Process for isolating lignin from an alkaline process stream

Abstract

A process for isolating lignin from an alkaline process stream of thickened black liquor which is introduced continuously into a lower region of at least one circulation reactor having two reactor zones in concentric arrangement, liquid level of the alkaline process stream in the interior of the reactor is at a level with an upper end of an inner tubular reactor zone, a CO.sub.2-containing gas is blown continuously from the bottom into the inner tubular reactor zone of the reactor, wherein the CO.sub.2-containing gas is absorbed by the alkaline process stream in the inner circulation reactor zone and offgas is drawn off with residual amounts of the CO.sub.2 at the top of the reactor, the process is run at 1 atm, and thickened black liquor with a reduced lignin content together with precipitated lignin present are drawn off optionally after settling at the base of the reactor.

Claims

1. A process for isolating lignin from an alkaline process stream of thickened black liquor, comprising introducing the alkaline process stream continuously into a lower region of at least one circulation reactor having an interior with an inner tubular reactor zone and an outer tubular reactor zone in a concentric arrangement so that a liquid level of the alkaline process stream in the interior of the at least one circulation reactor is chosen at a level with an upper end of the inner tubular reactor zone which level is higher than the liquid level of the outer tubular reactor zone and therefore prevents foaming; continuously blowing a CO.sub.2-containing gas from the lower region into the inner tubular reactor zone of the at least one circulation reactor, so that the CO.sub.2-containing gas is absorbed by the alkaline process stream in the at least one inner circulation reactor zone and offgas is drawn off together with residual amounts of the CO.sub.2-containing gas at the top of the at least one circulation reactor; running the process at ambient pressure of 1 atm, and drawing off a thickened black liquor with a reduced lignin content together with precipitated lignin present therein after settling at a base of the at least one circulation reactor.

2. The process as claimed in claim 1, wherein a dwell time for the CO.sub.2-containing gas in the circulation reactor is determined by a height of the at least one inner tubular reactor zone.

3. The process as claimed in claim 1, wherein a rate of the CO.sub.2-containing gas absorption in the thickened black liquor is managed or controlled by a CO.sub.2 concentration in the CO.sub.2-containing gas.

4. The process as claimed in claim 1, wherein dilute CO.sub.2, in offgas from a lime kiln CO.sub.2-producing unit is introduced into the at least one circulation reactor.

5. The process as claimed in claim 1, wherein the process is carried out at a temperature between 30° C. and 80° C.

6. The process as claimed in claim 1, wherein a quality of lignin to be precipitated is managed or controlled by adjusting a pH with CO.sub.2 as an acidifying agent to values between 13 and 8.

7. The process as claimed in claim 1, wherein a plurality of circulation reactors are arranged in succession in a cascade, so that the thickened black liquor which is drawn off at the base of the circulation reactor which is arranged respectively upstream and which has an at least partially reduced lignin content is passed to a respectively immediately downstream circulation reactor, and in that a pH drop is maintained in successive circulation reactors of the cascade from upstream to downstream.

8. The process as claimed in claim 7, wherein the fall in value of the pH is adjusted by CO.sub.2 concentration in the CO.sub.2-containing gas, wherein a lowest CO.sub.2 concentration is supplied to the circulation reactor which is furthest upstream.

9. The process as claimed in claim 1, wherein a settling of precipitated lignin is carried out in a separate conically-shaped settling tank.

Description

DESCRIPTION OF THE DRAWING

(1) The invention will now be described in more detail with the aid of an exemplary embodiment, the process protocol of which is shown by the schematic of the device required for carrying out the process, as shown in FIG. 1.

(2) FIG. 1 shows a schematic of a continuous circulation reactor with an attached settling tank which can be used for precipitating lignin from an alkaline process stream.

DETAILED DESCRIPTION OF THE INVENTION

(3) Black liquor from evaporation is supplied to the base region 2 of a circulation reactor 1 via the line 3. The thickened black liquor which is supplied to the circulation reactor 1 via 3 has a dry matter content of approximately 20% to 40% by weight here and originates from cellulose production. In order to be able to manage or control the inflow of the thickened black liquor, a control or regulating valve 4 is also provided in the feed line 3. The temperature of the thickened black liquor which is supplied to the base region 2 of the circulation reactor 1 is between 55° C. and 80° C. here and is temperature-controlled prior to supplying it. Normally, the thickened black liquor is introduced into the reactor at the temperature at which it leaves the evaporator. For the conversion with carbon dioxide, in particular the precipitation of lignin with the aid of CO.sub.2, it should be noted here that the higher the temperature of the thickened black liquor, and thus the temperature of the process being carried out in the circulation reactor 1, the faster the CO.sub.2 will be absorbed by the thickened black liquor.

(4) CO.sub.2 is also supplied to the base region 2 of the circulation reactor 1 via the line 5. In this regard, then, the line 5 discharges into the interior of the reactor 1 in a manner such that it essentially discharges at the bottom end of an inner tubular reactor zone 6. In this manner, i.e. with the line 5 for introducing CO.sub.2 discharging at the bottom region of the inner tubular reactor zone 6 of the circulation reactor 1, this ensures that CO.sub.2 is essentially introduced into this region only, for example by means of a sieve bottom, a frit or a regulating valve, and by means of the CO.sub.2 which is introduced, on the one hand the conversion of thickened black liquor with CO.sub.2 is begun, and on the other hand the circulatory movement in the circulation reactor 1 is begun. This occurs because the CO.sub.2 rises in the interior of the tubular reactor zone 6 and entrains black liquor with it as it rises, and subsequently, after the upper end 7 of the inner tubular reactor zone 6 is reached, flows over this upper end 7 into the outer annular region 8 of the circulation reactor 1 and thus starts the circulation of material. Because the process, i.e. the conversion of an alkaline process stream or of thickened black liquor, is carried out with CO.sub.2 under normal pressure, the extent of the conversion or the rate of acidification of the thickened black liquor with CO.sub.2 is dependent on the one hand on the temperature of the thickened black liquor which is introduced and of the gas which is introduced, and on the other hand on the dwell time of CO.sub.2 in the interior of the circulation reactor 1, in particular in the interior of the inner tubular reactor zone 6.

