Vapour phase esterification of free fatty acids

Abstract

A method is presented for producing biodiesel from a triglyceride feedstock. The feedstock is pretreated by thermal cracking or rapid pyrolysis to form a middle distillate fraction rich in fatty acids. The middle distillate fraction is then treated by vapour phase esterification under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a biodiesel stream. The biodiesel stream can be treated with a basic solution to convert residual free fatty acids to non-foaming metallic soaps, which are separated by known means. A method is also provided for producing a biodiesel/naphtha mixture, in which a triglyceride feedstock is pretreated by thermal cracking or rapid pyrolysis to produce a middle distillate fraction, a naphtha stream and a gas stream. The naphtha stream and the middle distillate fraction are then treated by vapour phase esterification under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a mixed biodiesel/naphtha stream.

1 . A method of producing biodiesel from a triglyceride feedstock, the method comprising:
 a. pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides to form a middle distillate fraction rich in free fatty acids;   b. conducting vapour phase esterification of the middle distillate fraction under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a biodiesel stream;   c. treating the biodiesel stream with a basic solution to convert residual free fatty acids to non-foaming metallic soaps; and   d. separating the non-foaming metallic soaps by vacuum distillation, centrifugation, filtering or combinations thereof.   
     
     
         2 . The method of  claim 1  wherein the basic solution is an aqueous solution of a compound selected from the group consisting of lithium hydroxide (LiOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH) 2 ), and calcium hydroxide (Ca(OH) 2 ). 
     
     
         3 . The method of  claim 1  wherein the triglyceride feedstock is selected from the group consisting of canola oil, palm oil, soy oil, corn oil, cottonseed oil, mustard seed oil, fish oils, algae oils, restaurant trap grease, rendered animal fats, waste greases, low-quality vegetable oils and combinations thereof. 
     
     
         4 . The method of  claim 1  wherein thermal cracking is conducted at a temperature of from 390° C. to 460° C. 
     
     
         5 . The method of  claim 1  wherein rapid pyrolysis is conducted at a temperature of from 480° C. to 600° C. 
     
     
         6 . The method of  claim 5  wherein the triglyceride feedstock is fluidized with steam. 
     
     
         7 . The method of  claim 6  wherein the steam to triglyceride feedstock ratio ranges from 0.5 to 1.5. 
     
     
         8 . The method of  claim 5  wherein an inert gas is used to purge any oxidizing agents during rapid pyrolysis. 
     
     
         9 . The method of  claim 5  wherein a catalyst is used during rapid pyrolysis to enhance the cracking of triglycerides to largely free fatty acids. 
     
     
         10 . The method of  claim 9  wherein the catalyst is selected from the group consisting of acid washed activated carbon, calcined sewage sludge solids and silica sand. 
     
     
         11 . The method of  claim 1  wherein esterification is conducted in the presence of methanol as the alcohol. 
     
     
         12 . The method of  claim 1  wherein the ratio of middle distillate stream to alcohol is in a range of from 3:1 to 0.1:1. 
     
     
         13 . The method of  claim 12  wherein the ratio of middle distillate stream to alcohol is in a range of from 2:1 to 1:1. 
     
     
         14 . The method of  claim 1  wherein residence time for esterification ranges from 6 to 425 minutes. 
     
     
         15 . The method of  claim 14  wherein residence time for esterification ranges 6 to 43 minutes. 
     
     
         16 . The method of  claim 1  wherein esterification is conducted at a temperature of from 150 to 350° C. 
     
     
         17 . The method of  claim 16  wherein esterification is conducted at a temperature of from 200 to 250° C. 
     
     
         18 . The method of  claim 1  wherein esterification is conducted in a vacuum range of from 0.1 to 1.16 psia. 
     
     
         19 . The method of  claim 1  wherein the solid acid catalyst is selected from the group consisting of a TiO 2  solid support doped with Zr(SO 4 ) 2 , SnO 2  doped with sulphuric acid, sulphated zirconium oxide (ZrO 2 /SO 4   2− ), sulphated iron oxide, halogenated alumina, sulphated tin oxide, trifluoromethyl-imines, tungstated zirconia-alumina (W/SiZr—Al) and silica-supported aluminum chloride. 
     
     
         20 . A method of producing a biodiesel/naphtha mixture from a triglyceride feedstock, the method comprising:
 a. pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides to produce a middle distillate fraction rich in free fatty acids, a naphtha stream and a gas stream;   b. conducting vapour phase esterification of the middle distillate fraction under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a mixed biodiesel/naphtha stream;   c. treating the mixed biodiesel/naphtha stream with a basic solution to convert residual free fatty acids to non-foaming metallic soaps; and   d. separating the non-foaming metallic soaps by vacuum distillation, centrifugation, filtering or combinations thereof.