The present disclosure provides a method for mechanical extraction of black liquor from a. digester and particularly a method for extraction of black liquor from a horizontal tube continuous digester at elevated temperature and pressure for efficient heat recovery.

BACKGROUND

A chemical pulp mill is designed to convert raw material such as wood, bamboo, agro-residues such as wheat straw or bagasse, cotton stalks etc. into pulp with the utilization of various chemicals such as sodium hydroxide, sodium hydroxide and sodium sulfide or sodium sulphite. Several of the chemical pulp mills operate on the principle of the Kraft process or sulfate process for conversion of raw material into pulp. In the recent years, non-wood raw materials have made significant contribution in worldwide production of paper pulp. This type of production of pulp has increased more rapidly than that of pulp from wood in the last two decades, particularly in China, India, Mexico & Latin America.

In general a chemical pulp mill comprises of various components including screw conveyor for feeding raw material to a digester, a batch or continuous digester (horizontal or vertical) for digesting raw material with the help of digesting liquid containing chemicals called white liquor or cooking liquor, a discharge device where the pulp with the black liquor obtained from the digester is mixed with dilution liquor

Various methods of heat and steam recovery are reported in the prior art. One of the methods includes reducing the temperature and pressure in a digester by releasing flash vapour from the digester from approximately 7 bars to approximately 1 bar. The flash vapour thus obtained is at atmospheric pressure with low heat potential. Therefore, total heat and steam recovered in this method is very low as compared to the heat and steam consumed. Several attempts have been made to address this problem, by maximizing the heat and steam recovery and reduce the overall heat requirement of the process. In cooking wood or bamboo a part of the chemicals solution is penetrated into the wood or bamboo chips, but a larger part is a free liquid between the chips. The free hot liquid at the end of cooking, called spent liquor, is displaced by a cold liquid and heat recovered from the hot displaced liquid. However, these methods cannot be applied to cooking of agro residues such as wheat straw or bagasse these are forming a structure with interstices in which almost all liquid is held by capillary forces. The interstitially bound liquid cannot be drained from the digester or displaced by another liquid. Therefore in cooking of agro residues such as straw or bagasse these methods fail to effectively address the two issues: 1) reducing the overall demand of steam, and 2) increasing the overall heat recovery from the system. Thus there is still a requirement to develop an efficient system and method for extraction of hot black (spent) liquor from the pulp mill especially when cooking agro residues in a horizontal tube continuous digester and heat recovery from the hot black liquor while being cost effective.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 and 2 illustrate various components of a pulp mill cooking fibrous raw material in a horizontal tube continuous digester in accordance with an embodiment.

Figure 3 a illustrates a system of heat recovery from extracted hot black liquor comprising combination of flash cyclones in accordance with an embodiment. Figure 3b illustrates a system of heat recovery from extracted hot black liquor comprising a combination of a heat exchanger and a flash cyclone in accordance with an embodiment.

Figure 3 c illustrates a system of heat recovery from extracted hot black liquor comprising a combination of heat exchangers in accordance with an embodiment.

SUMMARY

A method for extraction of black liquor from a horizontal tube continuous digester is disclosed. The method comprises of passing a mixture of pulp and black liquor from the horizontal tube continuous digester through a screw press at substantially the same pressure as that of the horizontal tube continuous digester. The screw press configured to separate the black liquor, including interstitially bound black liquor, from the pulp. The method further comprises of storing the black liquor obtained from the screw press in a tank maintained at substantially the same pressure as that of the horizontal tube continuous digester and the screw press.

A system for extracting black liquor from a horizontal tube continuous digester is also disclosed. The system comprises of a screw press capable of integration with an outlet of the horizontal tube continuous digester for receiving a mixture of pulp and black liquor from the horizontal tube continuous digester. The screw press configured to separate the black liquor, including interstitially bound black liquor, from the pulp. The system further comprises of a tank in fluid communication with an outlet of the screw press and configured to receive the black liquor from the screw press, wherein the screw press and the tank are at substantially the same pressure as that of the horizontal tube continuous digester. DETAILED DESCRIPTION

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the disclosed process, and such further applications of the principles of the invention therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.

