Abstract

The dehydration of natural gas cannot be neglected in processing the gas to consumer specifications. Glycol dehydration (the conventional dehydration method used in Nigeria) has its associated problem - the venting of pollutants such as Methane, VOCs (Volatile Organic Compounds) and HAPs (Hazardous Air Pollutants). This constitutes an environmental hazard and also loss of valuable gas that could be sold to generate revenue.

This problem need not continue, as improved solid desiccant technology has proven to be capable of solving it. This paper presents this view in a systematic analytical approach. To validate this view, environmental and economic analyses were carried out and the results obtained demonstrate that desiccant dehydrators are preferred in terms of cost savings and environmental friendliness. Thus they are viable alternatives to the conventional glycol dehydrators.

Introduction

As the nation's petroleum industry gradually transits from a predominantly crude oil exploration and production industry to an oil and gas exploration and production industry, more focus is given to natural gas engineering projects. The removal of its water content is very important in bringing natural gas to pipeline specifications. If not removed, the water can condense and/or freeze in the gathering, transmission, and distribution pipeline causing plugging, pressure surges, and corrosion. Various methods have been used to dehydrate natural gas over the years. Four of the most common are

  1. Glycols,

  2. Regenerative adsorption systems,

  3. Membrane filters, and

  4. Deliquescents, commonly referred to as dry bed desiccants.

Of all the processes mentioned above, glycol process is the most used. Glycol systems have been in operation throughout the petroleum industry since 19496, and have provided fairly reliable dehydration. However, they are labor intensive, the associated technology has not advanced significantly, operating costs can vary dramatically, and there are substantial environmental problems due to emissions, soil contamination, and fluid disposal problems6. Also, harmful compounds such as Methane, VOCs (Volatile Organic Compounds) and HAPs (Hazardous Air Pollutants) are vented into the atmosphere when the glycol is regenerated.

Also, deliquescent desiccants have been used for gas dehydration for years, but only under certain conditions6. Deliquescents are salts, which due to their chemistry have an affinity for water. Their use has been limited by pressure, rate, required outlet moisture content, economics, and primarily by poor design and application. Recent advancements6 in the chemistry of these salts, deliquescent dehydration equipment, and most importantly, correct application, have now made these salts viable alternatives to glycol systems.

Studies2 have shown that replacing glycol dehydrators with desiccant dehydrators reduces methane, VOC, and HAP emissions by 99 percent and also reduces operating and maintenance costs. In a desiccant dehydrator, wet gas passes through a drying bed of desiccant tablets. The tablets pull moisture from the gas and gradually dissolve in the process. The analysis presented in this paper was done with calcium chloride as the desiccant.

This shows that harnessing this technology would improve natural gas processing in Nigeria.

Process/Equipment Description

The type of process and equipment depends on the solid desiccant being used. There are several types of desiccants9 but only Calcium Chloride is considered in this paper.

Calcium chloride has been used to dehydrate natural gas and air since the 1920's and 1930's4. Since it is hygroscopic, it has the ability to attract and remove water vapor from the surrounding environment. The ability of the salt to remove water vapor is based on the vapor pressure difference between the hydrate of that salt and the vapor pressure of water in the environment. The salt naturally attracts and absorbs moisture (hygroscopic), gradually dissolving to form a brine solution. The amount of moisture that can be removed from hydrocarbon gas depends on the type of desiccant as well as the temperature and pressure of the gas3.

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