raw gas flow rate from the gas flow.
• Temperature transmitter (TI-1023) and a pressure transmitter (PI-1023): these pressure/temperature transmitters provide data which allows the basic flowrate to be converted to the compensated raw gas flow rate.
• Raw Gas Flow Control Valve: The feed gas flow rate for each amine contactor is controlled by a pre set flow control valve. The flow control valve (FV-1021) maintains the calculated set point that is used from the Ultrasonic flow transmitter and respective temperature and pressure transmitters.

Both compensated raw gas flow rates are adjusted based on the inlet raw gas flow transmitter (FT-8006) located in the inlet/outlet building. The Raw gas flow control valves for each contactor will automatically adjust their position to equally share the inlet raw gas flow measured at FT-8006.

All metered compensated raw gas flow rates are displayed via the DCS in the control room. A back pressure control valve (PV-8025) provided on the dry sweet gas line off the TEG unit in the plant Inlet/Outlet building, will maintain the required pressure in the amine and TEG contactors high pressure gas systems. Without this back pressure control the gas treating sections of the plant will not operate as designed.

Purpose and Operation

The amine contactor (figure 4) has 28 two pass valve trays throughout the tower evenly spaced at 2 feet apart to remove acid gas from the raw gas inlet stream. Lean amine enters from the discharge of the amine high pressure pump normally at tray #1 but it can also be introduced into tray #5 and #9 respectively through the manual operation of isolation valves on each inlet leg. Normal operation is to have one tray receive the full amine flow at any given time.

At the same time as the lean amine enters the top of the contactor to flow downwards through the trays, raw gas enters the bottom of the contactor and rises up through each tray via tray valves. The rising gas passing through the valves bubbles through the amine flowing crosswise on each tray. This intimate mixing and high pressure condition causes the MDEA to absorb the H2S and C02 from the raw gas. The gas then leaves the top of the contactor sweet but wet.

Each 2 pass valve tray has weirs and downcomers. The amine flowing downwards in the downcomer is the first pass and the second pass is when the amine flows along the top of the tray to the next downcomer(see Figure 3).

• weirs maintain the level of amine liquid on the tray
• downcomers direct amine flowing over the weir down to the tray below:

-odd-number trays have 2 downcomers; one at each end

-even-number trays have a centrally mounted downcomer

• down flowing amine liquid in the downcomer is a seal to prevent gas from bypassing the tray valves through the downcomer

During amine-sour gas contact at each tray, amine absorbs and reacts with acid gases to form water-soluble salt compounds. Heat is released during this exothermic reaction, and, as a result, rich amine leaving the contactor is 20-30oF warmer than lean amine entering the contactor. Lean amine proportionately removes more H2S than CO2 because amine absorption of H2S is much faster than the absorption of CO2. The amine becomes richer as it flows downward in the Amine Contactors, while the gas becomes sweeter as it rises through the valve trays. Rising gas also becomes saturated with water vapour when in contact with the water-based amine solution on the contactor trays. Sweetened, wet gas leaves the top of the contactor through a demister pad that coalesces entrained amine droplets. The gas then flows to the Sweet Gas Cooler(HT-1101).

Note:

TCPL sales gas specifications stipulate the H2S and CO2 content:

• H2S must be almost fully removed (i.e., to a maximum 16 ppm trace concentration) because of the severe toxicity hazard that H2S would pose to the end users of the sales gas
• CO2 does not present the same toxicity hazard (as H2S) and is therefore tolerated in higher concentrations in the sales gas. CO2 can be present to a maximum 2% concentration. In higher concentrations, the inert CO2 is a diluent that reduces the combustion ability of the sales gas.

MDEA’s ability to limit the amount of CO2 it absorbs (i.e., leave more CO2 in the sales gas) is known as CO2 slip. For the FNNPF, CO2 slip has the following benefits:

• CO2 slip is an effective business practice, i.e., removing more CO2 than is required by the sales gas specifications needlessly results in a lower volume of sales gas

The amine (now termed rich) descends from the bottom tray into the enlarged bottom storage section of the contactor. This bottom section is filled with packing rings. The packing rings (Pall) provide a large surface area and agitation that removes some of the methane absorbed by the Amine at the trays. Rich Amine leaves the contactor through, the (LV-1028) level control valve, through the contactor ESD valve (XV-1030) which prevents the contactor from bottoming out, and then flows to the Amine Flash Drum.

Sweet gas leaves the top of the contactor through a demister which removes any entrained amine in the gas. It then travels to the sweet gas cooler through an open or closed control valve actuated locally or through the DCS. This valve is open to allow gas to flow to the sweet gas cooler or closed with the gas flowing through ESD control valve (XV-1031) which allows off-spec to be flared or during start-up/shutdown activities.

Anti-foam is added to the inlet of the contactor from the amine antifoam injection package. Anti-foam knocks down excessive bubbles and foaming in the contactor trays by lowering the amine’s surface tension. Foaming will create a high pressure differential pressure across the contactor which may lead to damaged or collapsed trays. Inlet hydrocarbon carryover in the raw inlet gas and dirty solution are the main causes of foaming.

