The present invention relates to a process for controlling microbial growth in a production line for cellulosic products with the aid of gases. The invention also relates to the use of gases such as carbon dioxide, nitrogen, a noble gas and/or non-naturally occurring mixtures thereof for controlling microbial growth.
In the production of cellulosic products such as cellulose, paper and board (hereinafter referred to as paper), cellulosic fibers are treated in aqueous suspensions under varying conditions. The amount of water in the production line is huge and the water is continuously recirculated in smaller or larger loops. The conditions in the production line are often susceptible to microbial growth. This is especially so during storage of the aqueous material.
For growth all microbes require that the system should include sufficient nutrients and that the pH, temperature, moisture, oxygen level etc. is adequate.
For breeding the microbes need time to grow and propagate in the system. The retention time must be long enough; otherwise, the cells will be washed out of the system. In a papermaking system the microbes can easily find locations with a sufficient retention time, e.g. water storage tanks, stock chests, treatment of broke and long pipelines.
Normal papermaking conditions are suitable for the growth of many kinds of microbes. The recirculating white water in the short and long circulation contains enough carbohydrates and other essential ingredients such as inorganics and trace elements. Also chemicals used in the papermaking themselves represent an ideal nutrient source, e.g. starch, or contain as impurities quite a lot of nutrient material, e.g. kaolin. The incoming raw water can also contain considerable amounts of nutrients.
The microbes found in a papermaking system can be divided into three main groups, bacteria, fungi and algae. The bacteria are either spore forming (anaerobic) or non spore forming (aerobic); the fungi comprise moulds and yeasts; and the prevailing algae are blue-green or green algae.
Many different kinds of problems can be caused by microbial growth, such as slime problems, runnability problems, corrosion problems, additive problems, and product problems.
Unless microbial control agents are added to the papermaking system, the general growth requirements for microbes are usually well satisfied in papermaking systems. Modem microbial control agents can roughly be divided into groups, which act in the following ways: oxidative decomposition of microbes (O2, ClO2, O3, peroxides); biocides, which inhibit or kill the microbes (organic, synthetic chemicals); and enzymes.
Oxygen is primarily used to prevent anaerobic conditions, while chlorine dioxide, ozone and peroxide work as biocides and disinfectants. They are quite effective but considered to be costly. Conventional biocides can be used alone or in combination with oxidative biocides. They are effective, but toxic. They can also be environmentally dangerous and hazardous for the working environment. A relatively new method of slime control is the use of enzyme. Enzymes are active at pH 3.5-10, which is an advantage compared with conventional biocides. However, enzymes have limited effect on bacteria.
Oxygen and oxygen-rich gases have primarily been used for preventing the formation of hydrogen sulfide and other volatile gases by anaerobic bacteria in waste waters. Robichaud, W. T., Tappi Journal, February 1991, pp 149-153, has reported on the use of aeration for controlling anaerobic bacteria to improve product quality and mill safety in papermaking systems.
Use of carbon dioxide in paper making has been suggested in the prior art for various reasons mainly connected with specific needs for adjusting the pH or influencing the carbonate or bicarbonate chemistry. Examples of patents related to the use of carbon dioxide in papermaking systems are U.S. Pat. No. 5,378,322 (Canadian Liquid Air); U.S. Pat. No. 5,262,006 (Mo och DomsjöAb); EP 0 296 198 (AGA Aktiebolag); EP 0 281 273 (The BOC Group); GB 2 008 562 (J. M. Voith GmbH); WO 99/24661 (AGA Aktiebolag); and WO 99/35333 (AGA Aktiebolag). None of these publications relate to the problem of microbial growth.
Mixtures of gases have been used in the packaging of foodstuffs in order to retain the foodstuff's original taste, texture and appearance. The gas mixtures usually consist of carbon dioxide, nitrogen and oxygen, but also other gases such as nitrous oxide, argon and hydrogen have been used. Carbon dioxide inhibits microbial activity on the foodstuff by reducing the pH and by penetrating biological membranes, causing changes in permeability and function. Nitrogen is primarily used to replace oxygen in packaging. Oxygen helps to preserve the oxygenated form of myoglobin, which gives meat its red colour. Oxygen is also required for fruit and vegetable respiration.
Amanatidou, A.; Smid, E. J.; Gorris, L. G.; J Appl Microbiol, March 1999, pp 429-38, have reported on the effect of elevated oxygen and carbon dioxide on the surface growth of vegetable-associated micro-organisms. Consistently strong inhibition was observed only when the two gases were used in combination.
As mentioned above, problems with microbial growth are conventional in papermaking systems, especially in storage tanks and in long pipe lines. Moreover, protection of the environment by the closing of the white water systems and increased recirculation of process waters as well as the increased use of waste paper has caused a marked increase in the microbial growth in the papermaking systems.
The papermaking industry consequently has an increasing need for means for reducing the microbial growth in a technically feasible, inexpensive and environmentally friendly way.
