AN IMPROVED METHOD IN GAS PROCESSING

Abstract: The disclosure relates to a method in processing, in particular the treatment of cellulosic materials, in a treatment zone with a gas in a liquid, the gas being added to the liquid and being transported therewith to the treatment zone. The method according to the present invention is characterized in that, to increase the capacity of the liquid to retain and transport the gas, the foaming properties are improved and/or particles are added to the liquid with a capacity to retain gas bubbles. The foaming properties of the liquid may be increased by adding foaming agents, by deactivating and/or removing ions which hamper or hinder present foaming agents from forming foam, by deactivating and/or removing ions by means of complex formers, deposition agents or ion exchangers or by adding cellulosic fibres.

Description: TITLE OF INVENTION: AN IMPROVED METHOD IN GAS PROCESSING TECHNICAL FIELD The present invention relates to a method in processing, in particular the processing of cellulosic materials in a processing zone with a gas in a liquid, the gas being supplied to the liquid 5 and being conveyed together therewith to the processing zone. BACKGROUND ART It is previously known in this art that oxygen which is added to ligninous paper pulp under alkaline conditions exercises a delignifying effect on the pulp. The so-called oxygen bleaching 0 process " is one example of this, oxygen-enriched alkali extraction is another. On the breakthrough of the oxygen bleaching process, this process was carried out at a relatively high pulp consistency (25-35 per cent), since this was the only possibility of creating acceptable contact between oxygen gas and paper pulp fibres. With 5 the development of the so-called high-intensity mixers - which make for the admixture of oxygen gas into paper pulp suspensions of between 8 and 12 per cent pulp consistency, developments have steadily pointed towards the execution of delignification by oxygen of medium consistency. This has also led to oxygen-enriched 0 alkali extraction, which implies that oxygen is added to and caused to react with paper pulp in the alkali extraction stage of the bleaching process, where the pulp concentration is normally between 8 and 12 per cent. Since that volume which is consumed both in oxygen bleaching and in oxygen-enriched alkali extraction 5 is considerably greater than that which would be possible to dissolve into the available volume of liquid, the greater part of the oxygen must be supplied in gas form and this oxygen volume must then be caused to remain in contact with the paper pulp for a sufficient time to allow for the necessary reactions. It is also important that the oxygen be distributed as uniformly as possible, in the form of minute gas bubbles, since this favours the reaction speed between oxygen and paper pulp. In the so-called high-intensity mixers, the desired effect is achieved by an intensive shearing of the paper pulp suspension, this suspension being subjected to a powerfully turbulent state. The paper fibres are released from one another and float freely in the suspension, and, in this state, the suspension assumes-, properties which very closely approximate those of a liquid. As a result of the violent turbulence, if oxygen or other gas is supplied to a pulp suspension in this state, the gas will be distributed extremely uniformly throughout the suspension in the form of minute bubbles. Once the suspension has flowed out of the high-intensity mixer and is no longer subjected to mechanical agitation, it will reassume the state prior to the mixer, i.e. the fibres attach to one another and create a relatively fixed network. This network is capable of retaining the " gas bubbles which were formed, until such time as the necessary reactions have taken place. Since its breakthrough roughly eight years ago, oxygen- enriched alkali extraction (which entails an oxygen addition in the alkali extraction stage of the bleaching plant) has developed apace. Considerable cost savings have been achieved at low investment costs. In so-called filter bleaching plants - in which the pulp is conveyed in conduits to the subsequent bleaching stage at a pulp concentration of between 8 and 12 per cent, it has become possible to apply this technique, since a high-intensity mixer may readily be installed in the conduits. In so-called displacement bleaching towers, the situation is, however, quite different. In these, transport of the paper pulp suspension is effected from the bottom and upwardly in the tower, while the pulp is subjected to the action of different bleaching chemicals. The different bleaching stages are, hence, executed in one and the same bleaching tower and the different bleaching stages are achieved in that one bleaching chemical displaces and replaces another bleaching chemical. This procedure is attained by means of screens and rotary distributors in the so-called dynamic zones located in the transitional regions between the different bleaching stages. In this type of bleaching tower, it is extremely important that the pulp suspension moves in plug-flow, and any form of agitation in the pulp suspension will considerably disrupt operations. It is, hence, impossible to employ the same technology for the supply of gas to the paper pulp as in filter bleaching plants. In order to be able to introduce the technology of oxygen- enriched alkali extraction into this type of bleaching plant, it- has, consequently, been necessary to utilize the liquid flow of incoming extraction liquid as the carrier of the oxygen. The gas may be supplied to the liquid through apertures in a perforated tube in the liquid flow. The liquid may also flow through a tube of sintered metal through whose pores the oxygen is supplied. One precondition for these methods to function