WO2021255613A1

WO2021255613A1 - Recycling of formic acid

Info

Publication number: WO2021255613A1
Application number: PCT/IB2021/055200
Authority: WO
Inventors: Jari KAVAKKA; Christian EHN
Original assignee: Stora Enso Oyj
Priority date: 2020-06-16
Filing date: 2021-06-14
Publication date: 2021-12-23
Also published as: SE2050722A1

Abstract

The present invention is directed to an improved method for recirculation of formic acid in a process for preparing dissolving pulp, in which sugars are extracted from pre-hydrolysis liquor.

Description

RECYCLING OF FORMIC ACID

Field of the invention

Background

Dissolving pulp consists nearly exclusively of pure cellulose, compared to typical paper pulp, in which hemicelluloses play an important role in paper furnish. The alkaline Kraft pulping process alone has limited ability to dissolve hemicelluloses completely, due to re-precipitation of xylan on fibers in the end of the delignification process. Thus, in dissolving pulp production, wood hemicelluloses must be depolymerized either to soluble form or to make them easily alkali soluble before alkaline Kraft delignification process.

Therefore, sugar extraction from the hemicelluloses removed from the dissolving pulp production, for example by pre-hydrolysis, is of interest.

In current processes, pre-hydrolysis is achieved using steam, but if a reasonable amount of hemicellulose sugars is to be extracted, then a liquid- based pre-hydrolysis must be performed.

In current dissolving pulp processes, biomass, such as wood chips, are typically heated at approximately 170°C in vapor phase to degrade hemicelluloses or to make them degradable in subsequent alkaline cooking process. Moreover, in case of birch wood, released acetic acid from the xylan backbone further enhances hemicellulose degradation. After the hot vapor treatment, degraded and de-acetylated xylan will easily dissolve in subsequent highly alkaline Kraft delignification process, producing a pulp with low hemicellulose content. In highly alkaline Kraft process all degraded/dissolved hemicelluloses form different hydroxy acids and they cannot be recovered as sugar oligo- or monomers. However, their heat value can be exploited to generate high pressure steam in recovery boiler to be used in power generation and create lower pressure steam flows that may serve as the pulp mill’s process heat source.

In liquid phase pre-hydrolysis, the aqueous phase is naturally acidic due to acetic acid which is released from e.g. hemicellulose side chains. This process is called autohydrolysis and it partially hydrolyses the hemicellulose.

If the pre-hydrolysis liquor is removed from the digester, it mainly contains hemicellulose (oligo- and monomeric), acid soluble lignin, extractives, and inorganics.

It has been found that wood hemicelluloses can be degraded also by using acidic hot-water (e.g. 160°C; 60 min), instead of vapor phase treatment in the pre-hydrolysis stage, without negative effects on the pulping process or on the dissolving pulp properties. The acid used is formic acid (5-20 g/l of formic acid to ensure hydrolysis of dissolved hemicellulose).

In the acidic aqueous pre-hydrolysis stage, depending on conditions, 15-35 % of wood dry solids are dissolved to pre-hydrolysis liquor, resulting in an aqueous solution of hemicellulose sugars, acid soluble lignin, organic acids, and inorganics. Free pre-hydrolysis liquor can be recovered by a combination of drainage and liquor displacement, leaving hemicellulose-poor chips in the digester for further processing. However, a major challenge for valorization of the pre-hydrolysis liquor is the scaling of the machines and pipelines caused by lignin when the pre-hydrolysis liquor is cooled down from pre-hydrolyzation temperatures (typically 150-170°C) to temperature of 80-60°C, at which the sugar purification process is typically operated. Traditional purification techniques in sugar production is continuous chromatography which is operated at high dry solids e.g. 30-50%. As the dry solid level of the pre hydrolysate out from digester is less than 10%, a concentration step is needed prior to the chromatography. If this liquor would be directly concentrated to high dry solids, acid soluble lignin would precipitate and cause scaling. Also, the pH of the solution remain at low level risking decomposition of xylose and arabinose to furfural.

EP2336194 is directed to a process for treating lignocellulosic biomass material, in which the biomass is subjected to cooking in the presence of a mixture comprising water and formic acid.

Brief description of the figures Figure 1 : illustrates the process according to the present invention. The reference numerals in figure 1 are referred to below.

Summary of the invention

It has been found that base can be added to cooled and filtrated pre hydrolysis liquor, to increase pH prior to evaporation. Surprisingly, it has been found that even if the pH of the solution is increased substantially above the pK_a value of the formic acid (pK_a 3.8), it is possible to evaporate and recycle a considerable proportion of the acid, resulting in lower make-up of the formic acid being required, thus reducing the total formic acid consumption of the process.

According to the present invention, the pre-hydrolysis liquid is cooled from pre-hydrolysis temperature of 150-180°C to about 60°C and solids are removed using methods known in the art, such as filtration or centrifugation. After solids removal, the pH of the pre-hydrolysis liquor is increased by addition of base. If this neutralization is accomplished such that a reasonable part of formic acid / sodium formate equilibrium lies on the formic acid side, such as a ratio of formic acid / sodium formate of from about 1.5:1 to 2.5:1 , such as about 2:1 . it is possible to evaporate formic acid while securing that lignin is not precipitating.

