FIELD

[0001] The present invention relates to prehydrolysis of lignocellulosic material and the use of the method. In particular, the present invention concerns a method of prehydrolysing lignocellulosic chip material with S02-steam mixture and the use of the method to produce dissolving pulp.

BACKGROUND

[0002] Mankind is looking for possible avenues to replace fossil raw material based products by renewable lignocellulose based products. More particularly this means that the lignocellulosics’ main components, namely cellulose, hemicelluloses and lignin must be separated from each other and the fractions to be turned into corresponding intermediate bioproducts, cellulose fiber, carbohydrate solution (hydrolysate) and lignin solution or solids.

[0003] As described by Sixta, H.,”Handbook of Pulp” prehydrolysis is generally used to selectively remove the hemicelluloses from the lignocellulosic biomass by hydrolysis in water at 160-180 °C, in dilute acid (0.3-0.5% H2SO4) at 120-140 °C or in concentrated acid (20-30% HC1) at 40 °C. Dilute mineral acids are efficient in hemicellulose hydrolysis, but they bring along corrosion and material problems and have an adverse effect on lignin - deactivation by acid catalyzed condensation reactions - which makes it more difficult to remove lignin and bleach the cellulosic fibers. So far, all the few industrial continuous prehydrolysis-kraft pulp processes use water prehydrolysis prior to the kraft cooking stage.

[0004] Prehydrolysis products from wood are considered to be a source for chemicals, fodder, food additives and pharmaceutical products. Both the high yield of the prehydrolysis released carbohydrate compounds and the efficient production of the high value products are prerequisites for a feasible industrial process.

[0005] However, at present there is no commercial utilization of the water prehydrolyzate during the course of a prehydrolysis-kraft pulping operation. The running of a water prehydrolysis step, without extensively utilizing the degraded wood by- products, is economically not feasible because recovery of the large amounts of prehydrolysate requires a lot of additional equipment and evaporation capacity. In addition, the water based wood prehydrolysate contains many chemical compounds which cause severe solid depositions and scaling on process equipment metal surfaces, especially on heat exchange surfaces, thus requiring special control and maintenance lowering the overall process efficiency.

[0006] One way to improve the above described situation of water prehydrolysis or dilute mineral acid prehydrolysis is to use sulphur dioxide as a low concentration water solution at 140-150 °C. According to Radiotis et al. and Sixta, H., “Handbook of Pulp” it has several advantages: the acidity by bisulfite ions in water is higher than that in water prehydrolysis. Thus the hemicellulose degradation and dissolution is higher and the resulting monomeric sugar proportion is higher. However, the acidity is still lower than that of the reactive protons of dilute mineral acids which damage lignin and lower the pulp yield. SCk-water based prehydrolysis has a unique advantage: it sulphonates the low- molecular phenolic compounds to some extent, thus making them hydrophilic and lowering the scaling plugging tendency in prehydrolysis as well as in the hydrolysate refining operation.

[0007] Documents US 3,132,051; US 5,139,617 and FI 121237B use SCk-water, more precisely water solution of SO2 gas containing HSO3- bisulfite ions and dissolved SO2. However, the solutions in these documents add 8 - 12 tons of water per ton of pulp to be processed and evaporated.

[0008] In addition to the evaporation and energy usage burden of large water volume, the water dilutes the SO2 charge. However, chemical reactions, i.e. hydrolysis, are controlled by reagent concentration which requires large quantity of charged SO2. For example in the publication by Radiotis, T., et al. a SO2 dosage of 0.5 - 1.0 % on dry wood was used to achieve satisfactory hydrolytic reaction. This is 5 - 10 kg SO2 per ton of dry wood and about 15 - 30 kg SO2 per ton produced pre-hydrolysed pulp. The excess sulphur which cannot be further utilized in the pre -hydro lyse-pulp plant goes into the plant’s chemicals recovery cycle and accumulates disturbing the normal chemical balance. Eventually the plant must invest in sulphur removing technology increasing cost and complexity of the plant.

