The present invention relates to the use of hydro¬ phobic zeolites for cleaning air which contains mercap- tans and H2S. More specifically, the present invention relates to the use of mordenite and/or ZSM 5 (silicalite) for cleaning air containing mercaptans and H2S.

Zeolites, aluminium silicates with space lattice structure, have lately been used to a great extent as adsorbents for organic compounds, such as solvents, for cleaning industrial emissions. Moreover it is well known that some types of industry, such as the paper industry, emit large quantities of CH3SH and closely related com¬ pounds.

Zeolites consist of a group of crystalline aluminium silicates. The crystalline structure is of the space lat- tice type, which makes the zeolites microporous, having a pore diameter which is well defined for each zeolite, typi¬ cally 3-7.5 A. The ratio of silicon to aluminium, which for some zeolites may vary within very wide limits, is decisive of the adsorptive properties of the zeolite. A silicon-rich zeolite has pronouncedly hydrophobic properties, whereas an aluminium-rich zeolite is correspondingly hydrophilic. Silicon-rich zeolites may thus be used as adsorbents for hydrophobic materials in hydrophilic surroundings.

Russian Patent Specification No. 1,528,541 discloses the use of hydrophobic zeolites for cleaning petroleum gases containing mercaptans and H2S, and the special type of zeolite which is mainly used is a Russian variant of ZSM 5 (silicalite) .

In zeolites, molecules are adsorbed on the basis of their size, i.e. if a zeolite has large pores, large mole¬ cules are adsorbed. Small molecules are also adsorbed by zeolites with large pores, but not at all as efficiently as by zeolites with small pores.

In simultaneous removal of different mercaptans and H2S, there is thus the problem that it is difficult to

obtain efficient removal of both mercaptans and H2S. In fact, H2S is much smaller than the different mercaptans and thus is not adsorbed particularly efficiently by a zeolite which has larger pores and which is adapted to the different mercaptans.

We have found that the zeolites ZSM 5 (silicalite) and mordenite result in excellent and efficient removal of both H2S and mercaptans when cleaning air which con¬ tains mercaptans and H2S. Mordenite is one of the largest, known zeolites with a pore size of 7.5 A, and silicalite has a pore size of about 5.5 A.

The pore size of the silicalite is such that both H2S and mercaptans are well adsorbed, but the pore size of the mordenite should not result in a particularly efficient adsorption of H2S.

The structure of the mordenite, which is composed of twelve-membered-rings, is such that the twelve-membered- rings are linked to eight-membered-rings . It has been found that these eight-membered-rings, which have a smal¬ ler pore diameter, are excellently adapted to adsorb H2S.

The structure of the silicalite is composed of ten- membered-rings only.

In the paper industry and in sewage treatment plants, there are problems of nasty-smelling emissions of air, and such emissions contain mercaptans and H2S, H2S besides being very toxic.

The air in many computer rooms also contains mercap¬ tans and H2S, and it is known that H2S causes corrosion in the contactors of the computers. By cleaning the air of mercaptans and H2S, great economical profits can be made.

One object of the present invention thus is the use of hydrophobic zeolites for cleaning supply/exhaust air, which contains mercaptans and H2S.

A specific object of the present invention is the use of mordenite and/or ZSM 5 (silicalite) for cleaning supply/exhaust air which contains mercaptans and H2S.

These objects are achieved by using hydrophobic zeo- lites for cleaning air which contains mercaptans and H2S, the air being caused to contact a hydrophobic zeolite which has a zeolite space lattice structure [ (AIO2) _x (Siθ2) y], wherein x and y are integers and y/x > 15, in a quantity which is sufficient to remove in a sufficiently effective manner mercaptans and H2S.

The hydrophobic zeolites used according to the present invention are composed as follows Na _x [ (AIO2) _x (Siθ2) y], wherein x and y are integers and y/x > 15, preferably y/x > 20, most advantageously y/x > 45. Two preferred hydrophobic zeolites are mordenite and silicalite, especially mordenite. The hydrophobic zeo¬ lites can be used in the form of powder on carriers, such as in rotors, or in the form of pellets of sintered zeo¬ lite crystals. In pellet form, generally used binding agents are silica, aluminium oxide or certain clays. In the Examples given below, the binding agent is aluminium oxide in a content of 15% by weight.

