Method and arrangement for generating process steam

Abstract

A method and an arrangement for generating process steam at a chemical pulp mill. Water is heated by subjecting it to an indirect heat exchange contact with steam in a heat exchanger. The water is heated with live steam produced in a steam boiler for generating process steam, whereby the live steam is condensed and the generated condensate is recovered. The process steam is subjected to a direct heat exchange contact with a material for heating the material. The water used for process steam production is obtained from secondary condensates, purified waste water and/or raw water. Process steam can be used in the treatment of cellulosic fibrous material, such as chips.

Claims

1. A method of producing process steam at a chemical pulp mill, the method comprising: heating water in a heat exchanger in which the water is subjected to indirect heat exchange contact with live steam produced in a steam boiler, wherein the water is free of volatile compounds and the heated water is output from the heat exchanger as process steam, condensing the live steam in the heat exchanger and recovering condensate resulting from the condensing of the live steam, and heating a material by direct heat exchange contact between the material and the process steam output from the heat exchanger.

2. The method according to claim 1, wherein the water to be heated in the heat exchanger to produce the process steam is at least one of: purified waste water and raw water.

3. The method according to claim 1, further comprising preheating the water to be heated to produce the process steam with a hot liquid stream before the step of heating the water.

4. The method according to claim 1, further comprising separating the water to be heated to produce the process steam from the process steam in a separation tank, wherein an upper part of the separation tank includes a steam space and a lower part of the separation tank includes a liquid space.

5. The method of claim 4, wherein the process steam is discharged from the separation tank and the step of heating the material includes direct heat exchange contact between the process steam and chips in a chip bin.

6. The method according to claim 1, wherein the step of heating a material includes using the process steam to directly heat cellulosic fibrous material.

7. The method of claim 1, wherein the step of heating the material includes direct heat exchange contact between the process steam and chips in a chip bin.

8. A method of producing process steam at a chemical pulp mill, the method comprising: heating water in a heat exchanger in which the water is subjected to indirect heat exchange contact with live steam produced in a steam boiler, and outputting the heated water from the heat exchanger as process steam, condensing the live steam in the heat exchanger and recovering condensate resulting from the condensing of the live steam, heating a material by direct heat exchange contact between the material and the process steam output from the heat exchanger, and preheating the water to be heated to produce the process steam with black liquor discharged from a digester or flash steam from the black liquor.

9. The method according to claim 8, wherein the water is fed into the heat exchanger from a separation tank, whereby the process steam flows from the heat exchanger to the separation tank and the water flows from the separation tank to the heat exchanger.

10. A method to produce process steam in a chemical pulp mill, the method comprising: feeding live steam generated in a boiler into a heat exchanger, condensing the steam in the heat exchanger, and outputting condensate of the steam from the heat exchanger; feeding water free of volatile compounds into the heat exchanger, heating the water in the heat exchanger to produce process steam using heat energy extracted from the steam condensed in the heat exchanger; outputting the process steam from the heat exchanger into a conduit which directs the process steam into a separation tank in which a portion of the process steam condenses into water; outputting the process steam from the separation tank into a conduit which directs the process steam to directly heat a process material being processed in the chemical pulp mill; and outputting from the separation tank the water condensed in the separation tank as the water fed into the heat exchanger.

11. The method of claim 10 further comprising circulating the water and process steam between the heat exchanger and the separation tank.

12. The method of claim 10 further comprising adding additional water to the water being fed into the heat exchanger, wherein the additional water is sourced from at least one of a secondary condensate, raw water and purified waste water.

13. The method of claim 12 further comprising heating the additional water with heat extracted from black liquor output from a digester in the chemical pulp mill.

14. The method of claim 13 wherein the heating of the additional water is in a second heat exchanger which receives the black liquor and separates the black liquor from the additional water.

15. The method of claim 10 further comprising regulating a flow of the steam fed into the heat exchanger based on a desired pressure and/or amount of the process steam outputted from the separation tank.

16. The method of claim 10 wherein the heat exchanger separates the live steam fed into the heat exchanger from the water and process steam flowing through the heat exchanger.

