Reduction Of Glycol Loss From Gas Dehydration Unit At Offshore Platform in Bombay Offshore - A Case Study.
Glycol dehydration is most common liquid dessicant for removing water from natural gas. Under normal operating conditions, make up glycol rate is about 16 litres/MMSCM (0.12 gal/MMSCF) of gas treated. In the present case make-up glycol consumption exceeded four times normal value at gas dehydration unit (GDU) at one of the offshore platform in Bombay High.
Chemical analysis of circulated glycol was carried out. The plant operating parameters were simulated. Sensitivity analysis of important process parameters was done to evaluate their effect on plant performance and operating parameters with a view to reduce glycol loss while achieving dehydration efficiency.
Chemical analysis of glycol has shown presence of contaminants. The analysis of operating parameters have shown glycol foaming tendency, higher pressure in glycol reboiler and high stripping gas rate. Based upon analysis and simulation studies, troubleshooting was carried out which has resulted in significant saving in makeup glycol consumption.
ACG is one of the process platform in Bombay High which receives wet gas from oil process platform for compression, dehydration and export. It is processing about 4 MMSCMD of gas. The inlet gas is compressed from 7 kg/cm2g to 90 kg/cm2g in three stage compressors. The compressed gas is dehydrated using Triethylene Glycol (TEG) in two identical parallel trains of gas dehydration units (GDU) to meet pipeline transportation specification with maximum of 112 kg/MMSCM (7lb/MMSCF) water vapor in gas. This gas is transported to land base through the trunkline.
The process schematic of the gas dehydration unit (GDU) is given in Fig. 1. It consists of a contactor tower having eight bubble cap trays for absorption of water vapor from gas by using TEG, and a TEG regeneration unit. The operating data of one of the train is given in table 1.
Normal make up glycol rate for such types of gas dehydration units is about 16 litres/MMSCM. But in this plant glycol consumption was about 70 litres/MMSCM resulting in a significant loss of revenue during plant operation. Many workers mention normal glycol consumption rate of 13.4 litre/MMSCM (0.1 gal/MMSCF) of gas. In another analysis Ghoshal indicated equilibrium glycol loss of 17.4 to 22.7 litre/MMSCM (0.13 to 0.15 gal/MMSCF) at 54.4 C (130 F) gas inlet temperature. In similar work, Weininger recommended to carry out an audit of the unit if glycol consumption exceeds 7.2 - 28.6 litre/MMSCM (0.5-2 lb/MMSCF) of gas. All these parameters therefore indicated that present plant of Bombay offshore requires a thorough audit of GDU to identity the causes of glycol loss and prevent the same.
Glycol losses from GDU are grouped into three categories
Vaporisation loss takes place both in glycol contactor as well as in glycol still of regenerator. The equilibrium vaporisation loss from contactor is normally 5% of normal loss and is primarily due to higher gas inlet temperature.
The vaporisation loss in glycol still is due to higher still temperature and high stripping gas rate.
The carry over loss in absorber is generally due to foaming, high gas velocity and inadequate mist eliminator at gas outlet. The carryover loss in glycol still is due to high stripping gas velocity and foaming in still column.