satisfactorily is, however, that the minute oxygen bubbles which are initially formed do not combine to form larger bubbles, and that they are held floating in the liquid substantially in their original size until such time as the liquid comes into contact with the paper pulp within the displacement bleaching tower. From that point where the oxygen is supplied, the extraction liquid flows vertically downwardly in a central pipe from the top of the bleaching tower down to the distribution nozzles of the extraction stage. The extraction liquid is distributed from the nozzles into the paper pulp. It is known, from, for instance, waste water treatment, that this type of liquid transport is ill-suited if it is to be avoided that the oxygen bubbles combine to form larger bubbles. Consequently, the formed oxygen bubbles must be stabilized, for example, by means of surface tension reducing and/or viscosity increasing compounds present in the extraction liquid. In a displacement bleaching tower, considerable recirculation of different bleaching chemicals takes place, which also implies that a portion of the organic substances released in the bleaching stage are recycled. In this way, for example resin acids will, in the alkali extraction stage, be found in the extraction liquid which is supplied to the alkali extraction stage. At the high pH levels which prevail in the liquid, the resin acids form corresponding dissolved sodium salts in accordance with the following equation: RCOOH + NaOH === RCOO " + Na + + H2O These resin acid salts have surface active properties. The liquid receives frothing properties with the result that supplied gas bubbles may be stabilized. Hence, there are normally present in the extraction liquid substances which allow for supplied., oxygen to be maintained floating in the form of minute bubbles until such time as the liquid is conveyed into the paper pulp suspension. OBJECT OF THE PRESENT INVENTION One object of the present invention is to devize a method for increasing that volume of gas which may be retained in a liquid and be conveyed therewith. A further object of the present invention is to device a method for obtaining a finely-divided admixture of gas in a liquid which is stable for a relatively lengthy period of time. SOLUTION These objects are achieved according to the present invention in that the frothing properties of the liquid are improved and/or in that particles capable of retaining gas blisters or bubbles are added to the liquid. Preferred embodiments of the present invention will be apparent from the characterizing features as set forth in the appended subcl ims. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The nature of the present invention and its aspects will be more readily understood from the following brief description of the accompanying drawings, and discussion relating thereto. In the accompanying drawings: Fig. 1 shows the defoaming as a function of time for T-liquor and U-liquor before and after centrifuging in table centrifuge; Fig. 2 shows defoaming as a function of time for liquors from A, U and T; Fig. 3 shows defoaming as a function of time for the same liquors as in Fig. 2 with an addition of EDTA; Fig. 4 shows defoaming as a function of time for T-liquor in different degrees of concentration; Fig. 5 shows defoaming as a function of time for T-liquor, before and after pH adjustment and addition of EDTA; Fig. 6 shows defoaming as a function of time for T-liquor and U-liquor before and after ultra-centrifuging. It has proved that, certain displacement bleaching towers in which oxygen is supplied to the alkali extraction stage function unsatisfactorily. Relatively minor volumes of oxygen may be added before operational disruption occurs within the bleaching tower. Fig. 1 shows a number of characteristic properties for extraction liquid from three mills, designated A, U and T. The liquor from mill T may only absorb relatively slight amounts of oxygen, while the liquors from mills A and U have a considerably greater capacity to absorb and transport oxygen. Table 1 Factory A U T Properties Dry content {%) of sample 1.1 1.2 0.8 Extract content (%) of sample 0.013 0.010 0.006 Extract content {%) of dry content 1.2 0.8 0.7 Calcium ion content (mg/1) 68 30 15 pH 11.2 12.7 12.7 Clason lignin {%) 1.0 1.3 0.7 Acid-soluble lignin (%) 3.0 3.6 2.8 Chloride ion content (mmol/1) 44 59 26 The reason why operational disruptions occur in the bleaching tower in plant T is that the extraction liquid lacks the necessary foaming properties. Larger oxygen additions would give gas accumulations in conduits etc. When the gas bubbles become sufficiently large, they accompany the extraction liquid into the tower. The result will be an extremely uneven and intermittent supply of oxygen, which causes disturbances within the tower. Fig. 1 shows the foam stability for liquors from plant T and from plant U. Liquor from the latter plant displays excellent foaming stability and is capable of absorbing a sufficient amount of oxygen for its purpose, while the liquor from plant T suffers from poor foam stability and displays an insufficient capacity to absorb and transport oxygen. It will be apparent from this Figure that, by simple foam tests, it is possible to measure significant differences in the samples correlated to the problem under consideration. One point of uncertainty was that the degree of acidity between the samples was extreme, i.e. pH 7 for liquor from plant U and pH 13 for liquor from plant T. A modification of the pH for the liquor from plant U to pH 13 did not, however, change the foam stability. Fig. 2 shows the foam degradation as a function of time for liquors from plants A, U and T. The l quor from plant T gives a less voluminous and stable foam than foam from either plant A or plant U. On the addition of a complex foaming agent for