METHOD FOR THE ACETYLATION OF WOOD

Abstract

The present invention provides a method for the acetylation of wood comprising treating the wood with an acetylation medium under wood acetylation reaction conditions and drying the acetylated wood, wherein the drying comprises at least two steps, wherein the wood is first dried with a first drying medium and then with a second drying medium.

Claims

1. A method for the acetylation of wood comprising treating the wood with an acetylation medium under wood acetylation reaction conditions and drying the acetylated wood, wherein the drying comprises at least two steps, wherein the wood is first dried with a first drying medium and then with a second drying medium.

2. The method according to claim 1, wherein the first drying medium has a higher heat capacity than the second drying medium.

3. The method according to claim 1, wherein the first drying medium comprises an acetylation medium vapour.

4. The method according to claim 1, wherein the second drying medium is an inert gas, preferably nitrogen.

5. The method according to claim 1, wherein the acetylation reaction conditions comprise a pressure of at least 1 barg.

6. The method according to claim 1, wherein the drying in the first and/or the second step is carried out at a pressure lower than the acetylation reaction pressure.

7. The method according to claim 1, wherein the acetylation medium comprises acetic acid, acetic anhydride or a mixture thereof.

8. The method according to claim 4, wherein the acetylation medium comprises a mixture of acetic acid and acetic anhydride, preferably comprising at least 50 wt. %, more preferably at least 85 wt. % of acetic anhydride.

9. The method according to claim 1, wherein the acetylation conditions comprise heating to a temperature in the range 160-190? C.

10. The method according to claim 1, wherein the drying temperature in either one or both of the drying steps is in the range 150-190? C.

11. The method according to claim 1, wherein in the first drying step the residence time is 5-25 min and, optionally, the gas to solid flow rate is 1-6 mass ratio.

12. The method according to claim 1, wherein in the second drying step the residence time is 15-40 min and, optionally, the gas to solid flow rate is 0.7-5 mass ratio.

13. The method according to claim 1, wherein the liquid content in the acetylated wood after acetylation and before drying is at least 55 wt. % on dry basis.

14. The method according to claim 1, wherein the liquid content in the acetylated wood is reduced during the first drying step to at least 2-8 wt. % on dry basis and in the second drying step to max 0.5 wt. % on dry basis.

Description

EXAMPLE

[0035] A mixture of wood chips with dimensions 2.5 cm?1.5 cm?1.5 cm consisting of 80% Sitka Spruce with 20% Loblolly Pine has been dried to a moisture content of 2.5%. This mixture is fed to an acetylation step where it is sprayed with liquid acetic anhydride/acetic acid mixture and heated with superheated acetic anhydride/acetic acid mixture (where the liquid to gas ratio is 1:2.5 wt:wt) to a temperature of 180? C. at a pressure of 2.5 barg. The weight ratio of acetic anhydride to acetic acid in the acetylation medium mixture is 90:10. After a residence time of 30 minutes at these conditions the pressure is released and the wood chips are transferred to the drying step, which proceeds at 0 barg. The first drying step occurs with a gas flow to solid flow ratio of 3.2:1 (mass:mass) with a gas phase composition of 33% acetic acid, 65% acetic anhydride and 2% nitrogen. The residence time for this step is 15 minutes, where the chips are heated with a temperature of 170? C. Subsequently the chips are transferred to the second drying step, where the chips are heated with nitrogen at 170? C. for 25 additional minutes. This process resulted in acetylated wood chips with an acetyl content (AC) of 23.2% and a residual acid content of 0.3 wt. %.