GB2060653A - A method of manufacturing superficially hydrophilized filling bodies - Google Patents

Abstract

The method comprises the steps of bringing polypropylene filling bodies in a vessel into contact with a gas containing from 0.5 to 10 volume per cent of sulphur trioxide and less than 5 volume per cent of free oxygen, at a filling body temperature between 10 and 50 DEG C, the gas temperature being not more than 10 DEG C higher and not more than 35 DEG C lower than the filling body temperature, and subsequently bringing the filling bodies into contact with a gaseous or liquid medium containing a substance reactable with sulphur trioxide.

Description

SPECIFICATION Method of manufacturing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units The invention relates to a method of manufacturing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units by the action of gaseous sulphur trioxide, such as, water-wettable filling bodies for absorption and extraction columns, cooling towers and like plants.

In the Czechoslovak Inventor's Certificate No. 169,054 there have been disclosed filling bodies for mass and heat exchange units, which are made of thermoplatic materials of substantially hydrophobic type such as polyethylene, polypropylene, polyvinyl chloride or the like, the surface of which is coated with a thin layer of another, water-swelling polymeric material. The surface of such filling bodies is better utilized because of the fact that, on the one hand, a larger surface portion of the bodies is covered with a flowing down aqueous film than in case of conventional filling bodies made of hydrophobic thermoplastic materials, and, on the other hand, that the hydrodynamic film flow conditions are improved thereby.The final result thereof is in that the active interphase surface on which a mass and/or heat exchange takes place, if compared with geometrically analagous filling bodies made of hydrophobic thermoplastic materials, is approximately double, or, in other words, the height of the respective exchange unit can be reduced to about a half with practically the same output maintained.

Thermoplastic filling bodies having a hydrophilic surface layer thereon have preferably been prepared in that the surface of the body made of hydrophobic thermoplastic material is subjected to an appropriate chemical reaction such as, for instance, sulfonation, or oxidation. The reaction and the conditions thereof have to be chosen so as to give rise to a continuous resistant coating of a hydrophilic, water-swelling but insoluble polymer. Since most of the hydrophilizing reactions, above a certain degree of conversion, lead up to water-soluble products, and since between the unswollen hydrophobic layer and the swollen hydrophilic one, a tangential tension arises which latter may even lead to peeling off of the hydrophilic layer, it is evident that the suitable reaction conditions are considerably limited.As a rule, such a hydrophilizing reaction is to be supplemented by simultaneously cross-linking the hydrophobic layer, and a diffusion of reagent into the hydrophobic polymer is to be promoted so that a swelling power gradient obtains between the two layers, and the tangential tension on the interfacial surface may decrease. Further limitation in the choice of an appropriate reaction is imposed by the price of filling bodies, in view of the manufacturing costs connected with the hydrophilizing process.

Thermoplastic materials mostly used for this purpose are polyolefins, especially polyethylene and isotactic polypropylene. The material which is particularly preferable because of its specific weight and suitable thermo-mechanical characteristics is isotactic polypropylene which, in addition, is relatively inexpensive. Its disadvantage, however, lies in its considerably crystalline character so that low molecular weight reagents diffuse thereinto too slowly while the hydrophilizing reaction takes place at a relatively high speed. It is why in various processes such as, for instance, superficial oxidation by chromic acid, the surface is exposed to an etching effect so that immediately after being formed the superficial thin hydrophilic layer is dissolved and washed off whereby the hydrophobic surface is being steadily denuded.For this reason, it is not possible to use with polypropylene some of the technologically suitable processes usable with polyethylene, such as, for example, oxidation in liquid phase. The only practically viable process has been constituted by cool sulfonation by oleum, the reaction having been accompanied by oxidation to a small extent only.

However, the sulphonation by liquid oleum has various disadvantages which particularly result mainly from the fact that the sulfonated surface well absorbs oleum and retains a large amount thereof even after the reaction has ended. The manipulation of large amounts of oleum and the disposal of its remnants adhering to the filling bodies is troublesome as regards both safety and hygiene.

Because of the adherence of considerable amounts of liquid reagent to the filling bodies it is evident that it would be more advantageous, in general, to carry out the reaction in a gaseous phase. In view of the shape of the filling bodies, however, it is impossible to use the mathod used with polypropylene foils, such as oxidation initiated by corona discharge, or the like. Apart from this, such methods of hydrophilization would be insufficient for filling bodies since they have been developed for a specific purpose, viz. to enable wrapping sheets to be printed, where the requirements as to hydrophilic properties are incomparably less demanding.

