CA1157001A

CA1157001A - Method of manufacturing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units

Info

Publication number: CA1157001A
Application number: CA000361717A
Authority: CA
Inventors: Zdenek Krivsky; Miroslav Strnad; Vaclav Linek; Petr Benes; Jiri Sulc; Artur Stoy
Original assignee: Vysoka Skola Chemicko Technologicka V Praze
Current assignee: Vysoka Skola Chemicko Technologicka V Praze
Priority date: 1979-10-18
Filing date: 1980-10-07
Publication date: 1983-11-15
Also published as: DE3035483C2; GB2060653B; JPS5695320A; JPS6249095B2; IT8025042A0; GB2060653A; CS210175B1; DE3035483A1; FR2467619B1; IT1133638B; FR2467619A1

Abstract

ABSTRACT OF THE DISCLOSURE:
A method of manufacturing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units.
In accordance with the invention, polypropylene filling bodies in a vessel are brought into contact with a gas containing 0.5 to 10 vol. % of sulphur trioxide and less than 5 vol. %
of free oxygen, at a filling body temperature ranging between 10 and 50 °C. The gas temperature is at most 10 °C higher or at most 35 °C lower than the filling body temperature. After interruption of the contact between the filling bodies and the gas which can be constituted by a contact gas present in the sulphuric acid production, the filling bodies are brought into contact with a gaseous or liquid medium which is apt to react with sulphur trioxide and which can be selected from the group comprising water, ammonia, alkali carbonate or hydrogen car-bonate, and alkali or alkali earth the metal hydroxide. During the reaction which can be carried out in a rotatable vessel, the position of individual filling bodies is intensively varied.
This method permits to advantageously solve the problem of homogeneity of the hydrophilizing reaction.

Description

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The present invention relates to a method of manufac-turing superficially hydrophilized filling bodies for interphase mass and/or heat exchange units by the action of gaseous sulphur trioxide, such as, for example, water-wettable filling bodies for absorption and extracting 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 thermoplastic materials of substantially hydrophobic types such as polyethylene, poly-propylene, 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, on the one hand, a larger surface portion of the body 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, the hydrodynamic film flow conditions are improved thereby. The final result is that the active interphase surface on which a mass and/or heat exchange takes place, if compared with geometrically analogous filling bodies made of hydrophobic thermoplastic materials, is ap-proximately 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 up to now by subjecting the surface of the body made of hydrophobic ther-moplastic material to an appropriate chemical reaction such as, for instance, sulfonation, or oxidation. The reaction and the conditions thereof are chosen so as to give rise to a continuous resistant coating of a hydrophilic, water-swelling but insoluble polymer. Since most of the hydrophilizing -1- ~, UO~

reac:tions, when surpassing a certain degree of conversion, leacl 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 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 between the two layers a swelling power gradient may rise, and the tangential tension on the interfacial surface may decrease. Other limitations in the choice of an appropriate reaction result from the requirements laid on the price of filling bodies, in view of the manufacturing costs to be expended on the hydrophilizing process.
Thermoplastic materials mostly used for this purpose are polyolefins, especially polyethylene and isotactic poly-propylene. The material that is particularly preferable because of its specific weight and suitable thermomechanical charac-teristics is isotactic polypropylene which apart from this,is relatively inexpensive. Its disadvantage, however, lies in its considerably crystalline character so that low molecular weight reagents can 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 the superficial thin hydrophilic layer, immediately after its rise, gets dissolved and washed off whereby the hydrophobic sur~ace is being steadily denuded. For this reason, it is made impossible to apply to polypropylene some technologically suitable processes usable with polyethylene, such as, for example, oxidation in liquid phase. A sole, f~O~

