Residence Time Distribution in Gravity Oil-Water Separations
Authors: Zemel B. , Bowman R.W. 
Published: December 2007

An investigation was conducted of the hydraulic characteristics and effectiveness of commonly used oilfield oil-water separators. Measurements were made in a number of separators by injecting a radioactive tracer using a pulse technique. Residence time distributions were calculated to deduce the hydraulic characteristics of the separators, and these were compared with the effectiveness of the separator operations.

Introduction

The principal method used for separating oil from water in oil production operations is gravity separation. Although simple differences in density are often adequate, gravity commonly is assisted by the use of gas flotation, heat, chemicals, and high-surface-area coalescer sections. These separators come in many different mechanical configurations. Despite this variety in design and configuration, there are remarkably few engineering criteria for properly designing such separators. A notable exception to this is the "API separator," widely used to separate free oil from refinery and chemical-plant waste waters. The design principle's of this separator are based on a 3-year study instituted at the U. of Wisconsin in 1948. A significant part of this study consisted of determining the hydraulic characteristics of the separator by using tracer response techniques. The data obtained from this study, however, apply only to the specific mechanical configuration of the rectangular API separator and free oil (droplet size above 150 microns). A 1956 survey of some circular oil-water separators suggested that, while they often appear to work, "a rational design procedure had not yet been developed." This applies procedure had not yet been developed." This applies even more to the cylindrical oil-water separators so widely used in oilfield separation, particularly to those separators with ratio of width to length approaching unity.

As part of our effort to develop a more rational design procedure, the hydraulic characteristics of a number of procedure, the hydraulic characteristics of a number of different oil-water separators used by Shell Oil Co. were measured. These included many different designs and oilfield locations. These measurements were made under operating conditions by injecting a radioactive tracer using a pulse technique. The tracer response was monitored through the walls of the exit pipes by means of suitable detectors. From these data, residence time distributions (RTD's) were obtained, from which the hydraulic characteristics of the separator could be deduced.

This paper defines the terms used in these analyses in accordance with current usage. The principles involved are discussed briefly and the necessary equations are given in terms of our applications. The results obtained from field tracer tests in a number of different separators are described. The tracer injection method has been presented fully in a previous paper and only is described presented fully in a previous paper and only is described here. Ref. 4 also included simplified descriptions of the basic terminology.

Principles and Terminology Principles and Terminology Tracer response data have been used widely to obtain residence time distributions in nonideal systems. These data then can be used to characterize the hydraulic behavior of a given vessel. A large collection of literature on the interpretation of such data is available. Some examples are given in the figures.

A consistent language for expressing hydraulic characteristics in terms of their RTD's first was developed by Danckwerts and extended by Levenspiel, Himmelblau and Bischoff, and others.

JPT

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