(702c) Membrane Assisted Liquid Absorbent Regeneration (MALAR):Innovative Use of Membranes in Liquid Absorbent Based CO2 Capture

CO₂ capture and geological storage (CCS) is a necessary option to limit CO₂ emissions from existing power stations. In the CCS chain the capture and compression of CO₂ to pipeline or injection pressure represents around 70% of the costs and all of the energy requirements of the process. Typical output reductions for 90% capture of CO₂ at a power station are around 30% and these are very large indeed. The leading capture technology is based on a liquid absorbent which chemically reacts with CO₂. The liquid absorbent is regenerated thermally and it will be re-used during many cycles. The CO₂produced from this process is also of high purity and can easily be stored in the underground after compression.

The energy required for the liquid absorbent regeneration is extracted as steam from the power station. This steam is then not available for power generation. Hence the output of the power station is reduced, which is undesirable. It is therefore necessary to develop absorbent regeneration processes which have much lower energy requirement. The starting point is to analyse the thermal energy requirement in more detail. It has the following contributions:

• The energy required to break the bond between CO₂ and the active component in the solvent as determined by the chemical formulation of the liquid absorbent.
• The heat required for bringing the solvent up to the regeneration temperature, which is a function of the CO₂ loading of the liquid absorbent.
• The evaporation enthalpy for the stripping steam which leaves the stripper together with the CO₂, which is determined by the temperature of the steam-saturated CO₂-product leaving the stripping column.

The overall energy penalty also incorporates the electricity use of the capture and compression process. This penalty is however to a large extent determined by the heat required to release the CO₂ from the liquid absorbent, typically around 4 MJ/kg CO₂. Dependent on the fuel type, this represents between 20 and 40% of the energy contained in the incoming fuel.

The ongoing development of new liquid absorbents will very likely reduce the first two contributions, i.e. new amines with lower binding energy and higher CO₂-loadings have been identified. However the energy required for the production of stripping steam is still needed. In fact all the energy needed for the regeneration is brought in via the stripping steam and is transferred to the liquid absorbent via condensation. Hence efficient control of the steam balance is important for the energy performance of the capture process.

The MALAR process builds on this foundation, aiming to reduce the energy requirement to less than 2 MJ/kg CO₂ using a Membrane Assisted Liquid Absorbent Regeneration process. This process concept lends itself for application with any type of liquid absorbent.

The proposed innovation aims to efficiently integrate heat recovery in the liquid absorbent regeneration process. This is achieved by the use of membranes for transfer of heat using condensation and evaporation processes. The process concept is an entirely new one utilising membranes as gas-liquid contactors in such a way that the (common) occurrence of liquid leakage through the membrane is not an issue.

In the MALAR process energy is recovered via transfer of steam through a membrane. In the membrane condenser latent heat is recovered from the wet CO₂ product into a portion of the cold CO₂-rich liquid absorbent; in the membrane evaporator latent heat is recovered from the hot CO₂-lean absorbent into the stripping column. The use of membranes for direct heat transfer via evaporation or condensation enables quite small approach temperatures which would be unachievable and/or uneconomical with conventional heat exchangers. The compactness of the membrane equipment further adds to the achievement of small temperature differences and thereby a very energy efficient process. Two porous membrane configurations using either hyrodophobic or hydrophilic are proposed for the exchange of latent heat.

This application of membranes is a so-called membrane contactor application, i.e. the membrane is used to contact a gas flow with a liquid flow. Such operations often suffer from minor amounts of liquid permeating through the membrane to the gas phase. This liquid is subsequently carried away by the gas flow, which is generally not desired, and particularly if the gas enters the atmosphere. It will furthermore hamper the efficient transfer of components between the two phases. In the MALAR application envisaged here, liquid leakage is not critical as the heat transfer function is not affected and the liquid will be contained in the stripper column and mixed with the liquid absorbent already present.

A 50% reduction in thermal energy required for the liquid absorbent regeneration can be achieved by using the MALAR concept in combination with improved liquid absorbents, resulting in a thermal energy requirement below 2 MJ/kg CO₂.  The improvements are coming from:

• A lower gas temperature exiting the stripper column reducing the evaporation loss
• A closer temperature approach through the use of the membrane heat exchangers
• The liquid absorbent having a double CO₂-loading and lower reaction enthalpy, which a feature of new liquid absorbents developed over the last 2 decades.

In summary: the benefits of the MALAR are:

• Energy efficient capture process through novel heat recovery method
• Smaller size equipment through use of high specific surface area membrane equipment
• Applicable to a wide range of liquid absorbents
• Different membrane types (hydrophobic and hydrophilic) are feasible
• Use of a membrane contactor in a non-critical application with respect to liquid leakage