[0009] Steam-phase prehydrolysis has been one way to decrease the water usage. The heat needed to heat-up wood chips to prehydrolysis temperature, 170-175 °C, is introduced as direct “medium pressure” 10-12 atm mill process steam and hydrolysis takes place in steam phase, without liquid water in the voids within the wood chip charge . For 1 ton dry wood 0.3 t steam is needed. Chip moisture being 0.65 t H2O, the liquid-to-wood ratio is 0.3+0.65 = 0.95 t FhO / I t dry wood. As wood chips can carry up to 2 t FkO per 1 1 dry wood, this in an amount that the chips carry inside and there is practically no free water in the void volume of the chip column. In contrast, in water prehydrolysis the liquid-to- wood ratio is 4:1 - 5:1 t FkO/t dry wood. Also, many of the commercial hardwood dissolving pulp have a pentosan content level of 5 % or under and a more accurate removal is hard and expensive to achieve even with modem process technology.

[0010] So far, in prehydrolysis kraft batch cooking, the steam-phase prehydrolysis has provided for the technical and economic feasibility of the production. The prehydrolyzate and degradation products have occurred only inside the chips in the reaction vessel, where the following alkaline cooking has neutralized and dissolved them away together with the yield loss in the kraft cooking part. Thus, all dissolved solids and liquids are gathered without extra burden in the black liquor, which is evaporated and ran through the kraft chemical/energy recovery cycle with only a minor capacity increase demand. The drawback of the steam-phase prehydrolysis is that it is technically only possible as a batch operation.

[0011] The above described segregates prehydrolysis technology into two areas, the continuous prehydrolysis-kraft process using the water based prehydrolysis and the discontinuous batch prehydrolysis-kraft process using steam-phase prehydrolysis. The steam-phase has been a winner so far, but it has one major drawback: it is not possible to use any hydrolyzing agents such as acid SCk-water, etc., because there is no aqueous phase to enable transfer, carrying and mixing of chemicals.

SUMMARY OF THE INVENTION

[0012] The invention is defined by the features of the independent claims. Some specific embodiments are defined in the dependent claims.

[0013] It is the aim of the invention to remove at least some of the problems relating to the known technical solutions.

[0014] According to a first aspect of the present invention, there is provided a method for prehydrolysing lignocellulosic material in a reaction vessel by a prehydrolysis reaction carried out in the presence of steam at a reaction temperature. The method comprises the steps of:

- heating the contact surfaces of the reaction vessel above the boiling point of water by feeding a steam flow into the reaction vessel,

- introducing a dosage of liquid SO2 flow into the steam flow and vaporizing the liquid SO2 flow to form gaseous SO2 in the steam flow,

- continuing the steam feed into the reaction vessel after said preselected period of time, and

- maintaining the reaction temperature in the reaction vessel to achieve prehydrolysis of the lignocellulosic material.

[0015] According to an aspect of the invention, the method for prehydrolysing lignocellulosic material is used to produce dissolving pulp.

[0016] Considerable advantages are obtained by this invention. The present invention provides a method that reduces the use of highly corrosive SCh-water and introduces a feasible prehydrolysing method with the use of SCh-steam mixture in controlled conditions.

[0017] It is to be noted the new unexpected founding: the SCh-steam prehydrolysis does not reduce the cellulosic yield, viscosity or kappa number. Especially is surprising that the cellulosic yield does not decrease when using the SCE-steam prehydrolysis.

[0018] It is to be noted here, that the use of SO2 according to the present invention is the most efficient in view of the SO2 concentration in the small amount of condensate and chip water (about 1 ton water per 1 ton of dry biomass); this is quite contrary to the situation in the water liquid-phase prehydrolysis, where the SO2 dilutes in the big amount of water filling the entire reactor (4-5 tons water per 1 ton dry biomass). In process technology the reactions are most effective when the reagent concentration is high. Achieving high concentration in small amount of diluting reaction media also enables the lowest possible reagent dosage and chemical cost as well as saving in the heating energy needed.

