WO2009011639A2

WO2009011639A2 - Diesel range fuels from carboxylic acids with plant origin

Info

Publication number: WO2009011639A2
Application number: PCT/SE2008/000457
Authority: WO
Inventors: Lars Stigsson
Original assignee: Sunpine AB
Current assignee: Sunpine AB
Priority date: 2007-07-19
Filing date: 2008-07-18
Publication date: 2009-01-22
Anticipated expiration: 2010-01-19
Also published as: WO2009011639A3

Abstract

The present invention provides a process for manufacturing renewable diesel fuels from carboxylic acid rich organic material originating in plants, by treatment of said carboxylic acid rich material in at least three process stages; a) a distillation step under vacuum b)a decarboxylation step in the presence of an heterogeneous decarboxylation catalyst forming a renewable diesel fuel with a high cetane number c)a separation step wherein carbon dioxide is separated from the renewable diesel formed in step b).The renewable diesel fuel produced by the treatments of the present invention may be further treated by esterif ication for removal a traces of carboxylic acids, by adsorption or by treatment with hydrogen for deep removal of sulphur.

Description

DIESEL RANGE FUELS FROM CARBOXYLIC ACIDS WITH PLANT ORIGIN

Introduction

The present invention relates to a process for manufacturing a diesel range fuel from raw materials of plant origin such as crude tall oil. The process comprises treatment of a feed stream rich in carboxylic acids in at least three process steps a) a vacuum distillation step b) a decarboxylation step and c)separation of gases comprising carbon dioxide to form a renewable diesel range fuel stream. The renewable diesel can be used directly as an automotive diesel fuel or be further treated with hydrogen for saturation and deep desulfurization. Background

In times of high cost for fossil fuels and greenhouse gases emission considerations the interest for producing automotive fuels and chemicals from "green sources" such as plants and wood has increased.

Production of biodiesel by transesterification of vegetable oils such as palm oil, soy oil and canola to form FAME (fatty acid methyl ester) type biodiesel fuels have gained much attention and several commercial plants have been built. This approach often denoted "first generation biofuels", however, suffers from several drawbacks including competition for raw materials for food application, deforestation in tropical countries and issues relating to diesel engine performance.

Biodiesel fuels of the FAME type create deposits in the fuel and combustion system and furthermore FAME fuels have poor cold flow stability, low stability towards heat and oxidation, high water absorption, higher emissions of NOx and shortens the lifetime of engine oils. These drawbacks can be eliminated if the oxygen functionality is removed from the vegetable oils or FAME fuels. By decarboxylation and/or hydrogenation paraffinic, isoparaffinic and cycloalkane rich fuels with very good properties for direct use as diesel fuels in modern diesel engines can potentially be produced. For example, National Resources of Canada (NRCan) has developed a process for the catalytic hydrogenation of vegetable oils over conventional NiMo/AI2O3 and CoMo/AI2O3 catalysts. Although the biogasoil products of this process purportedly have cetane numbers in the range of 70-90, the yield of the desired diesel range hydrocarbons are lower than about 60 % (the byproducts are carbon oxides, short chain hydrocarbons, gasoline range hydrocarbons and very heavy high boiling hydrocarbons). By the denotation "diesel range hydrocarbons" inhere is understood hydrocarbons boiling in the 180-370 C range.

Crude tall oil is a renewable raw material originating from wood. The tall oil comprises organic compounds that can be converted to combustion engine fuels such as diesel fuel. The tall oil is recovered as a by-product from pulping of softwood in kraft pulp mills and typically consists of carboxylic acids whereof approximately 35-60% are fatty acids, including oleic, linoleic, linolenic and palmitic acids, 15-55% are rosin acids, including abietic, dehydroabietic and neoabie- tic acids and 5-35% unsaponifiable and neutral material including sterols such as beta-sitosterol. Hardwoods also contain extractives including fatty acids and neutrals (beta-sitosterol, betulin) but no resin acids.

