Process Optimization in Acid Gas Removal System
Authors: Suebsook Jiranoot , Eksaengsri Achapan , Lertsakulsup Supot , Suwanvesh Korakot , Iswigrai Jeerisuda , Sillapacharn Thitinun 
Published: March 2015

This paper mainly describes challenges in process design and operation optimization of Acid Gas Removal System with membrane separation technology in developing high CO2 gas fields. It concentrates on techniques and applications being used in CO2 Removal System of Arthit Offshore Processing Plant located in the Gulf of Thailand in which PTT Exploration and Production Public Company Limited (PTTEP) is an operator. This plant was designed based on export gas at 363 MMscfd with feed CO2 up to 40% and lower down to 23% in Sales gas.

It is imperative for optimized CO2 Removal operations since it can minimize revenue losses as well as hydrocarbon releases to environment resulting in lower greenhouse gas emission. Formal methods are established for troubleshooting in case of %CO2 feed deviation from design, failure to meet sales gas specification, and high hydrocarbon losses. Membrane area adjustment was firstly implemented to match with feed CO2, and then performed operating condition optimization. The effects of the following factors which are very sensitive to the performance were examined such as: feed temperature, feed pressure, permeate pressure, feed flow rate and membrane area. Several performance tests were performed not only to fine-tune the best operating point, but also to analyze the effect of changing operating parameters in order to develop operating guideline for CO2 Removal process adjustment.

This paper also discusses in the methodology to corroborate performance of membrane element which are carried out in order to minimize the loss of hydrocarbon gas as well as the costs of membrane element change-out.

It is worthwhile for operators and engineers to translate the information gathers to serve as operating guidance to optimize process condition and maximize performance of Acid Gas Removal Units which can be certainly benefit to other fields with high CO2.

1. Introduction

Membrane technology is used for removal of acidic component like CO2 from natural gas in order to achieve the sales gas specification. In the separation mechanism of spiral wound membrane element, the nonporous membrane layer allows selective removal of fast gases (CO2, H2, He, H2S and water vapor) rather than slow gases (CO, N2, CH4, C2H6 and other hydrocarbons). Therefore, when the sour natural gas is flow through to the membrane element, impurities including CO2 and H2S are dissolved and diffused through membrane and eventually released through flare. This stream is called "Permeate" or "Acid gas". The low-CO2 gas will then leave the membrane on "Residual" stream. The driving force for separation is the differential pressure between high pressure feed side and low pressure permeate side. The permeate gas enters the permeate tube through holes in the tube inside the membrane element representing in Figure 1. The membrane unit equip with the membrane element connected together and inserted into a tube as shown in Figure 2. Multiple tubes are then mounted on skids in either a horizontal or vertical orientation.