[0019] The present invention combines the prehydrolysis reactor input streams, the heating direct steam serves simultaneously as the carrying media for the SO2 to enter and spread into the reactor content; a very simple operation saves process time, extra equipment and operational steps.

[0020] Another benefit of using SO2 in prehydrolysis is that the hydrolysate liquid contains much more monomeric carbohydrates than is the case in water-based auto- hydrolysis. This is technically an important advantage as the water-hydrolysis dissolved poly- and oligomeric carbohydrates must be hydrolysed to monomers later anyway.

[0021] The SO2 charge needed for the prehydrolysis is allocated so that the last part of the direct steam input flows without SO2 in order to flush the input pipe/inlet connection and prehydrolysis reaction temperature is reached. In this new way, the SO2 addition becomes a part of the heating media, i.e. of the input flow of the steam, and is being transferred on the chip particles with condensing steam and thereby absorbed into the chip particles.

[0022] Therefore, the method according to the present invention decreases the pentosan content of the dissolving pulp and simultaneously significantly mitigates the process conditions of the prehydrolysis. The current invention can decrease the process time, which enables the more pulp production with the same process equipment. The use of SO2 decreases the process temperature compared to water hydrolysis, which may enable energy savings. Decreasing the temperature of the neutralizing by white liquor -step following the prehydrolysis has an influence on the energy usage of the cooking department and the degree of polymerization of the cooked cellulose, in other words, the viscosity remaining on a higher level, which means having a better cellulose yield.

[0023] Next, embodiments will be examined in more detail with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWING [0024] FIGURE 1 illustrates a batch reactor in accordance with at least some embodiments of the present invention. EMBODIMENTS

DEFINITIONS

[0025] In the present context, the term “hydrolysate liquid” is where the hydrolysis reaction products accumulate. [0026] In the present context, the term “odw” means oven dry wood weight. It is the weight after all the moisture has been evaporated.

[0027] In the present context, the term “EA” means effective alkali. It is the sum of the sodium hydroxide and half of the sodium sulphide in the white liquor.

[0028] In the present context, the term “dry” means non-aqueous. [0029] In the present context, the pressure unit “bar” is absolute pressure. When referring to pressure difference “bar” is the differential pressure.

[0030] In the present context, the term “contact surfaces” is the piping, valves, connections, screens and any other metal parts of the reaction vessel system in contact with the heating steam flow. [0031] In the present context, the term “prehydrolysis” is used for the chemical reaction of removal of hemicelluloses from lignocellulosic material before a pulp cooking process.

[0032] As briefly discussed above, presented is a method for hydrolysing lignocellulosic material by using SCk-steam in controlled conditions. A flow of SO2 is fed into the heating steam for a period of time. The SO2 is vaporized immediately so that it mixes into the heating steam. This mixture is further fed into a batch reaction vessel so that the temperature of the wood chip bed in the reaction vessel and the process equipment is over 100 °C, but so much under the target temperature so that pure heating steam (without SO2) is fed into the reaction vessel as end steam. This 1) prevents the condensation onto the metal parts of the reactor equipment of water, which would, in combination with SO2, be highly corrosive, and 2) the end steam flushes the pipes and the reactor equipment from any SO2 residue. In this way gaseous SO2 is fed onto the wood chip bed within the normal prehydrolysis steam heating and it reacts with the moisture of the wood chips and a small amount of steam condensate to intensify the prehydrolysis and especially the removal of xylan. Examples below show that with embodiments of the present invention doses of SO2, as small as 0.2 - 0.3 % odw, are enough to produce the dissolving pulp having a reduced content of petosan to a level of 3 % or less.