In addition the tall oil contains a small fraction of contaminants from black liquor such as sulphur compounds (up to 3000 ppm as S), lignin components and fibers. The sulphur compounds include a wide range of organic and inorganic sulphur compounds including sulphate, sulphite, polysulfide, elemental sulphur, mercaptans, organic sulphides and organic sulfones and sulfonates. The sulphur compounds are connected to both the fatty and diterpenic moieties of the crude tall oil.

Tall oil is very corrosive due to its content of carboxylic acids (fatty acids) and sulphuric acid entrained with the tall oil from the tall soap acidula- tion stage in the pulp mills. The diterpenic acids present in the CTO are orders of magnitude less corrosive compared to fatty acids. The tall oil acidity is normally quantified as the acid value. Acid value (or "neutralization number" or "acid number" or "acidity") is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of the tall oil. Beyond any entrained sulfuric acid, the acid number of tall oil is a measure of the amount of carboxylic acid groups (in fatty acids and resin acids). In a typical procedure, a known amount of sample dissolved in organic solvent is titrated with a solution of potassium hydroxide with known concentration and with phenol- phthalein as a color indicator. The acid value of crude tall oil varies but is normally in the range of 120 mg/KOH to 160 mg/KOH.

The corrosivity of a biofuel is directly connected to the acid number and acid numbers lower than 1 mg KOH/g are typically the norm.

Crude tall oil contains a significant portion of C 20 carboxylic acids of diterpenic type including abietic acid. Diterpenic acids have two functional groups, carboxyl group and double bonds. Nearly all diterpenic acids have the same basic skeleton: a 3-ring fused system with the empirical formula Ci₉H₂₉COOH.

Diterpenic acids occur in pines in a number of isomeric forms having the molecular formula C₁₉H₂₉COOH and in some related structures. The most prevalent diterpenic acids in tall oil are: Abietic-type acids abietic acid abieta-7,13-dien-18-oic acid 13-isopropylpodocarpa -7,13-dien-15-oic acid neoabietic acid dehydroabietic acid palustric acid simplified formula C₂₀H₃₀O₂, or C₁₉H₂₉COOH represents the majority 85-90% of typical tall oil. structurally shown as (CH₃)₄C₁₅Hi₇COOH molecular weight 302 Pimaric-tvpe acids pimaric acid pimara-8(14),15-dien-18-oic acid levopimaric acid isopimaric acids simplified formula C_2C)H₃₅O₂ or Ci₉H₃₄COOH structurally represented as (CH₃)_S(CH₂)Ci₅H₂₃COOH molecular weight 307 CTO has been proposed as a source of raw material for the production of diesel range fuels over hydrogenation in hydroprocessing plants, however the presence of contaminants have led to fast catalyst deactivation and un- desired high yield of + C20 complex hydrocarbons boiling outside the diesel range. Furthermore the high content of fatty acids in crude tall oil inevitably leads to high corrosion in existing hydroprocessing plants normally not designed for fatty acid rich feedstocks. Feeds to petroleum hydroprocessing plants normally have acid values below 2 (acid value normally connected to the content of naphtenic acids in the crude feedstock).

Distilled and/or depitched tall oil or the use of clean tall oil within a certain specification has been proposed as a feasible alternative feedstock for the production of diesel range fuels over hydrogenation. Traditional depitching and distillation are however costly procedures performed at high temperature (above 200 C) and a large portion of the CTO is lost as a low value pitch. Furthermore, both distilled and depitched tall oil still contains the corrosive fatty acids which prevent the safe use of this feedstock to standard petroleum refinery hydrotreaters.

There is at present an intensive research and development effort in finding fast growing non-food type of oil bearing vegetables for use as raw material for biodiesel and renewable diesel. Jatropha is one such non-food plant with a great potential for exploitation as a biofuel source. The oils recovered from plants such as jatropha often have a large content of free fatty acids (carboxylic acids). Traditional transesterification routes for making bio- diesel are not suitable for highly acidic oils comprising carboxylic acids and new methods are needed to efficiently convert these oils to renewable diesel fuel.

