FR2691375A1 - Purificn. of waste industrial gases - by adsorption on a hydrophobic zeolite contg. platinum or palladium - Google Patents

Abstract

A process for the removal of ill-smelling and/or toxic gases from gas mixts. comprises the following stages: 1) an absorption stage in at least one vessel, in which the gases are passed through an absorbent composed of at least one hydrophobic zeolite contg. Pt and/or Pd or mixed with a catalyst which will initiate catalytic combustion. 2) a combustion-desorption stage, in which the hot gases are passed through the zeolite contg. the absorbed substances so as to de-sorb and burn them on the zeolite. USE/ADVANTAGE - The invention seeks to eliminate all cpds. causing atmos. pollution, which emanate from chemical and food manufacturers, e.g. styrene from plastic workshops. Techniques up to now, have mainly used activated carbon as absorbent which is however, difficult to re-generate due to its combustion, thus limiting regeneration temps.

Description

The present invention relates to a method for treating smelly and / or toxic gases.

In recent years, more and more attention has been paid to the discharge into the atmosphere of smelly and / or toxic compounds from various industries.

Thus, the tanks and reservoirs of effluents from chemical manufacturing or foodstuffs, the ventilation vents of plastic workshops or paint booths are the source of releases to the atmosphere of foul-smelling and / or toxic gases at levels more or less important.

The purpose of the process according to the invention is to ensure the elimination of the compounds responsible for this atmospheric pollution.

One of the techniques most used in the prior art is to adsorb smelly and / or toxic compounds on activated carbon. However, activated carbon has difficult regenerability because it is combustible and does not withstand high temperatures.

The method according to the invention is a method for treating smelly and / or toxic gases which therefore contain smelly and / or toxic substances responsible for the bad odor and / or the toxicity of these gases, for example styrene, said method including the following successive steps
1. An adsorption step: in this step, we pass into a
surrounds said gases through at least one sub-capture mass
smelly and / or toxic stanzas, formed by at least one zeolite
hydrophobic, so that said substances contained in said gases are
adsorbed by this zeolite; moreover, this zeolite contains platinum
and / or palladium or is mixed with a catalyst capable
initiate catalytic combustion: thus the combustion phase of
the following step 2) is carried out inside the same enclosure on
zeolite.

2. A combustion-desorption step: in this step, we pass
hot gases through said zeolite which contains the wrong substances
odorous and / or toxic adsorbed, in order to desorb said substances,
and burn them on the zeolite.

The zeolite used has advantageously been made hydrophobic by a dealumination treatment with a natural or synthetic zeolite, preferably with a natural zeolite; any dealumination treatment known to a person skilled in the art can be used, for example a direct acid treatment. This hydrophobic zeolite preferably has an atomic ratio of at least 25, generally at least 30 and for example between 30 and 40. It is usually chosen from the group formed by mordenite and clinoptilolite.

The hot gases used in step 2) are introduced into the zeolite, in general at a temperature between 200 and 3500 C. They are generally inert combustion gases (for example a mixture of carbon oxides, oxides nitrogen and water in particular), from a boiler and into which an increasing amount of oxygen is added. After a certain time, these hot gases can consist of hot air. Step 2) in particular makes it possible to regenerate the zeolite used and which has reached saturation.

 The effluent from step 2) can optionally be then subjected to an ultimate combustion step, generally at a temperature above 8000 C, for example in a boiler which may be the one used to heat the gases used in the stage 2) of combustion-desorption.

 The effluent from step 2) or from the ultimate combustion step (when this is present in the process according to the invention), may advantageously be subjected to a step for removing sulfur oxides contained in said effluent. This step is advantageous when the smelly and / or toxic gases to be treated contain sulfur compounds, for example mercaptans.

It lowers the sulfur oxide content to a value usually below 150 ppm. It can be carried out according to any method of removing sulfur oxides known to those skilled in the art, for example according to the methods described in patents US-A-4,725,417, US-A-4,781,903 and US
A-4 855 117 in the name of the plaintiff.

