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Apr-2009

Treating high CO2 gases with MDEA

Using MDEA as a solvent for high levels of CO2 removal requires careful 
process modelling using accurate simulation

Jenny Seagraves, Ineos Oxide
Ralph H Weiland, Optimized Gas Treating
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Article Summary
Generic N-methyldiethanolamine (MDEA) is commonly used as a highly selective solvent to treat sour gases down to parts-per-million levels of H2S while slipping a large proportion of the CO2 in the feed gas from the system. It is also the major constituent in many speciality amine formulations developed for deeper CO2 removal in applications such as synthesis gas production and treating high CO2 natural gases found in several regions of the world. However, in recent years, attempts have been made to use solvents containing MDEA alone for CO2 removal from high concentration gases, usually at high pressure.

A number of treating plants have been designed and built around using generic MDEA for treating high CO2 gases. However, there is a limit to how much CO2 removal can be achieved using generic MDEA in a column of reasonable height or tray count. Unfortunately, several CO2-only plants have failed to meet treating requirements or have encountered treating difficulties as a result of the choice of solvent. In many cases, these plants have had to be retrofitted to speciality amines, and sometimes they have had to undergo expensive tower revamps in order to correct the problems.
This article is intended to help prevent future failures by exploring the limitations of using generic MDEA for CO2 removal applications. Through case studies that use plant performance data, it demonstrates what is possible and what is not.

Understanding the process
In most treating applications for removing CO2 as the only acid gas constituent, the choice of MDEA as the sole active ingredient in the solvent is likely to be unsatisfactory, except in cases where only a small amount of CO2 removal is needed. To make correct, rational decisions on solvent selection, it is helpful to understand why this is the case.

MDEA is a tertiary amine whose amine group lacks even the single proton that is so essential to react directly with CO2. In terms of its chemistry, the most that MDEA can do is provide a sink for the hydrogen ions produced when CO2 hydrolyses in water:

CO2 + H2O ⇔ H+ + HCO3-

HCO3- ⇔ H+ + CO3=

H+ + R3N ⇔ R3NH+

Although CO2 does not react with MDEA, when you measure the effect of MDEA on the absorption rate of CO2, the results can be interpreted in terms of chemical reaction rate parameters for the so-called “apparent” reaction between CO2 and MDEA. This apparent reaction is found to be first order in both MDEA and CO2 with a very small, albeit non-zero, value for the reaction rate constant. There is no question that MDEA is unable to react directly with CO2, because it cannot form a reaction product (such as the carbamated form that results with primary and secondary amines). Instead, it is said to catalyse the hydrolysis reaction of CO2. But even when catalysed, the CO2 hydrolysis reaction is extremely slow, so slow in fact that it barely affects the CO2 absorption rate at all. In other words, unlike primary and secondary amines, the apparent reaction kinetics of MDEA does not have much impact on the rate of the absorption process.

MDEA’s real role is to provide an enormous sink for protons produced by slow CO2 hydrolysis. Thus, while the capacity of MDEA solutions for CO2 is very high, the absorption rate is so low that this capacity is rarely realised in practice, unless the chemistry is enhanced by means of a promoter. The reason for low absorption rates can be clarified by considering the effect of chemical reaction on absorption, and what the driving force for absorption really is.

There are two kinds of solubility that are important: the physical solubility of CO2 in the solvent (the solution is rarely less than 90 mol% water in commercial MDEA solvents); and the equilibrium solubility of CO2 in the treating solution as it is measured in the laboratory (which we will call the chemical solubility). The physical solubility is quite low and is properly calculated from Henry’s Law for CO2 in water, modified somewhat for the effect the amine has in replacing up to 10 or 12 mol% of the water with a component (the amine) in which the physical solubility of CO2 is considerably higher. The chemical solubility, on 
the other hand, is extremely high because MDEA protonation allows very high bicarbonate and carbonate concentrations to be formed.

The absorption of CO2 occurs in a series of steps: diffusion from the gas to the gas-liquid interface; gas dissolution into the liquid at the interface; possible chemical reaction with the amine; and diffusion of the dissolved gas in both its free and reacted forms away from the interface and into the bulk of the liquid. Figure 1 shows typical concentration gradients of CO2 diffusing through the gas and the liquid when there is no reaction (shallow gradient) and relatively fast reaction (steeper gradient that enhances the diffusional mass transfer rate).

The process of dissolving is physical and its rate is determined by the physical solubility of the gas right at the interface. Diffusion away from the interface through the liquid takes place under a concentration driving force, which is the concentration gradient of the unreacted (but maybe still reacting) gas. Reaction steepens the concentration gradient and so it allows the diffusional process to occur must faster. In other words, reaction accelerates mass transfer rates in the liquid (roughly in proportion to the square root of the reaction rate constant). But if the reaction is too slow, as it is in the case of CO2 hydrolysis, there is no mass transfer enhancement, although the solvent’s capacity is still enormously high. What are the implications of this for MDEA as a solvent specifically for CO2 removal?

For those who think in terms of theoretical stages, the chemical solubility is used as the basis for stepping off the number of theoretical stages, whether this is done on a piece of paper or within a computer simulation. The problem comes in translating the resulting NTS count into actual trays or bed depths for packing. The solvent capacity is extremely high, but the driving force for absorption is just as extremely low because of the poor physical solubility. The result is extraordinarily low tray efficiencies and enormous height equivalent to theoretical plate (HETP) and height of transfer unit (HTU) values. The theoretical stage approach to simulation of such a process is utterly inappropriate; the process is completely controlled by the slow absorption rates, not by chemical equilibrium. The only way to approach this type of problem is through a true mass and heat transfer rate-based simulation that is grounded in the real physics and the real chemistry of the process. ProTreat is the only such simulator commercially available that is robust enough for general everyday use in amine treating.
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