Direct Fired Heating of Fats and Oils for Deodorization

Abstract

The heating method which has been widely used commercially for deodorizing oils is condensing Dowtherm or high pressure steam heating. Direct fired heating has been kept away because of danger of overheating.If oil is heated in the direct-fired tubular furnace, the heat balance at any cross section of the radiant tube givesRD=h (tw-tb) Di (1) ortw=tb+ (R/h) (D/Di) (2) The overheating of oil must occur at the point where tw is too high. Eq. (2) indicates that tw depends on tb and R/h, if the inside and outside diameters of the tube are given.Consider a tubular furnace, employing 3/8 in. (2mm thick wall) heating tube, which absorbs heat at the rate of 20, 000kcal/m2hr. Let oil be heated by direct firing in this furnace up to 250°C. The maximum value of tw is calculated as follows.Since, in ordinary case, tw will get to the highest at the outlet of the heating tube where tb is the highest, tb=250°C. D=0.0173m, Di=0.0133m for 3/8 in. tube and h=1, 500kcal/m2hr°C.The radiant rate is non-uniform at various points around the circumference of the tube. The radiant flux distribution on the tube circumference has the maximum value at the point facing just opposite to the heat source. For the common tubular furnace in which bare tubes are arranged in a single row in front of a refractory wall, the ratio of the maximum point flux to the average flux on the tube surface is πD/lF12. It varies with l/D as shown in Table-1. Assuming l/D is 2.0, the maximum flux is 1.78 times of the average flux, and the maximum radiant rate is also 1.78 times of the average.Substitution in Eq. (2) gives the maximum value of tw=250+ (20, 000×1.78/1, 500) (0.0173/0.0133) =281°C.This temperature is not so high that it seems the effect is not injurious to the product. It suggests that the direct fired tubular furnace can be used to heat oils without danger of overheating.The author modified a usual semicontinuous deodorizer having a capacity of 18 tons oil per 24 hour day to test the direct fired heating system on a commercial scale. Fig.-1 shows a flowsheet of the modified deodorizer with an external tubular furnace instead of Dowtherm heating system. The modified plant was operated satisfactorily with refined cottonseed oil for over 300hr including a 200-hr continuous run. Both commercially and laboratory-deodorized samples were tested for initial flavor and AOM. An example of the test results is shown in Table-2.The direct firing system has been demonstrated on a commercial scale to be able to heat oils for deodorization without overheating if the tubular furnace is properly designed.