HYDROLYSIS OF LIGNOCELLULOSIC MATERIAL

Abstract: A continuous hydrolysis process for the hydrolysis of wood or other lignocellulose material into sugars and other products has an overall countercurrent flow of liquids and solids but an integral co-current flow of the liquids and solids as part of the process. As shown in Figure 2, woodchip or other feedstock is formed into a slurry which is acidified, pressurised and heated before being hydrolysed in reactors J. Three heat exchangers L1, L2 and L3 form a closed circuit in which exchanger 2 recovers heat from the slurry, L1 returns heat to the slurry and L3 makes up lost heat. The slurry is cooled before pressure reduction by pressure reducing means N and separation of the solids and liquid. The cooling prevents flashing to steam of part of the liquid in the slurry so that the process is single phase where generation of steam is avoided. After separation the solids can proceed to further processing or to discharge as lignin as indicated by arrow B. The liquid can proceed to further processing or discharge as indicated by arrow D.

Description: BACKGROUND OF THE INVENTION This invention relates to improvements in and relating to an hydrolysis process and in particular to the hydrolysis of wood or other lignocellulose material, in particular the conversion of cellulose and hemicellulose into glucose, xylose, and other 5 and 6 carbon sugars. There is a growing interest throughout the world in the utilisation of lignocellulose material as a feedstock for the manufacture of chemical and fuels. Woodchips, shavings, waste, waste-paper or other residues, offer a useful, but not the only, raw material for this purpose. The hydrolysis of wood or lignocellulose material has been proposed by various routes, including the use of acids and enzymes. The process of hydrolysing involves breaking down the carbohydrate molecule, either cellulose or hemicellulose, into simple sugars. The process which has further evolved in New Zealand over recent years is the hydrolysis of wood using a high temperature, weak, sulphuric acid solution and a plant based on this process has been built in New Zealand under my direction. In this process a hot, weak, acid solution is percolated through wood chips in a reactor vessel when the carbohydrate breaks down to simple sugars. The process recycles; the fresh sugar-free solution at highest temperature is first percolated through one reactor with wood previously hydrolysed to remove the hemicellulose; when under set conditions, hydrolysis of the remaining cellulose naturally occurs. The resulting sugar is discharged with the acid solution from this first vessel into a second vessel then containing fresh feedstock. The resulting acid solution, with sugar from both cellulose and hemicellulose hydrolysis, is then discharged and neutralised, this solution being called the hydrolysate. One possible processing route for this hydrolysate is to innoculate it with a suitable yeast able to ferment the sugars in solution into ethanol and carbon dioxide; then to concentrate the ethanol for sale. The hexose sugars are fermented; however, the pentose sugars may not be used so easily and pass through the system as a pollutant in the effluent. One possible processing route for these pentose sugars is to digest them thereby significantly cleaning the effluent before discharge and using the methane produced from the digestion as an energy source for the total process, i.e. hydrolysis, fermentation and distillation. The hydrolysis process is not new having evolved prior to World War 1, the first plant being built in South Carolina, United States of America. The Germans and Russians acquired the technology in the 1920's and 1930's and a higher yielding process named the Scholer process was developed in Germany. Further development of the process occurred at Forest Products Laboratory, Madison, Wisconsin, United States of America and Eugene, Oregon, United States of America, in the 1940's and the Madison process was reassessed in the late 1970's by the New Zealand Forest Research Institute at Rotorua with a pilot plant being commissioned in 1979. The New Zealand Forest Research Institute process is described in the Forest Research Institute publication No. 69,1979, "What's New in Forest Research", and is shown in Figure 1. The description in this publication is as follows: "Hydrolysis: Water, with sulphuric acid as a catalyst, is used to break down wood cellulose into its component sugars, hexose and pentose. Wet sawdust or wood chips are loaded and sealed into a reactor vessel. Water is then superheated to 170-200 o C, sulphuric acid added and the solution percolated through the wood for about 3 hours. During this time the sugar solution is continually drawn off. An insoluble residue, lignin, remains in the reactor and is removed at the end of percolation. The sugar solution is cooled rapidly by flashing it to lower pressure, this releases the volatile materials, furfurol and methanol, along with steam and causes tars to be precipitated to the bottom of the tank. The remaining sugar liquid is further cooled to 30 o C. The sulfuric acid is now removed by adding lime to the solution. The gypsum resulting from this reaction can be filtered off." The process described above is a batch process and reconfirms work done at the Forest Products Laboratory in