METHOD FOR PRODUCING CELLULOSE CARBAMATE

Abstract

A method for controlling discharges of nitrogen compounds in the production of cellulose carbamate (CCA). Dissolving pulp (DP) or kraft pulp is produced at a pulp mill, and the DP or kraft pulp is activated. The activated pulp is reacted with urea to produce cellulose carbamate whereby ammonia is released. The cellulose carbamate production is integrated into the pulp mill having a flue gas system such that carbon dioxide from the flue gases is reacted with released ammonia to produce urea, which is used in the carbamate production.

Claims

1. A method for controlling discharges of nitrogen compounds in production of cellulose carbamate (CCA), in which the method includes: dissolving pulp and/or generating kraft pulp from cellulosic fibrous material at a chemical pulp mill, activating the dissolved pulp and/or the kraft pulp to decrease a degree of depolymerization of the pulp, and reacting the activated dissolved pulp and/or the activated kraft pulp with urea to produce cellulose carbamate, wherein ammonia is released in the reacting step, wherein the cellulose carbamate production is integrated into the pulp mill having a flue gas system such that carbon dioxide from the flue gases is reacted with the released ammonia to produce the urea used in the carbamate production.

2. The method according to claim 1, wherein the flue gas system in associated with a lime kiln, and the carbon dioxide is obtained from the flue gases of the lime kiln.

3. The method according to claim 1, wherein the released ammonia and/or a wastewater with the released ammonia is used to neutralize prehydrolysate from the step of dissolving pulp and/or the kraft pulp and/or in acidic wastewater from the pulp mill.

4. The method according to claim 1, wherein at least a portion of a nitrogen-containing wastewater from the production of CCA is led to an evaporation plant and concentrated to produce dry nitrogen fertilizers.

5. The method according to claim 1, wherein a nitrogen-containing wastewater from the production of CCA is conveyed to a wastewater treatment plant of the pulp mill to be used as nutrient.

6. The method according to claim 1, wherein the activation is carried out at a fiberline of the chemical pulp mill, and/or at a separate stage between the chemical pulp mill and the CCA plant, and/or in the CCA plant.

7. The method according to claim 1, wherein the activation method is at least one of the following: acid hydrolysis, alkali hydrolysis (merceration), ozone stage, peroxide treatment and peroxide addition in an oxygen stage.

8. The method according to claim 1, wherein the activation is carried out by using chemicals produced or formed at the pulp mill.

9. A method comprising: activating pulp to decrease depolymerization of the pulp, wherein the pulp includes a cellulosic fibrous material; reacting the activated pulp with urea to produce cellulose carbamate, wherein ammonia is released by the reaction of the activated pulp with the urea; generating a flue gas in a chemical pulp mill; and reacting carbon dioxide in the flue gases with the released ammonia to produce the urea to be reacted with the activated pulp.

10. The method of claim 9, wherein the activation of the pulp includes one or more steps of: acid hydrolysis of the pulp, alkali hydrolysis of the pulp, treating the pulp in an ozone stage, treating the pulp with peroxide and treating the pulp with peroxide in an oxygen stage.

11. The method of claim 9, further comprising dissolving the pulp such that the dissolved pulp includes less than five percent of hemicellulose, and the dissolved pulp is the pulp activated in the activation step.

12. The method of claim 9, further comprising dissolving the pulp such that the dissolved pulp includes at least ninety percent α-cellulose, and the dissolved pulp is the pulp activated in the activation step.

13. The method of claim 9, further comprising producing kraft pulp in the chemical pulp mill and the kraft pulp is the pulp activated in the activation step.

14. The method of claim 9, wherein the chemical pulp mill includes a lime kiln, the flue gas is discharged by the lime kiln, and the carbon dioxide is obtained from the flue gas from the lime kiln.

15. The method of claim 9, further comprising using the released ammonia and/or a wastewater with the released ammonia to neutralize prehydrolysate in the pulp mill.

16. The method of claim 9, further comprising concentrating a nitrogen-containing wastewater generated in the production of the cellulose carbamate in an evaporation plant to produce a dry nitrogen fertilizer.

17. The method of claim 9, further comprising using nitrogen-containing wastewater generated in the production of the cellulose carbamate as a nutrient in a wastewater treatment plant of the chemical pulp mill.

18. The method of claim 9, wherein the activation of the pulp is carried out at a fiberline of the chemical pulp mill.

19. The method of claim 9, wherein the activation of the pulp is performed chemicals produced or formed at the chemical pulp mill.

Description

[0045] FIG. 1 is a schematic illustration of CCA production by integrating it into a chemical pulp mill where dissolving pulp or kraft pulp is produced.