When a majority of the formic acid has been evaporated, the pH of the remaining liquor can be further increased to a final desired value with additional base and the stream can then be concentrated up to the dry solids as required for the subsequent purification stage.

Thus, the present invention is directed to a process for treating acidic biomass hydrolysate, comprising the steps of a) providing acidic biomass hydrolysate comprising formic acid (1); b) mixing the acidic hydrolysate with a base (2) so that the pH of the mixture (3) is increased to 3 to 4; c) carry out evaporation of the mixture (3), by which formic acid in neutral form is evaporated (4) to a larger extent than water, resulting in an increased pH on the concentrated hydrolysate (5) compared to the pH in the inlet stream; d) increasing pH of the concentrated hydrolysate (5) to 3 to 5 by addition of base (6); e) carry out evaporation of the concentrated hydrolysate (7) from step d), thereby increasing pH of the concentrated hydrolysate (9) to obtain a concentrated mixture for subsequent sugar purification.

The present invention is also directed to recovering and recirculating the formic acid (4) evaporated in neutral form in step c) back to the pre-hydrolysis stage. The present invention is also directed to recovering and recirculating the formic acid (8) evaporated in neutral form in step e) back to the pre-hydrolysis stage.

Detailed description

The acidic biomass hydrolysate comprising formic acid is preferably pre hydrolysis liquid prepared as part of a process for producing dissolving pulp. The biomass can for example be bagasse, softwood, hardwood or a mixture thereof. Preferably, the biomass is provided in the form of wood chips.

Preferably, the pre-hydrolysis liquid has been prepared by subjecting wood chips to cooking in hot water (e.g. 160°C; 60 min), in the presence of formic acid. Preferably, the amount of formic acid in the pre-hydrolysis liquid is 5-20 g/l.

The base used in the process according to the present invention is preferably sodium hydroxide or oxidized white liquor. The oxidized white liquor is preferably filtrated before or after oxidation, such that the oxidized white liquor is oxidized and filtrated white liquor.

The white liquor used in the preparation of oxidized white liquor is preferably oxidized white liquor from a process for preparing dissolving pulp.

The base used according to the present invention is preferably provided as an aqueous basic solution or suspension.

The resulting pH of the mixture (3) in step b) can be monitored using methods known in the art.

The evaporation in step c) is carried out using methods known in the art. The formic acid evaporated (4) can be recovered using methods known in the art and recycled back to the pre-hydrolysis stage. Preferably, the amount of formic acid evaporated is at least 20%, preferably at least 25%, more preferably at least 30%. At the end of the evaporation in step c) the amount of components other than water in the concentrated hydrolysate obtained is typically in the range of from 8% to 15% by weight. Acetic acid is also present in the mixture (3). Most of the acetic acid remains in the hydrolysate in the first evaporation step.

The resulting pH of the concentrated hydrolysate (5) in step c) can be monitored using methods known in the art.

The evaporation in step e) is carried out using methods known in the art. The formic acid evaporated (8) can be recovered using methods known in the art and recycled back to the pre-hydrolysis stage. During evaporation the pH of the concentrated hydrolysate typically increases to approximately pH 5.

Acetic acid is recovered to a higher extent in step e) than in step c).

The concentrated mixture obtained in step e) is used for subsequent sugar purification. The amount of components other than water in the concentrated mixture obtained is typically in the range of from 30% to 50% by weight.

Optionally, the steps of increasing pH and evaporating can be repeated. For example, steps b) and c) can be repeated.

In view of the above detailed description of the present invention, other modifications and variations will become apparent to those skilled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

Claims

1. A process for treating acidic biomass hydrolysate, comprising the steps of a) providing acidic biomass hydrolysate comprising formic acid

(1 ); b) mixing the acidic hydrolysate with a base (2) so that the pH of the mixture (3) is increased to 3 to 4; c) carry out evaporation of the mixture (3), by which formic acid in neutral form is evaporated (4) to a larger extent than water, resulting in an increased pH on the concentrated hydrolysate (5) compared to the pH in the inlet stream; d) increasing pH of the concentrated hydrolysate (5) to 3 to 5 by addition of base (6); e) carry out evaporation of the concentrated hydrolysate (7) from step d), thereby increasing pH of the concentrated hydrolysate (9) to obtain a concentrated mixture for subsequent sugar purification.

2. A process according to claim 1 , wherein the formic acid (4) evaporated in neutral form in step c) is recovered and recirculated back to a pre-hydrolysis stage in which the acidic biomass hydrolysate is prepared.

3. A process according to claim 1 or 2, wherein the formic acid (8) evaporated in neutral form in step e) is recovered and recirculated back to a pre-hydrolysis stage in which the acidic biomass hydrolysate is prepared.

4. A process according to any one of claims 1-3, wherein the base is sodium hydroxide or oxidized white liquor.

5. A process according to claim 4, wherein the base is oxidized and filtrated white liquor.

6. A process according to any one of claims 1 -5, wherein the amount of components other than water in the concentrated hydrolysate obtained in step c) is in the range of from 8% to 15% by weight.

7. A process according to any one of claims 1 -6, wherein the amount of components other than water in the concentrated mixture obtained in step e) is in the range of from 30% to 50% by weight.