Flash tank steam economy improvement

Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With heat recovery

Reexamination Certificate

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C162S239000, C162S240000

Reexamination Certificate




In the pulp and paper art it is highly desirable to improve the steam economy of the flash tanks utilized (which flash tanks are, for example, shown per se in U.S. Pat. Nos. 5,172,867, 4,551,198, and 5,700,355, the disclosures of which are hereby incorporated by reference herein). It may be possible to get such improved economy by decreasing chip bin or flash tank pressure to be able to get more flash steam from the flash tank. [Though the term “flash tank” is used throughout this discussion and is a term of the art, it is to be understood by those familiar with the art that this term includes any apparatus in which a hot pressurized liquid is exposed to a lower pressure and allowed to evaporate, typically violently, in an enclosed container to produce a source of steam and liquid at a lower temperature and pressure.] Black liquor could be directly flashed for example to a temperature 90° C. instead of 107° C. Then there would not be need for additional cooling of black liquor and also evaporation loading would slightly decrease. For example, the sub-atmospheric pressure in a flash tank could be maintained by a vacuum pump, such as shown in patent publication WO 97/29236.
However, there is a better solution. One can get more flashed steam from a flash tank by using a steam jet ejector. For example in some continuous digesters it is normal to use much low pressure (LP) steam for steaming even in the summer time. By using some LP steam in a steam jet ejector one can produce more flashed steam and the total LP steam consumption would decrease. A steam jet ejector is a very simple device without any moving parts. The lower LP steam consumption utilizing a steam ejector may give hundreds of thousands of dollars of savings per year for continuous digesters. The investment cost of the ejector should be less than 20% of that and there are essentially no additional operating costs.
There are also some other ways to use a steam jet ejector to improve energy efficiency in the digester area. For example LP steam pressure could be increased by medium pressure (MP) steam to be able to use the LP steam in digester heaters or a digester steam phase.
A steam jet ejector (as seen in
FIG. 1
) is a venturi jet device that uses the energy available in steam to either a) create a vacuum, b) boost the pressure of a gas, or c) a combination of a) and b). Single stage ejectors can be used to create vacuum levels of about 75 torr (1.5 psia), when discharging to atmosphere. Steam ejectors per se in the pulp and paper art are disclosed in U.S. Pat. Nos. 5,139,620 and 4,692,214. In co-pending application 09/195,444 filed on Nov. 18, 1998 [attorney. ref. 10-1268] a jet ejector is used to increase the efficiency of a spent cooking chemical heat recovery system having reboilers.
Black liquor is normally flashed in one or several stages against atmospheric pressure. Flashed steam is typically used to heat and expel air from the chips arriving to the process. This can be done for example in an atmospheric steaming vessel such as a DIAMONDBACK® chip bin (available from Ahistrom Machinery Inc. of Glens Falls, N.Y.), or in a conventional pressurized steaming vessel. Typically flashed steam is not enough to completely steam the chips and fresh low pressure steam is needed to complete steaming. Due to some friction losses and increased boiling point, black liquor temperature is typically about 107° C. after flashing. Before sending the flashed black liquor to an evaporation plant, the black liquor is typically cooled in a heat exchanger by water to temperature of about 90° C.
By using a conventional steam jet ejector (see
FIG. 1
herein) to enhance the use of flash steam in an atmospheric steaming vessel, the flash tank pressure could be decreased to a pressure of about 0.5-1.1 bar absolute (abs.) pressure, preferably about 0.7-1.0 bar abs., and the black liquor would then be flashed to a temperature of about 80-102° C., preferably about 90-100° C. This way the total amount of usable flashed steam would increase and the use of valuable fresh steam could be decreased. The “high pressure” steam utilized as a motive fluid of the steam jet ejector could be low pressure fresh steam, or flashed steam at a higher pressure from one or more previous flashing stages, or high pressure fresh steam. Additional benefits are that there is no longer a need to cool the black liquor going to the evaporation plant and the amount of black liquor to be evaporated would be slightly lower.
A steam jet ejector could also be used in other environments in a Kraft (or other chemical pulping) cooking plant:
Flashed steam pressure could be increased to be able to use it in a pressurized steaming vessel.
Flash tank pressure could be decreased to get more steam to a flashed steam condenser to produce more hot water.
Flashed or fresh low pressure steam pressure could be increased by higher pressure steam to be able to use it in the liquor heaters of a continuous digester.
Flashed or fresh low pressure steam pressure could be increased by higher pressure steam to be able to use it in the steam phase of a continuous digester.
Low pressure steam pressure could be increased by higher pressure steam to be able to use it in the direct or indirect liquor heaters of a batch digester.
The broadest embodiment of the invention comprises a method of treating hot spent cooking liquor, having a first pressure and a first temperature, using a flash tank, having a high-pressure liquid inlet, a low-pressure liquid outlet, and a steam outlet; and an ejector, having a high-pressure gas inlet, a low-pressure gas inlet, and a gas discharge, to recover energy from the liquor, consisting of or comprising: (a) introducing the hot spent cooking liquor at the first pressure into the high-pressure liquid inlet of the flash tank; (b) exposing the liquor in the flash tank to a second pressure, lower than the first pressure, so that at least some of the liquor evaporates to form steam and a cooler liquid at about the second pressure and at about a second temperature, lower than the first temperature; (c) removing at least some of the steam from the flash tank in a first gaseous stream; (d) introducing the first gaseous stream to the low-pressure inlet of the ejector; (e) introducing a second gaseous stream having a third pressure, greater than the second pressure, to the high-pressure inlet of the ejector; and (f) discharging a third gaseous stream at a fourth pressure, higher than the second pressure, from the discharge outlet of the ejector; and wherein (a)-(f) are practiced so that the second pressure in the flash tank is lower (e.g. by at least about 0.1 bar abs) than the pressure that would be present in the flash tank without the presence of the ejector under otherwise substantially identical conditions. The present invention also includes practicing (a)-(f) so that the second pressure and the second temperature are lower than the pressure and temperature would be in the prior art without the presence of the ejector under otherwise substantially identical conditions.
The method as recited above may further comprise (h) discharging concentrated hot spent cooking liquor from the flash tank at a temperature at least 2° C. lower than would be present without the utilization of the ejector under otherwise substantially identical conditions.
The hot liquor is preferably hot spent extraction liquor removed from a kraft pulping process, for example, a continuous or batch pulping process. The liquor typically has a temperature of between about 100° C. to 180° C., preferably between about 110° and 160° C., that is, about the temperature of the pulping process, and a pressure ranging from about 5 to 15 bar gage (that is, 6 to 16 bar abs.)
The present invention may also further include, prior to (a), (g) cooling the liquor from the first temperature to a third temperature, lower than the first temperature (e.g. by at least about 5° C.). The cooling process (g) is preferably practiced by passing the hot liquor in heat ex


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