FI60072B - FOERFARANDE OCH ANORDNING FOER FOERBAETTRING AV VAERMEEKONOMIN VID TORKNING AV CELLULOSAMASSA - Google Patents

Description

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fc) r, y 1'tliy 13 11 l: n ^ T ^ (51) K ».ik.3 / int.ci.3 F 26 B 17/10 FINLAND - FINLAND (21) P» t «nttlh * k * mu «- Patwitameknlnf 750793 (22) HtkamltpUvt —Ara0knl« i | * dag 18.03.75 '* (23) Initial - Glhl (h «t * dtf 18.03-75 (41) Tullut (ulklMluI - Bllvlt off.mll * 23 -09 to 75

Patent and Registration Office .... .. , . . . . . (44) Date of issue and date of issue. - Q,

Patent- och registerstyrelsen AntMun ucl. * D och utl.skrifc.n pubiicurad 31-07.81 (32) (33) (31) Pyydetty * Tuoik * us —- Be |

Sweden-Sweden (SE) 7 ^ 03903-3 (71) MoDo-Chemetics AB, Box 313, 3-891 01 Ömsköldsvik, Sweden-Sweden (SE) (72) Bengt Olof Arvid Hedström, Gothenburg, Sweden-Sweden (SE) (7 ^) Ruska & Co (5M) The present invention relates to a method and an apparatus for drying cellulose pulp to dry in a loose form, so-called flakes are fed against a higher pressure to a pressurized drying unit, to which a carrier gas is possibly introduced in the form of possibly superheated steam under pressure and the flakes and steam are passed through the drying unit, after which the flakes and steam are separated and separated dried product or a further drying step is carried out a.

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The production of chemical pulp and paper consumes large amounts of energy. In addition to electrical energy, large amounts of thermal energy are required, usually in the form of steam for cooking, evaporation, drying, etc., and possibly also for bleaching. In each of these sub-methods, it has been possible to gradually improve the thermal economy, but no radical co-operation in the true sense of the different units of energy consumption has been achieved.

In addition, in certain cases, energy consumption has increased as one technology has been transitioned, for example from web drying of pulp to flake drying. Although the pulp is usually mechanically dried to a higher dry matter content prior to the usual flake drying of the pulp compared to conventional web drying of the pulp, the thermal economy is a disadvantage of flake drying. In conventional flake drying, in which the gases from oil combustion are usually used entirely or as the largest branch at atmospheric pressure as an energy source, the heat consumption is considerably higher, for example about 3.8 MJ / kg evaporated water compared to about 2.9 MJ / kg evaporated water in web drying. One important fundamental difference between the two drying systems, when arranged in the usual way, is that there is no possibility of producing a back pressure force (electric force) in oil-heated flake drying. Conventional flake drying uses relatively large amounts of air, which in large quantities contributes to the deteriorating thermal economy. In flake drying, it is in principle conceivable to simulate web drying operations in order to achieve better thermal economy, so that a number of drying steps consisting of a steam-heated heat exchanger plus moist air and stock separators are arranged in a suitable manner. It is also possible to arrange these steps in such a way that the air is in principle directed countercurrent to the stock. Further improvements in flake drying have recently been proposed and explained in Swedish Patent Publication 341,909. In this system, the heating surface extends over the entire length of the drying equipment. In this case, it is not necessary to separate the moist air and the stock between the different drying steps. In this case, in each drying step, the mixture of air and stock is heated indirectly by means of steam-heated heating surfaces. This makes it possible to maintain a relatively large average temperature difference between the dry heating surface and the air used as the carrier gas for the wet stock.

It has also been proposed in the past to use a more or less commonly used method of improving temperature economy in conventional drying systems that operate substantially at atmospheric pressure and with air as the drying medium. A well-known system for drying peat (Bauart VEB, Kneule, Das Trocknen, Aarau, Switzerland, 1959, page 290) utilizes, for example, the heat of moist air obtained from steam-heated drying steps to produce hot water. adapted, i.e. operating at atmospheric pressure with air as the drying medium.The thermal economy is reported to be improved to about 1.75 MJ / kg evaporated water.

5 60072 In a heat pump drying system (Kneule cited above, page 291), which likewise operates under normal conditions, i.e. essentially at atmospheric pressure and with air as the drying medium, comp. the more or less moist drying air expands, transferring heat to a work step which, in principle, mimics the usual work step in steam processes, for example heat pump evaporation.

