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DEA degradation in heat exchanger tubes

University of British Columbia

Aqueous diethanolamine ("DEA") is widely used for the removal of acid gases such as CO₂ and H₂S from natural gas as well as refinery gases. In addition to the desired absorption and desorption reactions, some side reactions occur between CO₂ and DEA resulting in the formation of degradation compounds. Degradation not only represents a loss of valuable DEA, but may also lead to operational problems such as corrosion, foaming and fouling. DEA degradation is a complex process and depends on solution concentration, raw gas composition, solution flow rate and (most importantly) temperature. Carefully controlled DEA degradation experiments were carried out in a coiled heat exchanger tube (2.032 mm ID, 3.175 mm OD and 4.8 m long) heated by means of a constant temperature heat transfer fluid. The operating conditions covered are: 1379 to 4137 kPa CO₂ partial pressure, 60 to 200 °C, 20 to 40 wt% DEA solutions and 0.011 L/s to 0.0172 L/s (3.4 m/s to 5.3 m/s) solution flow rate measured at 60 °C. The DEA degradation rate was found to increase with temperature, CO₂ partial pressure and DEA concentration and decrease with solution flow rate. Degradation resulted in severe fouling of the heat exchanger tube. The viscosity as well as foaming tendency of the solutions were found to increase with the concentration of degradation products. The following simple mathematical model for the prediction of DEA degradation in heat exchangers was developed : [See thesis for diagrams] Potentiodynamic corrosion studies as well as conventional weight loss tests were carried out and degraded DEA solutions were found to be corrosive towards AISI-SAE 1020 carbon steel. 3-(hydroxyethyl)-2-oxazolidone ( "HEOD") was identified as one of the corrosive components. Severe pitting of AISI-SAE 1020 carbon steel by HEOD was detected by electron micrographic analysis. Minor pitting was also noticed in the case of BHEP and DEA. Use of activated carbon filters and soda ash treatment were both found to be incapable of removing major degradation products. A purification method consisting of NaOH injection was developed and found to be effective in converting HEOD and N,N,N-tris-(hydroxyethyl)ethylenediamine ("THEED") back to DEA. However, conversion of HEOD to DEA apparently depends on the presence of other degradation compounds.

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2010-05-17

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http://hdl.handle.net/2429/24807

Chemical and Biological Engineering

University of British Columbia

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