practically viable process has been constituted by cool sulfonation by oleum, the reaction having been accompanied by oxidation to a small extent only.
Ilowever, the sulphonation by li~uid oleum has to cope with various disadvantages which particularly result from that the sulfonated surface well absorbs oleum and retains a large amount thereof even after the reaction has ended. The manipu-lation of large oleum amounts and the disposal of its remnants adhered to the filling bodies is troublesome with regard to both safety and hygienic rules.
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. With the filling body shape in view, however, it is impossible to avail of the method used with polypropylene foils, such as oxidation initiated by corona discharge, or the like. Apart from this, such methods of hy-drophilization would be insufficient in case of filling bodies since they have been developed to a specific purpose, vlz. to enable wrapping sheets to be printed, in this case the claims laid on hydrophilic properties are evidently incomparably modest.
The sulfonation by means of gaseous sulphur trioxide has been accompanied by great difficulties as yet. At a relatively high concentration of sulphur trioxide, blisters or isles arise in high-degree reaction regions where the hydrophilized layer consequently peels off after having been swollen. During the dilution of sulphur trioxide by air, an induced oxidation by oxygen occurs, giving rise to dark coloured tarry products which are soluble in water, and partic-ularly in alkaline solutions. The main difficulty results from an autoacceleration of the reaction, especially if ac-companied by oxidation, and in a high activation power. A

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combination of such effects causes the reaction to take place for a time without any change in the hydrophilic surface character /the so-called induction phase/ whereupon the reaction goea on at a considerable speed up to a high conversion degree.
At such a course, the reaction is hardly controllable since there arise substantial differences in reaction degrees between the regions that get as 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 ~ se and, on the other hand, by a sorption of sulphur trioxide in the layer subjected to the reaction and on the surface thereof.
In a usual mode of the reaction when, e.g., sulphur trioxide vapours are introduced into a reactor or a column containing the polypropylene filling bodies, the result is practically negative since at the inlet portion of the res-pective 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 polypropylene bodies remains practically hydrophobic. If adjusting the reaction time - by means of temperature and sulphur trioxide concentration - in such a manner that the conversion may take place in the main, i.e. intermediate region of the reactor at a sufficient and optimal extent, that part of the bodies exposed to the reagent at the inlet for a bit longer time and at a bit 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 trloxide concentration, are still in the induction period region, are non-reacted and _~_ 610~

practically hydrophobic.
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 exchanged units by the action of gaseous sulphur trioxide.
The method according to the invention consists in bringing the polypropylene filling bodies in a vessel into contact with a gaseous medium containing from 0.5 to 10 % by volume, preferably from 1.5 to S % by volume of sulphur trioxide and less than 5 ~ by volume of free oxygen, at a filling body temperature varying between 10 and 50 C, the gas temperature being at most 10 C higher or at most 3~ C lower than the fil-ling body temperature; subsequently interrupting the contact between the filling bodies and the sulphur trioxide containing gas, and finally bringing the filling bodies into contact with a gaseous or liquid medium containing a substance apt to react with s~lphur trioxide.
In accordance with a preferred embodiment of the invention, the substance apt to react with sulphur trioxide is selected from the group comprising water, ammonia, alkali carbonate or hydrogen carbonate, and alkali or alkali earth metal hydroxide. During the reaction, it is possible to in-tensively change the position of individ~al filling bodies, both relative to the vessel and to one another. The reaction can be carried out in a rotatable vessel.
In accordance with another preferred embodiment of the invention, the sulphur-trioxide containing gaseous medium is a contact gas present in the sulphuric acid production and preferably a gas having passed through at least one absorber.
This invention will now be described in greater detail having reference to the attached drawing which diagrammatically 115~YC~O~

illustrates the situation that arises in the inode of reaction used up to now. In the diagram curve a corresponds to the reaction at the gas inlet; curve b in the middle of charge;
and curve c at the gas outlet; ordinates tal tb~ tc indicate the respective reaction times and abscissae alphaa, alphab, alphac the respective conversion grades. As apparent from the diagram, even negligible, seemingly unimportant differences between the body/reagent contact periods can result in consider-able 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 to bring about a substantial exothermic induced oxidation so that moreover a temeprature gradient occurs inside the grouped filling bodies. Such negative effects may even combine themselves. The result is very marked so that the filling bodies have a flamed appearance even if the entire contact period is, by order, tens of seconds, or several minutes, and if the gas advances at such a translational velocity that the difference in contact begin is given in seconds at the most.
The body bulk portions adjacent the gas inlet, outer edges and projections of the individual filling bodies are "burnt", dark coloured whereas the notexposed areasof the surface, the parts turned away from the gas flow or farther from the inlet remain practically not reacted. It is evident from the foregoing that such a reaction of exposing the filling bodies to sulphur trioxide containing gas flow - even if advantageous on principle ~
is unusable when practised in the afore-described way.
The effect of the invention resides in the elimination of infavourableinfluences as certained 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 ~Sf~O~

