DESIGNING CUSTOMIZED DESULFURIZATION SYSTEMS

FOR THE TREATMENT OF NGL STREAMS

 

Neil Eckersley, Johnson Matthey Catalysts, Chicago, IL

James A Kane, American Mobile Research Inc, Casper, WY

 

Abstract

 

The purpose of this paper is to define and evaluate the key sequential steps required in transmitting, sampling and speciating sulfur laden NGL streams, in order to recommend and install absorbents capable of passing pipeline copper strip tests. An evaluation of the sulfur adsorption characteristics of specific sample containers will be given along with sample container material recommendations. A description of speciation instrument options will be followed by a description of the method for choosing and testing absorbents from an appropriate range to remove the sulfur species present in the speciated NGL.

A case study will be discussed demonstrating sulfur absorbent selection after a sour liquid is sampled, speciated and treated using an absorbent in a plant slipstream treater in the field.

The effect of sulfur species, concentration, temperature and pressure on the candidate absorbent will be discussed.

 

Introduction

 

Purification processes for natural gas, refining and petrochemical feed stocks are of primary importance to the modern chemical and fuel industries. Various contaminants are commonly found in NGL streams and their presence can result in a failure to meet pipeline and copper strip specifications. The growing trend towards applying more severe emission/environmental standards and the economics of refining and petrochemical process optimization, require that NGL streams be purified to ever more stringent standards.

A primary contaminant found in most virgin NGL streams is sulfur, present as H2S, elemental sulfur and often in various organic forms. These sulfur compounds are known poisons for the majority of refinery processes, even when present at low ppb levels [often below the level of detection for many facilities] and are significant contributors to atmospheric pollution. Plant and pipeline operators may experience design, operational and maintenance issues when sulfur levels exceed set catalytic, environmental or transmission specifications.

Desulfurization of fuels is a self-evident requirement to control acid gas emissions and meet legislation standards. The continuing trend toward lower emission fuels is evident with recent changes in legislation mandating low sulfur gasoline and diesel in many parts of the world; price differentials are evident between low and high sulfur fuel oils.

The base technology of non-regenerable fixed bed desulfurization has not changed significantly over the years, continuing to rely on classical chemical and physical reactions. However, the activity of desulfurization processes has continuously developed in response to the increasing demands of downstream processes and emission legislation. The use of fixed bed systems for purifying NGL streams is both practical and cost effective.

NGL streams can vary significantly in terms of their hydrocarbon and sulfur content. Matching the appropriate absorbent to purify a specific NGL is key in designing any fixed bed system. Sequential steps should be taken to ensure an appropriate match is made between candidate absorbent and specific NGL. The steps include sampling the NGL in question, transportation from plant site to laboratory, speciating its hydrocarbon and sulfur content, screening an appropriate absorbent to test under realistic conditions either in-lab or at-site.

The Duke Energy Field Services Patrick Draw facility is a fractionation plant processing NGL components for sale to refining and petrochemical customers in Wyoming. In 2002 Duke approached Johnson Matthey Catalysts to supply a fixed bed absorbent in order to meet a total sulfur specification in an NGL stream. After full NGL characterization, absorbent screening commenced utilizing a fully instrumented slipstream test reactor located at the depropanizer bottoms section of the process plant. After taking the sequential steps described, an appropriate absorbent was identified and successfully tested in the slipstream test reactor. This case study is designed to illustrate the steps required when designing a customized desulfurization system for the treatment of an NGL stream.

 

NGL Sulfur Specifications

 

In many areas of the world the transportation of lower molecular weight hydrocarbon fractions such as NGL is dependent upon a pipeline specification being achieved. Such specifications are tending towards lower sulfur levels, with new ceilings being imposed by governments and associated legislative agencies on both upstream operators and downstream refineries. Reactive sulfur [H2S] are soon to be replaced by total sulfur specifications, meaning the traditional technology gap for treating “non-reactive” sulfurs must be bridged prior to the implementation of these tighter specifications. One of the most commonly used methods of product specification testing is the copper strip test for lower molecular weight liquid hydrocarbons [ASTM D-1838-84 and D-130-83]. The test is ubiquitous amongst operators since it is cheap, simple to perform and generates instant results. However, there is a degree of subjectivity associated with the test in terms of requiring the correct lighting, angle of strip inspection etc. The test has been designed to limit the corrosion of pipeline materials by H2S, hydrolyzed COS, elemental sulfur and polysulfides

Often after passing a copper strip, liquids transported down a pipeline, or by rail car, will fail subsequent strip tests after either blending or hydrolysis or some combination of both. Elemental sulfur lay down in storage containers caused by the presence of iron on their inner surfaces is also a known source of copper strip failure after previous strip passes.

Existing solid absorbents [both regenerative and non-regenerative] used on gas and fractionation plants are capable of removing H2S to levels below which a transportable liquid will pass a copper strip test. Currently though there is a lack of appropriate products to remove certain less reactive species present in levels, often equal to and exceeding those of H2S. In future the tighter specifications outlined above will require a greater absorption efficiency in the removal of less reactive S species often found to co-exist with H2S in NGL streams.

 

 

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