The Utilization of a Flash-Differential Process to Increase Stock-Tank Recovery
Authors: Kalish Paul J. , Endean Howard J. 
Published: December 2007

Introduction

This paper presents the results of laboratory and field investigations of the differential separation processing of primary separator liquids to the stock tank. The process basically consists of removing dissolved gases from primary separator liquids by both flash and differential vaporization rather than by conventional flash separation methods. When the gases are so removed stock-tank recovery is increased, since the differential process affords the equivalent of an infinite number of flash separation stages. While the process is applicable to any primary separator liquid, the largest stock-tank recovery increases generally are obtained with rich liquids having a high propane-hexane content.

The laboratory phase of the study was concerned primarily with devising a simple method for evaluating the relative merits of the flash and differential processes. Utilization of the method is shown in connection with three primary liquids of widely differing characteristics. Recovery by differential separation was from 4 to 7.5 per cent greater than that obtainable by optimum two-stage flash separation. In one case analyses were made of the tank oils obtained by both processes to determine the distribution of the recovery increase.

The laboratory method developed was instrumental in the design of a prototype differential separator for field testing. It was found that a combination flash-differential separation process is essentially as efficient as complete differential separation. Consequently, design simplification of a field prototype was possible, and the diversion of high-pressure gas from sales for operation was unnecessary.

The prototype separator used for field testing was constructed to permit either three-stage flash or flash- differential operation. The separator was tested on a condensate well stream. using the alternate methods until sufficient data were obtained for reliable evaluation. Stock-tank recovery increases by the differential process of some 5 per cent were obtained. Concurrent laboratory tests included stage and differential separations and relative weathering rate tests. The laboratory tests generally substantiated the field results, indicating that such tests can be used to evaluate the differential separation potential of a specific well stream.

The Flash and Differential Processes

The evolution of gas from complex hydrocarbon liquids during a pressure decline may occur by either a flash or differential process. In the flash process gas is evolved as a result of a pressure decline, and it remains in contact with the residual liquid. The composition of the system as a whole does not change in a flash process during the pressure decline. In a differential process gas evolves as a result of a pressure decline and is removed continually from the system. Removal of gas in this manner causes a change in the overall composition of the system. In a complete differential process the pressure decline. gas evolution and gas removal are continuous. The differential process is introduced first in field separation when gas or liquid is removed from the primary separator. In each subsequent stage of separation the liquid initially undergoes a flash vaporization to equilibrium gas and oil, followed by a differential process as actual separation occurs.

JPT

P. 998^