[0046] The numbers and letters in FIG. 1 refer to the following streams and processing stages: [0047] A. Chemical pulp mill [0048] B. Cellulose carbamate (CCA) plant [0049] 1. Raw materials, chemical and process utilities needed to produce pulp [0050] 2. Cellulose carbamate [0051] 3. Urea [0052] 4. Dissolving pulp or kraft pulp [0053] 5. Slightly alkaline nitrogen-containing filtrate from CCA plant to neutralization, or to a wastewater treatment plant, or to an anaerobic digestion plant to biogas production at the pulp mill [0054] 6. Steam, water, electricity, and CO.sub.2 (urea recovery) from the pulp mill to the CCA production [0055] 7. Acidic or alkaline activation agent if the activation is done at the CCA plant [0056] 8. Ammonia to flue-gas systems to reduce NOx-emissions

[0057] Raw material, such as wood chips, and chemicals, typically cooking chemicals, as well as process utilities are introduced (line 1) to a chemical pulp mill A, where dissolving pulp or kraft pulp is produced in a way known per se.

[0058] Dissolving pulp (DP) or kraft pulp is produced (line 4) at the chemical pulp mill in a way known per se. It is activated in the chemical pulp mill fiberline or at the CCA plant B.

[0059] A plant for a cellulose carbamate (CCA) production is integrated into the chemical pulp mill (line 6). CCA is produced by reacting [0060] a) activated DP or activated kraft pulp with urea at an elevated temperature. The dry matter content of DP or kraft pulp is about 40-90%, and it is mixed efficiently with urea (from line 3), or [0061] b) DP or kraft pulp with urea (line 3) and with an activation agent (line 7) at an elevated temperature so that the activation of DP or kraft pulp is performed at the same time than CCA is produced. The dry matter content of DP or kraft pulp is about 40-90%, and it is mixed efficiently with urea (from line 3) and with activation agent (line 7).

[0062] Then the final carbamation reaction is made in a steam-heated mixing reactor at a temperature of 130-160° C. Ammonia is generated in the reaction. A small portion of the urea may not react, and it may be removed from the CCA product by washing. The CCA product is cooled, washed and dried, and then it is led to further processing (line 2).

[0063] Ammonia released from the carbamation process is scrubbed out of the reactor with steam and used to produce urea, which is recycled to the carbamation process. The amount of the urea supplied from an external source through line 3 can be decreased by producing urea internally. This is carried out by reacting ammonia with the carbon dioxide of flue gases from the pulp mill. An advantageous source for carbon dioxide is flue gases from a lime kiln, in which lime mud (CaCO.sub.3) is burnt to lime (CaO). The urea regeneration plant is preferably part of the CCA plant.

[0064] Carbon dioxide is preferably captured from the flue gases. This can be performed by using conventional well-known methods, such as the monoethanolamine (MEA) absorption process and pressure swing adsorption (PSA) process.

[0065] The urea production is a two-step process where the ammonia and carbon dioxide react to form ammonium carbamate, which is then dehydrated to urea. Ammonia and CO.sub.2 are introduced in gaseous form. Both components are liquefied in a high-pressure condenser and led to a high-pressure reactor where ammonium carbamate is formed at an elevated temperature, e.g. 180-190° C., reaction equation (5). This reaction is fast and exothermic. The second reaction (6) is endothermic and does not go to completion. A solution comprising urea and ammonium carbamate is obtained. The ammonium carbamate present in the solution is decomposed to CO.sub.2 and NH.sub.3 in a recovery unit and recycled to the urea synthesis reactor. The urea process solution is led to the carbamation reactor.

[0066] A part of the released ammonia can be utilized also in pulp mill flue gas systems to minimize NOx emissions according to reaction equations (5) and (6). Ammonia can be in gaseous form or in dissolved form when it is led to the pulp mill (line 8).

[0067] Slightly alkaline waste streams (line 5) comprising nitrogen compounds, such as unreacted urea, can be led fully or partly to a chemical pulp mill wastewater treatment plant. These waste streams can optionally or alternatively be led to an evaporation plant and produce dry fertilizers or to be led to biogas production or to be used as neutralizing agent to neutralize [0068] acidic hydrolysate from the dissolving pulp production (prehydrolysate), [0069] acidic washing filtrates from bleaching washers, [0070] acidic condensates from NCG gas condensation systems of the pulp mill evaporation plant, or [0071] other acidic process streams.

[0072] These neutralized waste streams containing nitrogen can be further led to a chemical pulp mill wastewater treatment plant, where they act as a nutrient source for micro-organisms, or to anaerobic digestion to produce biogas.

[0073] The new method provides an efficient way to utilize available ammonia and carbon dioxide and thus control their emissions from the integrated CCA plant and pulp mill. The raw material, dissolving pulp or kraft pulp, is activated in the fiberline or at the CCA plant preferable by using chemicals produced by the pulp mill.