GB1558944A

GB1558944A - Treatment of waste

Info

Publication number: GB1558944A
Application number: GB51710/77A
Authority: GB
Original assignee: NITTETU CHEMICAL ENG Ltd; Nittetsu Kakoki KK
Current assignee: NITTETU CHEMICAL ENG Ltd; Nippon Steel Eco Tech Corp
Priority date: 1976-12-27
Filing date: 1977-12-12
Publication date: 1980-01-09
Also published as: FR2375548A1; CA1100819A; FR2375548B1; DE2757783B2; DE2757783C3; JPS5382066A; NL7714402A; JPS5510803B2; DE2757783A1

Description

(54) TREATMENT OF WASTE (71) We, NITTETU CHEMICAL ENGINEERING LIMITED, a Japanese body corporate of Asakaze No 2 Building, No 16-9 Sotokanda 1-chome, Chiyoda-ku, Tokyo, Japan do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the treatment of liquid waste and especially liquid waste, containing organic and/or inorganic compounds.

Because the combustion of a liquid waste which contains a large amount of water and a small amount of combustible components, requires a large amount of an auxiliary fuel, such liquid waste is suitable subjected to a concentration treatment prior to the combustion to save the consumption of the auxiliary fuel. It is advantageous to utilize the heat produced by the combustion of the concentrated liquid waste for the concentration of the liquid waste. Thus, we disclose in our British Patent Specification No 1321387 and in its United States equivalent (U.S. Patent 3912577) a process wherein a concentrate of a liquid waste is subjected to a submerged combustion and a moisture saturated exhaust combustion gas evolved from the submerged combustion zone is introduced into a heat exchanger to utilise the heat contained in the moisture laden combustion gas by indirect heat exchange with the liquid waste, thereby producing the concentrate of the liquid waste. In this process, the heat utilized in the heat exchanger is mainly the latent heat of the steam in the waste combustion gas. To utilize additionally the sensible heat including that of non-condensible gases in the exhaust combustion gas (such as N2, CO2, 02) it is necessary to increase the area of the heat transfer surface of the heat exchanger. Thus, though the process described in the British and U.S. patents is effective in utilizing the heat of the exhaust combustion gas, it requires a large sized heat exchanger and thus, involves substantial construction costs. In order for the exhaust combustion gas satisfactorily to flow through the gas-liquid indirect heat exchanger, which inevitably has a complicated structure, a blower providing substantial out-put pressure is needed, involving additional energy consumption.

In accordance with a first aspect of the present invention we provide a process for the treatment of a liquid waste, comprising: heating the liquid waste by indirect heat exchange with a hot recycling liquid, thereby also cooling the recycling liquid; concentrating the heated liquid waste under reduced pressure; combusting the liquid waste so concentrated, thereby producing combustion products; bringing the combustion products into contact with a liquid in a liquid contact zone; and bringing at least some of the resultant vapour laden exhaust combustion gas into direct contact with at least some of the cooled recycling liquid resulting from said indirect heat exchange, thereby heating said liquid; and at least a portion of said hot recycling liquid being being derived from the hot liquid which results from said direct contact.

The whole of the process may be performed in a continuous manner.

According to a second and alternative aspect of the present invention, there is further provided an apparatus for the treatment of a liquid waste, comprising a heat exchanger of the indirect contact type; an evaporator connected to the heat exchanger for concentrating heated liquid waste derived therefrom; means for combusting the thus concentrated liquid waste; a gas-liquid contactor connected to receive combustion products from the combustion means; and a heat exchanger connectd to receive cooled liquid from said indirect heat exchanger and to heat the same by direct contact with vapour laden exhaust combustion gas emitted from the contacts; means being provided for introducing heated liquid from the direct contact heat exchanged into the contactor, and for introducing a portion of the liquid in the contactor into the indirect heat exchanger to heat said liquid waste.