When using hydrophobic zeolites for cleaning air which contains mercaptans and H2S, the air is caused to contact the hydrophobic zeolites, preferably mordenite and/or silicalite, most advantageously mordenite, in a quantity which is sufficient to effectively remove mer¬ captans and H2S.

The hydrophobic zeolites may be present in the form of pellets and may be used in a filter bed. The advantage of the zeolites being present in the form of pellets in¬ stead of powder or granulates is that a small pressure drop is obtained across the filter bed. In the industry, thousands of m^ air/h may flow through the filter bed. On the other hand, zeolites in powder form can be used, if carriers are used, and an example of carriers is a rotor.

The enclosed Figs 1-4 concern the experiments according to Examples 1-4.

Fig. la shows adsorption of H2S at 20°C while using mordenite. Fig. lb shows adsorption of H2S at 45°C while using mordenite.

Fig. 2 shows the breakthrough curve of mordenite in the adsorption of CH3SH, when the concentration of CH3SH at the inlet is 700 ppm. Fig. 3 shows the breakthrough curve of mordenite in the adsorption of H2S, when the concentration of H2S at the inlet is 45 ppm.

Fig. 4 shows the breakthrough curve of mordenite in the adsorption of H2S, when the concentration of H2S at the inlet is 70 ppb. Example 1

Adsorption of H2S, CH3SH and C2HgS2 while using mordenite and silicalite

Pellets of the hydrophobic zeolites mordenite and silicalite with Si:Al > 50 and 90, respectively, are used in an experiment for adsorption of pure H2S, CH3SH and ^c 2 ^H β ^s 2 ^at different temperatures. The results are shown in Table 1.

TABLE 1

Zeolite Temperature Gas Adsorption

(20°C) (% by weight)

Mordenite 20°C H ₂ S 5.6

45°C TT 3.8

Silicalite 20°C TT 1.0

45°C TT 0.1

Mordenite 20°C CH3SH 12.7

45°C TT 9.1

Silicalite 20°C TT 10.0

45°C TT 7.5

Silicalite 20°C C ₂ H ₆ S ₂ 10.2

45°C TT 7.5

As is evident from Table 1, mordenite is superior to silicalite in respect of adsorption of H2S, but also the silicalite adsorption of H2S is satisfactory. The reason for these experiments at 45°C is that emissions of air from the paper industry have higher temperatures, about 20-60°C, and we thus want to show that mordenite and silicalite also function in this range of temperature. Example 2 Breakthrough tests for mordenite with CH3SH Pellets of mordenite (from Tosoh, Japan) with Si:Al > 50 were used in a filter bed (having a volume of 0.3 dm^) , and air having a relative humidity of 60% and a CH3SH con¬ centration of 700 ppm was passed through the bed with a dwelling time of 0.6 s. The exhaust air had a CH3SH con- centration of less than 1 ppm for 50 minutes. Example 3 Breakthrough tests for mordenite with 45 ppm H2S

Pellets of mordenite (from Tosoh, Japan) with Si:Al > 50 were used in a filter bed (having a volume of 0.3 dm^), and air having a relative humidity of 60% and an H2S con¬ centration of 45 ppm was passed through the bed with a dwelling time of 1.5 s. The exhaust air had an H2S con¬ centration of less than 1 ppm for 18 minutes. Example 4 Breakthrough tests for mordenite with 70 ppb HoS

Pellets of mordenite (from Tosoh, Japan) with Si:Al > 50 were used in a filter bed (having a volume of 0.3 dm^), and air having a relative humidity of 75% and an H2S con¬ centration of 70 ppb was passed through the bed with a dwelling time of 1.5 s. The exhaust air had an H2S con¬ centration of less than 1 ppb for 38 minutes.

This experiment shows the effective adsorption of H2S, which mordenite has even at low H2S concentrations. The use according to the present invention is highly applicable to the paper industry for eliminating nasty- smelling emissions of mercaptans and the very toxic H2S. H2S which is present in the air in sewage treatment

plants is also readily removed according to the present invention. A- further field of application is cleaning of air which contains mercaptans and H2S in computer rooms. In fact, it has recently been found that H2S or other sulphur compounds cause corrosion of the computers.