Cellulose acid hydrolysis system

Acid hydrolysis of cellulosic materials has been studied for many years (13,33). Although it is a relatively straightforward process, it has the problems of requiring acid-resistant equipment and yielding a poor grade of sugar (because the product contains many reaction product impurities). However, in terms of practical application, acid hydrolysis of cellulosic material is by far the most commonly used hydrolysis system. 

Conditions selected so that the copolymer contained about the same amount of cellulose, 77-82% AN = acrylonitrile DMF = N,N-dimethylformamide. b Initial molecular weight of cellulose 7.1 X 105 for ionizing radiation molecular weight of cellulose was determined at dosage indicated no determination was made on possible oxidative depolymerization of cellulose by chemical redox systems. c Based on poly (acrylonitrile) recovered from acid hydrolysis of copolymer and on intrinsic viscosity method. 

Acidity. For many years the pH of the paper has been considered an important characteristic affecting peramnence. Permanency specifications exist which call for paper with a pH of 6.5 or higher. Other specifications say that the pH should not be lower than 5.5. There is no disagreement, however, about the poor permanency of paper when it is under pH 5.5 (4). In this context, paper pH is determined by the extraction procedure already described. The acidity that is present in the paper will promote acid hydrolysis of the glucosidic bonds of cellulose. To prevent this type of degradation and to produce paper with a high extract pH, the paper machine process water system must be controlled at a neutral to alkaline pH with a minimum of total acidity, preferably with an excess of alkalinity. The term total acidity is used to describe the concentration of all dissolved ions and particles in the process water that contribute to acidity in the system and a low extract pH in the paper. For example, papermaker s alum dissociates to alumi-... 

In general, acid hydrolysis produces similar degradation. Since acids usually involve a much smaller molecule as the attacking agent, however, acids penetrate the amorphous area of cellulose more easily than enzymes and produce a faster drop in DP. Diffusion into the crystalline areas proceeds more slowly. Because acid hydrolysis is more easily controlled and is faster than enzymatic attack, acid hydrolysis was selected as the second aging system to be examined. 

Water used in this supercritical state behaves very differently from water under normal pressure and temperare." In such a supercritical state, the water can be expected to act as an acid or base, but by returning the system to ordinary conditions before pyrolysis occurs, glucose and its derivatives could be obtained in water from cellulose. Therefore, supercritical water treatment can be superior to enzymatic saccharification or oidinaiy acid hydrolysis mentioned above, for the chemical conversion of biomass to useful chemicals. 

A continuous counter-current reactor system has shown great promise as a process reactor in the dilute acid hydrolysis of cellulose. However, the findings on this unique reactor system have been limited to the theoretical aspects and the proof-of-concept laboratory experiments. It has to be developed into an upscale process reactor before it is adopted into the biomass conversion process. To this end, a pilot-scale process study is being conducted at NREL. This is only the first step. To be noted here is that reactors of similar design are being used in industry it took years of developmental work, however, before they were put into commercial service. It would probably take about the same degree of investment in this case. In addition to the reactor issue, there are other important issues that need to be addressed in the dilute acid process. 

In acidic conditions, glycosidic bonds are cleaved in the cellulose structure. The rate of this reaction is controlled by the crystallinity of the cellulose and the temperature of the system (3, 5, 6). The resultant hydrolyzed cellulose has a shorter chain length than the original polymer, which greatly decreases the strength of the wood (7). Hackney (8) noted embrittlement and discoloration of sulfur dioxide exposed cellulose fibers. Such acid hydrolysis experiments have been conducted in aqueous systems, with the degradation rate dependent on the difiiision characteristics of the acidic solution into the wood structure. Because museum artifacts will not normally... 

Acid hydrolysis enhances the pore system by removing amorphous cellulose from the surface and revealing the macrofibrillar structure of cellulose fibres [5]. Drying results in an irreversible reduction of the pore volume as a result of the pores collapse arising from the capillary forces, a mechanism called hornification. 

The high yield and hence higher purity of sugar obtained by strong acid hydrolysis of cellulose makes it an attractive process, but the lack of a recovery system for strong acid complicates the outlook. The improvement of membrane technology will probably proceed because of potential applications to many problems. Application to cellulose hydrolysis adds justification for intensified work in the field. 

Data and kinetic models of dilute acid pretreatment ate vital to provide a foundation for undostanding hemicellulose hydrolysis and the cause of enhanced performance by flow system systems. Initial hemicellulose hydrolysis models were adapted from Saeman s first-order homogeneous kinetic model of cellulose hydrolysis in a dilute acid batch system (30) and later modified to include two different fractions of hemicellulose, one of which is more easily... 

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