Difference between revisions of "MDEA Sweetening"

Revision as of 14:21, 20 June 2018

Introduction

Sour gas from inlet separation and filtration is now divided into two equal flows for the two MDEA contactors. Flow to the contactors is controlled by individual flow control valves (FCVs). Gas enters the bottom of the contactor and flows upwards through 28 stainless steel trays coming into contact with lean amine (MDEA) flowing downwards. The lean MDEA absorbs the H2S and C02 from the gas flowing upwards through contactor through physical and chemical attraction. The MDEA leaves the contactor as rich amine to the regeneration section and the gas leaves sweet (no H2S or C02) to the TEG dehydration section. Lean amine is supplied by high pressure charge pumps described in module 8. Figure 1 block flow diagram highlights the amine location of the amine sweetening processes described in this module.

Raw Gas to 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. Refer to Amine Contactor A/B(PV-1101A/B) for more information.

Gas Cooling

After passing through the the hot amine and doing chemical reaction the gas is hot and needs to be cooled. The gas from the contactors passes through an air cooler. Refer to Sweet Gas Cooler(HT-1101).

Sweet Gas Scrubbing

The cooled gas from the sweet gas cooler flows into the sweet gas scrubber. Refer to Wet Sweet Gas Scrubber(PV-1105) for more information.

Sales Gas Analysis

H2S Analysis

On the 16” outlet line of the scrubber there is a take-off for 2” sample line where a gas sample is taken for a H2S sample. The H2S analyzer is also known as the 933 analyzer (the analyzer is an Ametek 933 model). The analyzer continuously samples sales gas to determine the H2S content and relays the results to DCS:

• for onscreen display
• to trigger alarms:

-a high-high alarm is triggered onscreen at 12 ppm H2S to alert the CRO

The analyzer is in a cabinet that has no external readouts for the Area Operator. The analyzer has local on/off switches.

The analyzer uses two gases for analyzer calibration:

• the zero gas cylinder holds pure Nitrogen . Every 12 hours, the analyzer automatically performs a zeroing calibration. The analyzer stops sampling sales gas and instead draws in a sample of the zero gas to confirm 0 ppm H2S is measured. The analyzer resets its measuring sensitivity if the zero gas analysis does not indicate 0 ppm H2S.
• the span gas cylinder holds a nitrogen/25 ppm H2S mixture. FNNPF instrumentation personal perform manual analyzer calibration tests with the span gas to confirm that the analyzer reads 25 ppm H2S.

The CRO will also have the ability to perform an auto zero from the DCS if they feel that the analyzer reading is not correct.

CO2 Analysis

On the same draw-off line as the H2S another sample is present where a continuous sample of sales gas is automatically withdrawn for use by the Sales Gas CO2 Analyzer. The withdrawn sample is directed through a pressure regulator that drops the gas from train pressure to the pressure required for sample analysis. Sales gas CO2 content is continuously determined to ensure the train’s MDEA-based amine sweetening operations (in the Contactors) have removed sufficient CO2 i.e., the sales gas remains within design specifications (less than 6%)]. The analyzer measures the CO2 content and relays the reading to DCS for onscreen display.