[0033] After the steam prehydrolysis phase, in one embodiment the process comprises conventional sulphate cooking, which removes most of the lignin of the wood material. Water-steam hydrolysis is performed in a higher temperature, about 170 °C, than the actual sulphate cooking, which takes place in a 150 - 160 °C temperature. In the neutralizing phase the steam phase prehydrolysis ends when the neutralizing white liquor dose is pumped through the cooker. It continues with a dose of hot black liquor, which is recovered from the previous cooking. The black liquor is fed into the cooking process together with a dose of white liquor needed for the cooking phase. The first phase of white liquor neutralizes all the acidity in the reaction vessel and the black liquor phase continues the displacement reaction. The neutralized substance from the neutralization step comes out of the reaction vessel with components from the neutralizing white liquor and the neutralized acidic components from the prehydrolysis and the small amount of SO2 that what fed into the reaction vessel in the steam heating phase.

[0034] Turning now to the drawings, Figure 1 presents a batch reaction vessel in accordance with at least some embodiments of the present invention. The reaction vessel is metallic, for example steel. In the starting situation the reaction vessel is empty. The process starts by wood chips filling, through the feeding hopper and reaction vessel capping valve 1. Often chip packing is intensified by steam jets giving more speed to falling chips. During the chip fill, gases are evacuated out from the middle screens 5, through the cooking circulation pipeline 6. In an embodiment, air is removed from the reaction vessel.

[0035] When the reaction vessel is full with chips the capping valve 1 is closed. Air removal can be intensified by steaming the chips by introducing low pressure steam through bottom screens 7 and valves 9, 10, 15. At the same time gases are evacuated from the top through degassing line 4, thus so far reaction vessel has been kept practically at atmospheric pressure.

[0036] The first heating stage towards reaction temperature and pressure is carried out by steam through top screens 2, bottom screens 7 and valves 3, 11, 15 and 9, 10, 15. The first steam heating stage elevates the temperature close to the corresponding saturated steam temperature at the prevailing pressure.

[0037] According to one embodiment the first heating stage pressure is 3 - 5 bar and the first heating stage temperature is 125 - 150 °C. The time for this heating stage is 15 - 45 minutes.

[0038] Then heating is continued by medium pressure steam in the same way as in the first heating stage. According to one embodiment the medium pressure is at least 1 bar higher than the corresponding pressure of saturated steam at reaction temperature of the prehydrolysis. According to one embodiment the medium pressure is 6 - 10 bar. The time for this heating stage is 5 - 25 minutes The total time for heating to the desired hydrolysis temperature is 20 - 60 minutes, preferably about 45 minutes The heating is performed until the temperature of the contact surfaces of the reaction vessel is above the boiling point of water, preferably over 110 °C.

[0039] According to one embodiment of the present invention SO2 is introduced into the flowing heating steam. First, the pure steam is allowed to fill the reaction vessel and condense onto the chips, after this liquid SO2 is injected to the steam, then ending the SO2 injection so that a clear pure steam feed phase is the last part of the heating steam flow. Thus, SO2 injection is placed into the steam flow securing that all the pipe lines and valves etc. are above condensing temperature, but on the other hand there is enough pure steam flow to purge all pipelines and valves etc. clean from SO2 before the end of steam flow.

[0040] According to another embodiment the predetermined SO2 dosage in the SO2 injection is 0.1 - 0.6 % odw, preferably 0.15 - 0.5 % odw, for example 0.15 - 0.4 % odw, such as 0.15 - 0.3 % odw.

[0041] According to one embodiment the predetermined SO2 dosage in the SO2 injection is at least 0.15 % odw, preferably at least 0.20 % odw, for example 0.25 % odw.

[0042] According to one embodiment, the prehydrolysis of the lignocellulosic material is partial.

[0043] According to one embodiment, the temperature is maintained until a predetermined degree of prehydrolysis of the lignocellulosic material has been reached.