Process Controls, Instrumentation and Shutdowns

Referring to figure 4, the Amine contactors (PV-1101A/B) are equipped with the following level controls, instrumentation, and shutdowns:

• level control valve (LV-1028) the level control valve is modulated automatically via DCS based level control to maintain a setpoint level in the contactor.

Note: In addition to maintaining a rich amine level setpoint in the contactor, the level control valve is the location of an important pressure drop. Across the level control valve, the rich amine drops from contactor pressure (between 1000 and 1070 psig) down to approximately 100 psi. Reducing the rich amine pressure is a prerequisite condition for achieving the low pressure required for downstream amine regeneration in the amine regenerator.

• local level indication—the contactor’s rich Amine level can be locally read from the following:
  • A gauge glass (LG-1028) on the level column equipped with a level transmitter (LIT- 1028).
  • A local digital readout from the level transmitter (LIT1022) (this transmitter reads directly off the vessel i.e., it is not mounted on the level column).
• DCS-based level shutdowns on the amine contactors (PV-1101A/B):
  • DCS based low level shutdown is triggered if the level falls to 20%; the contactor’s rich amine outlet ESD valve closes to prevent bottoming out and the potential of high pressure gas flowing into the low pressure sections of the plant.
  • a DCS based High level shutdown is triggered if the level rises to 100%; the gas inlet flow control valve closes

The amine contactor is equipped with a differential pressure transmitter (PDI-1051A/B) that relays the gas pressure drop across the trays in the contactor (i.e., below the bottom contactor tray and above the top contactor tray) to the control room. Increasing differential pressure is an indication that the contactor is foaming (foaming is described in section 7 of this module). Normal Differential pressure should be 5psi.

The amine Contactor (PV-1101A) is equipped with the following pressure instrumentation: the pressure transmitter (PIT-1027) at the raw gas inlet meter run (as described earlier in this module) that relays the pressure to the Control Room for onscreen display

• local pressure indication on the bottom of the contactor in the amine surge area (PI-1025)
• pressure Transmitter (PT-1027A) on the Top of the contactor above the top tray
• pressure Transmitter (PT-1027B)
• on the bottom of the contactor below the bottom tray a pressure differential transmitter (PDI-1027) takes the pressure differential from (PT-1027A) and (PT-1027B)

The Amine Contactor (PV-1101A): contactor is equipped with temperature elements that relay the temperature to the Control Room for onscreen display. The temperature elements are located as follows:

• at the raw gas surge area of the contactor (TI-1025) (as described in Section 8.1)
• at the sweet gas outlet (TIT-1034)
• at the rich MDEA outlet (TIT-1029)
• at Contactor tray #17 (TI-1026D)
• at Contactor tray #19 (TI-1026C)
• at Contactor tray #24 (TI-1026B)
• at Contactor tray #27(TI-1026A)

The amine contactor (PV-1101A): is equipped with lean amine flow controls. The feed of high pressure lean amine is flow controlled by a controller (FV-1021). The Flow Control Valve controls the flow of amine from the constant speed amine charge pump ensuring that the right amount of amine is fed to the contactors. Amine flow is based on the amount of gas being sold and the bottom temperature of the rich solution leaving the contactor. The rich amine leaving the contactor should be around 1650F – 1700F for efficient operation. Too much amine feed can cause over stripping of the gas noticed by a low rich amine outlet temperature. Not enough flow results in a high amine outlet temperature that can cause non spec Gas to enter TCPL because the gas is being under-stripped.

Note:

For the amine contactor (PV-1101A): a low level shutdown is triggered on the contactor level ESD (XV-1030) if the lean amine flow falls to 390 USGPM; the charge pump shuts down. Refer to module 8—Lean Amine Feed.

Additional Specialty Lines and Valves

The Amine Contactor (PV-1101A): is equipped with the following:

• Rich Amine outlet line ESD valve (XV-1030) This ESD valve is triggered to close automatically in response to a contactor low level, a amine flash drum tank high level, a sweet gas scrubber high level and when a train or plant ESD occurs. Because the up-stream level control valve (LV-1028) may pass, closing the ESD valve ensures a positive rich amine shutoff; the shutoff prevents rich Amine draining from the contactor. If amine were to pass through the level control valve and fully drain from the contactor, high pressure gas will overpressure the piping/equipment downstream of the ESD. The ESD valve can be remotely reopened by the Control Room Operator; using the onscreen reset controls enables the valve’s reopening.
• A pressure relief valve (PRV-1019) on the sweet gas inlet, set at 1300 psi, relieves to the HP flare system.
• A 65# steam purge line is used for startup/shutdown steam purging.
• A top vent valve for use during startup/shutdown steam purging.
• Level column drain valving. Both of the contactor’s level columns have manual bottom drain valves that tie into the amine drain Vessel.
• Manual drain valves to the amine drain Vessel from the base of the contactor.

Operations Monitoring

During monitoring rounds at the Amine Contactor the Area Operator:

• visually checks that the contactor pressure is approximately 1000 to 1070 psig
• ensures that the rich amine is draining correctly and checks the bottom level glass to confirm that the rich amine level is being held at setpoint
• when necessary, blows down the level glass and level columns to ensure they remain clear of debris (Always inform the CRO before blowing down levels and columns)