Furfural Processes

In the now-obsolete furfural process, furfural was decarboxylated to furan which was then hydrogenated to tetrahydrofuran (THF). Reaction of THF with hydrogen chloride produced dichlorobutene. Adiponitrile was produced by the reaction of sodium cyanide with the dichlorobutene. The overall yield from furfural to adiponitrile was around 75%. 

Lubricating Oil Extraction. Aromatics are removed from lubricating oils to improve viscosity and chemical stabihty (see Lubrication and lubricants). The solvents used are furfural, phenol, and Hquid sulfur dioxide. The latter two solvents are undesirable owing to concerns over toxicity and the environment and most newer plants are adopting furfural processes (see Furan derivatives). A useful comparison of the various processes is available (219). 

Furfural Process. A patent involving furfural for solvent extraction of lubricating oils was issued to Eichwald of the Royal Dutch Shell Co. in 1925 (10). The first commercial application of the furfural process was at the Lawrenceville, 111., refinery of the Indian Refining Co. in December 1933. 

This company never had a pilot plant for furfural. Work moved directly from the laboratory experiments to what was essentially a full scale unit. This was the result of the fact that the QUAKER OATS COMPANY had available in the plant at Cedar Rapids, where the first furfural processing plant was to be operated, a number of iron pressure cookers about 8x12 feet, which had been used in the manufacture of a cereal product which did not prove profitable. Since these cookers were available and since the process was to consist of the treatment of oat hulls with acid under pressure, it seemed advisable to try to use these digesters at least for the first attempts at large scale operation. 

With the sealed ampoule process used for their kinetic studies, Root, Saeman, Harris, and Neill [20] achieved fiirfiiral yields well in excess of 70 % at temperatures above 220 °C, whereas industrial furfural processes, operating at lower temperatures and featuring a continuous removal of the furfural by steam stripping, have typical yields below 60 %. By contrast, in analytical chemistry, at a proven yield of 100 % [21], the formation of furfural from xylose or pentosan is routinely used for the quantitative determination of these substances. It is of great importance to elaborate the reasons for this yield paradox . 

In more detail, this fundamental difference between the analytical furfural process and the industrial furfural processes is illustrated schematically in Figure 10 showing... 

Figure 10. Phase Diagram for Explaining the Difference between Analytical and Industrial Furfural Processes D and D Dew Point Curves E and E Boiling Point Curves...

The situation is quite different when a small furfural concentration is generated in the second case, and when the heating is effected by condensing steam of 100 °C (atmospheric analogue of an industrial furfural process) as this results in point B lying in the liquid field where furfural can react with itself and with the first intermediate of the xylose-to-furfu-ral conversion. 

It is instructive to compare the formation of furfural in a boiling xylose solution (analytical furfural process) with an injection of some ether into boiling water. The ether/wa-ter phase diagram is shown schematically in Figure 11. When ether is injected into boiling... 

Figure 17. Schematic of the Chinese Furfural Process (Plant in Shanying, 2500 tons/a with 6 reactors).

Finally, on another front, an understanding of the reasons for the present huge losses in industrial furfural reactors has shown a way towards the 100 % yield routinely obtained in the analytical furfural process discussed in chapter 7. 

The enormous advantage of the STAKE feeder lies in the fact that it can handle almost dry raw material. For the manufacture of furfural, water is required only to the extent as it is needed for hydrolyzing pentosan to pentose, and for dissolving this pentose. The subsequent conversion of pentose to furfural actually creates water. Thus, any excess input water is undesirable as it dilutes the acid catalyst and reduces the caloric combustion benefit of the residue. Against this background, a furfural process using the STAKE feeder has inherent advantages. 

All furfural processes lead not only to furfural but also to carboxylic acids. Depending on the raw material employed, the production of carboxylic acids may exceed the production of furfural. 

In the normal recovery schemes treated so far, the acid waste water of the furfural process was directly submitted to a separation so as to obtain the carboxylic acids as a byproduct, for sale. It is, however, instructive to realize that apart from the small percentage of... 

It is a tragedy that the first industrial furfural process, described in the introduction, had to be carried out in a total absence of these facts, and that its temperature was limited by the low pressure capacity of old reactors from an abandoned cereal process, so that severe loss reactions and correspondingly low yields became a trademark of the furfural industry from the very start. 

In the light of all the facts now available from many independent sources, new furfural processes, as the SUPRATHERM and STAKE processes, aim at the increased yields obtainable at high temperatures, even without removal of the furfural from the scene of the reaction. Although this leads to somewhat uncomfortable high pressures, it is certainly a correct route towards higher yields, based on a fundamental principle of thermodynamics, and in hindsight the circumstances at the birth of the furfural industry must be deplored. 

Inasmuch as all industrial furfural processes are carried out at elevated pressures, they all involve a depressurization (flashing) of the residue. In the case of processes where the raw material is steam-stripped to the point of exhaustion, the flashing of the residue yields very little if any furfural, but in the case of non-stripped single pass processes, such as the SUPRATHERM and STAKE processes, the flashing of the residue is an important part of the overall process in that it yields a vapor stream containing most of the furfural produced, and a residue still containing some furfural in its liquid phase. This is illustrated schematically in Figure 123. 

Figure 130. The Yield of a Hypothetical Furfural Process involving Resinification as the Only Loss. Graphical representation of equation (7) in dimensionless form.

This surreptitious atmosphere has not been conducive to progress as evidenced by the fact that the very first industrial furfural process, launched 78 years ago with old equipment of a defunct cereal plant, is still used today although its yield is poor, without necessarily being so. 

Furfural Process gas oils Carbon black feed, ... 

Of the 5.4 MMTPY of corn produced, approximately 40% consists of residue and the remaining 60% is used as grain (4). Total annual furfural consumption in the United States is 150 million pounds. For each pound of furfural processed, approximately 10 lb of residue is produced. The furfural residue contains approximately 35% moisture (. Availability of the wood residue is estimated by assuming that only 10% of the class l-IV sites will be used for hybrid poplar plantations and that half of the wood produced will be collected as wood residue. Availability of the three biomass feedstocks is summarized in Table I. 

Furfural Process (23, 37/, 41, 94, 103). Furfural as a selective solvent is used at relatively high temperatures, usually in the range from 150 to 250°F. The higher permissible temperatures allow for extraction of oils of high viscosity and waxy fractions even in packed towers without danger of clogging the packing. 

Miscellaneous Hops (beer), coal, coca bean, coke, cork, corn (cobs, stovers), citrus pulp, flax shieves, furfural process residues, hemp hurds, keratin (chicken feathers, cattle hooves, hog bristles), leather, milkweed products, peat moss, soybean meal, starch, corn, potato, rice, tapioca, wheat, wool fiber... 

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