[0044] In one embodiment, the pentosan content of the pulp (pulp after cooking) is - as a result of the prehydrolysis - less than 4.5 % by weight, in particular less than 4.2 % by weight, for example about 4 to about 3.5 % by weight of the pulp. [0045] In one embodiment, hydrolysis of the lignocellulosic material will lead to a pentosan content of 4 wt-% or under of the mass of the fiber.

[0046] The injection of SO2 charge takes place by pumping a predetermined mass- flow of liquid SO2 from pipeline or SO2 (1) storage 14, through valve 12 into the steam pipe 13 through injection pipe 12b over a predetermined period of time. As the mass flow of liquid SO2 is very small compared to the tons of steam flow, the liquid SO2 is immediately and completely vaporized in the steam flow. This means that the flow of steam and gaseous flow is dry, avoiding corrosion and difficult material selections. The steam flow into the chip bed reaction vessel carries the SO2 in all the space of the reaction vessel as the heating-by-condensing-steam is the main mechanism, local steam pressure decreases and the gradient is withdrawing and spreading steam and SO2 everywhere.

[0047] Once the target reaction temperature has been reached, that temperature will maintained for a predetermined time. According to one embodiment the target reaction temperature is 150-175 °C, preferably 160-165 °C and the desired reaction time is 15-60 minutes, preferably about 30 minutes.

[0048] Sulphur dioxide, SO2, in the steam, absorbed on the chip surface is quickly transferred to the chip interior together with the heat. A uniform SO2 enhanced prehydrolysis is achieved in all parts of the reaction vessel - without using any external SCk-water, water phase hydrolysis, re-circulations, indirect heating etc.

[0049] After prehydrolysis, the chips are subjected to kraft cooking. As a result of the prehydrolysis carried out as described in the fore-going using steam containing gaseous SO2, there is practically no free condensate or water-phase in the reaction vessel, and the burden of removing this liquid phase is completely avoided.

[0050] As mentioned above, in steam-phase batch reactor process the use of acid or SCk-water has not been possible, as there is no liquid reaction medium to inject and mix the acid or SCk-water into. However, now it has been discovered that also steam-phase prehydrolysis can be improved and intensified by using SO2 gas. Further, the use of SO2 gas can be made very simple and the preparation, storage and handling of highly corrosive SCk-water is completely avoided. Liquid dry SO2 is injected from a storage tank directly into the steam pipe/inlet connection into the prehydrolysis reactor (batch reaction vessel) and the in-flowing heating steam carries and distributes the SO2 all over the reactor space as a part of the steam flow into every part of the chip filled reactor pulled by the pressure gradient generated by the condensing steam on the wood chips. The spread SO2 gas absorbs on the moist chip particle surfaces and subsequently penetrates and diffuses into the wood chips.

[0051] The kraft cooking is carried out as known in the art. It can be a conventional kraft batch cooking, where first white liquor is added and then the reaction vessel is heated, next the white liquor is circulated through screen 5 to cooking circulation pipeline 6 and pumped up to valve 3 and down to valve 9 and fed back into the reaction vessel through screens 2 and 7. An option to the conventional kraft batch cooking is a so called “displacement kraft batch cooking” process where several filling and displacements take place. The prehydrolysed and cooked - chemically defibrated pulp - is discharged through bottom curve 17 and valve 8 to fibre line feed 16.

[0052] It goes without explaining that the prehydrolysis part may have different process equipment and instrumentation embodiments, such as separate steam connections and lines, different screen designs, flow of steam and S02(g) injection only from bottom or from top, or both and multiple points.

[0053] The present invention has preferred embodiments in batch reactor processes, but it is not limited to only batch process. It is evident that steam-phase prehydrolysis could be constructed to be a continuous process when the chip feeding and discharging machine organs are designed and constructed to operate continuously. In that case SO2 would be injected into the entering steam, in particular medium pressure steam.

