Continuous biodiesel production from acidic oil using a combination of cation- and anion-exchange resins

• A continuous process was developed to produce biodiesel from acidic oil.
• An optimal process of esterification–online separation–transesterification was suggested.
• NKC-9 cation and D261 anion-exchange resin all showed high catalytic activity.
• The conversions of oleic acid (above 98%) and soybean oil (92.3%) were achieved.

A continuous process was developed to produce biodiesel from acidic oil containing soybean oil and oleic acid, which combined esterification by cation-exchange resin NKC-9, online separation and transesterification by anion-exchange resin D261. The esterification was carried out with soybean oil/oleic acid weight ratio of 5/5, methanol to oleic acid weight ratio of 1.5/1, reaction temperature of 338 K and residence time of 126.6 min. After the reaction, the mixture was settled to online separate into two layers, and the methanol–water–oleic acid mixture at the top layer was reclaimed. The bottom layer, mainly containing soybean oil and methyl oleate, was transesterified under methanol/soybean oil weight ratio of 1/3 and n-hexane/soybean oil weight ratio of 1/2 at 323 K for the residence time of 112.0 min. The high conversions of oleic acid (above 98%) and soybean oil (92.3%) were achieved. The yield of biodiesel in this process reached up to 95.1%. The main parameters of the product met the Chinese Standard of biodiesel.