It has now unexpectedly proved possible, using other principles, to achieve further improved energy economy together with improved convective heat transfer, in so-called fluff drying in a pulp mill, for example a partially combined mill where the pulp part is further processed into paper. With respect to flake drying of pulp (cellulose pulp), no known system (including those disclosed herein) takes advantage of the combined potential for improved heat and plant economy provided by this invention. According to the invention, the improved thermal economy is achieved in that the carrier gas pressure is at least 0.12 MPa and that the drying unit is indirectly heated by water vapor, the saturation pressure of which exceeds the carrier gas pressure. The supply against a higher pressure is suitably carried out via a valve. The pressure is suitably at least 0.2-0.4 MPa due to the pressure in a conventional low pressure network, but higher pressures can also be used. A portion of the separated steam is suitably returned to the drying unit (s), optionally after superheating, for use as a carrier gas. The remaining part of the separated steam, the vapor pressure of which has been suitably increased to, for example, 0.2 to 0.5 MPa, is preferably used as secondary steam, for example for drying, evaporating, bleaching or the like paper. The fan is suitably arranged in a drying unit which is formed as a pressure vessel and into which flakes and steam are fed. The fan is preferably adapted to the flake supply point and the steam conduction point.

The cellulosic pulp to be treated according to the present invention may be, for example, mechanical, thermomechanical, quaternary, semi-chemical or chemical pulp. Above all, the pulp or stock is a chemical pulp, such as sulfate cellulose pulp.

The device according to the present invention comprises one or more drying units arranged in series with indirect heating and means for feeding and discharging fibrous material in the form of flakes 4,60072. The device is characterized in that the drying unit (s) is (are) formed as a pressure vessel and and the outlet devices for the fibrous material are formed in a manner known per se against a continuous or intermittent supply, against a higher pressure, for a lower pressure, e.g. via a valve, the possibly superheated steam being used as a carrier gas and the fan being adapted to convey the steam and fibrous material through the (units).

Examples of inlet and outlet devices that can be used are screws and star valves.

For example, the device according to the invention can in principle be provided with drying units such as those disclosed in Swedish patents 300190 and 341909. However, the drying devices must be modified so that they can operate under moderate internal pressure, for example at a pressure of up to 0.4-0.7 MPa. Devices for introducing a fibrous substance from atmospheric pressure to higher pressure are known per se. The same, of course, also applies to the devices for discharging the fibrous material from the drying unit. The fibrous material and the steam used as carrier gas must be separated before being discharged. This separation takes place in a device known per se, for example a cyclone. Of course, devices are also provided for introducing any superheated steam and for removing the steam separated from the fibrous material.

In the present device, the drying units are heated indirectly. This is done by means of pipes suitably arranged in the drying units, through which the steam flows. By transferring heat through the wall of the pipes to the mixture of steam and stock, the temperature of the steam in the pipes decreases and usually the steam condenses on them. The saturation pressure of this heating steam can be, for example, 0.2 to 0.3 MPa up to 1.2 MPa and even higher. Steam pipes adapted for heating may be arranged and formed as disclosed in Swedish Patents 300190 and 341909. According to another suitable embodiment of the invention, the stock is passed through the pipes from the lower end to the upper end, the pipes being surrounded by steam to heat the stock and the carrier gas in the pipes. The tubes can be fitted with a pre-filter with a triangular distribution. This drying unit is then connected to the next blower via a bend as shown.

According to another suitable embodiment, the device is arranged like a double heat exchanger, i.e. in three concentric tubes, so that the steam passes in the central tube and in the outer annular space and the pulp mixture in the inner annular space, which is thus surrounded by the heating steam.

As shown and as will be explained in more detail below, the present invention achieves very good thermal economy, i.e., operating costs remain low, but in addition, the present invention also provides improved convective heat transfer, i.e., increased heat transfer coefficient (increased values).