There is clearly an expressed need for an efficient process for the production of a low corrosivity, low oxygen content diesel range fuel with plant origin that can be used directly as a renewable blending component in mineral diesel fuels or be further treated by for example hydrogen to produce premium diesel range renewable fuels. Summary of the invention

The present invention relates to a process for manufacturing a diesel range fuel from raw materials of plant origin comprising carboxylic acids such as crude tall oil, highly acidic vegetable oils and hydrolysed vegetable oils. The process comprises treatment of a feed stream rich in carboxylic acids in at least three process steps a) a vacuum distillation step b) a decarboxylation step and c)separation of gases comprising carbon dioxide to form a renewable diesel range fuel stream. The renewable diesel can be used directly as an automotive diesel fuel or be further treated with hydrogen for saturation and deep desulfurization.

Detailed description of the invention

We have discovered that carboxylic acid rich material including hydrolysed vegetable oils with a free fatty acid content over about 90 %, crude tall oil, tall oil rosin, gum rosin oils, acidic oils with origin in vegetable oils and the likes can be transformed into a low corrosivity, low oxygen content and high cetane bio renewable diesel range fuel(in the following biocetane fuel. The biocetane can be used as directly as a diesel fuel or be fed to petroleum refinery hydrotreaters wherein the biocetane is treated with hydrogen for final desulfurization and saturation of unsaturated compounds. The biocetane fuel can be treated by an adsorbent (with or without hydrogen present) for removal of sulphur compounds and thereby form a high quality diesel fuel product ready for use in diesel engine vehicles.

In accordance with the process of the present invention carboxylic acid rich biocetane raw material originating in plants is treated in at least three process steps. Several types of carboxylic acid rich raw materials originating in plants, including hydrolysed vegetable oils where glycerol have been splitted of and separated, acid oils with high free fatty acid content (over 90 %) or acidic animal fats can be used in practising the present invention. A preferred carboxylic acid raw material is crude tall oil. In the first process step of the present invention a crude carboxylic acid rich feed material stream is distilled under vacuum for separation of high boiling pitch components from the lighter diesel range organic compounds. Optionally the distillation step is preceded by an esterification of carboxylic acids. A stream of purified biocetane fuel is recovered from the first process step of the present invention.

The second process step comprises a catalytic reaction zone wherein the purified biocetane fuel from the first process step is subjected to treatment with at least one decarboxylation catalyst forming a second stream of decar- boxylated biocetane fuel having an acid value below about 40. Preferably the acid value of the second stream of biocetane fuel have an acid value below 20 and even more preferred an acid value below about 10.

In the third process step carbon dioxide containing gases are separat- ed from decarboxylated biocetane by stripping thereby forming a biocetane product fuel stream with a low corrosivity and a low content of oxygen. An ₍ inert gas or carrier gas may be added to be present during decarboxylation in order to facilitate separation of carbon dioxide gases from the decarboxylated biocetane stream. Optionally the biocetane product fuel stream from the third process step is further treated with an alcohol in order to esterify any remaining carboxylic acids in the product biocetane fuel. The acid value of the biocetane fuel product is thereby lowered to a level below about 1 mg KOH/g.

In the following the unit operations of the present invention is described in more detail. Esterification sterification treatment is optional but normally used in the practise of the present invention, particularly if acid values of product fuel have to be lower than about 1 mg KOH/g. Esterification can be performed in two positions, namely prior to distillation and/or after decarboxylation. Esterification comprises treatment of the carboxylic acid rich feed material and/or decarboxylated biocetane with methanol or ethanol in one or more reactors at a temperature in the range from 70 - 130 C in the presence of an acidic catalyst. If esterification is performed prior to distillation, a homogeneous catalyst is used which comprises for example sulphuric acid or organic sulphonic acids. A heterogeneous acidic catalyst system is used if esterification is performed on the biocetane fuel after decarboxylation. Typical heterogeneous catalysts include acidic resins (Amberlysts), acidic clays etc. The alcohol is charged to the carboxylic acid rich feed material in a stoichio- metric ratio from about 1/1 to 4/1 relative to carboxylic acid content of the feed material stream. By esterification prior to distillation the acid value of the bio- cetane fuel is lowered from the range of 70-250 mg KOH/g to a level below about 70 mg KOH/g. By esterification after decarboxylation in process step 2 the acid value of the product biocetane is lowered to a level below about 1 mg KOH/g.

