Field of the invention

The present invention relates to method for producing a biofuel mixture and for upgrading thereof.

Technical Background

Lignin is a class of complex organic polymers that form key structural materials in the support tissues of vascular plants and some algae. Lignin’s are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity and do not rot easily. Chemically, lignin’s are cross-linked phenolic polymers.

Lignin is liberated in the traditional kraft process and discharged from the kraft digester in the form of an alkaline liquor called black liquor. The black liquor is an aqueous solution of lignin residues, hemicellulose, and spent inorganic chemicals used in the pulping process. Further, black liquor comprises tall oil soap. Lignin may also be liberated from wood or non-wood lignocellulosic plants by other processes than the kraft process, such as the sulfite process, organosolv or hydrolytic processes. The lignin obtained is therefore often referred to as kraft lignin, sulfite lignin (or lignosulfonates), organosolv lignin or hydrolytic lignin. Kraft lignin is the dominant source for lignin for a foreseeable future, the sulfite processes are declining. Organosolv and hydrolytic lignin are available in minute quantities only, as these processes are still only operating in pilot scale.

Kraft lignin extracted from black liquor is potentially available in tens of millions of tons per year. Most kraft lignin is today burned in the kraft pulp mill recovery boiler for raising steam, though there is an interest for a use of lignin providing higher economical values, i.e. profit. Recently, technologies for extracting and purifying kraft lignin from black liquor have been commercialized to provide for other uses of lignin than simply burning it in the pulp mill. These processes are inter alia based on acidulation of the black liquor with acids and subsequent recovery of lignin by filtration as exemplified by disclosure EP2247785. Another route for recovery of lignin is based on membrane filtration and subsequent washing of the lignin by acids and/or solvents. Such method is disclosed in SE 540451 included here as a reference in its entirety.

Tall oil pitch (herein after TOP) is an oily by-product obtained when processing crude tall oil to provide tall oil. Crude tall oil is obtained from the black liquor of alkaline digestion of coniferous wood, most notably the kraft process. The black liquor from the kraft process is typically concentrated and settled to yield soap skimmings that contain sodium salts of fatty acids, sodium salts of resin acids, and unsaponifiables (sometimes referred to as neutrals). The latter group of substances include fatty alcohols, free sterols, steryl esters, and fatty acid esters. In kraft pulp mills, the collected soap is routinely acidulated with a mineral acid, such as sulphuric acid, to yield an oil phase and an aqueous phase. The oil phase contains free fatty acids, resin acids and unsaponifiables; it is commonly known as crude tall oil. Typically, the amount of unsaponifiables can range from 10 to 35% by weight of the crude tall oil, depending on the species and quality of coniferous wood used. The water phase containing sodium sulphate and any lignin entrained in the original soap is normally recycled back to the pulp mill chemical recovery system.

In the subsequent recovery of desired fatty acids and resin acids, crude tall oil is typically evaporated under low pressure conditions to yield a light phase, known as depitched tall oil (or simply tall oil), containing mainly fatty acids and resin acids, and a heavy phase, known as tall oil pitch (TOP), containing a substantial amount of the original unsaponifiables. The TOP also contains varying amounts of fatty acids (straight hydrocarbon chain ranging from e.g. C12-C26 of varying unsaturation degree i.e. zero, one, two, three, etc. double bonds and containing a primary/terminal carboxylic acid group) and rosin acids (typical examples being Pimaric, iso-Pimaric, Sandaracopimaric, Levopimaric, Palustric, Abietic, Dehydroabietic and Neoabietic acids occasionally referred to as resin acids) as well as their derivatives.

TOP is typically characterized by having a high viscosity. TOP normally also contains a considerable amount of ash (being non-volatile), in particular calcium compounds and sodium ash compounds, such as sodium sulfate. The ash content of untreated TOP varies depending on the source of crude tall oil and the efficiency of upstream desalting steps, but is normally in the range from 0.1 % up to 2 to 3 % by weight of TOP.

