Enhanced vapor recovery modeling of condensate tank flaring system coupled with existing vapor recovery unit in Bibiyana gas field

Abstract

The Bibiyana gas field is one of the most prolific gas fields in Bangladesh. It stared production in March, 2007 with 200 MMSCFD. Production has steadily increased from this field, and currently it is producing more than 730 MMSCFD. In addition, Bibiyana field also produces about 3,500 barrels of condensate per day. There are six storage tanks for condensate final stabilization and storage. Significant amount of vapor is produced in these tanks due to shrinkage/flash, standing and working effects. This vapor is hydrocarbon, i.e., natural gas, which has heating value and therefore valuable. There is a three-stage vapor recovery system in the field to capture the vapor produced at different stages of production and processing. However, vapor from the storage tanks cannot be recovered by the existing system because its pressure is too low- nearly atmospheric. It is therefore regularly flared through the Low Pressure Flare line. Flaring of gas is a problem which entails both economical loss and environmental concerns. The hydrocarbon burning produces toxic gases, soot, acid rain, unburned hydrocarbons and a huge amount of CO2, which contributes to greenhouse effect. The economical impact is the cost of gas that is flared. This project demonstrates a method for recovering the low pressure vapor from the condensate tanks. This method uses a Gas ejector as a device to compress the low pressure natural gas from the condensate tanks to an intermediate pressure, which can be fed into the intermediated stage of the existing vapor recovery unit. Thus the natural gas will be saved which would have been otherwise flared. The amount of tank vapor is calculated by different methods, which shows a significant amount of gas which is now being flared. Gas ejector is a device which converts pressure energy of a motive stream into kinetic energy which entrains a secondary stream and discharges the combined stream at an intermediate pressure. This project uses the relatively high pressure gas from the third stage of the existing vapor recovery unit as the motive gas, and the low pressure condensate tank vapor as the suction gas for ejector. The combined discharge stream will be fed into the vapor recovery unit’s second stage.