the 1940's (see Ind. & Eng. Chem. Vol. 38 No. 9,p.890 (1946)). The alternative to a batch process for acid catalysed cellulose hydrolysis is by a continuous process. While in a batch process a discrete quantity of feedstock has an acid solution percolated through it, with the remaining solids then discharged, in contrast, in a continuous process the feedstock is fed continuously to a processing means, together with an acid solution and the resulting solid (lignin) and solution (hydrolysate) is discharged continuously. Various researchers have in recent times developed methods for achieving this continuous hydrolysis. Notable is work done by Grethlein et al at Dartmouth College, New Hampshire, U.S.A. (see U.S. Patent 4,237,226) and by Rugg et al at New York University (see U.S. Patent 4,316,747). Other research has been conducted by the American Can Company (see Church et al U.S. Patent 4,201,596). In addition, US 4384987 (Brink) appears to disclose a process for the continuous hydrolysis of a biomass which comprises continuously feeding the material as a feedstock to a receiver, continuously feeding the feedstock from the receiver to a pump which feeds the feedstock under pressure towards a first reactor loop of the main process line. Water is injected into the main process line to form a slurry with the feedstock and a weak acid solution is injected into the solution. The reactor loop is heated to raise the temperature of the pressurised feedstock to a temperature sufficient for hydrolysis of the feedstock to occur and this temperature is maintained for a sufficient time for leaching of the feedstock to occur. An object of the present invention is to overcome, or at least reduce, the disadvantage in wood hydrolysis batch processes and apparatus available to the present time for this purpose. In particular to provide an improved wood hydrolysis process operating as a continuous process. According to the invention, there is provided a process for the continuous hydrolysis of wood and wood derived products such as cellulose and starch material, comprising: continuously feeding wood and wood derived products as a feedstock to a receiving means of a main process line; injecting a presoak liquid into the receiving means to presoak the feedstock; continuously feeding the feedstock by a conveying means from the receiving means to a first pressurising pump which feeds the feedstock under pressure toward a first reactor of the main process line; creating in the main process line at an injection point a feedstock slurry by injecting water or a liquid phase into the process line; continuously feeding the slurry under pressure into and through one or more reactors whereby the feedstock in the reactors is the pressurised slurry; injecting a weak acid solution into the system prior to at least one of the reactors; heating each reactor to raise the temperature of the pressurised feedstock therein to a temperature sufficient for hydrolysis of the feedstock to occur; maintaining the temperature of the feedstock in the process line for sufficient time for leaching of the feedstock to occur and/or to maximise sugar production and separating the solid and liquid portions of the slurry using a separator; characterised by: the fact that the reactors are tubular reactors and the heating of each reactor is carried out by means of respective first heat exchangers, and further characterised by cooling the pressurised slurry in each tubular reactor with a second heat exchanger to avoid the generation of vapour and to ensure the process is single phase; recovering heat from the second heat exchanger and utilising the heat recovered to supply at least part of the heat requirements of the first heat exchanger; reducing with pressure reduction apparatus at an outlet of each tubular reactor the pressure of the cooled slurry therein while maintaining the pressure in the tubular reactor while the slurry is continuously discharged; discharging the solid portion which is lignin or passing it to the next reactor loop of the main process line; and discharging the liquid portion which is sugar rich acid-hydrolysate or injecting the liquid portion to the preceding tubular reactor of the main process line. Preferably, the weak acid solution injected forms part of or all of the presoak liquid. Alternatively, the presoak liquid may be water and the weak acid solution is injected into the main process line at the injection point or at other points along the main process line. Preferably, the temperature of the slurry in the process line is maintained in the first heat exchanger for a sufficient time to maximise sugar production. The process may also include providing several of reactor loops in a series within which the flow of solids and liquids is co-current; collecting the liquid portion of the slurry after discharge from each reactor loop after the pressure reducing apparatus of the series except for the first reactor loop where the liquid portion is discharged from the main process line; injecting the liquid portion into the main process line before the previous reactor loop of the series; collecting the solid portion of said slurry after discharge from each reactor loop except for the final reactor of the series where the solid portion is discharged as lignin; and feeding the collected solid portion into the main process line by