This invention relates to a process for producing hydrocarbons useful as diesel fuel or a diesel fuel blending component from renewable feedstocks such as the triglycerides and free fatty acids found in materials such as plant oils, fish oils, animal fats, and greases. The process involves hydrogenation, decarboxylation and/or hydrodeoxygenation and isomerization in one or more reactors. A selective hot high pressure hydrogen stripper is used to remove at least the carbon oxides from the hydrogenation, decarboxylation and/or hydrodeoxygenation zone effluent before entering the isomerization zone. A diesel range stream or a naphtha range stream, or a mixture of the two streams is used as an additional rectification agent in the selective hot high pressure hydrogen stripper to decrease the amount of product carried in the overhead thereby reducing the amount of n-paraffins in the diesel fuel.
As the demand for diesel fuel increases worldwide there is increasing interest in sources other than petroleum crude oil for producing diesel fuel. One such non-petroleum source is what has been termed renewable sources. These renewable sources include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean and algal oils, animal fats such as inedible tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these sources is that they are composed of triglycerides and Free Fatty Acids (FFA). Both of these compounds contain n-paraffin chains having from about 8 to about 24 carbon atoms. The n-paraffin chains in the tri-glycerides or FFAs can also be mono-, di- or poly-unsaturated. Some of the glycerides from the renewable sources may be monoglycerides or diglycerides instead of or in addition to the trigylcerides. Another class of components considered a renewable feedstock is fatty acid alkyl esters.
There are reports in the art disclosing the production of hydrocarbons from oils. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US 2004/0230085 A1 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
Applicants have developed a process which comprises one or more steps to hydrogenate, decarboxylate (and/or hydrodeoxygenate) and isomerize the feedstock. The performance of the isomerization catalyst is improved by removing at least carbon dioxide from the feed to the isomerization zone. The presence of oxygen containing molecules including water, carbon dioxide, and other carbon oxides may result in the deactivation of the isomerization catalyst. The oxygen containing molecules such as carbon dioxide, carbon monoxide and water are removed using a selective hot high pressure hydrogen stripper which additionally contains a rectification zone.