Aliphatic carboxylic acids such as for example oleic acid will be esteri- fied while cyclic carboxylic acids of the diterpenic type, if present in the feed stream, are not esterified at the esterification reaction temperature disclosed herein. The main reason for this is a steric hindrance and the tertiary carbon location of the diterpenic acid carboxylic group.

Monohydric alcohols such as methanol or ethanol are preferably used as esterification alcohols. Biomethanol produced as a by-product in kraft pulp mills is a preferred alcohol for use in the present invention. Vacuum distillation

Prior to distillation in the distillation step of the process of the present invention the carboxylic acid feed stream (whether it has been esterified or not) is treated thermally by steam or methanol in a stripper, flash vessel or evaporator in a temperature range of 150-250 C in order to remove volatile compounds boiling at a temperature below about 150 C.

After removal of volatiles the carboxylic acid feed stream is distilled in order to form a purified biocetane fuel stream substantially free from inorganic salts and high boiling carbonaceous compounds. By a foregoing esterification of carboxylic acids with an alcohol (as described herein above) the boiling point of esters has been lowered relative to the corresponding acids.

The distillation is advantageously performed in a packed bed column operating under vacuum (1-25 mbar) in the temperature range of 150-280 C. The packed column may comprise one or more beds of structured packing and may comprise a reflux arrangement in the upper part and a reboiler arrangement in the lower part. The temperature in the upper part of the column is from 150 to 250 C and in the lower part from 220 -280 C. The pressure drop over the column is lower than 20 mbar, preferably lower than 10 mbar. Three product streams are discharged from the distillation step; a) purified biocetane fuel stream b) a high boiling carbonaceous tall pitch stream c) a volatile low boiling gaseous stream. The latter gaseous stream comprises terpenes, carbon dioxide and water. By cooling in one or more condensers the low boiling compounds are condensed and removed prior to vacuum steam ejectors and/or vacuum pumps. Decarboxylation

In order to remove further acidic functionality and lower the oxygen content and acid value of the purified biocetane fuel the purified biocetane fuel recovered from the first process step is further treated in a second process step in a reaction zone comprising a decarboxylation catalyst.

Decarboxylation reactions are endothermic reactions ΔH₅₇₃ is 9.2 kJ/kmol for the reaction R-COOH + Heat — > R-H + CO₂ wherein R is a C17 alkane. Therefore energy has to be added to the decarboxylation step of the present invention. While energy can be supplied in the form of preheating of the biocetane fuel feed, energy can optionally be provided by adding hydrogen to the decarboxylation step. Hydrogen reacts exothermally with unsaturated structures in the biocetane feed providing at least a portion of the heat necessary to drive decarboxylation reactions. The quantity of hydrogen needed for saturation of unsaturated structures in the feed can easily be established by simple stoichiometric analysis wherein the amount of hydrogen based on carboxylic acids in the feed shall be lower than about 30 kg/ton feed, preferably lower than 20 kg/ton feed and most preferred lower than about 10 kg hydrogen per ton of carboxylic acid rich feed fed to the decarboxylation stage. Decarboxylation of biocetane in accordance with the present invention is performed in a reactor at a temperature from about 180 C to 380 C, at a pressure ranging from atmospheric (0,1 MPa) to 5 MPa, an hourly space velocity of 0,1 to 5 h^"1 and in the presence of an active decarboxylation catalyst. The decarboxylation reactions are performed with biocetane in the liquid phase at prevailing pressure and temperature. Decarboxylated biocetane fuel product obtained from the decarboxylation step may be recycled to the decarboxylation stage to, for example, provide a larger mass flow through the catalyst bed. One key objective of the decarboxylation step of the present invention is to remove oxygen functionality from the biocetane by decarboxylation of carboxylic acids and/or esters present in the biocetane.

