Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2000-06-19
2002-05-28
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S114000, C095S116000, C095S902000, C423S715000, C423S332000, C502S079000
Reexamination Certificate
active
06395069
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an aqueous solution and to its use in an ion-exchange process for the purpose of manufacturing zeolites exchanged with metal cations, in particular zinc cations.
BACKGROUND OF THE INVENTION
Gases and gas mixtures have applications in numerous industrial fields. Thus, the gases of the air, such as, in particular, oxygen and nitrogen, are commonly used in numerous fields, such as electronics, combustion, medicine, foodstuffs, welding, and the like. In addition, it is the same for other gases and gas mixtures, such as synthesis gases, also known as “syngases”, or hydrocarbons, such as, in particular, olefins.
One of the techniques currently used to produce or purify gases, in particular gases of the air, is the technique referred to as PSA (for Pressure Swing Adsorption), which technique encompasses not only PSA processes proper but also analogous processes, such as VPSA or VSA (Vacuum “Pressure” Swing Adsorption), TSA (Temperature Swing Adsorption) or MPSA (Mixed Pressure Swing Adsorption) processes.
According to this PSA technique, when the gas mixture to be separated is, for example, air and when the component to be recovered is oxygen, oxygen is separated from the gas mixture by virtue of Preferential adsorption of at least nitrogen on one or more materials which preferentially adsorb at least nitrogen and are subjected to cycles of given pressure in the separation region. Oxygen, which is not adsorbed or only slightly adsorbed, is recovered at the outlet of the separation region with a purity generally of greater than 90%, indeed even of greater than 93%.
More generally, a PSA process for the separation of a gas mixture comprising a first compound which is adsorbed preferentially on an adsorbent material and a second compound which is adsorbed less preferentially on the adsorbent material than the first compound, for the purpose of the production of the second compound, cyclically comprises:
a stage of preferential adsorption of at least the first compound on the adsorbent material, at an adsorption pressure referred to as “high pressure”, with recovery of at least a portion of the second compound thus produced;
a stage of recompression of the separation region comprising the adsorbent, by changing from the low pressure to the high pressure.
Similarly, in order to be able to be used industrially, some gases have to be purified beforehand, in particular by adsorption and/or by catalysis, in order to convert, transform or remove some of the compounds or impurities which they comprise.
Thus, atmospheric air is usually freed from all or a portion of the impurities which it comprises, in particular water vapour, carbon dioxide, carbon monoxide, hydrocarbons, nitrogen oxides and hydrogen, before being subjected to subsequent stages of fractionation by cryogenic distillation, in order to prevent these impurities from having a detrimental effect on the performance of the air separation units, in particular the cryogenic distillation columns. The removal of these impurities is usually carried out by means of one or more adsorbents, in particular zeolites, optionally in combination with alumina.
Similarly, it is also known to remove impurities, in particular metal residues, liable to be present in olefins, in order to prevent their degradation or their colouring, it being possible for the removal of these impurities to be carried out, for example, by means of an adsorbent.
It is known that the efficiency of the separation or of the purification of the fluid, in particular of a gas mixture, such as air, depends on numerous parameters, in particular on the composition of the fluid to be treated, on the type of adsorbent material used and on the affinity of the latter for the compounds to be adsorbed or to be converted, on the size of the adsorbent particles, on the composition of these particles and on their arrangement in the adsorption region or regions.
The size of these adsorbent or catalyst particles is generally highly variable, given that the adsorbent can have a size from several &mgr;m (powder) to several mm, and is most often of the order of 1 mm to 3 mm.
Zeolitic materials are currently the most widely used adsorbents in gas separation or purification plants employing an adsorption separation process.
In point of fact, in order to improve the adsorption efficiency, it is conventional to introduce mono-, di- and/or trivalent metal cations into the zeolite particles, for example alkali metal, alkaline earth metal, transition metal and/or lanthanide cations.
These metal cations are usually incorporated during the synthesis of the zeolite particles and/or are subsequently inserted by an ion-exchange technique, that is to say, generally, by bringing the particles of crude zeolite into contact with a solution of one or more metal salts comprising the metal cation or cations to be incorporated in the zeolitic structure and subsequent recovery of the particles of exchanged zeolite, that is to say of zeolite comprising a given amount of metal cations.
The proportion (in %) of metal cations introduced into the zeolitic structure with respect to the total exchange capacity is known as the degree of exchange.
Such zeoliltes are disclosed in particular in the documents EP-A-486,384, EP-A-606,848, EP-A-589,391, EP-A-589,406, EP-A-548,755, U.S. Pat. No. 5,268,023, EP-A-109,063 and EP- A-760,248.
Furthermore, the document U.S. Pat. No. 5,419,891 discloses a zeolite of X type exchanged with lithium and zinc cations which exhibits improved adsorption properties for the separation of polar gases.
According to this document, the ion exchange, which takes place during the process for the manufacture of the zeolitic samples, is carried out with an exchange solution of zinc salts having a concentration of zinc of 0.1 N and a pH of between 5.6 and 7.0.
In point of fact, such exchange conditions cannot be regarded as ideal or at the very least favourable or suited to operation on an industrial scale, because they result in certain problems.
This is because it is known that, in aqueous solution, zinc salts form, with OH
−
ions, a hydroxide Zn(OH)
2
of low solubility having a solubility product S of 1.8×10
−14
.
The solubility product S is defined hereinbelow by the equation (I):
S=[Zn
2+
]*[OH
−
] (I)
where: [Zn
2+
] and [OH
−
] are the concentrations in mol.liter
−1
in equilibrium with the precipitate Zn(OH)
2
.
It is then immediately apparent that Zn(OH)
2
can precipitate if the pH exceeds the value of 7.7 and if the concentration of zinc salt exceeds 0.1 N.
It then follows that the examples given in the document U.S. Pat. No. 5,419,891 are carried out under conditions very close to precipitation and are therefore difficult to carry out, in particular, under industrial conditions.
Furthermore, the zeolite exhibits basic properties. For example, an agglomerated zeolite 13× (binder+zeolite), immersed in water, can raise the pH of an aqueous solution to a value which can reach approximately 10.
Bringing an industrial zeolite into contact with a zinc salt solution initially with a pH of about 5.5 to 7 hence necessarily results in an increase in the pH of the solution, the result of which is to precipitate zinc hydroxide, which zinc hydroxide will then seal off the macropores of the zeolite agglomerates and be deposited on the surface of the zeolite crystals.
An important consequence of this phenomenon is that the kinetics of adsorption of the zeolite will then necessarily decrease, due to the presence of this deposit in the activated agglomerates, and the adsorption properties of the zeolite particles thus produced will then also be found to be detrimentally affected thereby.
In attempting to solve this problem of precipitation of Zn(OH)
2
, it may be envisaged to dilute the saline ZnCl
2
solution, so as to eliminate or minimize the precipitation of Zn(OH)
2
.
However, such a dilution poses the problem of the industrial use of very dilut
L'Air Liquide, Societe Anonyme a Directoire et Conseil de S
Spitzer Robert H.
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