The heat transfer coefficient in the device according to Swedish patent publication 341909 with air as the carrier gas can be calculated as

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63 W / m. K. If steam were the carrier gas, the heat transfer coefficient p can be calculated to be 70 W / m. K. In the present method, this value is 190 W / m. K. at a pressure of 0.4 MPa. (a prerequisite in all calculations has been a gas velocity of 27 m / s with a hydraulic diameter of 0.176 m). A prerequisite for this method to be implemented and for the surprisingly large improvement in convective heat transfer presented here is that the impregnation pressure of the steam used for indirect heating of the drying units exceeds the pressure of the possibly superheated water vapor acting as a carrier gas. It has previously been mentioned here that the pressure in the drying unit is at least 0.12 MPa and preferably at least 0.2 MPa. This means that the pressure of the carrier gas, i.e. possibly superheated water vapor, is also at least 0.12 * MPa. As previously mentioned, the lowest saturation value of the steam used for indirect heating is 0.2-0.3 MPa and the saturation pressure can always be increased to 1.2 MPa and above. The fact that the carrier gas pressure in the process according to the invention is kept considerably above atmospheric pressure, i.e. at a pressure of at least 0.12 MPa, preferably at least 0.2 MPa, provides said large increase in heat transfer coefficient, which in turn leads to surprisingly high gains in hardware and operating costs. Thanks to the improved convective heat transfer, the required heating surfaces can be made smaller. Thus, the calculation shows that the total heat required in the drying system according to the present invention becomes approximately 6,60072 less than what is required in the imaging system according to Swedish Patent Publication 3 * 11909. Thus, the present imaging system as a whole becomes smaller than known drying systems having the same drying capacity. As a result, the cost of capital also becomes lower, although the drying units must be made into pressure vessels and the auxiliary equipment must also be made to operate under pressure. No known system for drying cellulose pulp operates under overpressure.

The invention will be explained in more detail below with reference to the accompanying drawings, in which Figures 1 and 2 show diagrammatically and by way of example various embodiments of the devices according to the invention.

in a leaf mill, a portion of the pulp, for example about 60%, is made into paper and the rest is dried for sale, the pulp being dried in the apparatus of Figure 1. This figure shows two drying tower-shaped drying units connected in series. These are equipped with conveying fans 1 with directly connected drive motors 3 · A conical supply pipe 5 is connected to the suction side of the fans. , is provided on the inside with steam pipes 11 for indirect heating. The upper ends of the drying towers are connected to the pipe elbows 13 and connected to the transport pipes 1? By conical connection pieces 15. The conveying pipe 17 is again connected as cones to the supply pipe 5 of the next drying unit. shown is a pipe elbow 35, a cyclone 43 with a pulp outlet valve 44 and a pulp outlet pipe 46, and a secondary steam line 45.

According to the invention, the pulp is now fed in flakes through the shut-off valve 41 to the suction side of the conveying fan 1 at the same time as superheated steam with a temperature of, for example, 144 ° C is fed via line 42. The conveying fan 1 then conveys the mixture of steam and stock through the conical body 9 and the conveying pipe 7. The steam pipes 11 arranged inside the transport pipe 7 are heated with steam,

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7 ’60072 with a pressure of 1MPa. Due to the temperature difference between the mixture of steam and stock and the outer surface of the steam pipes 11, heat is transferred from the steam pipes to the mixture of steam and pulp. The heat given off by the steam pipes 11 is transferred to the mixture of steam and stock and, above all, to steam, which quickly absorbs heat. There is a continuous heat transfer from the steam to the stock. Since the stock cannot absorb heat as fast as the steam, a temperature difference is created between the steam and the stock as they pass through the transport tube 7. After passing through the transport pipe 7, the steam and stock mixture is directed to the pipe generation 13 and passes through a temperature equalizing section of the drying unit comprising a pipe elbow 13, conical bodies 15 and 5, transport pipe 17 and transport fan 1. In this temperature equalizing section the stock and steam temperature difference decreases. The treatment of this mixture is then repeated in a series of drying units, after which the mixture of steam and stock is discharged from the apparatus through an outlet pipe 35 and passed to a cyclone 43 to separate the pre-dried stock from steam. The stock is then passed through a shut-off valve 44 and steam at a pressure of 0.4 MPa to a secondary steam line 45. A portion of the steam is passed from the secondary steam line 45 for superheating through a heat exchanger to the steam supply pipe 42 in the first drying unit. through the drying tower to the secondary steam line 45 and back to the supply pipe 42 is substantially unchanged. The amount of steam obtained from the drying of the moist stock can thus be used substantially entirely for other purposes, for example for drying, evaporation or bleaching. In this case, the obtained steam is used for drying in a paper machine.