Catalyst design is of great importance in the decarboxylation step and methanization reactions should be suppressed while decarboxylation and saturation reactions are desired. As the biocetane may comprise anywhere from 20 to 1000 ppm sulfur compounds the decarboxylation catalyst should be sulphur tolerant. Preferred heterogeneous sulfur tolerant catalysts include acid clays and alkaline earth compounds (including calcium or magnesia carbonate. The most preferred catalyst is based on alumina and silica aluminas which has been activated with and acid such as phosphoric acid or sulphuric acid. For example aluminas marketed under the trade name Pural or silica aluminas marketed by Sasol under the trade names SIRAL and SIRALOX can be used. While the decarboxylation stage may be performed in a separate reactor (such as a fixed bed reactor) located afterthe distillation stage, decarboxylation can also be integrated in the distillation stage described above. By combining the separation of high boiling organic compounds from biocetane feed material with decarboxylation in a distillation column a catalytic distillation process is established. Catalytic distillation is a unique process that combines two fundamental unit operations, namely reaction and distillation in a single piece of equipment.

. In a specific embodiment of the present invention the decarboxylation of carboxylic acids and esters is performed in the gaseous phase in a distilla- tion column where carboxylic acids in gaseous form are passed through a catalytic bed comprising an active heterogeneous decarboxylation catalyst. The catalyst can be selected among the same catalysts disclosed above for decarboxylation reactions in the liquid phase outside the distillation stage. The heterogeneous decarboxylation catalyst can be contained in the form of a structured packing (type Katapak a trademark of Sulzer or type Katamax a trademark of Koch-Glitch).

Catalysts based on noble metals are known to be effective decarboxylation catalysts. For example palladium and/or platinum catalysts supported on active carbon, metal oxides, aluminium oxides or silica oxides has been proposed for decarboxylation of vegetable oils. Catalysts based on noble metals are, however, known for their high sensitivity to sulphur. Sulphur is poisoning and deactivating most noble metal catalysts. Purportedly advanced sulphur tolerant noble metal catalysts (comprising Pd or Pt, bimetallic) have been developed and are available for commercial application by catalyst manufacturers. These types of bimetallic sulphur tolerant catalysts are expensive but can be used in the decarboxylation step of the present invention. The composition of a typical feed to the decarboxylation step of the present invention is given in Table 1. The raw carboxylic acids are crude tall oil which have been partially esterified and thereafter distilled in accordance with the invention.

The carboxylic acid rich feed originating from plants is by the treatments disclosed by the present invention converted into a pure decarboxy- lated liquid renewable diesel fuel having a low corrosivity and low oxygen content. Furthermore, the renewable diesel product has a high cetane number (in the range from 60-100) and has good physical properties for use directly as a blending component in mineral diesel fuels. Should very low sulphur content be desired (below about 20 ppm) the renewable diesel fuel product can be further purified by oxidative treatment with for example per- acids or ozone followed by alkaline wash and /or adsorption on for example silica gel. Sulfur can also be removed by selective adsorption in an adsorption bed comprising a metal or metal oxide, preferably ZnO, CuO or NiO.

The biocetane product can advantageously be further processed by treatment with hydrogen in one or more hydroprocessing reactors for saturation, final desulfurization and isomerisation forming a premium renewable diesel fuel.

The present invention thus provides a process for manufacturing renewable diesel fuels from carboxylic acid rich feed streams originating from plants, by treatment of said feed stream in at least three process stages; a) distillation under vacuum with or without prior esterification of carboxylic acids b) decarboxylation in the presence of an effective heterogeneous decarboxylation catalyst forming a renewable diesel fuel with a high cetane number c) separation of carbon dioxide from the renewable diesel formed in step b). The renewable diesel fuel produced in the three process steps of the present invention may be further treated by esterification for removal a traces of carboxylic acids, treated by adsorption with a metals for sulphur removal or be treated with hydrogen for deep removal of sulfur.