way of the feed and pressurising pump into the next reactor loop of the series so that the overall flow of solids and liquids within the series is countercurrent. Preferably, each of the reactor loops is comprised of a tubular member whose length is dependent upon the temperature, the velocity, the solid-liquid ratio and the pH in the slurry. Preferably, a third heat exchanger is provided in each closed reactor loop to supplement the heat requirement of the first heat exchanger, the first and second heat exchangers respectively introducing heat to and extracting heat from the slurry towards its entry into and exit from the closed loop and the third heat exchanger introducing additional heat as required to a heat transfer medium flowing in the reactor loop between the heat exchanger. According to a still further aspect of the invention there is provided an apparatus and/or method for the continuous hydrolysis of wood substantially as shown in the accompanying drawings. Further aspects of this invention which should be considered in all its novel aspects will become apparent from the following description given by way of example of one possible embodiment of the invention and in which reference is made to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of the prior art hydrolysis process as carried out by the New Zealand Forest Research Institute of Rotorua, New Zealand. Figure 2 is a schematic illustration of an hydrolysis process and apparatus therefor according to one possible embodiment of the invention and wherein in a continuous hydrolysis process feedstock such as woodchip and water can be introduced with hydrolysate and lignin being continuously produced. Figure 3 is a very diagrammatic and simplified illustration of the process of Figure 2. Figure 4 is a diagrammatic illustration of the process of Figure 2 with two sets of possible process flow temperatures included. DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENT The process according to the embodiment of the invention shown in Figures 2, 3 and 4 is seen to have some elements of a tubular reactor (see Perry Chilton - 5th Edition - Figure 4.4) and a continuous countercurrent leaching process (see Perry Chilton, page 19.54) with or without line mixers (Perry Chilton, Figure 19.39). Referring firstly to Figure 3, this shows very diagrammatically the simplified flow pattern of a continuous countercurrent leaching tubular reactor according to a preferred embodiment of the invention. Woodchip or other feedstock is fed into the process at arrow A and a main process flow line is then shown by the solid line extending to the output of lignin indicated by arrow B. Also on the right hand side of the schematic diagram in Figure 3, water is introduced as indicated by arrow C and at the areas indicated by arrow E will counteract with the main process flow line having an output of hydrolysate as indicated by arrow D at the left hand side of the schematic diagram. As will be immediately apparent from Figure 3 while in toto the respective solid and liquid flows are counter-current, within the reaction loops the solid and liquid flows are co-current in direction. Thus, the hydrolysis process of the present invention provides a continuous hydrolysis process which has both an overall countercurrent flow of liquids and solids but an integral co-current flow of the liquids and solids as part of the process. Turning now to Figure 2 of the accompanying drawings, feedstock such as wood or cellulose/starch is fed in a direction indicated by arrow A into a feedstock acid presoak container 1 from which acid may be drained or recycled as indicated by arrow F. The container 1 may be any suitable type but could for example be merely a walled storage area. This receives the feedstock A which may suitably have been previously screened. In the container 1 a weak acid solution will be sprayed over the feedstock and allowed to soak through it for a predetermined time. Any excess solution will be drained away as mentioned previously for re-use along arrow F. Alternatively, the feedstock may be wetted with water only and an acid solution may be pumped under pressure into the main process line at either or all of the possible alternative acid injection points indicated by G in Figure 2. The saturated feedstock is then conveyed by a suitable conveying means H, for example a screw-feed conveyor, to a main pump I, being, in the process shown, one of several main feed and pressurising pumps used in the system. The conveyor H and the pump I may suitably be of stainless steel or some other non-corrosive or non-reacting material. The main pump I will force the feedstock into the main tubular reactor J raising the pressure in the reactor J well above the later saturation pressure. The reactor J, one of several reactors in the process of the embodiment of the invention as shown, will suitably be a pipe made of copper, monel, titanium, hastalloy or other suitable material or coated with such materials or for example a material such as Teflon (registered trade mark). Counterflowing liquid indicated by arrows X can be injected into the main process pipe using one of a series of fluid injection pumps K, either before a main feed and pressurising pump I, or after it as indicated by the dotted line, the feedstock and the liquid combining to form a slurry. The slurry passes