WASHING

Preferential dissolution washing columns function according to a simple principle of physics which takes into account the different solubiities of gases in a liquid. Very roughly speaking, since CO2 is 25 times more soluble than methane, it will dissolve 25 times more easily in contact with water.

Washing thus consists in passing the mixture of gas to be purified through a current of pure water. In order to optimise gas-liquid exchange there must be sufficient gas in the liquid : the gas enters, either through a perforated plate, or through a lining which favourises the formation of innumerable tiny bubbles, thus increasing the surface area for the gas-liquid exchange. Since CO2 has a higher solubility than CH4, it will dissolve preferentially and thus the proportion of methane will increase.

Principle of a four storey washing column with perforated plates

Our experimental modelling has shown the pointlessness of extreme washing of the methane gas. In fact, during the washing process the CO2 solublises. This process increases the concentration of methane gas extracted.

As during the separation process, the phenomena of the dissolution of the different gases are not independent. The CO2 does not first completely enter into solution before the methane. The two events are simultaneous, the difference in solubility of these two gases means only that the CO2 is dissolved preferentially.

For a methane-rich gas the washing procedure will have this unwanted effect: the higher the concentration in methane the more will be put into solution - a scaling factor to be taken into account. Even if methane is 25 times less soluble than CO2, to enrich the gas to over 85% methane would result in to large a loss of that methane. Taking into account the relatively small loss of calorific power between pure methane and a gas with 85% methane there is no justification for wanting to enrich the gas above this latter level and in so doing wasting methane in the lake water.

Different solutions have been considered for washing the gas. It has been decided to adopt the choices made for the Cape Rubona plant, except that the washing columns will be submerged. Washing will be done using running water, gas passing through a metal grid in order to obtain a fine dispersion of the gas which would favour the solublisation of the CO2. Columns with lining, like Raschig rings or Pale rings, were excluded, they being incompatible with the size of the equipment.