For the effect of hydrolysis, the lignocellulosic materials may be described as a celullosic, hemicellulose and lignin complex further containing lower organic components, such as taninnes, waxes, oils, etc, said "extractive" and mineral substances (silica, calcium, potassium, sodium, etc, the ashes). The cellulose (or glicane, 36 to 40% in mass) is a glucose polymer found in the amorphous form (its greater part) and in the microcrystalline form. The hemicellulose (34%) is a complex amorphous polymer containing glicane (-8%), xylane (22%), arabinane and galactane (total 4%). It has been shown that hemicellulose hydrolyzes almost instantly, microcrystalline cellulose is quite resistant to acid attacks and that amorphous cellulose is intermediary. The lignin (a polymer derived from phenyl propene containing active phenolitic functions) is not soluble in an exclusive acid medium, but may be dissolved by certain organic solvents. The ashes constitute of silica and aluminum and iron oxides that are very little soluble in hydrolytic means, and of potassium, sodium oxides, etc., that are soluble in acids. Such characteristics require desirable conditions for the hydrolysis apparatus and processes.
Acid hydrolysis processes of lignocellulosic materials produce, among others: hexoses (sugars with 6 carbons), such as glucose, galactose and mannose; pentoses (sugars with 5 carbons), such as xylose and arabinose: lignin; furfural; 5-hydromethil furfural; acetic acid; and methanol among others, in variable proportions, depending on the raw material being processed. Known acid hydrolysis processes of lignocellulosic materials are divided into two major groups: processes using concentrated acids and processes utilizing dilute acids.
From the first group, the "Bergius" and "Udic Rheinau" processes stand out, which use 40-45% hydrochloric acid, and the "Riga" process, which uses 75% sulfuric acid.
Although such processes present elevated hydrolysis yields (approximately 94% of the estoichiometrical value), high investments in equipment are required once they should be constructed with material resistant to such strongly concentrated acids. In addition, the manipulation of said acids makes the operation of the process extremely difficult.
Among processes which use dilute acids developed to overcome the drawbacks exposed hereinabove, the "Schoeller" process should be mentioned. According to this process the wood is heated in percolators at 134.degree. C. with the aid of sulfuric acid, thus obtaining, through repeated extractions, sugars with concentration of 2 to 4% in the resulting hydrolysis solution.
It is evident that this process, which is carried out in a batch procedure, presents a yield that is rather below that commercially expected.
In order to improve the process with dilute acid described above, a process namely "Madison" has been developed by the Laboratory of Forest Products of the United States, which uses 0.6% dilute sulfuric acid within the range of 18 m.sup.3 by ton of the dried treated material and is carried out in 3 to 5 hours, reaches a maximum yield of 67% of its estoichiometrical value. Although the "Madison" process is substantially improved relatively to the "Scholler" process, it presents a yield still below the desired. Further, due to the elevated temperature at which it is carried out, the equipment, even when dilute acid is used, must be made from special materials, such as titanium and zirconium, thus elevating the investment value therefor, even though below the investment made in processes wherein strong acids are used.
Apart from the drawbacks evidenced above, which are specific to each type of process, both present a common problem: during the saccharification operation of the acid hydrolysis process there is virtually no delignification, the lignin being retained in the equipment in a viscous state, from which it is taken in a batch process, thus creating the additional problem of managing the continuous step of saccharification and discontinuous step of withdrawing the lignin, consolidating them as a single process routine.
Endeavors carried out to obtain a continuous process for acid hydrolysis of lignocellulosic materials that would provide simultaneous delignification and saccharification have resulted in a continuous countercurrent process for the production of lignin and sugars from wood and other lignocellulosic materials through delignification and saccharification by organossolvent at elevated temperatures and pressures, such process basically comprising: continuous introduction, by a reactor end, of comminuted lignocellulosic material; countercurrent introduction, by another end of the reactor, of a cooking liquor comprising a larger quantity of organic solvent and a smaller quantity of water, and a slight quantity of inorganic acid; contact of said lignocellulosic material with said cooking liquor, and withdrawal of the latter after being mixed with and having dissolved sugars and the remaining substances of the comminuted lignocellulosic material.
Although such process achieves elevated degrees of lignin recovery and sugar conversion, such performance has not been quantitatively disclosed. Further, the execution of said process in laboratory scales have shown many possibilities of improvements: once greater delignification or saccharification occurs in a distinct manner at each level of the reactor, and considering that the process in question provides cooking liquor feeding and withdrawal of the product from the liquor under unique flows, the temperature and concentration conditions of the solvent and reagent along the reactor's height are practically aleatory, thus making it difficult and even impossible to adequately control the process in terms of obtention of the total conversion of the lignocellulosic material, without overage of the reacting acid, as well as with respect to the prevention of decomposition of sugars obtained from the hydrolysis of the cellulosic material.
USPTO Backgrounds
USPTO Backgrounds