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General Problems of Saccharification Processes |
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General Problems of Saccharification Processes
The problems associated with the development of an economic saccharification process for cellulosic materials are numerous and complex but they appear no more difficult than those faced by many of the present chemical industries during their development. The saccharification processes developed to date have met these problems in various ways but in none has complete cognizance been taken of all the difficulties, with the result that none have been economically successful without subsidy.
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The following discussion will cover the major points that should be borne in mind while weighing the merits of proposed processes and also point to the general direction in which research could be done.
The cellulosic raw materials may be broadly grouped into two classes: agricultural residues, which are harvested annually, and wood residues, which may be harvested continuously as desired. Both are available in continuous supply at low cost in the locality where they are grown. However, the annual crops usually are an expensive raw material when the cost of collection and storage is considered. Most of them also have a very low bulk density and large equipment and facilities are required. Annual crops have a further disadvantage in being subject to seasonal variations which require minor process variations. Wood is superior as a raw material in the general aspect to the annual crops but it too has disadvantages. It has a low bulk density, and the cost of handling and preparing wood for the saccharification step is high, even when using the best methods and equipment available. For instance, the handling of logs and conversion to chips in the Scholler process amounts to approximately one-half cent per pound of sugar produced.
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The process should be matched carefully to the physical and chemical nature of the wood available. The strong acid process sets limitations on the raw material; sawdust often available at low cost (including handling) is unsuitable. Any process which requires bark-free chips incurs a large increase in raw material cost.
The bulk density of wood is an important factor in the plant cost. Saving could perhaps result from the development of a continuous process.
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The only practical path from cellulose to sugar appears to be acid hydrolysis and this fact requires that all processes use acid at least throughout the hydrolysis step. This is a critical point for consideration, as the type of acid and its concentration has a very important effect on plant cost. In the strong hydrochloric acid process, the major portion of the plant must be acid-resistant and estimates of plant cost indicate a depreciation cost amounting to at least one-half cent per pound of sugar production on the basis of a large plant. This amounts to more than $200 of plant investment per annual ton of product, putting it in the class of expensive chemical plants. The plants using dilute acid require much less corrosion-resistant equipment and investment costs are somewhat less than half those quoted above.
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Plant heat requirement depends largely on the end use of the hydrolysate. In the case where the product is crystalline glucose or molasses, the heat load is enormous. In the Bergius process, heat is required to recover the strong acid used for the primary hydrolysis, while in the dilute acid process the main consumption is in the evaporation of the dilute solutions to molasses. Methods for increasing the sugar concentration in the hydrolysates of the dilute-acid process have been employed and this gives an economic advantage to the dilute-acid process.
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In the event that the sugar is to be used in solution for the production of yeast, alcohol, or other product easily separable from dilute solution, the heat load is greatly decreased, giving dilute-acid processes a decided advantage over strong-acid processes.
Chemical costs for the dilute acid processes are small, amounting to less than one-fourth cent per pound of sugar. In the strong acid process, they become significant, as more than one-half cent per pound of sugar produced even after full advantage is taken of modern recovery methods. The chemical cost of the strong sulphuric acid process makes it unattractive except under certain circumstances where the acid might have further use.
Considering the high costs of handling the raw material, the chemical costs and the heating load, it is apparent that every effort should be made to obtain full utilisation of all the products available and these should be obtained in the highest possible quality compatible with cost. No commercial ventures have succeeded in such utilization. In no case4 has the lignin, which amounts to 20 to 30 percent of the dry wood substance, been used successfully other than as fuel. The hemicellulose fraction in some cases has been recognized as needing much milder treatment than the resistance cellulose, but no advantage has been taken of the fact that at least one unique chemical, furfural, may be obtained from this pentosan fraction.
In most cases the hexosan sugar product has been utilised as a crude molasses containing large quantities of unknown impurities. Some of these undoubtedly could be profitably removed as higher priced organic chemicals, resulting in a twofold gain. In the case of the strong acid process, a high-quality sugar fraction is obtained.
Recently a scheme has been proposed for manufacturing crystalline dextrose which would market approximately 55% of the sugar product as food, the rest being marketed as molasses. The production of high-quality sugars from dilute acid wood hydrolysates is poorly understood and should be the subject of research. Another method is suggested for purifying the sugar by the addition of sodium chloride which forms a binary crystal with glucose. The yields of sugar from all processes are high. New research could profitably be done in the direction of increasing the yield of total products.
Waste disposal problems in an efficient saccharification process could be made small; molasses for stock feed and the concentration and burning of most of the other unwanted organics would solve most of the problems at moderate costs. One point worthy of mention here is the production of alcohol by the current Scholler process. The organic solubles and unfermented sugars in the still bottoms have an intolerably high biological oxygen demand. This is a case where the waste disposal problem would be nearly eliminated if the pentosan fraction were utilised.
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