It has now been found that a gaseous inhibitor in the form of carbon dioxide, nitrogen, a noble gas or a gas mixture containing one or more of said gases may be used for controlling microbial growth in a papermaking system.
Consequently, the present invention provides a process for controlling microbial growth in a production line for cellulosic products, which comprises providing an aqueous material containing water as well as suspended pulp fibers and/or additives therefor, maintaining said aqueous material under conditions susceptible to microbial growth, providing a gaseous inhibitor comprising a gas selected from carbon dioxide, nitrogen, noble gases and non-natural gas mixtures containing the same, and introducing said gaseous inhibitor to said aqueous material in an amount sufficient to significantly retard or inhibit the growth of microorganisms therein.
The gaseous inhibitor is preferably added immediately prior to and/or during storage of said aqueous material, since storage provides a sufficient time for the microbes to propagate.
The gaseous inhibitor should preferably consist of or contain a significant amount of carbon dioxide, nitrogen and/or a noble gas such as helium, neon, argon, krypton, xenon and/or radon. Carbon dioxide is the preferred gas according to the present invention. Among the noble gases argon is preferred.
If a gas mixture is used, said mixture should be a non-natural one. The gases of the mixture may be mixed before introduction into the aqueous material, or they may be added separately, simultaneously or sequentially. Although the gas mixture may contain oxygen, it should not have the composition of air, since air is known to inhibit only the growth of anaerobic microbes.
The gaseous inhibitor of the present invention may be used in combination with oxygen either by combining oxygen in the mixture of carbon dioxide, nitrogen and/or argon, or by adding oxygen separately from the other gases. When oxygen is added to the gas mixture, the amount of oxygen may vary from 10 to 90% of the total gas volume. However, according to another embodiment of the invention, an oxygen-rich gas is introduced into the aqueous material separately from the gaseous inhibitor of the present invention. Such an oxygen-rich gas may be added either before or after the addition of the inhibiting gas mixture.
The present invention will now be described in greater detail with reference to paper-making systems. It is, however, clear that the gaseous inhibiting system of the present invention may be used also in the production of cellulose, board, etc. As used in the context of the invention, a production line for cellulosic products comprises a line for the production of pulp, paper, board or the like. The production line will typically include at least a portion of reprocessed recovered paper and/or broke and will include loops of recirculating waters. The production line has a more or less closed water system, it being understood by those skilled in the art that the problems with microbial growth are prone to escalate in closed systems with an increasing and accumulating mass of microbes circulating in the system.
The temperature in the papermaking systems usually varies from 30 to 60° C. Because of recycling, the temperature in the white water often exceeds 50° C. Fungi and yeasts generally do not tolerate temperatures above 40° C. Contrary to this, many bacteria thrive well in the high temperature range. pH usually varies from 3 to 10. Acidic conditions, pH 3-6, are very convenient for fungi and yeasts. Bacteria dominate under neutral and alkaline conditions, viz. at pH levels from 7 to 10. Anaerobic conditions can be found in many places throughout the production line, such as storage after dithionite bleaching, pulp chests and white water tanks.
Slime helps microbes to adhere onto surfaces and provides a food reserve. Microbial growth causes operating problems by plugging filters and screens, by reducing wire and felt life and by causing a decline of productivity due to breaks, wash-ups, etc. Corrosion induced by microbes is a consequence of vigorous microbial activity on surfaces.
The most important microbial species in this area include sulphate-reducing bacteria, which are anaerobic in character. There are, however, also a number of aerobic species which are harmful. Additives, such as starch, can deteriorate due to microbial activity, not to mention that a contaminated starch slurry can constitute a heavy inoculation of the white water system. When masses of microbes get loose from the actual growth place, the result may be seen as spots, holes or dirt specks in the final paper product. Spore-forming bacteria tolerate much heat and usually survive the drying stage. Thus they remain alive in the product and can be harmful later on.
In the working of the present invention note should be taken of the special circumstances of a papermaking system with its huge volumes of fluids, all the time on the move and having no definite surface where the microbes are prone to exist. This is in sharp contrast to, for instance, the packaging of food in protective atmospheres. The food moves nowhere within the package, its surface is solid and the gaseous atmosphere surrounds the product. In a papermaking system the aqueous material has a surface which changes continuously and it cannot be surrounded by the gas.
The term microbe or microorganism as used in the context of the present invention is intended to mean bacteria, fungi and/or algae such as described above. It should be understood that all of the microorganisms present in a papermaking system will not be influenced by the gaseous inhibitor of the present invention and that the gaseous inhibitor of the invention may therefore be used in combination with other inhibitors, such as oxygen-rich gases and biocides of various forms, as long as these do not interfere with the working of the invention itself.
