The Measurement Of Trace Element Concentrations In Sour Gas Plant Process Streams
Authors: St. Pierre C.C. , Gnyp A.W. , Smith D.S. , Viswanathan S. , Thimm H.F. 
Published: March 2010

The design of an all- Teflon impinge train, suitable for the sampling of trace elements from high-pressure gas process streams, is described. The method was used to sample streams that might be diverted to flare slacks at two sour gas plants in southern Alberta. The results from this study suggest that most of the detected trace elements have an affinity for streams with high hydrogen sulphide contents.

Introduction

While the trace element contents of Alberta's crude oils and bitumens are reasonably well known from a number of studies into maturation and migration of oil(1–5), the same is not true for Alberta's natural gases. The limited work with gases appears to have been motivated primarily by specific environmental questions. In a study of mercury concentrations in gases, condensates and formation waters by the Energy Resources Conservation Board, Shaw et al.(6) reported an inability to detect mercury in most samples, with a detection limit of 0.1 ppb (w/w) for natural gases. Where the presence of mercury was indicated, results were sufficiently close to the limit or detection (LOD) to be doubtful. In only one case a concentration of 1.5 ppb was found in a gas from the Mississippian age Shunda formation. In condensates and formation waters, mercury was similar difficult to find, with no result greater than 4.5 times the LOD for these fluids. Because no concentrations that might give rise to environmental concerns could be found, this work was abandoned after a survey of eighteen gas fields.

Hsu and Hodgson(7) have reported a depletion of selenium in stack gases compared to the acid gases at sour gas processing plants. Their findings suggest the possibility of retention of this element in the elemental sulphur product.

In another paper(8), the authors have described a technique for estimating levels of twenty-nine trace elements in the gases exhausted from sour gas plant incinerator stacks.

Sampling Requirements and Method

In the previous work, representative samples were collected from two-phase flows in large diameter ducts at low pressures and very high temperatures. The isokinetic sampling technique employed in that study, however, is not applicable to natural gas streams flowing in ducts, typically of small diameters, at lower temperatures, and at pressures approaching 6900 kPa (1000 psig). In addition, the gas streams to be sampled might contain more than 70% hydrogen sulphide.

A schematic of the sampling process developed for this study is shown in Figure 1. As far as was possible, all parts of the train ahead of the impingers were constructed or Teflon at the University of Windsor, Department or Chemical Engineering Workshop. The sampling line consisted of 9.5 mm (1/8,-in.) I.D. Teflon tubing with 1.6 mm (1/6-in.) wall thickness.

A filter holder, also made of Teflon, provided support for the filter material used in the previous study. However, filter use was discontinued because filter deterioration could not be prevented at the sampling pressures encountered in the initial tests.