int al calcium, EDTA, the foam stability increases for liquors from all plants. Moreover, the foam volume markedly increases for liquor from plant T, as will be apparent from Fig. 3. These results are reproduceable. If liquor from plant T is evaporated to higher dry contents, both foam volume and foam stability increase, as is apparent from Fig. 4. In a concentration of liquor from plant T by a factor of 3.6, the pH increases from 12.9 to 13.5 and the surface tension is reduced from 46.8 to 38.4 mN/m. In Fig. 5, it has been shown that the foam increase and surface tension -reduction are not dependent upon any change of the degree of acidity of a liquor. After ultra-centrifuging and separation of bottom phase arid top phase, no change took place in the foam formation for liquor from plant T, while the foam stability for liquor from plant U was significantly reduced. It will be apparent from Fig. 6 that the half-time for the stability of the U-liquor was reduced from 20 to 5 minutes. On centrifuging, an upper phase and a bottom phase were formed in samples from both plants U and T. In the upper phase of the sample from plant U, two typical observations could be made. The one in the form of needle crystals which may be assumed to be calcium soaps, and the other in the form of a typical crystalline phase for lamellar liquid crystalline phases. These are probably formed by resin and fatty acids -in the system. From liquor from plant T, minor amounts of the upper phase were separated off and the same types of crystal formations could be detected as in liquor from plant U. In the bottom phase of the U liquor, considerable amounts of fibre were to be found, as opposed to the bottom phase of the T liquor. It is apparent from the results that the following factors influence foam stabilization of liquors: 1. Lamellar liquid crystalline phases, probably of sodium soaps and of resin and fatty acids 2. Fibres 3. Calcium soaps of resin and fatty acids. In the liquor from plant U, a greater lamellar phase has been found, more fibres and more calcium soap than in liquor from plant T. This was ascertained by extraction determination, dry content determination, ultra-centrifuging • (non-quantitative) and by calcium analyses. On evaporation, the capacity of the liquor from plant T to form foam increases, as well as the foam stability. In this instance, the surface tension is reduced, which would seem to indicate an increase of the surface-active resin and fatty acid soaps. The rδle played by the calcium soaps in the system is less clear. Normally, the calcium soaps act as anti-foaming agents. They do not form lamellar phases to the same extent as sodium soaps and are insoluble in their nature. In a system in which both lamellar phases and calcium soaps co-exist, it is possible that the foam-damping effect of the calcium soaps is "masked" by the stabilizing effect of the lamellar phase. The liquor from plant T contains, after ul ra-centrifuging, a minor amount of calcium soap and lamellar phase. Liquor from plant T reacts much more powerfully to an addition of complex-forming agents, for example EDTA, which are assumed to complex-bind calcium ions. This may be because calcium soaps in liquor from plant T are not "masked" as effectively by lamellar phases and, as a result, have a foam- damping effect in this liquor. When calcium ions are complex- bonded by, for example, EDTA sodium soaps are formed which have a favourable effect on foam formation. It will be apparent from the foregoing that particles, in the given example, fibres, with a capacity to retain gas bubbles,-. contribute to the transport of gas to the reaction zone. Naturally, it is advantageous if the same particles as are present in the treatment zone are used for gas transport to this zone. Since the "" particles are solely for the purpose of transporting gas, they should not derange the process proper, i.e. be present in relatively small amounts. On treatment in the processing zone of a cellulosic fibre material, in which gas is transported to the zone by means of fibres, a negligible amount of fibres may be added to the processing zone. The content of fibres in this gas- transporting liquid should be less than 0.1 per cent.

Claims: WHAT WE CLAIM AND DESIRE TO SECURE BY LETTERS PATENT IS: 1. A method of treatment in a treatment zone with a gas in a liquid, in which the gas, and possibly a frothing agent, is added to the liquid outside the treatment zone and is conveyed therewith to the treatment zone, characterized in that prior to the addition of the gas, the capacity of the liquid to retain the gas is increased by inactivation and/or removal of ions in the liquid which hamper or hinder present foaming agents from forming foam, and/or addition of particles to the liquid with a capacity to retain gas bubbles. 2. The method as claimed in Claim 1, characterized in that such treatment relates to treatment of a cellulosic material. 3. The method as claimed in Claim 1 or 2, characterized in that the ions are deactivated and/or removed by means of complex- forming agents, deposition agents or ion exchange agents. 4. The method as claimed in Claim 1 or 2, characterized in that the particles are cellulosic fibres. 5. The method as claimed in any one or more of Claims 1-4, characterized in that the gas is oxygen. 6. The method as claimed in any one or more of Claims 1-5, characterized in that the liquid is a treatment liquid for oxygen- enriched alkali extraction. 7. The method as claimed in any one or more of Claims 1-6, characterized in that the liquid is an extraction liquid for bleaching, in particular for displacement bleaching.

Inventors: BOKSTROEM MONICA, SVENSSON SVEN-GOERAN, BOKSTROEM, MONICA, SVENSSON, SVEN-GOERAN
Applicants: AGA AB, AGA AKTIEBOLAG
Application year: 1988
Main classification: D21C9
IPC codes: B01J19, D21C9