The TOP residue fraction represents typically between 20 and 30 % by weight of crude tall oil processed. The TOP typically has an acid value in the range of 30 - 60 (mg KOH/g) and comprises a significant amount of valuable carboxylic acids in their free acid form. However, even larger amounts of these valuable components are present within the TOP as carboxylic acid derivatives, i.e. carboxylic acids in bound form, such examples include but are not limited to fatty acid steryl esters, fatty acid esters with fatty alcohols, etc.

The present invention is directed to a method for providing a substantially homogeneous low ash content mixture of TOP and lignin that can be used as a renewable feedstock in petroleum refineries. Use of such a biofuel mixture as a renewable feedstock in petroleum refinery is deemed to represent an interesting option, as it would provide a green feedstock which could be processed via de-oxygenation in an existing petroleum refinery, employing the established infrastructure, to provide various hydrocarbon mixtures, e.g. liquefied petroleum gas (LPG), gasoline/petrol, kerosene, jet fuel, diesel oil, and/or fuel oils.

Further, such a substantially homogeneous low ash content mixture of TOP and lignin can also be used as a biofuel for power and/or heat generation, as a fuel for limekilns or kilns for iron ore processing.

Summary of the invention

A first aspect of the invention relates to a method for producing a low ash content biofuel mixture comprising tall oil pitch (TOP), lignin and optional additives for use as renewable feedstock in a petroleum refinery. The method comprises the steps of;

- providing a stream of purified TOP with an ash content below 0.1 %;

- providing a stream of organosolv lignin, hydrolytic lignin, or purified kraft lignin, said lignin having an ash content below about 0.1 %, preferably an ash content below about 0.01 %, wherein the lignin has been purified to remove ash from the lignin to a level below 0.1 % by one of: acid refining with an acid, solvent extraction, or solvent dissolution;

- dissolving the purified lignin in an organic solvent;- mixing the purified lignin dissolved in an organic solvent, a surfactant increasing the solubility of the lignin, and the purified TOP in a reactor followed by evaporation of the organic solvent upon heating;

- discharging a pumpable and substantially homogeneous low ash biofuel mixture comprising TOP and lignin from the reactor. Preferably, TOP and lignin is discharged as a pumpable and substantially homogeneous slurry with lignin dispersed on a molecular level stabilized by the surfactant from the reactor.

According to another aspect, the biofuel mixture provided by the present method is used as a renewable feedstock in petroleum refineries, as a biofuel for power and/or heat generation, as a fuel for limekilns or kilns for iron ore processing or as a feedstock for fine chemicals or carbon fiber manufacturing. Preferably, the provided biofuel mixture is used as a renewable feedstock in a petroleum refinery.

Further details of the Invention

According to one specific embodiment of the present invention, the original TOP and components therein has an average molecular weight substantially above 350 g/mol, e.g. in the range of 350 - 800 g/mol, and any fatty acids and/or rosin acids, and/or derivatives thereof, and/or unsaponifiables and their derivatives present in the TOP have an average molecular weight below 350 g/mol, such as molecular weight in the range of 200 - 330 g/mol. The latter compounds, i.e. fatty acids, rosin acids, unsaponifiables, and/or derivatives of any these classes of compounds, having an average molecular weight below 350 g/mol, may be separated by fractionation, e.g. by further distillation, prior to use of the residue TOP as blending stock for lignin, such compounds may be used in other applications.

Mixing lignin with TOP improves the processability of the lignin, as the TOP may serve as carrier liquid. Further, TOP is a low value by-product in the pulp mill. However, similar to lignin, it may be upgraded by de-oxygenation in a petroleum refinery. Thus, there is no need to separate TOP before processing lignin in the petroleum refinery. On the contrary, it may be beneficial to process lignin mixed with TOP in the petroleum refinery. As an example, the higher hydrogen/carbon ratio of TOP compared to lignin, being aromatic, is advantageous.