Reference throughout this specification to "one embodiment" "an embodiment" or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase "in one embodiment", "in an embodiment" and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The present disclosure provides a method for extraction of black liquor from a digester. Particularly a method for extraction of black liquor from a horizontal tube continuous digester for efficient heat recovery is disclosed. The method comprises of passing a mixture of pulp and black liquor from the horizontal tube continuous digester through a screw press at substantially same pressure as that of the horizontal tube continuous digester and storing the black liquor obtained from the screw press in a tank maintained at substantially the same pressure as that of the horizontal tube continuous digester and the screw press. Figure 1 illustrates a pulp mill in accordance with an embodiment comprising a pair of horizontal tube continuous digesters (8,9) in series. A screw press (1 1) is connected at the end of the horizontal tube continuous digester (9). The teachings of this disclosure may equally be applied to a single horizontal tube continuous digester or a series of horizontal tube continuous digesters. Raw material is discharged from a bin (1) and metered into a screw feeder (2). The screw feeder (2) feeds the raw material into an inlet chamber (4) of the horizontal tube continuous digester (8). The raw material is compressed to a plug to withstand the pressure in the horizontal tube continuous digester, such that in case of a loose plug the screw feeder (2) is closed by a blow back valve (5). Steam and cooking liquor are injected through piping (6) and (7) respectively. Processed raw material from the horizontal tube continuous digester (8) is passed on to the horizontal tube continuous digester (9) for further processing. At the end of the horizontal tube continuous digester (9) the raw material is transformed into fibres (pulp) and falls through a chute (10) into the screw press (1 1) driven by a motor (12). The screw press (1 1) is maintained at substantially the same pressure as that of the horizontal tube continuous digesters (8,9). A tank (14) is connected to the screw press (11) for receiving the black liquor. The tank (14) is maintained at substantially the same pressure as that of the horizontal tube continuous digesters (8,9). The screw press (1 1) presses out the hot black liquor having substantially the same temperature as the liquor in the horizontal tube continuous digesters (8,9). The pulp leaving the screw press (1 1) is cooled by cold liquid entering through piping (13) and falls into a discharge device (17) equipped with an agitator (20), such that the pulp is diluted with wash liquor entering through inlet (19) in the discharge device (17) and pumped by a pump (22) to pulp chest through piping (23). The discharge device (17) includes a safety valve (18) and a junk outlet flange (21). In accordance with an embodiment, the horizontal tube continuous digester comprises of at least one perforated draining plate positioned proximate the bottom of the horizontal tube continuous digester for extraction of free black liquor. By way of specific example, as illustrated in figure 2, the horizontal tube continuous digester (9) comprises of draining plates (24) having flow control valves (25) for draining free black liquor from the horizontal tube continuous digester (9) into . a pressurized storage vessel (27). The pressurized storage vessel is at substantially the same pressure as that of the horizontal tube continuous digesters (8, 9). Alternatively, the free black liquor from the draining plates (24) is directly transferred to the tank (14) for storage.

In accordance with an embodiment, as illustrated in figure 2, the hot black liquor obtained from the draining plates (24) and from the tank (14) is pumped with pump (15) into the pressurized storage vessel (27) for storage, such that the vessel is maintained at substantially the same pressure as that of the horizontal tube continuous digesters (8, 9) and the screw press (1 1).

In accordance with an embodiment, the method further comprises of recovering heat from the black liquor by passing the black liquor from the pressurized storage vessel (27) through piping (16) into a heat recovery system (Figure 3a,3b and 3c). Alternatively, as illustrated in figure 1, the black liquor stored in tank (14) is passed by pump (15) through piping (16) into the heat recovery system (Figure 3a, 3b and 3c) for recovery of heat.

In accordance with an embodiment, as illustrated in figure 2, the extracted hot black liquor from the pressurized storage vessel (27) is pumped by pump (28) to the heat recovery system (Figure 3a, 3b and 3c). By way of a specific example, as illustrated in figure 3a the heat recovery system comprises of a combination of flash cyclones (29, 30). The hot black liquor is injected throtrgh piping (31) into flash cyclone (29) releasing low pressure steam through piping (32). The black liquor leaving from flash cyclone (29) flows through another flash cyclone (30) releasing steam at atmospheric pressure through piping (33). The black liquor is then pumped by pump (34) through piping (35) to filtering and evaporation.

