GB2049468A - Gas purification - Google Patents

Abstract

A gaseous impurity is removed from a gas by bringing the gas to be purified into contact with an extraction mass formed by an inert porous carrier impregnated with an inert selective organic solvent of low vapour pressure, regenerating the extraction mass by means of such an organic solvent and distilling or stripping the resulting regeneration solution to remove the impurity.

Description

SPECIFICATION Gas purification The present invention concerns a process for removing one or more gaseous compounds present in small quantities in a gas.

The invention can be used in particular, but not exclusively, for the treatment of natural gas, synthetic gas, combustible gases resulting from the-refin- ing of gasification of hydrocarbon or coal, hydrocarbon charges intended for vapour reforming, and recycle gas from hydrotreatment processes. Such gases may contain a small proportion of compounds such as H2S, CO2, COS, CS2 and water vapour.The usual methods of extraction (solvent extraction, in particular the conventional process of washing gas by means of solutions of alkanoiaminesl cannot be economically used in the case of gaseous effluents that contain only very small amounts of impurities; indeed, such methods would require costly and pointless investment and the expenditure of excessive amounts of energy and moreover would not always be suitable for overcoming the problem raised.

In accordance with the present invention, one or more gaseous impurities are removed from a gas by bringing the gas to be purified into contact with an extraction mass formed by particles of an inert porous carrier impregnated with an organic solvent immobilised in the pores of the carrier, the said solvent having a low vapour pressure and a good selective solvent capacity in regard to the-compound to be extracted and being chemically inert with respect to the carrier, then regenerating the extraction mass by passing over it an organic solvent as defined above, and distilling or stripping the regeneration solution thus produced in order to separate the dissolved gaseous compound from it.

One advantage of this process is that it consumes only a small amount of solvent using an extraction procedure that can attain a level of efficiency that could not be achieved with the conventional gasliquid extraction methods. Another advantage is that it is simple, reducing to a minimum the amount of handling of the products while producing effective contact between them.

The process also makes possible easy regeneration of the extraction agent.

The mechanism of this extraction agent seems basically to be as follows: the organic compounds present in the gas would be dissolved (by simple dissolution or by reaction) in the suitable organic liquid (referred to as the solvent) which is immobil ised in the pores of a porous solid acting as the carrier.

The solvent has a low vapour pressure, in order to avoid substantial losses by vaporisation. Its capabil ity for preferential absorption in regard to the impurity or impurities to be extracted means that in the equilibrium condition the ratio between the concentration of the compound extracted in the solvent and the concentration of the same product in the gas is as high as possible. Preferably the solvent may have chemical affinity for the impurity or impurities, thereby facilitating their extraction; thus, it may be a basic compound if the impurities are acid in nature.

Particularly suitable carriers are those that have internal-porosity (or grain porosity) of greater than or equal to 0.1,for example from 0.1 to 0.8.

Internal porosity is defined as the ratio of the volume of internal empty space to the actual volume of the particles of solid; it is measured for example by means of a mercury porosimeter. Internal porosity is therefore distinguished from bed porosity, which is the ratio between the volume of intergranular space and the apparent volume of the bed of particles. This porosity permits the carrier to be impregnated by a liquid which remains trapped in its pores.

It is unnecessary for the carrier to have its own selectivity with respect to the compound to be extracted. Good results have been obtained for example with pumice, kieselguhr, bauxite, alumina, carbon or silicates, which permit a substantial volume of solvent to be immobilised, while being of small apparent volume. These materials may be used in the form of powder, granules, balls or extrusions of various shapes. The grain size is preferably from 0.1 mm to 5 cm.

Preferred carriers are those that have good inertia with respect to the solvent and the components of the gas to be treated. Mineral carriers generally comply with all these conditions. The solvent for impregnating the porous carrier will be one that has the above-indicated properties and is suitable for the particular situation.

The liquids advantageously used as immobilised solvents are those usual in known processes for extraction of gaseous compounds, as referred to above: for example, aqueous solutions of alkanolamines (for example monoethanolamine, diethanolamine, triethanolamine and methyldiethanolamine) for the extraction of compounds such as H2S, CO2, COS AND CS2, and glycols (for example ethylene glycol, diethylene glycol, triethylene glycol and polyethylene glycol) for the dehydration of gaseous effluents.

The amount of solvent used depends on the internal porosity of the porous agent and the porosity of the bed of such material. It may represent for example from 20 to 75% by volume of the apparent volume of the filling.

It has also been found that, when the amount of impurity in the gaseous effluent collected had reached a value that was predetermined as the acceptable maximum, it was possible to regenerate the extraction material by bringing it into contact with the same solvent as that used for the impregnation step or with a different solvent that nonetheless complied with the above-indicated requirements.

Thus, the extraction material is subjected to a washing operation during which the solvent at least partly replaces the previously absorbed compound.

It would be possible to use other methods or regeneration, for example, putting the extraction material under a reduced pressure or carrying out a stripping operation. However, the preferred methods are washing or re-extraction so that the extraction material can be immediately re-used without any other expenditure or energy.

For the purposes of carrying the invention into effect, operation may be for example as follows: the porous carrier is dried and placed (for example) in a column. It is then impregnated with a suitably selected solvent, which fills the pores of the carrier.

The excess of solvent is drained off as far as possible and the gas to be treated is then passed through the impregnated carrier. If desired, the first gaseous fractions produced, which have entrained droplets of solvent which were in the intergranular spaces in the carrier, are eliminated.

When it is found that saturation of the impregnated carrier is approaching, that is to say, when the extraction rate falls below the selected limit value, regeneration is carried out, as set out above.

The amount of solvent required to produce regeneration is never very substantial.