While certain representative embodiments and details have been disclosed for the purpose of illustrating the present invention, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

Table 1.

Claims

1. A process for the manufacturing of a low corrosivity renewable diesel range fuel from a liquid stream comprising carboxylic acids with plant origin c h a r a c t e r i z e d in that the carboxylic acid liquid stream is subjected to at least three process step wherein;

a) a liquid stream of carboxylic acids is charged to a distillation unit operating under vacuum in the range of 1 to 100 mbar and at a temperature between 150-280 C thereby forming a stream of distilled and purified carboxylic acids

and

b) distilled and purified carboxylic acids formed in step a) is subjected to a treatment in the presence of an heterogeneous decarboxylation catalyst in a reactor or reaction zone at a temperature from about 180 C to 380 C, at a pressure ranging from 0,1 MPa to 5 MPa and under an hourly space velocity of from 0,1 to 5 h^"1 thereby forming a stream of decarboxylated renewable diesel range fuel with an acid value below about 50 mg KOH/g

and

c) carbon dioxide containing gases are separated from the decarboxylated liquid renewable diesel range fuel formed in step b) by stripping or by flashing

2. The process of claim 1 wherein at least a part of the carboxylic acids are esterified with a monohydric alcohol prior to distillation in step a)thereby lowering the acid value of the carboxylic acid stream to a level below about 70 mg KOH/g.

3. The process of claim 1 wherein carboxylic acids present in the decarboxy- lated renewable diesel range fuel formed in step b) of claim 1 are esterified with a monohydric alcohol thereby lowering the acid value of below about 1 mg KOH/g.

4. The process of claim 1 wherein hydrogen gas is present during the decarboxylation in step b) of claim 1 in a quantity sufficient to saturate at least a portion of the unsaturated moieties present in the carboxylic acids and/or diesel renewable fuel produced during decarboxylation in step b).

5. The process of claim 4 wherein hydrogen gas is present during the decarboxylation in step b) of claim 1 and is consumed by chemical reaction in a quantity corresponding to less than 30 kg per metric ton of carboxylic acids, preferably in a quantity of less than 20 kg per metric ton and most preferred in a quantity less than 10 kg per metric ton of carboxylic acids.

6. The process of claim 1 wherein carboxylic acids with a plant origin charged to step a) of claim 1 comprises cyclic diterpenic carboxylic acids.

7. The process of claim 6 wherein cyclic diterpenic acids are present in a quantity between 2 and 25 % by weight of the total quantity of acids charged to step a) of claim 1.

8. The process of claim 1 wherein carboxylic acids with a plant origin charged to step a) of claim 1 comprises fatty acids originating from hydrolysis of vegetable oils.

9. The process of claim 1 wherein carboxylic acids charged to step a) of claim 1 originates in crude tall oil.

10. The process of claim 1 wherein the decarboxylated liquid renewable diesel range fuel from step b) of claim 1 is subjected to treatment with hydro- gen in a separate hydroprocessing unit, thereby forming low sulfur content saturated renewable diesel fuel.

11. The process of claim 1 wherein the acid value of the of decarboxylated diesel range fuel formed in step b) of claim 1 has an acid value below about

20 mg KOH/g preferably an acid value below about 10 mg KOH/g.

12. The process of claim 1 to 3 wherein an additional esterification step is performed after step c) of claim 1 wherein the decarboxylated diesel range fuel formed in step b) is esterified with a monohydric alchohol in the presence of an heterogeneous esterification catalyst thereby forming a low corrosivity renewable diesel range fuel with an acid value below about 1 mg KOH/g.

13. The process of claim 1 wherein an additional treatment step for removal of sulfur is performed after step c) of claim 1 wherein the decarboxylated diesel range fuel formed in step b) is treated with an adsorbent thereby lowering the sulphur content of the decarboxylated diesel range fuel to a level below 20 ppm, preferably to a level below about 10 ppm.

14. The process of claim 12 wherein an sulphur adsorbent is selected among Zn ,Cu or Ni compounds.