Technology
HF key to improved cellulose-cracking process Use of hydrogen fluoride to produce glucose from biomass offers low reagent cost, less pretreatment of feedstock, and high yields The use of hydrogen fluoride to convert wood and other lignocellulosic biomass to glucose—for further conversion to ethanol or for other uses—may offer significant advantages over other cellulose conversion processes. That's the opinion of chemical engineers Martin C. Hawley and Susan Selke, plant research scientist Derek T. A. Lamport, and coworkers, all of Michigan State University. The MSU scientists note that the idea originated in the 1930s in Germany, where some development work took place. After that, however, the technique generally was neglected until recently. The three basic cellulose-to-glucose approaches that are farthest along in
development are dilute acid hydrolysis, concentrated acid hydrolysis, and enzymic hydrolysis. But according to the MSU team, each of these processes has its particular shortcomings. Dilute acid processes (the only ones to have operated successfully commercially) afford glucose yields of only about 50% of theoretical; also, the lignin is seriously degraded by the acid treatment. Concentrated acid processes using sulfuric or hydrochloric acid can achieve glucose conversion rates of 85 or 90% but material costs are high and, again, the reactivity of the lignin is affected adversely. Enzyme processes yield pure products and don't damage the lignin, but their glucose conversion rate is only about 50%, the feedstock usually requires extensive pretreatment, and long reaction times are the rule. In contrast, the group says, the use of hydrogen fluoride to saccharify cellulose offers the prospects of minimal feedstock pretreatment, high glucose conversion, low chemical costs, and undamaged lignin. In the approach now under study at MSU, small chips of aspen wood are
HF saccharification minimizes pretreatment Lignocellulosic material Sugar solution to fermentation or other use Drying
Anhydrous hydrogen fluoride
4 Saccharification
I
+> Excess moisture CaFto regeneration « of HF with sulfuric acid and recycle Calcium carbonate-
Removal
t Filtration
t Neutralization
t
• HF to water removal and recycling
Posthydrolysis
Water -
Washing and filtration
Lignin Sugar solution
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C&EN Feb. 1, 1982
dried, then treated with anhydrous hydrogen fluoride for about an hour in a vacuum distillation apparatus. The reaction forms glucosyl fluorides that further react with water to produce glucose and regenerate hydrogen fluoride. After that, the hydrogen fluoride is removed either by evacuation or by applying low-temperature heat. Water is added to form a water-soluble sugar fraction and an insoluble lignin fraction that can be separated by filtration or centrifugation. Sugar yields range from 45 to 99% of theoretical. However, a large percentage of the sugar monomers formed during the solvolysis stage recombine during the process of hydrogen fluoride removal to form oligomers. Fortunately, more than 90% of the sugar can be reconverted to monomer form by "posthydrolysis" treatment for one hour at 140 °C with either dilute sulfuric acid or dilute hydrofluoric acid. The MSU team says that if the latter were used in a commercial process, it would be neutralized with calcium carbonate after the posthydrolysis stage. The resulting calcium fluoride would be removed from the sugars by filtration and treated with sulfuric acid to regenerate hydrogen fluoride. (Recycling most of the hydrogen fluoride would be essential to the commercial success of the process.) Another matter of concern is the amount of fluorine retained in the reaction products. In tests, retention has been found to depend on evacuation time, temperature, and water content. The lowest fluorine content obtained was 4 mg per g wood for the sugars fraction and 0.1 mg per g wood for the lignin fraction. The team believes that most residual fluorine stems from the formation of fluorides with metals in the wood ash. The wood sugars obtained by the hydrogen fluoride saccharification process can be fermented successfully by the yeast Saccharomyces cerevisiae, with ethanol as the major product. The MSU workers note that the yeast can grow and ferment wood sugars with fluoride concentrations as high as 100 ppm. Team members also investigated the reactivity of the lignin produced by the process. They found that flu-
