TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The system further comprises one or more additional reaction vessels comprising the first solid catalyst in fluid communication with the first reaction vessel; one or more additional reaction vessels comprising the second solid catalyst in fluid communication with the second reaction vessel; a heat exchanger configured to heat or cool fluid flowing into at least one of the first reaction vessel and the second reaction vessel; and a first heat exchanger configured to heat or cool fluid flowing into the first reaction vessel and a second heat exchanger configured to heat or cool fluid flowing into the second reaction vessel, where the heat exchangers are configured to provide fluid to the first reaction vessel at a temperature different from that of fluid provided to the second reaction vessel. The fluid communication between the first reaction vessel and the second reaction vessel is provided by a pipe connection between the first reaction vessel and the second reaction vessel. The first and the second reaction vessels comprises a column reactor, fixed packed bed, fluidized bed, bubbling bed, boiling bed, moving bed, elutriated bed, circulating bed and/or an agitated bed.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: (II) is a material such as oils and/or fats. The alcohol is a 1-4C alkanol (preferably methanol or ethanol). The first solid catalyst comprises a cation exchange resin. The second solid catalyst comprises an anion exchange resin.Preferred Process: The method further comprises removing at least a portion of one or more components of (A) prior to contacting it with the second solid catalyst. The step of mixing comprises using a cosolvent to produce a single phase mixture. The cosolvent is tetrahydrofuran. The contacting steps are conducted at a pressure of 50-5000 (preferably 1000-5000) kPa gauge. In the method, any given portion of the feedstock contacts the first solid catalyst for less than one hour; and any given portion of the first reaction mixture contacts the second solid catalyst for less than one hour. In the method of converting a triglyceride/free fatty acid to one or more other esters, the alcohol is a 1-4C alkanol (preferably methanol); the mixing step comprises mixing a fat or oil feedstock with the alcohol and cosolvent; and any given portion of mixture contacts the anion exchange resin/cation exchange resin for less than one hour. EXAMPLE - Animal fat (100 g) containing 15% w/w free fatty acids was placed in a flask. Methanol in the ratio of 60:1 and tetrahydrofuran (THF) (336 g) were added and completely mixed to a single phase. The entire contents of the flask were transferred into the autoclave and a strong acidic cation exchange resin (C381BH) (375 g) was added. The autoclave was pressurized to 2068 kPa gauge (300 psi gauge) and heated to 120degreesC for 1 hour during which time the mixer was operated at 600 rpm. The autoclave contents were then emptied into a vacuum filter to filter out the cation resin catalyst. The filtrate was transferred into a clean autoclave for the second stage reaction and a strong basic anion exchange resin (A674BOH) (375 g) was added. This mixture was pressurized to 2068 kPa gauge (300 psi gauge) and heated to 60degreesC for 1 hour during which time the mixer was operated at 600 rpm. After an hour, the entire contents of the autoclave were transferred into a vacuum filter to separate the anion resin catalyst. The filtrate was then transferred into a flask and mounted on a vacuum roto-vap separator to distill out the excess methanol and THF. After about 10 minutes, separate glycerol and ester layers appeared in flask. The contents were then centrifuged and the top ester layer was decanted from the glycerol, which remained as the bottom layer. The results indicated that more than 99.9% conversion (free fatty acids to esters) was obtained.