According to the present invention microbial growth in the papermaking system is reduced by a gaseous inhibitor. The gaseous inhibitor of the present invention is a gas or a gas mixture capable of inhibiting, partly or totally, the growth of microorganisms present in the papermaking system. The preferred single gas is carbon dioxide. The gaseous inhibitor may also comprise nitrogen and/or argon. Said gases may also be used alone, but they are more preferably used in combination with carbon dioxide. A suitable gaseous inhibitor consists of a mixture of carbon dioxide, nitrogen and argon. The mixture preferably contains at least 10% carbon dioxide.
The gaseous inhibitor may additionally contain oxygen. The oxygen should not be used in a combination resembling air, since such a mixture is effective only against anaerobic microorganisms. In a mixture of carbon dioxide and oxygen, the proportion of carbon dioxide should be between 90% and 10% and the proportion of oxygen should be between 10% and 90%. It should be noted that normal air contains about 21% oxygen and about 0.03% carbon dioxide.
A preferred embodiment of the invention comprises the use of a gaseous inhibitor consisting essentially of carbon dioxide, nitrogen, argon or mixtures thereof, which is introduced into a liquid flow of the aqueous material entering a storage tank for said aqueous material or into said storage tank itself. An oxygen-free gaseous inhibitor is preferably added in an amount sufficient to purge said aqueous material of oxygen and thereby inhibiting the growth of aerobic bacteria contained therein. Most aerobic bacteria are sensitive to the lack of oxygen and will eventually be killed off by such a procedure while the carbon dioxide will adversely affect many of the anaerobic species.
In an improvement of this embodiment, oxygen is used separately from the gaseous inhibitor of the present invention. Thus, after a suitable retention time the introduction of the gaseous inhibitor is followed by and/or preceded by an introduction of an oxygen containing gas into said storage tank in an amount sufficient to kill anaerobic bacteria in the aqueous material.
The introduction of the gaseous inhibitor and the introduction of oxygen may be repeated in an alternating manner during storage of said aqueous material.
The inhibitor is preferably added in a position where the risk for microbial growth is largest, i.e. in storage tanks for aqueous pulp suspensions or aqueous additives susceptible to microbial attack. However, the gaseous inhibitor may also be added to liquid flows of the aqueous material, to recirculating waters and to fresh water prior to its entering the system. The main principle of the present invention is to reduce the microbial growth at any position where it would otherwise rise to harmful proportions. It is not necessary to kill all the microbes but it is essential to reduce the microbial growth to such proportions that the harmful accumulations are minimized in the production line and in the final product.
In a preferred embodiment of the present invention the gaseous inhibitor is introduced into a liquid flow of the aqueous material or into a liquid flow of a diluent or additive for said material just prior to storage thereof. The gaseous inhibitor may also be introduced into any storage tank containing said aqueous material by bubbling the gas into the aqueous material and/or by filling the void space above the fluid.
In a typical process according to the present invention the aqueous material comprises a pulp suspension in a papermaking system and the suspension is treated with the gaseous inhibitor before it enters and/or as it is retained in a pulp storage tower, a stock chest, a broke tower or the like storage tank. The pulp suspension may also be stock in the stock preparation of a paper making system.
In a preferred embodiment the aqueous material to be treated comprises white water in a papermachine, preferably white water stored in the long circulation.
In another embodiment the aqueous material comprises a slurry of an additive chemical such as starch, coating, pigment, filler, or the like. Such additives are usually retained in aqueous suspension in readiness for use in the papermaking process and many of the additives contain nutrients making them susceptible to microbial attack. Typically this is true of starch, which in itself is a nutrient for many microorganisms. Many other additives, although inert in themselves, contain sufficient amounts of impurities to make them, with time, susceptible to microbial attacks. Treating such additive tanks with intermittent introductions of the gaseous inhibitor will effectively reduce the amount of microbes entering the system that way.
In a preferred embodiment of the present invention, the gaseous inhibitor is added at a late point, preferably just prior to the point where microbial attack is expected to be most severe, such as in a storage tower. Additional gaseous inhibitor (carbon dioxide/nitrogen/argon) should, if necessary, be added to the head space of the tower.
If oxygen is used in combination with the gaseous inhibitor, the oxygen should preferably be added directly after a pump feeding the aqueous material to the storage tower to make use of any turbulence to achieve a high rate of mixing. Additional oxygen should, if necessary be added to any of the tower's recirculation pipes to avoid creating any anaerobic areas in the storage tower.
It should be noted that although gases such as carbon dioxide and oxygen have previously been used in papermaking, the gaseous inhibitor of the present invention, comprising carbon dioxide, nitrogen or argon alone or in a non-natural gas mixture has not previously been used in papermaking systems for controlling microbial growth.
The aqueous materials of the cellulosic production line are processed to cellulosic products such as paper, board, dried pulp or the like material in a manner which is conventional in all other ways except for the biocidal treatment of the present invention.
The present invention will now be illustrated with the following examples.