Selective adsorption of carbon dioxide on zeolites.

0001The present invention relates to a process for selective adsorption of carbon dioxide from gaseous mixtures.

0002Several methods are known for the removal of carbon dioxide from gaseous mixtures. These include, for instance, absorption in solvents by chemical or physical mechanisms, separation using membranes and adsorption over zeolites.

0003Solvent absorption is most widely used of these processes as it can achieve bulk C0₂ separations with high recovery. Examples of chemical solvent systems are monoethanolamine and hot potassium carbonate. More recently, physical solvent systems such as Selexol have been used due to their reduced energy costs; however, capital costs are still high.

0004Membrane processes are increasingly used in the oil/gas field separations of C0₂/CH₄ where the recovered C0₂ can be used for enhanced oil recovery. In this case, capital cost and equipment size are both low but C0₂ recovery and purity are also low.

0005In the zeolite adsorption technique zeolite A is used for removing trace levels of C0₂ from gas streams. Substantially complete C0₂ removal is possible due to the high sorption energy between zeolite A and C0₂. However, regeneration must be carried out by thermal swing, hence this process is not suitable for bulk C0₂ separations.

0006Accordingly, the present invention is a process for selectively adsorbing carbon dioxide from a mixture thereof with non-acidic gases by passing the mixture over a zeolite and subsequently recovering the adsorbed carbon dioxide during a regeneration cycle of the zeolite characterised in that the zeolite has a faujasite type structure which has been ion-exchanged with akali or alkaline earth metal ions and has a silicon to aluminium atomic ratio from 1.2-3.

0007Zeolites of the faujasite type are described in standard texts including "Zeolite Molecular Sieves, Structure, Chemistry and Use" by Breck, D.W., pp 92/93, published by John Wiley & Sons Inc. (1974) and in "Molecular Sieve Zeolites-1", Advances in Chemistry Series 101, edited by Gould, R.F., pp 171 et seq. and published by the American Chemical Society (1971). These zeolites are classified as having an FAU-type structure as characterised by their X-ray diffraction pattern and listed in the book by Meier, W.M. and Olsen, D.H. entitled, "Atlas of Zeolite Structure Types", p 37, published by the Structure Commission of the International Zeolite Association (1978) and distributed by Polycrystal Book Service, Pittsburgh, Pa., USA.

0008Examples of the zeolites of the FAU-structure type that may be used include zeolites X and Y provided that they have a silicon to aluminium atomic ratio from 1.2 to 3. Zeolites having a silicon to aluminium ratio from 1.5-3, especially those having a ratio from 2 to 3, eg zeolite Y are preferred.

0009The zeolites used may be ion exchanged with alkali or alkaline earth metal ions, such as sodium, by any of the conventional techniques. For instance, sodium ions may be introduced into the zeolite during synthesis by using sodium hydroxide as the alkali. Alternatively, the hydrogen form of the zeolite may be exchanged initially by treatment with sodium hydroxide solution, washed, dried and then granulated with colloidal silica and then dried. The resultant sodium ion exchanged zeolite having a silicon to aluminium atomic ratio from 1.2 to 1.5 (hereafter termed "NaX") or 1.5 to 3 (hereafter termed "NaY") is used for the selective adsorption of carbon dioxide from mixtures thereof with non-acidic gases.

0010Exmples of the non-acidic gases from which carbon dioxide can be separated by the present process include carbon monoxide, nitrogen and methane. Separation of mixtures of C0₂ and methane is particularly important in the petroleum industry especially as the C₀₂ recovered can be used in the enhanced oil recovery process.

0011The selective adsorption is suitably carried out 'uy passing the gaseous mixture containing carbon dioxide over the alkali/alkaline earth metal ion exchanged zeolite at ambient temperature eg 20°C and pressure eg 1 bar absolute. The adsorbed carbon dioxide may be recovered and the ion exchanged zeolite regenerated by applying a moderately low vacuum e.g. 10 mbar absolute at ambient temperature or by reducing the C0₂ partial pressure by use of a gas purge. For instance, using NaY with a silicon to aluminium ratio of 2.4, carbon dioxide was adsorbed from a 50/50 mixture thereof with nitrogen at 20°C and 1 bar absolute. The adsorption capacities for carbon dioxide and other non-acidic gases are shown in Table 1. The advantage of using a NaY zeolite is that it is highly selective for C0₂ in the presence of N₂, CO and CH₄ and it can be regenerated by the application of a vacuum or by a gas purge which enables a bulk C0₂ separation to be carried out.

0012The adsorption and regeneration cycles can be carried out at pressures from 1 mbar absolute to 40 bar absolute, preferably from 10 mbar absolute to 20 bar absolute. The loading on the zeolite increases with the partial pressure of the adsorbed component. Desorption occurs rapidly as the partial pressure of C0₂ is reduced.

0013Adsorption can be carried out in the temperature range -80°C to 150°C, preferably -50°C to +100°C. Loading is increased at lower temperatures whilst rapid adsorption and desorption rates are maintained.

0014It is preferable that the gaseous mixture being treated does not contain moisture as this will reduce the adsorption capacity of the zeolite.

0015The present invention is further illustrated with reference to the following Examples.

EXAMPLES

0016A standard NaY zeolite powder (obtained from Laporte Industries plc) was grapulated with colloidal silica (Ludnx AS 40.

0017Registered Trade Mark obtained from Du Pont) to give granules containing 30% silica. Ihe granuies were activated by heating at 350°C for four hours. The adsoprtion capacity was determined by passing gas at 20°C through the granules at atmospheric pressure. The zeolite was regenerated by applying a vacuum of 1 mbar absolute for 10 minutes. Capacities of NaY for CO₂, N₂, CO and CH₄ are given in Table 1. Table 2 lists adsorbed gas compositions for various C02/N₂ feed gas mixtures. C0₂/CH₄ and C0₂/CO separation factors were found to be similar. <tables id="tabl0001" num="0001"><img file="EP0173501A2_D0001.tif" /></tables><tables id="tabl0002" num="0002"><img file="EP0173501A2_D0002.tif" /></tables>