(5) When the circulation reactor 1 is operated under normal pressure, the height or length of the inner tubular reactor zone 6 is directly proportional to the dwell time of CO.sub.2 inside this zone, and thus of the period which is available for CO.sub.2 to convert the thickened black liquor. The longer this conversion period is, the lower will be the pH of the thickened black liquor and this management or control of the pH further means that the quality of the precipitated lignin can be influenced. When the inner tubular reactor zone 6 is selected to be sufficiently high or long, a maximum absorption of CO.sub.2 inside this reactor zone 6 may take place, so that offgas which exits the upper end 7 of the inner reactor zone 6 contains substantially no more CO.sub.2. Because the height of the inner tubular reactor zone 6 is used as a management or control element, it is consequently important to know the height of the volume of liquid inside the circulation reactor 1, because the height of the liquid volume in the reactor, i.e. how much liquid will be introduced before the reaction is started, or the ratio with respect to the outflow rate of the introduced thickened black liquor, will manage or control the process protocol and in particular will manage the levels in the reactor. As shown as an optional element in FIG. 1, the levels may be managed here by measuring the level of liquid 10 in the interior of the circulation reactor 1 by means of a level managing or control sensor 9, such that when the level of liquid 10 has reached a predetermined maximum value, the supply valve 4 for supplying more fresh thickened black liquor is closed and the excess level of thickened black liquor is recycled via the bypass line 11 into the supply line 3 for thickened black liquor. With this level management or control device, the level in the interior of the circulation reactor 1 is always kept constant and thus a specific, permanently consistent conversion of the introduced thickened black liquor by CO.sub.2 is obtained, and thus a consistent product quality is obtained for the precipitated lignin.

(6) Precipitated lignin together with thickened black liquor with a reduced lignin content are drawn from the base of the circulation reactor 1 via the line 12 and in the schematic 1 of FIG. 1 are transferred into a settling tank 13 in which lignin is allowed to sediment out. The sedimented lignin is withdrawn from the settling tank at 15 and the supernatant liquid will, for example, be supplied to further treatment via line 14. The lignin-rich sludge which is withdrawn from the settling tank 13 via line 15 will undergo further purification and the lignin will be supplied to an end use. Instead of the process protocol shown in FIG. 1, it is clearly possible to design the reactor of the circulation reactor 1 with a conical base, for example, to allow lignin to settle out there, in order to draw off the residual thickened black liquor with a reduced lignin content in the upper region of the reactor, approximately at the height at which the black liquor is introduced, to supply it to a further circulation reactor and to carry out the process as a cascade, wherein by means of stronger and stronger acidification of the thickened black liquor, other qualities of lignin can be precipitated each time.

(7) Finally, in order to be able to manage or control the quality of the precipitated lignin even more precisely, the process may be carried out in the circulation reactor 1 by monitoring the pH. To this end, in the outer reactor zone 8, a continuous measurement of the pH is carried out at 16 and when the measurement of the pH shows that the pH is too low for the desired lignin quality to be precipitated, a valve 17 provided in the discharge line 12 is opened in order to withdraw as much as possible of the product stream as quickly as possible. At the same time, for example, the supply of CO.sub.2 could be stopped. If, on the other hand, the pH in the interior of the reactor 1 after the CO.sub.2 has reached the top of the inner tubular reactor zone 6 is still too high to obtain the desired lignin quality, the valve 17 in the discharge line 12 can be adjusted in a manner such that a further discharge of black liquor with a reduced lignin content originating from the circulation reactor 1 is prevented and the circulation is continued by blowing in CO.sub.2 until the desired pH is obtained. It hardly needs to be mentioned that in this case, the valve 4 in the supply line for thickened black liquor must be closed, because otherwise, a surplus quantity of thickened black liquor would be present in the interior of the circulation reactor 1.

(8) When the process in accordance with the invention is carried out in a cascade of circulation reactors 1, then the pH in the individual circulation reactors 1 can be specifically adjusted by the process protocol described above, and thus a specific lignin quality can be precipitated in each individual circulation reactor 1. As is known in the art, lignin with a high molar mass is precipitated at relatively high pHs, whereas lignin with a low molar mass is precipitated at lower pHs.

(9) In summary, it therefore appears that with the process protocol in accordance with the invention, it is firstly possible to manage or control the CO.sub.2 absorption by means of the height of the inner tubular reactor zone 6, secondly to manage or control the rate of CO.sub.2 absorption with the aid of the temperature which prevails inside the circulation reactor 1, wherein at temperatures of more than 80° C., the glass transition temperature of lignin has already been exceeded and the quality of the product will deteriorate. Preferably, then, the precipitation is carried out at temperatures between 65° C. and 75° C. Thirdly, it is possible to manage or control the process by controlling a level, fourthly it is possible to manage or control the process by managing or controlling a pH, and fifthly, it is possible to configure the process as a cascade of a plurality of circulation reactors 1, whereupon a fractionated precipitation of lignin from the alkaline process stream, in particular thickened black liquor, can be obtained; this lignin can then be supplied for very specific uses. As an example, after precipitating it, the lignin may be oxidized, whereupon water-insoluble or concentrated water-insoluble lignin can be obtained. In the same manner, the process may be carried out using an ion exchanger, so that the lignin can be recycled to the lime kiln as fuel in known manner.