By way of another specific example, as illustrated in figure 3b, the heat recovery system comprises of a combination of a heat exchanger (36) and a flash cyclone (37). Hot black liquor entering through piping (31) into the heat exchanger (36) is used for heating white (cooking) liquor entered through piping (38), such that the heated white liquor leaves the heat exchanger (36) through piping (39). The black liquor leaving from heat exchanger (36) flows through flash cyclone (37). Flash steam at atmospheric pressure is then released through pipe (40) and the black liquor is then pumped by pump (41) through piping (42) to filtering and evaporation.

By way of another specific example, as illustrated in figure 3 c, the heat recovery system comprises of a combination of heat exchangers (43 and 44). Hot black liquor entering through piping (31) into the heat exchanger (43) is used for heating white (cooking) liquor entered through piping (45), such that heated white liquor leaves the heat exchanger (43) through piping (46). The black liquor leaving from the heat exchanger (43) flows through another heat exchanger (44). The heat exchanger (44) including an inlet (47) for cold wash liquor and an outlet (48) for heated cold wash liquor. The black liquor is then pumped by pump (49) through piping (50) to filtering and evaporation.

In accordance with an aspect, steam consumption in pulping of raw material in the horizontal tube continuous digester (8, 9) according to the present invention is reduced as compared with conventional pulping. By way of a specific example, a comparison of heat and steam consumption in conventional method of pulping and method in accordance with the present invention is described below:

Conventional method:

The heat and liquor balance in accordance with the conventional method of pulping in a horizontal tube continuous digester (using straw as a raw material):

Parameters:

- Cooking temperature: 175 °C

- Washed wheat straw moisture content entering screw feeder: 63 %

- Straw moisture volume entering screw feeder: 1,702 litre/tonne dry straw

- Straw moisture volume drained in screw feeder: 87.5 litre/tonne dry straw

- Straw moisture volume entering digester: 1614.5 litre/tonne dry straw

- Straw moisture volume entering digester: 3228/tonne unbleached pulp

- Washed wheat straw temperature: 3 °C

- White liquor temperature: 79°C

- White liquor concentration: 100 g/1

- Alkali charge on straw: 13% as NaOH

- Volume of white liquor: 2600 litre/tonne of unbleached pulp

- Total liquor at beginning of cook: 5828 litre/tonne pulp

- Material (straw) to liquor ratio at beginning of cook: 1 :2.91

- Pulp yield unbleached: 50%

- Steam consumption 1710 kg/tonne unbleached pulp

Calculation of steam consumption per tonne of unbleached pulp:

(Slight draining of washed straw in screw feeder calculated; heat losses included) Heat demand of straw:

Temperature difference between cooking temperature and the washed straw temperature: 140°C

Heat demand of straw (dry) 2 tonnes: 2000 x 1.53 x 140 = 428.4 MJ

Heat demand of moisture in straw 3229 litre: 3229 x 4.19 x 140 = 1894.1 MJ

Therefore, total heat demand of moist straw: 2322.5 MJ

Heat demand of white liquor: (Temperature difference: 96°C)

2600 x 1.1 x 3.77 x 96 = 1035.1 MJ

Total heat demand: 3357.6 MJ/tonne unbleached pulp

Therefore, Steam demand: 1689 kg/tonne

Assumed heat losses as steam: 41 kg/tonne

Therefore, total steam demand: 1740 kg/tonne

Spent liquor (black liquor) volume at the end of cooking per tonne of unbleached

Moisture in Straw : 3229 litre

White liquor : 2600 litre

Steam condensate : 1740 litre

Total spent liquor volume in digester: 7569 litre/tonne of unbleached pulp (7.569 New Method:

By way of specific example, the heat and liquor balance in accordance with the present disclosure is illustrated below (using straw as a raw material):

Heat and liquor balance: The hot spent liquor (black liquor) is used for pre-heating of white liquor to 160°C in a heat exchanger before injecting into the horizontal tube continuous digester. Hot water from flashing hot black liquor after pre-heating white liquor is used for washing and increasing temperature of straw to 90°C. The washed warm straw including moisture is pre-heated by expanded steam from flashing spent liquor to 95°C before feeding into the horizontal tube continuous digester.