EXAMPLE 1

[0054] Experimental results obtained in the following examples show the effect of gas phase SO2 - water steam hydrolysis compared to a reference process without SO2 in water steam hydrolysis. The process was the same in all examples, the prehydrolysis conditions were varied and the kraft cooking procedure and conditions were constant. The varying prehydrolysis conditions are shown in table 1. The process steps were as follows:

- chip steaming and air evacuation at 100 °C for 10 min

- direct steam injection into reaction vessel up to desired hydrolysis temperature

- staying at temperature the desired reaction time

- charging first volume of kraft cooking white liquor (8.3% EA on odw) for neutralization of reactor content

- charging a volume of hot white liquor (7.4% EA on odw) and hot black liquor (20 gEA/1) for kraft cooking stage

- adjusting cooking temperature 160 °C by indirectly heated reactor re-circulation - staying at cooking temperature the desired cooking time 45 min

- stopping cooking reactions by displacing hot black liquor from the reactor using cold washing liquid

The results of a reference prehydrolysis-kraft cook with no SO2 gas and two tests with (1) 0.2 % SO2 gas charge and (2) 0.25 % SO2 gas charge are presented in table 1 below.Table 1

[0055] The results of these experiments are shown in table 2 below.

Table 2

[0056] The pulp technical results of these experiments are characterized by pulp pentosan content. This is the main quality parameter of this type pulp and it is controlled by the prehydrolysis reaction degree (hydrolytic removal of xylan hemicelluloses from the wood). The pulp buying textile industry’s standard for prime pulp is pentosan content 3.5- 4.0 %. It is evident that this pentosan range is well met at 0.2-0.25 % S02(g) charge.

[0057] Most interesting are the other pulp parameters: pulp yield, pulp delignification degree (kappa number), pulp viscosity are at quite the same level regardless of using S0 ₂ (g). This is a new advantage: the state-of-the-art water-based SO2 prehydrolysis-kraft cooks have a clear yield loss. However, the prehydrolysis conditions were significantly milder with S0 ₂ (g): hydrolysis temperature 10 degrees lower.

[0058] The examples demonstrate that good quality effects can be achieved by using clearly less S02(g)(0.2-0.25% on odw) when it is introduced with the steam into steam phase process. It also proves that the SO2 gas dosage achieves high enough concentration of SO2 in wood chips - reflecting almost exactly the difference in the total amount of water and dilution in the prehydrolysis. In the flow of steam and S0 ₂ (g) phase about 1 t liquid/lt wood versus about 4 t liquid/lt wood.

[0059] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps, or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0060] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0061] As used herein, a plurality of items, structural elements, compositional elements, and/or materials may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary. In addition, various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0062] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0063] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0064] The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.

INDUSTRIAL APPLICABILITY

[0065] The aim of the present invention is to produce high purity wood cellulose, so called dissolving pulp, which is used in chemical cellulose derivatives and man-made regenerated cellulose fibres regenerated from it. Especially the present invention focuses on wood and annual plants with a naturally high hemicellulose, i.e. xylan, content, which is the polymer of the monosaccharide xylose of C5 pentosan sugar. Most of this component has to be removed from dissolving pulp to a pentosan content level of 3 %.

REFERENCE SIGNS LIST

1 Reaction vessel capping valve 2 Top screens

3 Valve

4 Degassing line

5 Middle screens

6 Cooking circulation pipeline

7 Bottom screen

8 Valve

9 Valve

10 Valve

11 Valve

12 Valve 12b injection pipe

13 Steam pipeline

14 SO2 Storage pipeline

15 Valve

16 Fiber feed line

17 Bottom curve

CITATION LIST

Patent Literature US 3,132,051 US 5,139,617 FI 121237B

Non Patent Literature Sixta, H.,”Handbook of Pulp”, Vol.l, p. 325-365 Radiotis et ak, NWBC 2011, Stockholm, p.92-99