along the reactor pipe J when it is heated by a first heat exchanger L1 to hydrolysis temperature. The slurry then passes along the reactor pipe J and may usefully be continuously mixed by in-line mixers M. The length of the reactor pipe J will be determined by several factors including temperature, velocity, solid-liquid ratio, and pH of the slurry so that the hydrolysis reaction for a particular part of the process is optimised. The slurry will then be cooled in heat exchanger L2 and the cooled slurry then passes to a pressure reducing means such as a pump, valve or nozzle or a purpose made device or any combination of these, N. The purpose of the pressure reducing means is to allow the reactor to remain under pressure while continuously discharging slurry. A consequence of the removal of heat before the pressure drop takes place is that as the discharge from high pressure to low pressure takes place no flash steam is generated. Thus, the present process in contrast with prior art proposals is a single phase process where the generation of steam is avoided. After the pressure reducing means N, the slurry, then at low pressure and temperature passes on to a separating means P which may for example be a filter pipe, filter press, settling container or centrifuge. Once the separation has been effected, the solids can then pass forward to further processing or to discharge as lignin as indicated by arrow B on the right hand side of Figure 2. The liquid passes backwards to further processing or discharge as hydrolysate as indicated by arrow D on the left hand side of Figure 2. The number of stages required to effect an optimum sugar separation will be dictated by several non-linear varying parameters. As previously mentioned, the flow of liquid and solid material through the flow loops is in a co-current direction. A consequence of this is that as it is a slurry which is passing through the pipes of the reactors clogging as could result from a separated liquid/solid phase process can be avoided. Also, in these heat exchange loops, the heat exchangers L1, L2 and L3 are joined by pipes and pumps Q to form a closed circuit. Heat given up by heat exchanger L1 to the main flow slurry is recaptured later on at L2 when heat is returned from the slurry to the closed circuit fluid. The recovery or regeneration of this heat will of course reduce heat requirements for the process. Heat lost to the atmosphere or remaining in the slurry after exchanger L2 is made up by heat from the external heat source such as a hot oil heater or boiler R which supplies heated fluid such as hot oil to the heat exchanger L3. If the sugar rich acid-hydrolysate is to be neutralised using calcium carbonate or calcium hydroxide as a milk and introduced at Z as is illustrated in Figure 2, then the resulting calcium sulphate with its inverse solubility may preferably be removed at about 150 o C with filter presses or centrifuges and the process flow becomes similar to the hydrolysis flow. As illustrated in the left hand portion of Figure 2 the filter or centrifuge S may provide an output of calcium sulphate in a direction indicated by arrow Y as a slurry or cake. The separation of the liquid and solid portions of the slurry as it is continuously fed through the system continues until the continuous cycle has been completed with further hydrolysing, washing and/or neutralising. The liquid hydrolysate lines and pumps shown in Figure 2 may suitably be of stainless steel or be of the materials or have the coatings mentioned for use previously in respect of the tubular reactor pipes. It is thus seen that a continuous process has been achieved by the present invention with the continuous leaching and removal of sugar and lignins. Additional advantages of the present invention are as follows; 1. very low heat energy requirements perhaps 10 to 20% of that required for other continuous processes and perhaps 5% of that required for batch percolation processes; 2. simplicity of design and construction of the reactor vessels with no moving parts, either valves or pumps, in the high temperature, corrosive zone; 3. single phase flow throughout the system leads to improved heat transfer to the slurry and the elimination of energy losses from flashing to steam of part of the liquid in the slurry; 4. because of the improved heat transfer, larger size particles can be utilised reducing the need for a ground feedstock with its possible degeneration to a mud-like slurry leading to more difficult separation problems; 5. the efficiency of the system allows for a low liquid-solid feed slurry ratio giving high sugar concentrations in the hydrolysate and consequentially lower energy needs. Referring now to Figure 4, this shows possible process flow temperatures throughout the process of Figure 2. Two sets of process temperatures are indicated, both having been derived from computer models. A slurry having a liquid-solid ratio of 6:1 has been assumed and the pressure in the process will always be well above saturation pressure. It is seen that the temperature change across the reactors is 10 o C for one set of process temperatures and 5 o C for the other set. It is emphasised however that the temperatures given are only examples of an infinite set of possible temperature combinations for each of which there will be an optimum and critical design requirement.

