The majority of the papermaking pulp produced in the world today is produced by the so-called kraft method. Kraft pulping produces strong fibers, a fact that has given the method its name. This method, however, has the drawback of being very capital intensive. This is due to the need for a very complex system for chemicals recovery and very large unit sizes in the reactors. The reactors have in fact become so big that controlling the actual reactions and liquor circulations has become extremely difficult. The huge unit sizes in all parts of the process also leads to very large in-process inventory and a process that reacts very slowly to e.g. grade changes, etc. Any improvement that would lead to a faster process with shorter in-process delays would therefore have to be seen as a big step forward.
Another problem regarding the kraft method is the use of sulfur, which leads to larger amounts of chemicals being in circulation, odor problems, as well as making the recovery of spent chemicals extra complicated. A process without sulfur would make it possible to have much more efficient burning processes for the dissolved organic material in the process.
In order to address the problems of slow and cumbersome processes and to get rid of the sulfur, and often all inorganic chemicals in the process, several researchers have proposed the use of organic solvents to act as a cooking chemical and dissolve the lignin that holds the cellulose fibers together in wood.
According to J. Gullichsen, C-J Fogelholm, Book 6A, Papermaking Science and Technology, Fapet, 1999, Helsinki, Finland, p. B411, the pulping methods using organic solvents can be classified as follows:                Autohydrolysis methods, in which organic acids released from the wood by thermal treatment act as delignification agents        Acid catalyzed methods, in which acid agents are added to the material        Methods using phenols        Alkaline organosolv methods        Sulfite and sulfide cooking in organic solvents        Cooking using oxidation of lignin in organic solvent        
The basic idea in autohydrolysis, as explained for instance in U.S. Pat. No. 3,585,104 (Kleinert), is to cook the wood in a solvent at high temperature. The high temperature leads to hydrolysis of sugars present in the wood, thus releasing acids. These acids are then supposed to break down and dissolve lignin together with the solvent. The drawback of this process is that very harsh conditions are needed in order to properly delignify the wood. This leads to yield losses and low pulp quality. Others have attempted to improve on the basic idea in order to improve the pulp quality. One such attempt is the so-called IDE process described in EP 0 635 080. The idea is to limit the drop in pH in order to salvage pulp quality. The process is proposed to achieve this by cooking using solvent in a countercurrent manner, thus removing the acids as they are formed early in the cook, and by adding alkali to maintain the pH as desired. The method has never been possible to implement on a commercial scale, possibly due to the large amount of solvent needed to maintain the proposed countercurrent flow. Further, even in the laboratory it is not well suited for all wood species.
If pulp quality is not seen as a major criteria (emphasis on by-product value), acid can be added to the system to increase the speed of the pulping process. Processes have for instance been developed that use acetic and formic acid as delignification agents. The drawback for these processes is that there is no market for the inferior quality pulp, and that severe corrosion problems arise in the equipment.
The so-called Organocell process has been closest to large-scale commercialization of the solvent-using pulping methods. This process is a variant of alkaline organosolv pulping, using simultaneous action of soda-anthraquinone and organic solvent on the lignin. The process seemed to give acceptable pulp quality in the laboratory, but when tried on mill scale the results were not satisfactory.
All prior pulping methods employing organic solvents have been attempts to develop substitutes for the presently dominating kraft pulping method. However, kraft pulping has been constantly improved upon for the last 100 years and is today quite efficient and thus hard to compete with. This can be seen from the fact that no solvent pulping method has proven to be commercially viable. There is, however, still room for improvement in the kraft process itself. For example, the odors of the process are seen as a problem, as is the fact that the reactors are becoming increasingly large and hard to control. Steps have been taken to improve alkaline kraft pulping. One such method is rapid steam phase pulping. The idea is to impregnate the wood with all the alkaline chemicals needed for the reactions in an impregnation stage, followed by heating in a water steam phase. This would make the reactors smaller and partly remedy the problems with odor as described in Canadian Patent No. 725,072. However, this method has not demonstrated enough improvement over the kraft process in liquid phase—yield increase has been very small and reactors still very big, leading to too high chip columns in vapor phase, in turn leading to compaction and collapsing of the digester content, thus plugging flows and destroying pulp quality.
In light of the current research it is clear that the previous research has failed largely because the true role of the organic solvent was not identified. In the current research it has been clearly seen that organic solvents do not participate in the reactions themselves as a solvent of lignin or active chemical, but in fact only have the impact of providing such a reaction environment as to boost the efficiency of other delignifying chemicals.