Sulfonation by means of gaseous sulphur trioxide has been accompanied by great difficulties. At a relatively high concentration of sulphur trioxide, blisters or isles arise in highdegree reaction regions where the hydrophilized layer consequently peels off after having been swollen. During the dilution of sulphur trioxide by air, induced oxidation by oxygen occurs, giving rise to dark coloured tarry products which are soluble in water, and particularly in alkaline solutions. The main difficulty results from the autoacceleration character of the reaction, especially if accompanied by oxidation, and from its high activation power.The combination of these effects causes the reaction to take place for a time without any change in the hydrophilic character of the surface (the so-called induction phase) whereupon the reaction proceeds at a considerable speed up to a high degree of conversion. Such a reaction can be controlled only with difficulty due to substantial differences in reaction degrees between the regions which get first into contact with the gaseous reagent, and where it acts somewhat longer and at a higher concentration, and those regions exposed to the reagent somewhat later and at a lower concentration thereof; this is caused, on the one hand, by the reaction per se and, on the other hand, by the sorption of sulphur trioxide in the layer subjected to the reaction and on the surface thereof.

When the reaction takes place in the usual manner, e.g. when sulphur trioxide vapours are introduced into a reaction or a column containing the polypropylene filling bodies, the result is practically negative since at the inlet portion of the respective unit, the filling body surface becomes prematurely "burnt", which means sulphonated to an excessive depth and reacted to a high conversion degree as well as oxidized while at the gas outlet, the surface of the polypropylene bodies remains practically hydrophobic. When the reaction time is chosen, at a given temperature and sulphur trioxide concentration, in such a manner that the conversion takes place in the main, i.e.

intermediate region of the reactor to a sufficient and optimum extent, that part of the bodies exposed to the reagent at the inlet for a longer time and at a higher reagent concentration, becomes reacted already to an undesirable, substantially higher conversion degree, whereas the bodies at the outlet portion coming into contact with the reagent a little later and at a little lower sulphur trioxide concentration, are still in the induction period region, are non-reacted and practically hydrophobic. The situation is schematically shown in the enclosed diagram where the curve a corresponds to the reaction at the gas inlet, the curve b in the middle of the charge, and c at the gas outlet; ordinates ta, tb, tc indicate the respective reaction times and abscissae 0:a' cry,, a, the respective conversion grades.As is apparent from the diagram, even negligible, seemingly unimportant#iffemnces in the length of the period of contact between the body and the reagent can result in considerable differences in the conversion grades achieved.

These differences further grow if the gas is warmer than the filling bodies, or if the gas contains oxygen in such a concentration that a substantial exothermic induced oxidation takes place so that moreover a temperature gradient occurs inside the filling bodies. Such negative effects may even be combined. The result is very marked so that the filling bodies have a flamed appearance even if the entire contact period is, of the order of tens of seconds, or several minutes, and if the gas advances at such a translational velocity that the time difference of initial contact is a matter of seconds at the most. The body bulk portions near the gas inlet, outer edges and projections of the individual filling bodies are -"burnt", dark coloured, whereas the not exposed areas of the surface, the parts facing away from the gas flow or-remote from the inlet remain practically not reacted.It is evident from the foregoing that such a reaction in which the filling bodies are exposed to a gas flow containing sulphur trioxide - even if advantageous on principle - is practically unusable.

It is an object of the present invention to eliminate the-drawbacks of prior art as hereinabove referred to and to provide an improved method of manufacturing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units by the action of gaseous sulphur trioxide.

In a method according to the invention polypropylene filling bodies in a vessel are brought into contact with a gaseous medium containing from 0.5 to 10% by volume, preferably from 1.5 to 5% by volume of sulphur trioxide and less than 5% by volume of free oxygen, at a filling body temperature varying between 10 and 500 C, the gas temperature being at most 1 00C higher or at most 350C lower than the filling body temperature, whereupon by interrupting the contact between the filling bodies and the sulphur trioxide containing gas, the filling bodies are brought into contact with a gaseous or liquid medium containing a substance reactable with sulphur trioxide.

In accordance with another feature of the invention, at least one substance selected from the group comprising water, ammonia, alkali carbonate-or hydrogen carbonate, and alkali or alkali earth metal hydroxide is used as the substance reactable with sulphur trioxide. During the reaction, it is possible to intensively change the position of individual filling bodies, both relative to the vessel and to one another. The reaction can be carried out in a rotating vessel.

Another feature of the invention consists in that a contact gas from sulphuric acid production, preferably a gas which passed through at least one absorber is used as the sulphur trioxide containing gaseous medium.

The effect of the invention resides in the elimination of unfavourable influences ascertained during the study of the process. The process makes it possible to average the reaction times for individual filling bodies as well as for their surface areas so that the time and concentration fluctuations in various parts of the area exposed to the reaction are neglible in view of the practical requirements on the homogeneity of hydrophilization. This can be achieved in practice in several ways; the filling bodies can be either agitated or kept air-borne by means of the reaction gas, or the reaction is carried out in a vessel rotatable about the horizontal or oblique axis while the inner wall of the vessel can be provided with weli-known projections, vanes or the like for facilitating the stirring step. The last mentioned mode has been found most viable and technologically most suitable since it can be used in conventional, only unsubstantially adapted plants such as rotary driers, mixers, granulators or the like; in this way the intensity of agitation does not depends neither on the translational gas velocity nor on the shape or size of the filling bodies.