Claims: A process for the continuous hydrolysis of wood and wood derived products such as cellulose and starch material, comprising: continuously feeding wood and wood derived products as a feestock to a receiving means (1) of a main process line; injecting a presoak liquid into the receiving means (1) to presoak the feedstock; continuously feeding the feedstock by a conveying means (H) from the receiving means (1) to a first pressurising pump (I) which feeds the feedstock under pressure toward a first reactor (J) of the main process line; creating in the main process line at an injection point (G) a feedstock slurry by injecting water or a liquid phase into the process line; continuously feeding the slurry under pressure into and through one or more reactors (J) whereby the feedstock in the reactors is the pressurised slurry; injecting a weak acid solution into the system prior to at least one of the reactors (J); heating each reactor to raise the temperature of the pressurised feedstock therein to a temperature sufficient for hydrolysis of the feedstock to occur; maintaining the temperature of the feedstock in the process line for sufficient time for leaching of the feedstock to occur and/or to maximise sugar production and separating the solid and liquid portions of the slurry using a separator (P) ; characterised by: the fact that the reactors are tubular reactors (J) and the heating of each reactor is carried out by means of respective first heat exchangers (L₁), and further characterised by cooling the pressurised slurry in each tubular reactor (J) with a second heat exchanger (L₂) to avoid the generation of vapour and to ensure the process is single phase; recovering heat from the second heat exchanger (L₂) and utilising the heat recovered to supply at least part of the heat requirements of the first heat exchanger (L₁); reducing with pressure reduction apparatus (N) at an outlet of each tubular reactor the pressure of the cooled slurry therein while maintaining the pressure in the tubular reactor (J) while the slurry is continuously discharged; discharging the solid portion (B) which is lignin or passing it to the next reactor loop of the main process line; and discharging the liquid portion (D) which is sugar rich acid-hydrolysate or injecting the liquid portion to the preceding tubular reactor of the main process line. A process as claimed in Claim 1 characterised in that the weak acid solution injected forms part of or all of the presoak liquid. A process as claimed in Claim 1 characterised in that the presoak liquid is water and the weak acid solution is injected into the main process line at the injection point or at other points (G) on the main process line. A process as claimed in any preceding claim further comprising: providing several tubular reactors (J) in series within which the flow of solids and liquids is co-current; collecting the liquid portion of the slurry after discharge from each tubular reactor (J) after the pressure reducing apparatus (P) of the series except for the first tubular reactor where the liquid portion is discharged from the main process line; injecting the liquid portion into the main process line before the previous tubular reactor of the series; collecting the solid portion of said slurry after discharge from each tubular reactor except for the final reactor of the series where the solid portion is discharged as lignin; and feeding the collected solid portion into the main process line by way of the feed and pressurising pump into the next tubular reactor of the series so that the overall flow of solids and liquids within the series is countercurrent. A process as claimed in Claim 4 characterised in that each of the tubular reactors (J) is comprised of a tubular member whose length is dependent on the temperature, velocity, solid-liquid ratio and pH of the slurry. A process as claimed in any preceding claim characterised in that for each tubular reactor (J), a third heat exchanger (L₃) is provided in a closed regenerative loop to supplement the heat requirement of the first heat exchanger, the first and second heat exchangers (L₁, L₂) respectively introducing heat to and extracting heat from the slurry towards its entry into and exit from each tubular reactor (J) and the third heat exchanger (L₃) introducing additional heat as required to a heat transfer medium flowing in the closed regenerative loop between the heat exchangers.

Inventors: JUST, JACK TAMA HAIGH
Applicants: JUST, JACK TAMA HAIGH, RAMSEY, FRANCIS JOHN
Application year: 1985
Main classification: C13K1
IPC codes: C13K1, C13K13