17. Furfuryl alcohol

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17. Furfuryl Alcohol Furfuryl alcohol is the most important derivative of furfural. At present, approximately 65 percent of all furfural produced is converted to furfuryl alcohol as there is a good demand for this product in the manufacture of foundry resins. The manufacture of furfuryl alcohol is a simple hydrogenation, with copper chromite used as catalyst: H

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17.1. The Vapor Phase Process The process employed almost universally is shown schematically in Figure 77. Furfural is fed into an evaporator system comprising a packed column 1, a circulation pump 2, and a heater 3 energized by steam to maintain the furfural temperature at 120 ~

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packing of column 1, a dosed quantity of hydrogen is introduced. In the countercurrent system of hydrogen flowing upwards and liquid furfural flowing downwards, the hydrogen gets saturated with the vapor pressure of furfural at 120 ~

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furfural vapor passes a demister pad 4 and a superheater 5 before it enters a tubular catalytic reactor 6 maintained at a temperature in the order of 13 S ~ by means of hot oil. The tubes are filled with copper chromite pellets catalyzing the desired reaction of furfural with hydrogen to form furfuryl alcohol. This reaction is slightly exothermic, releasing 14.5 kcal/mole (60.668 kJ/mole), so that the oil flowing around the reactor tubes must withdraw heat from the system. The gaseous mixture of reaction products enters a condensation system comprising a packed column 7, a pump 8, and a cooler 9. The pump circulates raw furfuryl alcohol through the cooler 9 and onto the packing of column 7 where it meets a countercurrent of the gaseous products. From the latter stream, most of the condensables are liquefied. The remaining

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portion, consisting of unreacted hydrogen and the saturation quantities of the condensables at the column temperature, is recompressed by a ROOTS pump 10 and added to the hydrogen feed to prevent losses. A small bleed stream prevents a buildup of impurities. The condensed portion is fed into a reboiler system comprising a tank 11, a circulation pump 12, and a heater 13 energized by steam. The vapor produced by this system enters a packed vacuum distillation column 14. The head vapor of this column is liquefied by a condenser 15 maintained at a reduced pressure by a vacuum pump 16. Most of the condensate is retumed to the column as reflux, while the rest represents a small head fraction consisting of 2-methyl furan, unreacted furfural, and reaction water from the 2-methyl furan formation and polymerization effects. The sump fraction is the purified furfuryl alcohol. High-boiling polymers remaining in the reboiler are withdrawn intermittently. In actual practice, the temperature of reactor 6 is gradually increased from 122 ~ to 152 ~ to compensate a progressing decrease in catalyst activity due to carbonaceous deposits. A typical rate of temperature increase is 3 ~

After ten days, when 152 ~ are

reached, the feed is tumed off, and the reactor is heated to 220 ~ to remove the deposits by oxidation. Then the catalyst is reactivated by hydrogenation at 160 ~

before a new pro-

duction cycle is started at 122 ~ The yield of the process is in excess of 92 percent. The principal by-product is 2methyl furan. Its formation increases when the reactor temperature is raised to compensate decreasing catalyst activity. At high temperatures, commercial quantities of 2-methyl furan can be produced.

17.2. The Liquid Phase Process An older, less elegant process for making furfuryl alcohol is shown in Figure 78. In this process, the catalyst is used as a slurry, and the hydrogenation is carried out at a pressure of 200 ATM and at a temperature of 120 ~

Furfural and a copper chromite catalyst

are mixed in tank 1 by means of a circulation pump 2. Pump 3 feeds the slurry continuously through a preheater 4 into a tubular bubble reactor 5. Hydrogen, usually from a water electrolysis plant, is injected by compressor 6. The mixture leaving the reactor flows through a cooler 7 into a cyclone 8 where excess hydrogen is separated from the slurry and reinjected into the reactor feed stream by means of compressor 9. The slurry is depressurized in tank 10, a relatively small quantity of hydrogen thereby released being vented into the ambient air.

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Pump 11 takes the depressurized slurry into an overflow sedimentation centrifuge 12 where most of the catalyst particles are separated from the liquid phase. Removal of the solids from the bowl is effected manually at appropriate intervals. The liquid phase flows into a still 13 topped by a rectification column 14. The head vapors of the column are liquefied in condenser 15, the resulting distillate being partly returned to the column to effect rectification and partly collected in tank 16. This distillate is pure furfuryl alcohol. Vacuum pump 17 maintains a reduced pressure to permit distillation at moderate temperatures. Catalyst fines and highboiling polymers inevitably formed in the reactor remain in the still and are discarded.

17.3. Comparison of Different Catalysts All catalysts used for the hydrogenation of furfural are made of copper chromite, but there are significant differences between various grades. For the customary vapor phase process, this is illustrated in Figure 79, where the temperature required to maintain 99 % furfural conversion is plotted versus the time of operation [71 ]. /8o"

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Figure 79. Comparison of Three Different Catalysts. 1 - HARSHAW Cu- 1132 2 - CALSICAT X-407 TU 3 - Modified CALSICAT X-407 TU

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All three catalysts compared are seen to gradually lose their activity so that maintaining a constant conversion of 99 % requires increasing the temperature, but catalyst 3 loses its activity much more slowly than catalyst 1, so that with catalyst 3 the 99 % conversion can be obtained at relatively low temperatures for a relatively long period of time. This is advantageous as with all catalysts the formation of 2-methyl furan, an unwanted by-product, increases with increasing temperature as shown in Figure 80. Thus, catalyst 3 leads to a product of greater purity which in the end is synonymous with a higher yield. ae

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Reference [71] D. G. Rodgers, Comparison of CALSICAT Modified X-407 TU, X-407 TU, and HARSHAW Cu-1132 for Vapor Phase Hydrogenation of Furfural, Publication of the MALLINCKRODT SPECIALTY CHEMICALS COMPANY, 1990.