The effluent from step 2) or from the ultimate combustion step (when it is present in the process according to the invention) or from the step of removing sulfur oxides (when that -this is present in the process according to the invention) perhaps advantageously subjected to a step of elimination of the nitrogen oxides contained in said effluent. These nitrogen oxides generally come from the hot gases used during stage 2) of combustion-desorption.

This nitrogen oxide removal step can be carried out according to any method known to those skilled in the art, for example by selective catalytic reduction of the nitrogen oxides with ammonia, it is possible in particular to use the method described in patent application EP-A-0 325 511 in the name of the applicant.

At least two zeolite beds may be optionally arranged alternately on the circuit of smelly and / or toxic gases to be treated. Thus, when the first bed of zeolite has reached saturation, the flow of smelly and / or toxic gases is sent to another bed of zeolite: the first bed of zeolite can then be regenerated without the treatment process being interrupted.

The method according to the invention can also be implemented on two different geographic sites: when the zeolite used, arranged on a first site, has reached saturation after step 1) of adsorption, it is replaced on this same site by another "new" zeolite and is sent to a second site where step 2) of combustion-desorption takes place and possibly the other steps described above. Once regenerated, it can then possibly be sent back to the first site in order to replace the other zeolite that has reached saturation. Thus, the installations used in the method according to the invention are not necessarily arranged on the same geographical site.

The single figure illustrates one of the possible variants of the invention.

A stream of smelly and / or toxic gases (1) passes through a 3-way valve (2) then through a pipe (3) to arrive in an adsorber (4) charged with a hydrophobic zeolite containing for example palladium which will adsorb the smelly and / or toxic substances contained in these gases. A pump (8) circulates these gases through the adsorber (4), the pipe (5), a valve which here by way of example is of the three-way valve type (6) and the pipe (7).

When the zeolite has reached saturation, on the one hand, the three-way valve (2) is closed in the driving direction (1) -conduction (3) and open in the driving direction (13) -conduction (3), d on the other hand, the three-way valve (6) is closed in the pipe (5) - pipe (7) direction and open in the pipe (5) pipe (9) direction. A gas stream (10) is then heated through a boiler (12) supplied with natural gas (11), then is supplied by the line (13), the three-way valve (2) and the line (3) towards the adsorber (4): the smelly and / or toxic substances are then burned inside the adsorber (4) and desorbed. The effluent leaving the adsorber (4) via the pipe (5) passes through the three-way valve (6) then the pipe (9) to arrive in a section for removing sulfur oxides (14). The effluent leaving this section (14) via line (15) is directed to a section for selective catalytic reduction of nitrogen oxides (16) supplied with ammonia through line (17).

The gases leaving the section (16) are then "clean", that is to say free from smelly and / or toxic substances.

Optionally, before being introduced into section (14), the effluent from the adsorber (4) can be sent via line (19) to the boiler (12) where it undergoes ultimate combustion at around 9000C . The effluent then leaves the boiler through the pipe (20).

Claims (5)

1- Process for the treatment of smelly and / or toxic gases, containing smelly and / or toxic substances, comprising the following successive steps  1. An adsorption step, in at least one enclosure in which the  said gases pass through at least one mass of sub capture  smelly and / or toxic stanzas formed by at least one zeolite  hydrophobic, said zeolite containing platinum and / or palladium or  being mixed with a catalyst capable of initiating catalytic combustion  lytic,  2. A combustion-desorption step, in which hot gases pass  through said zeolite containing said smelly substances  and / or toxic, in order to desorb them, and to burn said substances on  said zeolite. 2- The method of claim 1 wherein said hydrophobic zeolite is prepared by dealumination of a natural or synthetic zeolite. 3- The method of claim 2 wherein said hydrophobic zeolite is prepared by dealumination of a natural zeolite selected from the group formed by mordenite and clinoptilolite. 4- Method according to one of claims 1 to 3 wherein said hydrophobic zeolite has an Si / Al atomic ratio of at least 25. 5- Method according to one of claims 1 to 4 wherein, in step 2), the hot gases are introduced into said zeolite at a temperature between 200 and 3500C.