It is advantageous to perform the method according to the invention in a continuous process, and particularly because the product becomes more homogeneous and the reaction space is permanently filled up with the reaction gas. Such a process prevents corrosion effects which may be caused by sulphuric acid formed by the reaction of sulphur trioxide with air humidity so that the reactor need not be made of noble anticorrosive alloys, and may be made of e.g. iron.

The filling bodies continuously passing through the oblique rotary reactor cylinder on its wall along a helical path while all parts of their surface are successively exposed to the gas flow. The sulphur trioxide containing gas can be supplied in any manner, which means either co-currently or counter-currently, or through a perforated central tube adapted in such a way that the sulphur trioxide concentration in every portion of the reactor is alike. In such a plant, the reaction can also be effected discontinuously as, for example, in those cases where only a gas with a relatively low sulphur trioxide content is available which requires a longer reaction time.

It is preferred when the sulphur trioxide containing gas, at the inlet to the reaction space, is cooler than the filling bodies to be treated since those parts which react at a higher sulphur trioxide concentration and for a long time, react at a lower temperature. This leads to a partial straightening of the curves a, b, c (see diagram) which favourably influences the product homogeneity.

Such an optimum temperature difference depends upon the other parameters but should not exceed 350C; the most frequent value is 1 OOC.

Any inert gas may be used for the dilution of sulphur trioxide or of a gas containing sulphur trioxide in an excessively high concentration, such as nitrogen, carbon dioxide, or cool, filtered or otherwise purified flue gases with low oxygen content.

After the reaction, the surface layer contains sorbed sulphur trioxide so that the reaction could go on even when the filling bodies are no longer in immediate contact with the gaseous reagent. If the bodies are now exposed to dry air in which oxygen is not instantly disposed of by air humidity, an undesirable induced oxidation and consequently darkening of the product may occur.

Therefore an indispensable step of the process consists in the interruption of sulfonation which can be effected in that after the filling bodies have ceased to be exposed to the sulphur trioxide containing gas they are brought into contact with a gaseous or liquid medium containing the necessary amount of components reactable with sulphur trioxide to form compounds which are harmful for the intended purpose. Such a medium can be constituted, e.g., by humid air, water, water steam, liquid or gaseous ammonia (appropriately diluted), aqueous solutions of alkaliferous substances such as carbonates or hydrogen carbonates of alkali metals, hydroxides of alkali or alkali earth metals, or the like.

Aqueous alkali solutions are preferred because they convert sulfone, sulphate and also carbonyl groups in the surface layer into a neutralized and consequently more hydrophilic ionized form. Apart from this, the dark coloured oxidation products are soluble in alkaline solutions so that the filling bodies lose their dark colour, and simultaneously some small local differences in colour disappear; the latter, are not functionally harmful but, as a rule, are objectionable for commercial reasons.

The neutralization of the adhered sulphur trioxide is advantageous in that it simplifies the entire process since the filling bodies need not be dried and the reaction can be interrupted in a continuous process in separate sections, downstream of the reactor, or in a discontinuous process in the reactor by following the sequence: reaction, purging with a dry inert gas, neutralization by ammonia, purging with air. It is true that the body surface contains then a small amount of solid ammonium sulphate but this is not harmful in most applications. If necessary ammonium sulphate can be washed off whenever necessary.

The practically most suitable, most available and cheapest reagent for this purpose is contact gas present in the sulphuric acid production. Such a gas is available in large amounts and, by the selection of the point of withdrawal site, it is possible to choose both its composition and characteristics. The gas withdrawn from the first or the second absorber is preferred. The former corresponds in its composition to an equilibrium with oleum and is already cooler than before entering the first absorber. The gas, before having been introduced into the reactor containing the polypropylene filling bodies, has to be cooled, preferably to a temperature lower than that of the bodies.However, it is also possible to use final gases from the absorption, which contain a small amount of sulphur trioxide but are cool already and enable, with a larger reaction time, a higher homogeneity of the product to be achieved. Apart from this, final gases are waste material and are therefore free. It is therefore advantageous to carry out the hydrophilization process according to the invention directly in the plant for the manufacture of sulphuric acid where also ammonia for the manufacture of fertilizers is available.

The following examples are given as illustrative only without limiting the claimed scope of the invention to the details contained therein.

