Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By contact with solid sorbent
Reexamination Certificate
1999-09-10
2001-09-11
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By contact with solid sorbent
C585S824000, C585S825000
Reexamination Certificate
active
06288299
ABSTRACT:
The present invention relates to a new process for removing polymerization inhibitors from monomer mixtures, especially from ethylenically unsaturated monomers.
In the industry of polymerization of a large number of ethylenic monomers an important problem has to do with the storage and/or transport of these monomers. This is because an uncontrolled spontaneous polymerization of these monomers in the course of time, starting from free radicals, can be observed. These ethylenically unstable monomers are especially those which have a second unsaturation such as a COOH, C═O, C≡N, C═C, C═S or C═N functional group; they may be, for example, the following monomers: styrene, butadiene, isoprene, (meth)acrylic esters, acrylonitrile, acrolein, chloroprene, vinyl acetate, etc.
To avoid this degradation of the monomers it is known to stabilize them by means of inhibiting substances which prevent polymerization from taking place.
These substances, more generally known as “polymerization inhibitors” can be chosen from picric acid, nitroaromatics, quinone derivatives (hydroquinone, benzoquinone), naphthols, amines (p-phenylenediamine, phenothiazine), phosphites, p-methoxyphenol, p-tert-butylcatechol, etc.
When is it desired to employ inhibited monomers in order to polymerize them or to use them in chemical reactions, it is often necessary to remove the polymerization inhibitors. A number of means are employed for this purpose:
it is possible to add a large quantity of initiator to the reactor to combat the effect of the inhibitor; however, this technique is not suitable in every case,
the temperature can be raised considerably to produce the thermolysis of the inhibitor, but the monomer must have a high thermal stability,
the charge of monomers and of inhibitor can be distilled, but the monomer must exhibit good thermal stability; in addition, such an operation is difficult to carry out on an industrial scale, and the boiling point of the inhibitor is in many cases higher than that of the monomer,
the inhibitor can be removed by adding a dilute solution, for example of sodium hydroxide, the monomer charge being subsequently washed with water in order to remove all traces of caustic compounds; however, the treatment of the liquid effluents also presents industrial problems,
lastly, the inhibitor can be adsorbed with the aid of a compound such as alumina, silica gel, activated carbon, calcium oxide, aluminium silicate, talc, calcium sulphates, magnesium sulphates, copper sulphates, magnesium silicate clays, a resin etc.
Adsorption is one of the most advantageous methods because it does not exhibit any of the abovementioned disadvantages. Among the adsorbents employed, activated alumina is preferred.
The objective of the present invention is to propose a new alumina for the adsorption of polymerization inhibitors from ethylenically unsaturated monomers, exhibiting adsorption capacities which are improved in relation to the aluminas of the prior art.
To this end the invention relates to a process for adsorption of polymerization inhibitors from ethylenically unsaturated monomers, in which these inhibitors are placed in contact with an alumina, the said alumina exhibiting a volume of pores of diameter greater than 100 Å of at least 0.20 ml/g. preferably of at least 0.25 ml/g, still more preferably of at least 0.30 ml/g, and a specific surface of at least 30 m
2
/g, preferably of at least 60 m
2
/g, still more preferably of at least 80 m
2
/g.
The volume of the pores of diameter greater than 100 Å represents the cumulative volume created by all the pores of size greater than a diameter of 100 Å. These volumes are measured by the mercury intrusion technique, in which Kelvin's law is applied.
The specific surface indicated is a surface measured by the BET method. A surface measured by the BET method is intended to mean the specific surface determined by nitrogen adsorption in accordance with ASTM standard D 3663-78 established from the Brunauer—Emmett—Teller method described in the periodical “The Journal of the American Chemical Society”, 60, 309 (1938).
The sum total of these two characteristics ensures a high adsorption of the polymerization inhibitors by the alumina when compared with the aluminas employed in the prior art.
The alumina may, for example, be in the form of beads, extrudates or monoliths.
The processes for the preparation of the aluminas exhibiting the pore volume and specific surface characteristics which are necessary for making use of the process according to the invention are known to a person skilled in the art.
Insofar as the specific surface is concerned, this may be controlled especially by the calcination (or activation) temperature of the aluminas following their forming.
In the case of the pore volume, its control is due essentially to the choice of the starting alumina employed for the forming and to the operating conditions of forming of the alumina. A person skilled in the art knows these conditions. Some examples are given below.
If the alumina employed is in the form of beads, these beads may be produced by forming by a rotational technique or by drop (so-called oil-drop) coagulation.
Forming by a rotational technique is an agglomeration of the alumina produced by placing the alumina in contact and rotating it about itself. The rotary coating pan and the rotating drum may be mentioned as equipment used for this purpose.
This type of process makes it possible to obtain beads of controlled pore sizes and distributions, these sizes and these distributions being, in general, created during the agglomeration stage.
The porosity may be created by various means, like the choice of the particle size of the alumina powder or the agglomeration of several alumina powders of different particle sizes. Another method consists in mixing with the alumina powder, before or during the agglomeration stage, a compound known as a pore-former, which disappears completely on heating and thus creates a porosity in the beads.
Pore-forming compounds employed which may be mentioned by way of example are wood flour, charcoal, sulphur, tars, plastics or plastic emulsions such as polyvinyl chloride, polyvinyl alcohols, naphthalene or the like. The quantity of the pore-forming compounds added is determined by the desired pore volume.
The alumina powder employed as starting material can be obtained by conventional processes such as the precipitation or gel process and the process using rapid dehydration of alumina hydroxide such as Bayer hydrate (hydrargillite).
This last alumina is obtained especially by rapid dehydration of hydrargillite with the aid of a stream of hot gases, the entry temperature of the gases into the equipment generally varying approximately from 400 to 1200° C., the contact time of the alumina with the hot gases being generally between a fraction of a second and 4-5 seconds; such a process for preparing alumina powder has been described particularly in patent FR-A-1 108 011. This last alumina is that preferred in the invention.
The control of the volumes of the pores of given diameter can also be carried out during this agglomeration stage by suitable adjustment of the rate of introduction of the alumina powder and optionally of water, of the speed of rotation of the equipment or by the introduction of a forming initiator.
Following this agglomeration, the beads obtained may be subjected to various operations intended to improve their mechanical strength, such as maturing by holding in an atmosphere of controlled moisture content, followed by calcining and then an impregnation of the beads with a solution of one or more acids and a hydrothermal treatment in a confined atmosphere. Finally, the beads are dried and calcined so as to be activated.
Forming by drop coagulation consists in introducing drops of an aqueous solution based on an aluminium compound into a water-immiscible liquid (oil, kerosene, etc) such that the drops form substantially spherical particles; these particles are coagulated simultaneously with and/or subseq
Burns Doane Swecker & Mathis L.L.P.
Griffin Walter D.
Nguyen Tam M.
Rhodia Chimie
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