US 3661778 A
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May 9, 1972 L. A. PRADT 73 WE'I'T AIR )KKDIZATION SYSTEM FOR STRONG SLUDGES AND LIQUORS Filed Jan. 18, 1971 GASES TO ENERGY RECOVERY SEPA RATOR OXIDIZED LIQUOR TO POST TREATMENT FLASH TANK 36 26 REACTOR 32 sTEAM FROM BOILER 34 RAw LIQUOR 1O LL n. 5
HEAT EXCHANGER CONDENSATE I F I G 2 FLASH STEAM OUTLET ENTRAINMENT QVALL VE 1 STEAM INLET 40 LIQUIO I L .Ui LL 26 I LIQUID I REsERvOIR I INVENTOR LOUIS A. PRADT BY GM/F LIQUID OUTLET I ATTO NEY United States Patent Oflice 3,661,778 Patented May 9, 1972 3 Claims ABSTRACT OF THE DISCLOSURE Raw liquor is introduced to a heat exchanger and thence through a temperature control valve to a reactor and thence to a separator, and a flash tank is provided which receives oxidized liquor from the separator at an intermediate pressure resulting in flash steam used for preheating in the heat exchanger.
BACKGROUND OF THE INVENTION Wet oxidization systems designed to accomplish essentially complete oxidation of strong liquors and sludges containing a chemical oxygen demand of about 80 grams per liter or more are well known. In such a system all the necessary preheating can be accomplished by extracting heat from the oxidized liquor. Thus, the gases and steam can be used entirely for energy recovery. Such a system Will generally operate at pressures between 50 and 250 atmospheres, the preferred range being 150 to 200 atmospheres, and at maximum temperatures of about 250 to 375 centigrade, the preferred range being 300 to 320 centigrade. This type of system has been publicized as for instance in Appita, vol. 22, No. 3, the October 1969 issue of Pulp and Paper International; and attention is also directed to U.S. Pat. 2,824,058.
Disadvantages of this system are as follows:
(1) Suspended solids or scale forming compounds in the oxidized liquor can cause fouling and plugging of the shell of the process heat exchanger.
(2) The shell of the process heat exchanger must be designed for high pressure and high temperature and for maximum corrosion resistance. This often requires the use of expensive materials, such as nickel or titanium.
(3) The system has inherent control problems. For example, any change in a system variable which causes an increase in the reactor outlet temperature will raise the temperature of the oxidized liquor going to the process heat exchanger. This in turn will cause an increase in the temperature of the feed liquor to the reactor, causing a further increase in the reactor outlet temperature. Temperature control of the system is thus subject to inst-abilities.
(4) An auxiliary heat exchanger for start-up is required in addition to the process heat exchanger.
It is the general object of the present invention to provide a wet oxidation system for strong liquors and sludges operating substantially at the same or similar conditions as above outlined but in which the disadvantages enumerated and in which the following advantages are provided:
1) The shell side of the heat exchanger can be designed and built for lower pressures and of less expensive materials since corrosion difliculties are minimized.
(2) There is no fouling or scaling in the shell side of the heat exchanger.
(3) Reactor inlet temperature control is simplified.
(4) The need for a separate steam start-up heat exchanger is eliminated.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow diagram illustrating the invention; and
FIG. 2 is a diagrammatic showing of a form of flash tank that can be used in the system.
PREFERRED EMBODIMENT OF THE INVENTION As shown in the diagram, raw liquor and atmospheric air enter the system at 10 proceeding at 12 to the heat exchanger 14 from which condensate may be derived as shown. From the heat exchanger the preheated liquor or sludge proceeds to the reactor which has an inlet 16.
After passing through the reactor and through line 18 into separator 20, the gases are emitted as at 22 as before for energy recovery but the oxidized liquor which exits from separator 20 through bottom valve 24 to a flash tank 26, see FIG. 2, is reduced to an intermediate pressure in the approximate range of 10 to 20 atmospheres.
This evolves flash steam as a result of the pressure reduction, and this steam is used for preheating in the heat exchanger, passing from the top of the flash tank, through the valve at 28, to the heat exchanger 14. Additionally, steam from the boiler may be introduced into line 30 from the flash tank 26 to valve 28, as at 32. The valve 28 is controlled by a temperature control valve 34 controlling steam flow to the shell side of the heat ex changer.
The intermediate pressure in the flash tank 26 depends upon the required reactor inlet temperature. For example, if the required reactor inlet temperature is centigrade, the flash steam temperature must be higher than this or about 200 centigrade, fixing the intermediate pressure and flash tank 26 at about 15 atmospheres.
The reactor inlet temperature at 16 is easily controlled by the temperature control valve 34 which controls steam flow to the shell side of the heat exchanger 14. If the flash steam evolved is insufficient this can be made up by supplying steam from the boiler. If excess flash steam is evolved, this excess can be directed to the steam mains for general plant use.
The quantity of flash steam can be calculated in the usual way considering the initial temperature of the oxidized liquid and the final pressure to which this liquid is reduced or flashed. For practical purposes the oxidized liquid can generally be assumed to be water, although large quantities of dissolved salts may have some effect on the temperature-pressure relationship of the vapor of this liquid. The flash tank is in essence a device to separate the flash steam from the remaining liquid after pressure reduction. Design of such a tank is generally known in the art. This tank could be designed, for example, to give a liquid hold up time of 3 minutes, an upward flash steam velocity of 3-4 feet per second and could be equipped with a mesh type or baffle type of entrainment removal device in the top. The tank would also be equipped with a device to control the level of the liquid in the tank. This is a conventional device indicated at 40, and connecting the interior of the tank to the outlet, as shown in FIG. 2 in dotted lines.
While liquor and air can be mixed before admission to the heat exchanger as shown and described, liquor only can be passed through the heat exchanger and air admitted after the heat exchanger if this should become desirable.
Oxidized liquor from the flash tank can be sent directly to post treatment as at 36, 38 as shown, or can be used for additional preheating of raw liquor if desired and convenient. The oxidized liquor can be flashed to an even lower temperature and the resultant flash steam evolved used for heating instead of the oxidized liquor.
It is believed that it will be clear to those skilled in the art that the advantages of the invention has been accomplished and that the shell side of heat exchanger can be designed and built for lower pressures of less expensive material because corrosion difficulties are greatly minimized; and there will be no fouling or scaling in the shell side of the heat exchanger. Also the reactor inlet temperature control is greatly simplified and made more positive and the need for a separate steam start-up heat exchanger is eliminated.
I claim: 7
1. In a process for the wet oxidation of raw strong liquors and sludge'the steps which include introducing the raw liquor to a heat exchanger and preheating the raw liquor, introducing the preheated raw liquor to a reactor and oxidizing the raw liquor, and conducting the oxidized liquor thence to a gas separator, introducing the oxidized liquor from the separator to a flash tank, generating steam thereby, and passing the steam through a temperature control valve back to the heat exchanger while leaving the oxidized liquor from the flash tank for post treatment separately.
2. The process of claim 1 wherein the generated steam is at a higher flash steam temperature than the required reactor inlet temperature.
3. The process of claim 1 wherein the generated steam is at a higher flash steam temperature than the required reactor inlet temperature, being obtained by varying the pressure in the flash tank.
References Cited UNITED STATES PATENTS