EXAMPLE 1 A rotating cylindrical glass vessel having an inclination of 450 from the vertical was filled at a temperature of 250C with filling bodies of cylindrical configuration (Raschig rings) of polypropylene. The vessel was supplied with a gaseous medium at 200C containing 4% by volume of sulphur trioxide and 96% by volume of nitrogen, the medium having been prepared by allowing dry nitrogen to bubble through oleum.

After 5 minutes the vessel was purged with pure nitrogen, under continuous rotation, and the bodies were poured into a 5 per cent aqueous solution of sodium bicarbonate. The rings were washed by water and dried at 600 C. They were practically colourless and perfectly wettable with water and aqueous solutions which flowed down on them in the form of a thin film. Compared with the same but non-hydrophilized filling bodies, the active interphase surface was more than double so that the height of the respective absorption or cooling tower could be reduced to less than a half at the same output, or, at the original dimensions the output could be increased to more than double.

EXAMPLE 2 The example demonstrates a negative result when the above-mentioned reaction conditions have not been maintained.

Saddle-shaped polypropylene filling bodies were hydrophilized for 5 minutes at 230C in a stationary cylindrical glass vessel by means of a contact gas containing 8% by volume of sulphur trioxide, 7.5% by volume of oxygen (02), 1% by volume of sulphur dioxide and 83.5% by volume of nitrogen (N2), the gas having been cooled to 400 C.

The filling bodies, after having been purged with dry air, and washed with water, had a dark colour shade at the gas inlet, and a thin hydrophilic layer on their surface tended to peel off. On the other hand, the filling bodies adjacent the gas outlet were locally imperfectly hydrophilized.

EXAMPLE 3 A cylindrical reactor of stainless steel, rotating about a horizontal axis, was filled, at 250 C, with saddle-shaped filling bodies. The reactor was then filled with a gas at 250C containing 7% by volume of sulphur trioxide, 2.2% by volume of oxygen (02) and 90.8% by volume of nitrogen (N2). The reaction time was 10 minutes at 40 r.p.m. After the gas had been displaced by dry nitrogen, the reactor was filled with a mixture of 9 volume parts of dry nitrogen and one volume part of ammonia.

After being washed with water the polypropylene filling bodies were superficially hydrophilic and exhibited characteristics similar to those referred to in EXAMPLE 1.

EXAMPLE 4 Pall rings of polypropylene were exposed in a concrete mixer rotating at 30 r.p.m. for 20 minutes at 300C to a flow of final gases left in the production of sulphuric acid, containing about one percent of sulphur trioxide and approximately the same amount of oxygen, the gases having been cooled to 220C. The mixer was equipped with a gas-tight inlet and outlet for the gas. The mixer was then purged first with dry nitrogen, then with dry air or nitrogen containing 10% volume of ammonia, and finally with air. The rings were superficially hydrophilic similarly as described in EXAMPLE 1.

EXAMPLE 5 Raschig rings of isotactic polypropylene (15 mm in diameter and height) were hydrophilized for 70 minutes without agitation in a column, at 200 C, by a gas at 200C containing 0.5% by volume of sulphur trioxide and 99.5% by volume of nitrogen. The column was then purged with a mixture of 9 volume parts of nitrogen and one volume part of ammonia, and finally with air.

The filling bodies were found more uniformly hydrophilic than those treated in accordance with EXAMPLE 2.

Claims (8)

1. A method of manufacturing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units by the action of gaseous sulphur trioxide, the method comprising bringing polypropylene filling bodies in a vessel into contact with a gas containing from 0.5 to 10 volume per cent of sulphur trioxide and less than 5 volume per cent of free oxygen, at a filling body temperature between 10 and 500 C, the gas temperature being not more than 1 00C higher or not more than 350C lower than the filling body temperature, interrupting the contact between the filling bodies and the sulphur trioxide containing gas, and bringing the filling bodies into contact with a gaseous or liquid medium containing a substance reactable with sulphur trioxide.

2. A method according to claim 1, wherein the gas contains 1.5 to 5% by volume of sulphur trioxide.

3. A method according to claim 1 , wherein at least one substance selected from the group consisting of water, ammonia, alkali carbonate or hydrogen carbonate, and alkali or alkali earth metal hydroxide is used as the substance reactable with sulphur trioxide.

4. A method according to any one of claims 1 to 3, wherein the position of individual filling bodies is changed during the reaction, both relative to the vessel and to one another.

5. A method according to any one of claims 1 to 4, wherein the reaction is carried out in a rotating vessel.

6. A method according to any one of claims 1 to 5, wherein a contact gas from the production of sulphuric acid is used as the sulphur trioxide containing gas.

7. A method according to claim 6, wherein the sulphur trioxide containing gas is a gas which passed through at least one absorber.

8. A method of manufacturing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units by the action of gaseous sulphur trioxide substantially as herein described with reference to Example 1, Example 3, Example 4 or Example 5.