Figure 2 shows another embodiment of the device according to the present invention. It can be used analogously to the device of Figure 1 to implement the present method. The device shown in dry 2 comprises two drying tower-shaped drying units connected in series. The transport fans 1 with their immediately connected drive motors 3 are connected to the lower part of the drying towers. The suction side of the fans 1 is connected to the supply pipes 5. Above the outlets of the transport fans 1, the actual drying towers 7 are arranged via conical connections 9. The cross-section of the drying towers can be anything. The drying tower Ta is shown here as a straight tube. In contrast, the drying tower 7b is shown with a bend. It is suitable that several 8,60072 waves or bends have been made in the drying towers, because thus the relative velocity between the stock to be dried and the carrier gas increases, which causes an increase in the heat transfer coefficient. Drying towers of this type are known per se. In addition, each drying unit is connected to a cyclone 43 to separate the stock and the carrier gas. The carrier gas in the form of superheated steam is led to the fans 1 via pipelines 50. The carrier gas separated in the cyclones 43, formed by the substantially saturated steam, is collected in the secondary steam line 45. The secondary steam line may be a common secondary steam line throughout the plant. From this secondary steam line, steam is discharged along line 51 to heat exchanger 52, where the steam is superheated and passed as a carrier gas to the drying units via lines 50. The superheating of the steam in the heat exchanger 52 takes place at a higher pressure, for example by means of steam at a pressure of 1 MPa, which is passed along the line 53. Of course, depending on the circumstances, it is possible to provide a separate heat exchanger for each drying unit and to circulate the steam in the drying units and to discharge only so much into the secondary steam line 45 that the amount of steam in the drying unit remains substantially unchanged.

During the run, the wet pulp (normal dry matter content $ 40-50>) is passed as flakes to point 54 and fed through a star valve 55 against a higher pressure to the drying unit. The flakes fall and are then sucked through a pipe 5 to a blower 1, to which superheated steam is simultaneously introduced along a line 50. From the fan 1, the stock and steam are thrown through a drying tower 7a, where the heat exchange takes place between the steam and the stock, whereby the stock «dries the steam, giving off steam. From the drying tower 7, the stock is led to a cyclone 43 »where the stock and steam are separated. Heat exchange and drying can be continued during this separation. The separated steam is passed to a secondary steam line 45. The stock from cyclone 43 is then fed through a star valve 55 to a second drying unit, possibly operating at a different pressure than the first drying unit, for example at a somewhat higher pressure. The method is then repeated in the first drying unit, however, the drying tower 7b is provided with a plurality of bends 56 which increase the drying speed. The separated stock is fed to line 57. The stock is now dry (£ 90) and ready for sale.

For simplicity, only two drying units are shown in Figure 2, but it is of course possible to build a device according to the invention with a large number of steps (drying units) and usually 3-5 drying steps seem suitable.

Since this drying process produces very pure secondary steam at elevated pressure, for example about 0.4 MPa, as mentioned above, it is clear that the drying of the stock takes place in principle with very low energy losses, provided there is a suitable user for secondary steam, as is usually the case. production. As a whole, it can be seen that the steam used in the dryer for heating purposes is throttled from the process steam pressure, for example from about 1 MPa to a secondary steam pressure in the flake drying system of about 0.4 MPa. This drying method can thus be considered as a backpressure process analogous to the development of backpressure pressure and backpressure evaporation. Although both of these concepts and processes are known and previously used in the cellulose industry, for example, combined sulfite evaporation and alcohol separation, the back-pressure drying of this invention is a completely new concept.

As another example, the apparatus shown in the drawing is used to partially dry the stock. In this case, the stock was passed as shown, with a dry matter content of $ 45, and dried as shown to a dry matter content of about 87 5 *, separated in a cyclone 43, and fed out through a shut-off valve 44 and line 46. The stock was then allowed to swell to about 90%. In this case, the secondary steam obtained in line 45 was used to evaporate the lye obtained in cooking the stock.

In addition, according to one example, the apparatus shown in the drawings was used to partially dry the stock. The stock was passed as in the above examples, separated and fed off as described herein, and the secondary steam was utilized in the same manner. However, the stock was dried for only 60 sex and passed to an ordinary, oil-heated flake dryer or a flake dryer according to either Swedish patent publication 300190 or 341909.

According to another example, vertical tubes were used to transport the stock and carrier gas to the drying unit. In this case, the pipes were surrounded by heating steam and were attached to the upper and lower ends.