White liquor and straw pre-heating:

1. Heating up white liquor from 79°C to 160°C by the hot extracted spent liquor in heat exchangers: (Temperature difference: 81°C)

- Heat consumed: 2600 x 1.1 x 3.77 x 81 =873 MJ

2. Heating up the washed straw from 35°C to 95°C by expanded steam (Temperature difference: 60°C)

- Straw (dry) 2 tonnes: 2000 x 1.53 x 60 = 184 MJ

- Moisture in washed straw 3229 litre: 3229 x 4.19 x 60 = 812 MJ

Therefore, total recovered heat used for pre-heating: 1869 MJ

Heat and Medium pressure steam demand for heating digester:

- Heating up white liquor from 160°C to 175°C (Temperature difference: 15°C) Therefore, heat consumed: 2600 x 1.1 x 3.77 x 15 =161 MJ

1. Heating up the washed straw from 95°C to 175°C (Temperature difference: 80°C)

- Straw (dry) 2 tonnes: 2000 x 1.53 x 80 = 245 MJ

- Moisture in washed straw 3229 litre: 3229 x 4.19 x 80 = 1082 MJ

2. Heat losses: 81 MJ

Therefore, total heat demand: 1 69 MJ 3. Medium pressure steam demand: 789 kg per tonne of unbleached pulp

Steam consumption in straw pulping in a horizontal tube continuous digester according to the present invention is 45% as compared to the conventional method of pulping.

Spent liquor volume litreper tonne of unbleached pulp:

- Moisture in straw: 3229

- White liquor: 2600

- Steam condensate: 789

Total spent liquor volume: 6618 litre/tonne unbleached pulp (6.62 m ³ /t)

By way of specific example, the amount of extracted hot spent liquor by pressing out the pulp through the screw press at cooking temperature and pressure according to the present invention is shown in table 1.

Table 1. Spent liquor distribution before and after extraction by pressing out

SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW

A method for extraction of black liquor from a horizontal tube continuous digester comprising passing a mixture of pulp and black liquor from the horizontal tube continuous digester through a screw press at substantially the same pressure as that of the horizontal tube continuous digester; the screw press configured to separate the black liquor, including interstitially bound black liquor, from the pulp and storing the black liquor obtained from the screw press in a tank maintained at substantially the same pressure as that of the horizontal tube continuous digester and the screw press.

Such method(s), further comprising draining free black liquor from the horizontal tube continuous digester substantially at the same pressure in to a pressurized storage vessel.

Such method(s), further comprising draining free black liquor from the horizontal tube continuous digester in to the tank.

Such method(s), further comprising passing the black liquor stored in the tank to the pressurized storage vessel for mixing with the free black liquor contained in the pressurized storage vessel.

Such method(s), further comprising recovering heat from the black liquor by passing the black liquor from the tank through a heat recovery system.

Such method(s), wherein the black liquor sent to the heat recovery system is under elevated temperature and pressure.

Such method(s), wherein the heat recovery system is selected from the group of flash cyclone, heat exchanger or combinations thereof.

Such method(s), wherein the heat recovery system is used for heating cold cooking liquor. FURTHER SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW

A system for extracting black liquor from a horizontal tube continuous digester comprising a screw press capable of integration with an outlet of the horizontal tube continuous digester for receiving a mixture of pulp and black liquor from the horizontal tube continuous digester, the screw press configured to separate the black liquor, including interstitially bound black liquor, from the pulp, and a tank in fluid communication with an outlet of the screw press and configured to receive the black liquor from the screw press, wherein the screw press and the tank are at substantially the same pressure as that of the horizontal tube continuous digester.

Such system(s), wherein the horizontal tube continuous digester comprises of at least one perforated draining plate positioned proximate the bottom of the horizontal tube continuous digester for extraction of free black liquor.

Such system(s), further comprising a heat recovery system for recovery of heat from the black liquor in the tank.

Such system(s), wherein the heat recovery system is selected from the group of flash cyclone, heat exchanger or combinations thereof.

INDUSTRIAL APPLICABILITY

The method for extraction of black liquor from a horizontal tube continuous digester described above is easy to perform, energy-efficient and eco-friendly. The system and method for extraction of black liquor described herein can be used with variety of raw materials preferably agro residues. Particularly, the use of the system for extraction of black liquor makes the overall process of pulping economical, as overall heat and steam demand is reduced by 50-70% as compared to conventional methods of pulping and heat recovery.