SE2050722A1

SE2050722A1 - Recycling of formic acid

Info

Publication number: SE2050722A1
Application number: SE2050722A
Authority: SE
Inventors: Christian Ehn; Jari Kavakka
Original assignee: Stora Enso Oyj
Priority date: 2020-06-16
Filing date: 2020-06-16
Publication date: 2021-12-17
Also published as: WO2021255613A1

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

Background Dissolving pulp consists nearly exclusively of pure cellulose, compared totypical paper pulp, in which hemicelluloses play an important role in paperfurnish. The alkaline Kraft pulping process alone has limited ability to dissolvehemicelluloses completely, due to re-precipitation of xylan on fibers in the endof the delignification process. Thus, in dissolving pulp production, woodhemicelluloses 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 thedissolving pulp production, for example by pre-hydrolysis, is of interest. ln current processes, pre-hydrolysis is achieved using steam, but if areasonable amount of hemicellulose sugars is to be extracted, then a liquid-based pre-hydrolysis must be performed. ln current dissolving pulp processes, biomass, such as wood chips, aretypically heated at approximately 170°C in vapor phase to degradehemicelluloses or to make them degradable in subsequent alkaline cookingprocess. Moreover, in case of birch wood, released acetic acid from the xylanbackbone further enhances hemicellulose degradation. After the hot vapor treatment, degraded and de-acetylated xylan will easily dissolve insubsequent highly alkaline Kraft delignification process, producing a pulp withlow hemicellulose content. ln highly alkaline Kraft process alldegraded/dissolved hemicelluloses form different hydroxy acids and theycannot be recovered as sugar oligo- or monomers. However, their heat valuecan be exploited to generate high pressure steam in recovery boiler to beused in power generation and create lower pressure steam flows that may serve as the pulp mill's process heat source. ln liquid phase pre-hydrolysis, the aqueous phase is naturally acidic due toacetic acid which is released from e.g. hemicellulose side chains. Thisprocess is called autohydrolysis and it partially hydrolyses the hemicellulose.lf the pre-hydrolysis liquor is removed from the digester, it mainly containshemicellulose (oligo- and monomeric), acid soluble lignin, extractives, and inorganics. lt has been found that wood hemicelluloses can be degraded also by usingacidic hot-water (e.g. 160°C; 60 min), instead of vapor phase treatment in thepre-hydrolysis stage, without negative effects on the pulping process or onthe dissolving pulp properties. The acid used is formic acid (5-20 g/l of formic acid to ensure hydrolysis of dissolved hemicellulose). ln the acidic aqueous pre-hydrolysis stage, depending on conditions, 15-35 %of wood dry solids are dissolved to pre-hydrolysis liquor, resulting in anaqueous solution of hemicellulose sugars, acid soluble lignin, organic acids,and inorganics. Free pre-hydrolysis liquor can be recovered by a combinationofdrainage and liquor displacement, leaving hemicellulose-poor chips in thedigester for further processing. However, a major challenge for valorization ofthe pre-hydrolysis liquor is the scaling of the machines and pipelines causedby lignin when the pre-hydrolysis liquor is cooled down from pre-hydrolyzationtemperatures (typically 150-170°C) to temperature of 80-60°C, at which thesugar 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 isneeded prior to the chromatography. lf this liquor would be directlyconcentrated to high dry solids, acid soluble lignin would precipitate andcause scaling. Also, the pH of the solution remain at low level riskingdecomposition of xylose and arabinose to furfural.

EP2336194 is directed to a process for treating lignocellulosic biomassmaterial, 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 lt has been found that base can be added to cooled and filtrated pre-hydrolysis liquor, to increase pH prior to evaporation. Surprisingly, it has beenfound that even if the pH of the solution is increased substantially above thepKa value of the formic acid (pKa 3.8), it is possible to evaporate and recycle aconsiderable proportion of the acid, resulting in lower make-up of the formicacid being required, thus reducing the total formic acid consumption of the process.

According to the present invention, the pre-hydrolysis liquid is cooled frompre-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 Iiquor is increased byaddition of base. lf this neutralization is accomplished such that a reasonablepart 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 thatlignin is not precipitating.

When a majority of the formic acid has been evaporated, the pH of theremaining Iiquor can be further increased to a final desired value withadditional base and the stream can then be concentrated up to the dry solidsas required for the subsequent purification stage.

Thus, the present invention is directed to a process for treating acidicbiomass 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 themixture (3) is increased to 3 to 4; c) carry out evaporation of the mixture (3), by which formic acid in neutralform is evaporated (4) to a larger extent than water, resulting in anincreased pH on the concentrated hydrolysate (5) compared to the pHin the inlet stream; d) increasing pH of the concentrated hydrolysate (5) to 3 to 5 by additionof 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 theformic acid (4) evaporated in neutral form in step c) back to the pre-hydrolysisstage.

The present invention is also directed to recovering and recirculating theformic acid (8) evaporated in neutral form in step e) back to the pre-hydrolysisstage.

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 mixturethereof. Preferably, the biomass is provided in the form of wood chips.

Preferably, the pre-hydrolysis liquid has been prepared by subjecting woodchips to cooking in hot water (e.g. 160°C; 60 min), in the presence of formicacid. Preferably, the amount of formic acid in the pre-hydrolysis liquid is 5-20 The base used in the process according to the present invention is preferablysodium hydroxide or oxidized white liquor. The oxidized white liquor ispreferably filtrated before or after oxidation, such that the oxidized white liquoris 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 anaqueous basic solution or suspension.

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

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

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

The evaporation in step e) is carried out using methods known in the art. Theformic acid evaporated (8) can be recovered using methods known in the artand recycled back to the pre-hydrolysis stage. During evaporation the pH ofthe 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 sugarpurification. The amount of components other than water in the concentratedmixture obtained is typically in the range of from 30% to 50% by weight.

Optionally, the steps of increasing pH and evaporating can be repeated. Forexample, steps b) and c) can be repeated. ln 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 variationsmay be effected without departing from the spirit and scope of the invention.

Claims

1. Claims 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 ofthe mixture (3) is increased to 3 to 4; c) carry out evaporation of the mixture (3), by which formic acidin 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 in|et stream; d) increasing pH of the concentrated hydrolysate (5) to 3 to 5 byaddition of base (6); e) carry out evaporation of the concentrated hydrolysate (7) fromstep d), thereby increasing pH of the concentrated hydrolysate(9) to obtain a concentrated mixture for subsequent sugarpunﬁcaﬂon. A process according to claim 1, wherein the formic acid (4)evaporated in neutral form in step c) is recovered and recirculatedback to a pre-hydrolysis stage in which the acidic biomasshydrolysate is prepared. A process according to claim 1 or 2, wherein the formic acid (8)evaporated in neutral form in step e) is recovered andrecirculated back to a pre-hydrolysis stage in which the acidicbiomass hydrolysate is prepared. A process according to any one of claims 1-3, wherein the base issodium hydroxide or oxidized white liquor. A process according to claim 4, wherein the base is oxidized andfiltrated white liquor. A process according to any one of claims 1-5, wherein theamount of components other than water in the concentrated hydrolysate obtained in step c) is in the range of from 8% to 15%by weight. A process according to any one of claims 1-6, wherein theamount of components other than water in the concentratedmixture obtained in step e) is in the range of from 30% to 50% byweight.