At the end of the regeneration step, a solution of the extracted impurity in the solvent is obtained.

This mixture is treated by known means, for example stripping or distillation, in order to regenerate the solvent.

A preferred mode of operation comprises using at least two and preferably three columns, one or preferably two of which operate in an extraction mode so as to make the maximum possible use of the capacity of the impregnated carrier, the last column operating in a regenaration mode.

For the absorption operation, it is possible to use effective flow rates in respect of the gas to be treated ranging for example from 2 litres/cm2/hour to 100 litres/cm2/hour, preferably 10 litres/cm2/hourto 50 litres/cm2/hour (litre/cm2/hour means the volume of gas (S.T.P.) in litres per unit of section of bed and per hour). The amounts of gas to be treated and impregnated porous carrier used are advantageously in a ratio by volume of between 5:1 and 5000:1, preferably 100:1 and 2000:1.

In the regeneration step, the solent flow rate is for example from 10 to 500 cm3/cm2/hour. The flow rate values set out above are mean values and can be widely altered depending on the particular conditions under which the process is performed.

Usually, the gases to be treated contain small amounts of impurities, generally less than 5% by volume and in most cases less than 2% by volume.

These amounts are not limiting in regard to the invention; nonetheless, the treatment of gases that are not heavily charged is preferred.

The amount of impurities in a gas to be purified can be reduced to undetectable amounts, for example less than 1 ppm (volume).

The following examples illustrate the invention.

Example 1 A column with an inside diameter of 2 cm is filled, over a height of 45 cm, with pumice stone in grain form with a granulometry of from 0.08 to 0.1 cm. The resulting bed porosity is 00.3 and the grain porosity is 0.54.

A 25% by weight aqueous solution of diethanolamine (D.E.A.) percolates into the column, being introduced by way of the lower part thereof, until the porosity of the solid and the interstices between the grains are filled and the air bubbles eliminated; the excess of liquid is then drained off. A gaseous mixture comprising nitrogen and carbon dioxide (CO2) is then passed through the column, the CO2 content being 105% by volume and the flow rate being 50 1/h. The gas collected at the outlet of the column is analysed, and it is found that carbon dioxide is not detected (detection threshold: 0.01% by volume) until 120 litres of gas has passed; after 125 litres of gas has passed, the mean CO2 content is 0.01% by volume.Operation is then stopped and regeneration is effected by percolation of the 25% D.E.A. solution; after 60 cm3 of solution has passed, it is possible for CO2 absorption to be begun again under the same conditions and with the same results. The acid-gas-enriched D.E.A. solution is regenerated by vapour stripping.

Example 2 (Comparative) Operation is effected with the same apparatus, but the pumice stone is not impregnated with D.E.A.

solution. In this case, the carbon dioxide appears immediately in the gaseous effluent.

Example 3 Operation with the same apparatus and under the same conditions as in Example 1; however, the impurity to be removed is hydrogen sulphide; It is found that is possible to treat 58 litres of gas containing 1% by volume of H2S, reduce the amount of H25 in the gas to 0.01% by volume, and regenerate the absorbing filler with 53 cm3 of a 25% by weight solution of D.E.A.

Example 4 The mode of operation of Example 1 is followed, with a column which is 1 cm in inside diameter and 34 cm in height and which is filled with pumice stone with a granulometry of from 0.08 to 0.1 cm, impregnated with diethylene glycol (D.E.G.) by percolation of the diethylene glycol over the dry pumice stone; the bed porosity is 0.3 and the grain porosity is 0.54.

A 35 I/h flow of nitrogen containing water vapour, in a proportion of 1.55 g per 100 litres is passed through the column. After 90 litres of gas has passed, the water content of the gas is less than 0.02 9/100 litres; percolation of 25 cm3 of D.E.G. permits regeneration of the column and enables an identical absorption operation to be begun again.

The water is removed from the D.E.G. solution by distillation under normal pressure or under vacuum, or by stripping with a water-insoluble gas (for example natural gas). The D.E.G. from which the absorbed water has been removed is recycled.

Claims (9)

1. A process for extracting at least one gaseous compound contained as an impurity in a gas, comprising bringing the gas to be purified into contact with an extraction mass formed by particles of an inert porous carrier impregnated with an organic solvent immobilised in the pores of the carrier, the said solvent having a low vapour press ure and a good selective solvent capacity in regard to the compound to be extracted and being chemically inert with respect to the carrier, then regenerating the extraction mass by passing over it an organic solvent as defined above, and distilling or stripping the regeneration solution thus produced in order to separate the dissolved gaseous compound from it.

2. A process as claimed in Claim 1, in which the porous carrier is an inert mineral carrier having an internal porosity at least equal to 0.1.

3. A process as claimed in Claim 2, in which the carrier is pumice, alumina, bauxite, kieselguhr, carbon or a mineral silicate.

4. A process as claimed in any one of Claim 1 to 3, in which the gaseous compound or compounds is or are hydrogen sulphide and/or carbon dioxide and the solvent comprises an alkanolamine.

5. A process as claimed in Claims 1 to 3, in which the gaseous compound is water vapour and the solvent comprises a glycol.

6. A process as claimed in any preceding claim, in which the volume of gas to be treated and the volume of the impregnated carrier are in a ratio in the range 5:1 to 5000:1.

7. A process as claimed in Claim 6, in which said ratio is in the range 100:1 to 2000:1.

8. A process as claimed in any preceding claim, in which the particles of the impregnated carrier are disposed in a fixed bed and the hourly flow rate of gas through the bed is from 2 to 100 litres per cm2 of bed section.

9. A process for extracting at least one gaseous compound contained as an impurity in a gas substantially as hereinbefore described in any one of Examples 1,3 and 4.