oride retention in the lignin fraction could be reduced to 0.03 mg per g wood by crushing, washing, and dialyzing the lignin residue—indicating that there was no significant degree of fluorination of the lignin. Dioxane acidolysis rendered the lignin 93% soluble, suggesting that it was essentially noncondensed. "We have reason to believe that the lignin may retain a high degree of functionality and therefore will be valuable as a raw material for the production of aromatic hydrocarbons," the MSU scientists say. Of course, all the work is at a fairly early stage. There are several possible process flow schemes, differing mainly in reaction phase (gas or liquid hydrogen fluoride), the point of lignin removal, and the hydrogen fluoride recycle method. The MSU team notes that early removal of the lignin (before posthydrolysis) has two advantages. First, the lignin would be exposed to acid for a shorter time and thus probably would undergo less degradation. Also, early removal means that streams for further processing would be liquid rather than liquid-solid suspensions or slurries. That likely would make further processing easier. However, separating the lignin after posthydrolysis might lead to more complete sugar recovery: The insoluble oligomers created by reversion would be reconverted to monomers. It's also too early for detailed economic evaluations. However, preliminary studies suggest that the total feedstock and chemical cost would be about 5.4 cents per pound of glucose. Of that, 3.4 cents would be for the wood. Those figures may be optimistic, but they look attractive when compared to projected costs for other acid and enzyme hydrolysis processes. The MSU team notes that the high yield of the hydrogen fluoride process (90% glucose yield is assumed) plays a major part in keeping materials costs down. Two other characteristics of the hydrogen fluoride process work in its favor, the MSU workers note, although they can't yet quantify them. One is the fact that because of hydrogen fluoride's low heat of vaporization and low boiling point, the process generally operates at nearambient pressures and temperatures, so that energy requirements likely would be modest. And in contrast to other acids, anhydrous hydrogen fluoride can be handled in carbon steel equipment. Special acid-resistant materials would be needed in only a few areas. D
Efficient boiler control reduces fuel costs Six of the largest U.S. energy-using industries can save nearly $1 billion a year in fuel costs by improving the efficiency of their industrial boilers. That's the conclusion of a study by the Energy Management Information Center of process-controls manufacturer Honeywell Inc., Minneapolis. Boilers represent one of the easier targets for fuel savings. And in recent years, as fuel costs have risen sharply, a number of companies have offered control systems and/or operator training courses to help users of process steam to capture some of these savings. Chief among those users, as the Honeywell study points out, is the chemical industry. The study looked into the potential for savings by applying microprocessor control to existing industrial boilers. It showed the chemical industry could save about $272 million, pulp and paper about $258 million, petroleum $179 million, food processing $152 million, primary metals $58.3 million, and textiles $27.6 million. All told, the six industries could save nearly $94.8 million. The Honeywell study was based on data from the Department of Energy, Department of Commerce, Oak Ridge Associated Universities, General
Energy Associates Inc., National Association of Manufacturers, and the "Energy Future" report of the Harvard Business School energy project. Energy costs are for 1980. Microprocessor control, Honeywell notes, saves energy primarily through precise control of the fuel and air mix used in the boiler. Honeywell looked at 50 working microprocessor-controlled installations. It found that energy savings have been as high as 20%. However, it says, the typical saving is about 5%. Using data developed by General Energy Associates, Honeywell first determined the number of plants producing a total of 40,000 lb or more of process steam per hour. It calculated the annual steam production for 8400 hours a year, allowing for maintenance and downtime of boilers and making adjustments for new boilers or boilers with digital combustion controls. For example, it determined that the chemical industry had 494 such plants producing more than 706 billion lb of steam per year. At a usage of 1429 Btu per lb of steam produced, the Btu consumption for the chemical industry, for example, totals about 1 quadrillion Btu per year. At an average cost of $5.40 per million Btu, the total cost is almost $5.5 billion per year. Honeywell then calculated the potential savings at 5% of the annual cost. D
Detector cube to capture nuclear events "Crystal cube" is what it's called by its manufacturer, Harshaw Chemical Co.'s crystal and electronic products department, Solon, Ohio. The unit is a cubical configuration of 396 scintillation detectors intended for use at Los Alamos National Laboratory in high-energy physics research on subatomic particles and photons. The cubical arrangement, Harshaw says, provides the timing accuracy necessary to isolate and capture a one-in-a-trillion nuclear event. Here, Harshaw physicist Csaba Rozsa attaches a phototube to one of the detectors. Each detector is an individual thallium-activated sodium iodide crystal.
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