17. The method of claim 10 wherein the live steam fed to heat exchanger is supplied from a steam boiler and the condensate of the steam condensed in the heat exchanger flows into the steam boiler.

18. A method to produce process steam in a chemical pulp mill, the method comprising: feeding steam produced in a boiler into a heat exchanger, condensing the steam in the heat exchanger, and outputting condensate of the steam from the heat exchanger; feeding water into the heat exchanger, heating the water in the heat exchanger to produce process steam using heat energy extracted from the live steam condensed in the heat exchanger; outputting the process steam from the heat exchanger into a conduit which directs the process steam into a separation tank in which a portion of the process steam condenses in water; outputting the process steam from the separation tank into a conduit which directs the process steam for use in the chemical pulp mill, wherein the process steam output from the separation tank is applied to heat chips in a chip bin, and outputting from the separation tank the water condensed in the separation tank for use as the water fed into the heat exchanger.

Description

(1) The present method and apparatus are described in more detail with reference to the appended FIGURE.

(2) FIG. 1 illustrates schematically a preferred arrangement according to the invention.

(3) Process steam is produced in an indirect heat exchanger E-1, in which low pressure steam or intermediate pressure steam introduced from a steam boiler and water are subjected to an indirect heat exchange contact. Live steam is introduced via line 2 into the heat exchanger. The heat exchanger can be a plate heat exchanger, e.g. of the rising film type. The water to be heated is introduced via line 4. In the heat exchanger the heating steam heats the water for producing process steam, which is discharged via line 5.

(4) Clean condensate generated from live steam in the heat exchanger E-1 is led via line 3 into the clean condensate recirculation system of the mill and further into the feed water tank of the boiler.

(5) From the heat exchanger E-1 the process steam is led further via line 5 into a separation tank E-3 having two phase zones, both in liquid and in steam state. In the steam space of the tank, water droplets are separated from the steam stream, which droplets settle into the liquid space in the lower part of the tank. The process steam is led into further use via an outlet conduit 6 in the upper part of the tank E-3. The upper part of the tank is typically provided with a droplet separation device (not shown) for intensified water separation. In the outlet conduit the steam flow is regulated with a valve 13, which is controlled according to the temperature of the usage point, e.g. a chip bin. The flow of live steam is regulated with a regulation valve 14 for providing an adequate amount and pressure for the produced process steam in the separation tank. The required pressure and amount are dependent on the usage point of the process steam.

(6) The lower part of the separation tank E-3 is provided with a liquid space, from where water is led into the heat exchanger E-1 via line 4 for producing steam. Between the separation tank E-3 and the heat exchanger E-1 is arranged a liquid circulation formed of lines 4 and 5, since water is returned with the steam into the separation tank.

(7) The liquid level in the separation tank is controlled by introducing feed water via line 7 into the tank or into line 4. The liquid level regulation comprises a regulation valve 9 for controlling the water flow and thus for maintaining a suitable liquid level in the separation tank.

(8) The feed water of line 7 is heated in a preheater heat exchanger E-2 with a suitable hot process stream from line 10. The preheater can be e.g. a plate heat exchanger. The volume of hot process stream is regulated with a valve 11, which control is based on temperature measurement in the preheated water line 7. Advantageously this kind of process stream is a black liquor stream discharged directly from the digester, which stream is via line 12 led from the preheater E-2 into the evaporation plant. Upstream of the preheater, heat can be recovered from the black liquor in the heat recovery system of the digester, such as in flashing vessels or in hot accumulators.

(9) The water being fed into the preheater via line 8 can typically comprise condensate, raw water, purified waste water or other adequately clean waste water fraction from the mill. Thus, the steam produced in the heat exchanger E-1 contains a substantially smaller amount of non-condensable gases than steam produced by direct flashing of black liquor, which is also used as heating steam.

(10) The separation tank E-3 can be provided with a blow down line 15, via which water can be discharged for preventing the accumulation of salts and other undesired substances in the water circulation between the separation tank and the heat exchanger.

(11) Advantages provided by the invention: live steam condensate can be recovered, water consumption at the mill decreases, when clean live steam condensate is returned to be used as boiler water, production of demineralized water decreases, and waste water amount decreases.