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Degradation of diethanolamine solutions

University of British Columbia

Raw natural gas contains acid gases such as H2S and C02 which must be removed before the gas can be sold. The removal of these gases is called "sweetening" and the use of Diethanolamine (DEA) as a solvent has become widely accepted by industry. The process is simply based on the absorption and desorption of the acid gases in aqueous DEA. Side reactions can occur when DEA reacts with the C02 to produce degradation compounds. This degradation causes a loss in valuable DEA and an increase in plant operating costs. The reaction between DEA and C02 was studied experimentally, using a 600 ml stirred autoclave, to determine the effect of temperature, DEA concentration, and reaction pressure. Degraded DEA samples were analysed using gas chromatography. A fast, simple, and reliable technique was developed to analyse degraded DEA samples, which was ideally suited to plant use. Over 12 degradation compounds were detected in the degraded DEA solutions using gas chromatography and mass spectroscopy. Degradation mechanisms are proposed for the production of the various compounds. It was found that the degradation of DEA was very sensitive to temperature, DEA concentration, and C02 solubility of less than 0.2 g C02/g DEA. To study the effect of C02 solubility, which is a function of reaction pressure, simple solubility experiments were performed to cover the range of 100-200°C, 413.7-4137 kPa (60-600 psi) partial pressure of C02 and DEA concentration of 10, 20, and 30 wt % DEA. It was found that the reaction between DEA and C02 was extremely complex consisting of a mixture of equilibria, parallel, series, and ionic reactions. However, the overall degradation of DEA could be simply described by a pseudo first order reaction. The main degradation products were HEOD, THEED, and BHEP. It was concluded that C02 acted as a catalyst being neither consumed nor produced during the degradation of DEA to THEED and BHEP. HEOD was produced from DEA and C02, but was found to be unstable and could be converted back to DEA or react to form THEED and BHEP. The following simple kinetic model was developed to predict the degradation of DEA and the production of the major degradation compounds: [Figure 1] The model covered the ranges of DEA concentration 0-100 wt % DEA, 90-175°C, and C02 solubilities greater than 0.2 g C02/g DEA. Attempts were made to purify degraded DEA solutions. It has been claimed that activated carbon filters are useful in removing degradation compounds. However, tests with activated carbon proved it to be incapable of removing any of the major degradation compounds.

Text

2010-05-02

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http://hdl.handle.net/2429/24309

Chemical and Biological Engineering

University of British Columbia

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