Mixing MDEA, TEA shows benefit for gas-sweetening operations

Nov. 7, 2011
Studies have demonstrated how mixing primary-tertiary or secondary-tertiary amines have improved the efficiency of amine-sweetening processes in terms of gas purification and process energy requirements.

Wael A. Fouad
Abdallah S. Berrouk
Cornelis J. Peters
Petroleum Institute
Abu Dhabi

Studies have demonstrated how mixing primary-tertiary or secondary-tertiary amines have improved the efficiency of amine-sweetening processes in terms of gas purification and process energy requirements. This article discusses the use of amine solvents that consist of two tertiary amines, methyl-diethanolamine (MDEA) and triethanolamine (TEA).

A kinetic approach was used to explain results obtained from a process simulation of the Habshan gas sweetening unit in Abu Dhabi. Results show that up to a 3% reduction in unit running cost can be obtained with a mixture consisting of 40 wt % MDEA and 5 wt % TEA), while meeting sweet-gas specifications in terms of H2S and CO2 concentrations. Lean amine loading was fixed at a value of 0.005.

Results for the MDEA + TEA mixture were compared with results of the standardized (45 wt % MDEA) solvent used in Habshan and other possible primary-tertiary and secondary-tertiary amine mixtures. This reduction in cost was achieved through a decrease in plant raw materials cost and in both regenerator reboiler and trim-cooler energy requirements.

Sweetening processes

New technologies are being implemented recently to treat highly sour natural gas with different gas sweetening processes. As the gas acidity increases, the energy required by the process to achieve sweet gas specifications in terms of H2S and CO2 concentrations will increase. This is particularly true for gas sweetening processes that use such alkanolamine solvents as the primary amine methyl-ethanolamine (MEA), the secondary amines diethanolamine (DEA) and di-isopropanolamine (DIPA), and the tertiary amines triethanolamine (TEA) and methyl-diethanolamine (MDEA).

Many recent researches have focused on the mixing of different amine solvents and how their use affects both acid gas components' absorption and the energy price of the process. So far as the authors know, all these investigations have been on mixing primary and tertiary amines (MEA-MDEA, for instance) or secondary and tertiary amines (DEA-MDEA, for example).

The main reason behind mixing primary or secondary amines with tertiary is to benefit the sweetening process from the high reactivity of the former and the low energy requirement of the latter. Other reasons can also be mitigation of the effects of the relatively higher vapor pressure and corrosion rates of the primary and secondary amines as well as the relatively higher selectivity of tertiary amines towards H2S.

Usually, process simulations have been used to determine the proportionality of these different amines in the mixed solution that can meet the acid-gas-removal specification at the lowest energy requirement possible. The latter is estimated to be about 70% of the operating costs of an amine gas-sweetening unit, excluding labor.1 Thus, every option available for the process industry to lower the energy requirement should be investigated, in particular for sour gas feeds.

Abu Dhabi Gas Industries Ltd. (GASCO) has been running the Habshan sweetening gas units that treat gas streams with up to 10% of H2S. Newly developed gas reserves in Abu Dhabi are even more sour, with H2S concentrations reaching 30%. The amine used in these units has been MDEA at 45 wt %. This tertiary amine is known for its low vapor pressure and thus can be used at higher concentrations (up to 55 wt %) without appreciable vaporization losses for acid-gas removal from high-concentration natural gas.

MDEA is known for its selectivity towards H2S and its low energy requirement for regeneration compared with primary and secondary amines. However, MDEA slips some of the CO2, which can be a drawback only if CO2 specifications are strict. Triethanolamine (TEA) is another tertiary amine that has a lower energy requirement than MDEA but a lower loading capacity as well.

This article discusses the effect of a mixture of two tertiary amines, MDEA and TEA, on the performance of the Habshan gas-sweetening unit belonging to the onshore gas development (OGD) project Phase II. This unit is treating a sour gas feed with 4.26% mol of H2S and 5.34% mol of CO2. Performance is measured in terms of acid-gas removal and running cost. Enthalpies, kinetics, and acid gas loadings are used to examine the effect of such blend on the unit performance.

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