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From the upper end of the pipe, the stock and carrier gas were led through a bend to the next fan. According to a particular example, the pipes have an outer diameter of 129 nm, an inner diameter of 125 mm and are made of stainless steel. 20 such tubes are arranged in a 150 mm triangular section in a jacket with a diameter of 750 mm. With a pipe length of 20 ra and six such drying units (tower) in series, drying of 300 tons of pulp per day is achieved at a steam pressure of 1.0 MPa at a steam velocity of 30 ra / s in the pipes.

naturally, the drying of the stock can be divided into stages in a manner other than that shown. Thus, in principle, it is possible to carry out this process in two or more steps, adapted analogously to multi-effect evaporation. However, this form of application seems to be less relevant today.

The same result is achieved in an apparatus whose drying units consist of two concentric ones. stainless steel pipes with a wall thickness of the order of 2 mm, a smaller pipe with a diameter of 1000 mm and another pipe with a diameter of 1065 mm. The outer tube is surrounded by a jacket (pressure vessel) with an inner diameter of the order of 1080 mm. Steam is led to the outer annular space and to the central tube a mixture of stock and steam to another annular space, giving a hydraulic diameter of 2 x 65 = 130 mm. Ten such drying units with a length (height) of 15 m provide the heat exchange surface required to achieve the above result under all other conditions.

Extensive studies of the dried pulp (neutralized pine and birch sulphate pulp bleached to maximum whiteness) according to the present invention have shown that it has not been negatively affected by any quality aspect for as long a steam treatment as 4 minutes at a vapor pressure of 0.4 MPa. Surprisingly, it has also been found that the stock aggregates appear to be significantly reduced or completely eliminated. This can possibly be attributed to the presence of low-vigorous water at high temperature during a small part of the time that the stock remains in the drying system. It should also be mentioned that the residence time of the stock is also shortened compared to known methods carried out at atmospheric pressure. This has generally been considered an advantage for quality reasons.

Claims (9)

A method for drying a cellulosic pulp to achieve improved thermal economy and improved convective heat transfer in drying, wherein the cellulosic pulp in a loose form, so-called flakes, is fed under a higher pressure to a pressurized drying unit. , after which the flakes and steam are separated and the flakes are fed under reduced pressure to the finished dried product or passed to a further drying step, characterized in that the carrier gas pressure is at least 0.12 MPa and the drying unit is indirectly heated by water vapor saturation pressure. Method according to Claim 1, characterized in that the carrier gas pressure is at least 0.2 MPa. Method according to Claims 1 to 2, characterized in that the pulp flakes and the steam are passed through a plurality of drying units. Method according to Claims 1 to 3, characterized in that part of the separated steam is returned to the drying unit or drying units after possible superheating for use as a carrier gas. Method according to Claims 1 to 4, characterized in that part of the separated steam is used as secondary steam, e.g. for drying paper, at a pressure of, for example, 0.2 to 0.5 MPa. Apparatus for carrying out the method according to claim 1, comprising one or more drying units arranged in series with indirect heating and means for feeding and discharging fibrous material in the form of flakes, characterized in that the drying unit or units (1, 9, 7, 13, 15, 17; 9, 7a, 7b) are formed as a pressure vessel, that the inlet and outlet devices (41, 44, 54, 55, 57) for the fibrous material are formed per se in a manner known per se for continuous or intermittent inlet work. for discharging against a higher or lower pressure, e.g. via a valve, in which case superheated steam is used as a carrier gas, and that the fan (1) is adapted to transport the steam and fibrous material to the drying unit or units (1, 9, 7, 13, 15, 17; 9, 7a, 7b). Device according to Claim 6, characterized in that the device for separating the fibrous substance and the steam, suitably in the form of a cyclone, is arranged immediately from each drying unit (1, 9, 7, 13, 15, 17; 9, 7a, 7b) or at least in front of the fibrous material discharge device (44; 59) from the last drying unit of the series. Device according to Claim 6 or 7, characterized in that two or more drying units (1, 9, 7, 13, 15, 17; 9, 7a, 7b) are arranged in series, the lines being arranged in one step for the carrier gas and its thereafter for conveying the steam separated from the fibrous material and for use in another drying unit as a carrier gas. Device according to Claims 6, 7 or 8, characterized in that the two or more drying units are arranged in series in series, the latter preferably being arranged in a manner known per se with the air as a carrier gas, the air being optionally heated by oil. 60072 13 ✓