It is known to modify lignocellulose materials by acetylation to improve the dimensional stability of the obtained products.
U.S. Pat. No. 4,804,384 (Rowel et al.) discloses a method for modification of lignocellulosic materials by a catalyst-free acetylation by contacting the lignocellulosic material in the form of veneer, chips, flakes, fibres or particles with a liquid reactant of acetic anhydride and acetic acid; heating the reactant-contacted lignocellulosic material at a temperature of up to 120° C. for 1 to 5 hours; and, removing unreacted acetic anhydride and acetic acid from the resulting acetylated lignocellulosic material. The liquid reactant consists essentially of acetic anhydride and 0-55 vol % acetic acid, the preferred range being 10 to 30 vol %. The contact with the liquid reactant is carried out by simple dipping. Rowel does not teach to avoid contact with ambient moisture and oxygen. The unreacted acetic anhydride and acetic acid can be recovered and added back to the reactant bath until the concentration of acetic acid exceeds about 30 vol %.
EP 0650998 (Nelson et al. I) discloses a process for the acetylation of lignocellulosic fibres. The fibres are contacted with an acetylating agent comprising acetic anhydride at a temperature from 70 to 140° C. Then the acetylated fibres are contacted with a superheated chemical agent comprising acetic acid and/or anhydride at a temperature above 140° C. for removal of residual acetic acid or acetic anhydride content to below 10% by weight.
The process involves compaction of the fibres with a plug-screw feeder to reduce the permeability of the fibre to gas flow. The compacted fibres are dispersed and fed into a nitrogen purged first reactor after which a preheated acetylating agent of 10% by weight acetic acid and 90% by weight acetic anhydride is injected. The first reactor is heated at 120° C., and this temperature is maintained during the exothermic acetylating with vaporisation acetylating agent containing 70% by weight acetic anhydride, which is recovered and recycled.
The acetylated fibre emerging from the first reactor contains about 40% by weight liquid. This fibre is re-compacted in a further plug-screw feeder, and then dispersed and treated with a superheated vapour of acetic anhydride optionally containing some acetic acid from recycled streams at about 190° C.
The fibre so treated is entrained in the superheated vapour stream to a circulation stripper, where the chemicals adsorbed or occluded in the fibre are evaporated. The hot fibre entrained in the overheads from the stripper is recovered in a cyclone. After a further stripping in a steam stripper, in which any residual acetic anhydride in the fibre is hydrolysed to acetic acid and the acetic acid is stripped out, the treated fibre is recovered from the overhead by means of a cyclone.
Based on experiments made by the present inventors, it was found that the fibre treated in the plug-screw feeders as suggested in EP 0650998 (Nelson et al. I) is cut down to undesirable small particle sizes. Thus by preparation of fibre for filtration the obtained fibre had a length of about 1 mm, which is undesirably short. Moreover, the obtained fibre was far from being without unpleasant odours. Furthermore, the process is too complex and expensive for a commercial process at large scale.
WO 9523168 (Nelson et al. II) discloses a similar process for the acetylation of lignocellulosic materials using a heated inert gas in the first stripper. The remaining amount of acetic acid is stated to be below 0.5% by weight.
WO 9619526 (Nelson et al. III) discloses a further development of the above two processes using a superheated acetylating agent comprising at least 20% w/w acetic anhydride at a temperature of 140-220° C. The fibre is fed with a star feeder to a narrow chamber, wherein oxygen is displaced by purging with nitrogen followed by spraying with a mist of acetic anhydride. To avoid back flow of acetic anhydride the chamber is maintained at a pressure slightly below atmospheric. From the chamber the fibre is moved to an acetylation reactor, wherein it is treated with the superheated acetic anhydride. According to WO 9619526 (Nelson et al. III) this reactor is also a steam jacketed circulation stripper, where the chemicals adsorbed or occluded in the acetylated fibre are evaporated. As in the above mentioned processes, the acetylated fibre is recovered in a cyclone and stripped once more with steam and recovered in a second cyclone. Several systems for recovering and recycling the acetylating agent are involved in the process, which indicates that the agent at several points will contain more that 5% by weight of acetic acid.
WO 9409057 (Rogers et al.) discloses a reaction of lignocellulosic material with acetic anhydride vapour. The reaction is carried out in the absence of any co-solvent or added catalyst and without the need for distillation/rectification. Heated, partially dried or dry lignocellulosic material is treated with acetic anhydride vapour. The material is reacted and dried with or without gas flow. Acetic acid is only removed and further treated with ketene for re-vaporization. The process is described without essential technical features. Thus, inter alia, WO 9409057 (Rogers et al.) is silent about limits for acceptable amounts of acetic acid in the anhydride vapour.
U.S. Pat. No. 7,413,662 (Eriksen et al.) discloses a modified sorptive lignocellulosic fibre material with hydroxyl groups on the lignocellulosic fibres doubly modified by esterification with a combination of monocarboxylic and dicarboxylic acid ester groups. The esterification can be made with an aliphatic monocarboxylic anhydride and a cyclic dicarboxylic anhydride for example with acetic anhydride and maleic anhydride. The sorptive fibre material is effective for the removal of oils and other contaminants including heavy metals from a fluid such as contaminated water by a combined sorption of hydrophobic contaminants and ion exchange.
U.S. Pat. No. 7,413,662, belonging to the present applicant, discloses the preparation of the modified sorptive lignocellulosic fibre material in laboratory scale using the maleic acid in a solvent. In order to prepare this fibre on commercial basis in large scale it was necessary to find a suitable process. Based on the above mentioned U.S. Pat. No. 4,804,384 (Rowel et al.) and the further development by Nelson et al. (EP 0650998, WO 9523168 and WO 9619526), experiments have been carried out in pilot plan scale unsuccessfully due to several problems. Thus a severe emission occurred from the reactor feeding. The pressure came out of control and rose to about 200 kPag or more (≈300 kPaa (absolute) or above). The fibre was defibrated to an undesired small particle size. The chemicals were not removed sufficiently and the resulting fibre had an unpleasant odour. Moreover, the esterification was not efficient leaving a larger portion of the —OH groups in the lignocellulose non-esterified.
It appears that there is still a need for a suitable process and apparatus for the preparation of esterified lignocellulosic materials which meets the requirements of                efficient and controlled migration of the esterification agent and its access to the reactive —OH groups,        efficient and controlled esterification reaction,        efficient and controlled removal of excess of the esterification agent and by-products,        commercially acceptable costs, and        environmentally sound.        