According to an embodiment, the TOP is thermally treated before mixing it with the lignin. By thermally treating the TOP its viscosity may be reduced, thereby improving its applicability in mixing it with lignin and processing the mixture. The optional thermal treatment of TOP may be performed in the presence of water or steam. The TOP may be thermally treated at a temperature of at least 250°C to affect its viscosity, such as ta a temperature in the range 250 to 400°C, or in the range 300 to 350°C.

It is important to keep the TOP hot at all times as viscosity is a direct function of temperature. The TOP is thus kept at a temperature above 100 °C, and preferably at a temperature over 150 °C, at all times during processing in accordance with the invention.

Optionally, the thermal treatment of TOP is performed in the presence of water or steam. Thermally treating TOP in the presence of water or steam may further lower the viscosity of the TOP. One focus of the method according to the present invention is to decrease the viscosity of the TOP fraction produced and to further simplify purification from salts and other impurities and to facilitate blending with lignin.

The TOP may be recovered from any position in a crude tall oil fractionation plant or from a plant treating crude tall for the manufacturing of crude tall oil diesel. An example of a plant for treating crude tall for the manufacturing of crude tall oil diesel is disclosed in EP2935546A1.

Prior to the use as blending stock for lignin the TOP is typically purified in one or more steps. Purification of TOP is specifically targeting ash removal. It may be performed by acid refining with an acid such as sulfuric acid, filtration, solvent refining and/or distillation. The ash of the TOP content is lowered to a level of at least below about 0.1 %; preferably to an ash level below about 0.01 %.

The second component of the biofuel mixture of the present invention is pure lignin, i.e. lignin with an ash content below 0.1 weight percent (wt.%). The lignin may be organosolv lignin, hydrolytic lignin, or purified kraft lignin. According ot an embodiment, the lignin is purified kraft lignin.

While it is of great importance to use a pure lignin, the method of purification and processes upstream lignin purification may still vary to a great extent. Various methods for purifying lignin are indeed known in the art. The present method is not limit to a specific method for lignin purification.

US 2016/0137680 A1 discloses a method to obtain, fractionate, and purify lignin. The method was subsequently also disclosed in Chem. Commun., 2015, 51, 12855. The method comprises one or several separation steps where biomass containing lignin is extracted and fractionated with an aqueous solvent with may comprise an organic acid. The mixture separates and one obtains one liquid phase where high molecular weight lignin is abundant and one phase where the solvent is more abundant, but which phase also comprises low molecular weight lignin. The lignin phase can then be further treated with solvents to obtain a purified lignin. Though metal content of the lignin is lowered, the method does not allow for nearly complete removal of sodium. Further, some lignin is lost, lowering the overall yield in the process. Still, process may be used in the present method, although representing a less preferred method for purifying lignin.

In a process of repeated washing of solid, dispersed lignin, with an acidic aqueous phase (cf. WO 2018/004447) a threshold is reached. It seems that entrapped sodium will not be released even if the washing steps are repeated a number of times. Similar to the method of US 2016/0137680 Al, the method of WO 2018/004447 may be used in the present method, although representing a less preferred method for purifying lignin.

One particularly advantageous method for purification of lignin upstream the TOP lignin mixing step of the present invention is by a solvent refining process specifically targeting ash removal and in particular lowering the sodium content of lignin. The lignin may be lignin obtained from biomass, e.g. wood, in a kraft pulping process. As an example, the feedstock may be black liquor comprising lignin. Given the high pH of black liquor due to the use of sodium hydroxide in the kraft process, at least some of the phenolic hydroxyl groups of the lignin dissolved in black liquor are typically deprotonated with sodium acting as counter ion. At high pH, lignin is water soluble, whereas lignin will precipitate if the pH 10 is lowered as the phenolic hydroxyl groups will become protonated, lowering the water solubility of lignin.