parts of the area exposed to the reaction are negligible in view of the practical requirements laid upon the hydrophiliza-tion homogeneity. 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 well-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 effected in conventional, only unsubstantially adapted plants such as rotary driers, mixers, granulators or the like, in this way the intensity of agitation does not depend either on the translational gas velocity or on the shape or size of the filling bodies.
It is further preferable to perform the method of 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 arising by the reaction of sulphur trioxide with air humidity so that the reactor need not be made of noble anti-corrosive alloys (it can be made e.g. of iron). The filling bodies continuously passing through the oblique rotary reactor roll on its wall on 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 be alike.
In such a plant, the reaction can be also effected discontinu-ously as, for example, in those cases where there is not .

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available but a gas with a relatively low sulphur trioxide content which requires a longer reaction tlme to be applied.
The sulphur trioxide containing gas, at the inlet to the reaction space, is preferably cooler than the filling boclies to be treated since those parts which react at a higher su]phur trloxide concentration and for a long time, react at a lower temperature. This leads to a partial alignment of the curves a, b, c /see diagram/ which positively influence the product homogeneity. Such an optimum temperature difference depends upon the other perameters but should not exceed 35C; the most 'requent value is 10C.
For the dilution of sulphur trioxide or a gas having an excessive sulphur trioxide concentration there can be used any of inert gases such as nitrogen, carbon dioxide, or cool, filtered or otherwise purified flue gases with low oxygen content.
After the reaction, the surface layer contains a sorbed amount of sulphur trioxide so that the reaction could go on even when the filling bodies are no more in immediate contact with the gaseous reagent. If the bodies are exposed now to dry air wherein SO is not instantly disposed of by air humidity, an undesirable induced oxidation and consequently darkening of the product occur. It is why 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 g~seous or liquid mcdium containing a necessary amount of components apt -to react with sulphur trioxide so as to give rise to compounds not impairing the intended purpose. Such a medium can be constituted by, e.g., humid air, water, water steam, liquid or gaseous ammonia /appropriately diluted/, aqueous solutions of alkaliferous ~B 8 .

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substances such as carbonates or hydrogen carbonates of alkali metals, hydroxides of alkali or alkali earth metals, or the like.
~ queous alkali so]utions are preferred for the pur-pose since they are capable to convert sulfone, sulphate ac.
well as carboxyl 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 alka-line solutions so that the filling bodies lose their dark colour, and simultaneously some small local differences in colour disappear; the latter, however are not functionally harmful but, as a rule, are objectionable from the commercial viewpoint.
The neutralization of the adhered sulphur trioxide is advantageous above all 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 the reactor, or in a discontinuous process immediately in the reactor by following the sequence: reaction, scavenging by a dry inert gas, neutralization by ammonia, scavenging by air. It is true that the body surface contains then a small amount of solid ammonium sulphate but this is not harmful in most of applications. If necessary, a~monium sulphate can be whenever washed oEf.
The practically most suitable, most available and cheapest reagent for this purpose is contact gas present in a sulphuric acid production unit. Such a gas is av~ilable in large amounts and, depending upon the withdrawal site, it is possible to choose both its composition and characteristics.
The gas withdrawn from the first or the second absorber is mostly preferred. The former corresponds in its composition to an equilibrium with oleum and is already cooler than before entering the first absorber intake. The gas, before having g -f~O~

been introduced into the reactor containing the polypropylene fillins bodies, has to be cooled, preferably to a temperature lower than that of the bodies. However, it is also possible to apply final gases from the absorption, which contain a small amount of sulphur trioxide but are cool already and enable, to the detriment of a larger reaction time, a higher product homogeneity to be achieved. Apart from this, final gases are waste material and as that free of charge. It therefore is advantageous to practice the hydrophilization process according to the invention directly in the factory for manufacturing sulphuric acid where also ammonia for the manufacture of ferti-lizers is available.
The following examples are given as illustrative only without limiting the claimed scope of the invention to the details contained therein.
EXAMPLE
A rotatable cylindrical glass vessel having an in-clination of 45 from the vertical axis was filled at the temperature of 25 C with filling bodies of cylindrical configuration /Raschig rings/ made of polypropylene. The vessel was supplied with a gaseous medium /20 C/ containing 4 ~ by volume of sulphur trioxide and 96 % by volume of nitrogen, the medium having been prepared by allowing oleum to bubble through dry nitrogen. After 5 minutes the vessel was scavenged by pure nitrogen, under steady rotation, and the bodies were poured in an aqueous 5 per cent sodium bicarbonate solution.
The rings, once washed by water, were dried up at 60 C. They were found practically colourless and perfectly wettable by water and aqueous solutions which were capable to flow down on them in the form of a thin film. If compared with the same but non-hydrophilized filling bodies, it was ascertained that 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, or, at the original dimensions the output rose to more than double.

The example demonstrates a negative result if the above-mentioned reaction conditions have not been maintained.
Polypropylene filling bodies of sattle-like shape were hydrophilized at 23 C 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 /2/' one % by volume of sulphur dioxide and 83.5 % by volume of nitrogen /N2/, the gas having been cooled for 5 minutes to 40 C. I'he filling bodies, after having been scavenged by dry air, and washed by water, had a dark colour shade at the gas inlet, and a thin hydrophilic layer on their surface tended to peeling off. On the other hand, the filling bodies adjacent the gas outlet were locally imperfectly hydrophiliæed.

A cylindrical reactor of stainless steel, rotatable about horizontal axis, was filled, at 25 C, with filling bodies of sattle-like form. The reactor was then filled with a gaseous medium /25 C/ containing 7 % by volume of sulphur trioxide, 2.2 % by volume of oxygen /2/ and 90.8 ~ by volume of nitrogen /N2/. The reaction time was of 10 minutes at 40 rpm.
After the gas had been displaced by dry nitrogen, the reactor was filled with a mixture of 9 volume parts of dry nitxogen and one volume part of ammonia. The polypropylene filling bodies washed by water, were superficially hydrophilic and exhibited the similar characteristics as referred to in EXAMPLE 1.

Pall rings of polypropylene in a concrete mixer 7~0~L

rotating at 30 r.p.m. were exposed for 20 minutes at 30 C
to a flow of flnal gases left in the production of sulphuric acid, containing about one per cent of sulphur trioxide and approximate]y the same amount of oxygen, the gases having been cooled to 22 C. The mixer was equipped with gas-tight inlet and outlet for the gaseous medium. The mixer was then scavenged firstly by dry nitrogen, secondly by dry air containing 10 % by volume of ammonia, and finally by air.
The rings were found superficially hydrophilic similarly as described in EXAMPLE 1.

Raschig rings if isotactic polypropylene /15 mm diameter and height/ were hydrophilized for 70 minutes without agitation in a column, at 20 C, by a gas /20/ containing 0.5 % by volume of sulphur trioxide and 99.5 % by volume of nitrogen. The column was then scavenged by a mixture of 9 volume parts of nitrogen and one volume part of ammonia, and finally by air. The filling bodies were found more uniformly hydrophilic than those treated in accordance with EXAMPLE 2.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of manufacturing superficially hydro-phylized filling bodies for interphase mass and/or heat ex-change units by the action of gaseous sulphur trioxide, the method comprising bringing polypropylene filling bodies in a vessel into contact with a gaseous medium 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 varying between 10 and 50 °C, the gas temperature being at most 10 °C higher or at most 35 °C 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 apt to react with sulphur trioxide.

2. A method as claimed in claim 1, wherein as the substance apt to react with sulphur trioxide there is used at least one substance selected from the group comprising water, ammonia, alkali carbonate or hydrogen carbonate, and alkali or alkali earth metal hydroxide.

3. A method as claimed in claim 2, wherein the position of individual filling bodies is changed during the reaction, both relative to the vessel and to one another.

4. A method as claimed in claim 1, 2 or 3, wherein the reaction is carried out in a rotatable vessel.

5. A method as claimed in claim 1, wherein as gaseous medium, use is made of a contact gas present in a sulphuric acid production unit.

6. A method as claimed in claim 5, wherein the contact gas used as gaseous medium is a gas having passed through at least one absorber.

7. A method as claimed in claim 1, 5 or 6, wherein the gaseous medium contains from 1.5 to 5 volume percent of sulphur trioxide.