A specifically advantageous solvent refining process for purifying lignin, said lignin comprising metal cations, such as sodium and calcium ions, is disclosed in WO 2020/013752. According to an embodiment, the lignin is purified by this solvent refining process. This solvent refining process comprises the consecutive steps of:

(a) providing lignin to be purified, e.g. by acidifying black liquor, and subsequently separating the thereby formed raw lignin phase;

(b) dissolving the lignin in an acidic, aqueous solvent (e.g. aqueous methanol and/or aqueous acetic acid), at a temperature of at least 50 °C, such as at least 70 °C, 80

°C or 90 °C, to provide a liquid one-phase system comprising dissolved lignin;

(c) triggering phase separation by diluting the one-phase system by adding water, and/or by lowering the temperature (e.g. by at least 5°C, such as by at least 10°C, 15°C, 20°C, or 25°C), to provide a two-phase system, in which two-phase system the first phase is a lignin rich phase, and the second phase is a liquid, aqueous phase poor in lignin and comprising metal cations extracted from the lignin; and

(d) separating the lignin rich phase from the two-phase system to recover purified lignin.

It was found that by employing a step in which lignin is dissolved in a liquid one-phase system to subsequently be separated therefrom, a number of advantages are provided.

By employing a method in which lignin is dissolved in an acidic, aqueous solvent, efficiently extracts sodium and other metal ions, whereby metal cations efficiently are removed from the lignin, as they remain dissolved once the lignin is separated.

Secondly, if employing many, repeated washing steps, the overall yield of lignin may be lowered. In a process in which a step of dissolving lignin in a liquid one phase system is included, the overall number of steps may be reduced, whereby improving the yield.

Thirdly the washing efficiency is improved by completely dissolving lignin in liquid one-phase system and subsequently triggering phase separation to form a two- phase system, compared to processes in the art (cf. e.g. US 2016/0137680) relying on liquid/liquid two-phase system.

The inventors of the present invention have surprisingly found that a purified TOP with low ash content can be mixed with a pure lignin to form a substantially homogenous biofuel mixture that can be used in various applications as a renewable fuel or feedstock for fine chemicals. While the lignin typically is dissolved in an organic solvent before being mixed with TOP, it can, according to alternative embodiment, be mixed with TOP in pure powder form.

The mixing efficiency, stability of and rheological properties of the biofuel mixture can further be improved by the addition of one or more additives, e.g.

surfactants, to the lignin/TOP blending step or directly thereafter. Examples of such additives are standard surfactants with a selected hydrophilic lipophilic balance. The surfactants may be ethylene oxide adducts, anionic surfactants, cat ionic surfactants, zwitter ionic surfactants, modified polyesters or polyelectrolytes.

Also, water can be added to the mixture in low amounts where an amount of water corresponding to about 10 wt.% of lignin weight, has been shown to give improved mixing properties of lignin in organic solution. The amount of water is preferably kept as low as possible, such as in a range from 1 wt.% to 15 wt.% of the lignin content.

Other additives such as carboxymethylcellulose and/or guar gums may also be used to improve mixing or rheological properties of the TOP lignin biofuel mixture.

Also, solvents may be added to the biofuel mixture prior or during mixing, such solvents including green solvents such as methanol, ethanol, turpentine, desalted low acid value crude tall oil (for example tall oils with acid value below about 140 mg KOH/g), acetone or fossil blend stocks such as light cycle oil or vacuum gas oil (VGO). According to an embodiment, the solvent is methanol.

Additives, e.g. surfactants, are added in an amount from about 0.1 wt.% to 10 wt.% of the lignin weight and solvents may be added in any rage from 1 to about 60 wt.% of the lignin weight. The proportion between lignin and TOP may also be varied within a broad range from 10 wt.% lignin up to 90 wt.% lignin (relative to TOP in the biofuel mixture). The mixing of the components can be performed both in batch mixing reactors, semi batch reactors, or continuous reactors, such as continuously stirred tank reactors (CSTRs), tubular reactors with static mixers, or combinations of reactors designs.

The temperature during mixing should be kept high in order to facilitate for the proper mixing of lignin, additive and TOP into a homogeneous mixture. The

temperature in the mixing reactor(s) should be higher than 100 °C, and preferably higher than 130 °C.