In a third and yet further alternative aspect of the invention, we provide an apparatus for the treatment of a liquid waste, comprising: a heat exchanger of the indirect contact type; an evaporator connected to the heat exchanger for concentrating heated liquid waste derived therefrom, means for combusting the thus concentrated liquid waste; a gas-liquid contactor connected to receive combustion products from the combustion means; and a heat exchanger connected to receive cooled liquid from said indirect heat exchanger and to heat the same by direct contact with vapour laden exhaust combustion gas emitted from the contractor; means being provided for collecting liquid heated by said direct contact; and liquid for heating said liquid waste in the indirect heat exchanger being derived from liquid so collected.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic flow diagram of one embodiment of waste treatment apparatus in accordance with the present invention; Figure 2 is a schematic flow diagram for another embodiment of a waste treatment apparatus of the invention; and Figure 3 is a partial view schematically showing a modification of the embodiment of Figure 2.

Referring to Figure 1, a liquid waste contained in a storage tank 13 is, by means of a pump 14, introduced into a liquid waste circulation line including a circulation pump 10, a heat exchange zone preferably a heat exchanger 8 and a concentration zone preferably an evaporator 9. The liquid waste introduced into the circulation line is circulated as follows.

The waste is, by means of the pump 10, fed to the heat exchanger 8 where it is heated by indirect heat exchange with a hot recycling liquid which is a liquid introduced from a contacting zone preferably a submerged combustion vessel 5. The heated liquid waste is then introduced into the evaporator 9, where the heated liquid waste is concentrated under reduced pressure of preferably between 100 and 300 mmHg. The concentrated liquid waste is partly returned to the heat exchanger 8 together with a fresh liquid waste introduced from the storage tank 13.

The vapor generated at and evolved from the evaporator 9, is introduced into a condensation zone preferably a surface condenser 11, where the vapor is condensed with a cooling medium 23. The condensate is discharged through a line 24 while non-condensate is with- drawn through a line 21 and fed to a suitable device such as a steam ejector.

A portion of the concentrated liquid waste is withdrawn from the evaporator 9 and is introduced, by means of a pump 12, into a combustion zone preferably a combustion furnace 3 where the concentrated liquid waste is subjected to a combustion treatment to produce combustion products which are in turn introduced in to a contacting zone preferably the submerged combustion vessel 5. The concentrated liquid waste is supplied to an atomization nozzle 2 attached to the furnace 3 into which the liquid is sprayed for atomization. The temperature within the furnace 3 is kept, in general, at over 800"C by the combustion of an auxiliary fuel fed by a pump 16 from a tank 15 to a burner 1 provided at the top of the furnace 3. By the combustion in an excess oxygen atmosphere, the organic components contained in the liquid waste are oxidized. In case the concentrated liquid waste is combustible by itself, the use of the auxiliary fuel is unnecessary except at a time of the start of the combustion operation. In such a case, the waste may be sprayed from the burner 1. In order to maintain stable combustion, combustion air is supplied into the furnace 3 by means of a blower 17. The combustion products are introduced via downcomer tube 4 into the submerged combustion vessel 5 and are brought into direct contact with water in the vessel 5. Water is supplied from a line 22 into the vessel 5. More detailed process and device of the submerged combustion are shown in United States patent No. 3*912,577 (Akune et al), the disclosure of which is hereby incorporated by reference.

The vapor mainly moisture-saturated exhaust combustion gas emitted from the vessel 5 is introduced via duct 20 into a heat recovery zone preferably a gas-liquid direct, countercurrent flow type contacting tower 6, where the exhaust combustion gas is brought into direct contact with a cooled recycling liquid showered from the top of the tower 6 and introduced from the heat exchanger 8, thereby giving the heat contained in the exhaust combustion gas to the recycling liquid.

The thus cooled exhaust combustion gas is withdrawn from the top of the tower 6 and discharged into the air via duct 19 and a dust collector (not shown). The heated recycling liquid is introduced into the submerged combustion vessel 5 through a line 25. A portion of the liquid in the vessel 5 is, as the afore-mentioned hot recycling liquid, introduced by means of a pump 7 into the heat exchanger 8 to effect indirect heat exchange with the liquid waste, as described previously. The cooled recycling liquid discharged from the heat exchanger 8 is recycled into the heat recovery tower 6.

Thus, the heat contained in the exhaust combustion gas emitted from the submerged combustion vessel 5 is utilized to heat and concentrate the liquid waste.

In case inorganic components are contained in the liquid waste, they are converted into ashes by combustion, which are then collected as a slurry in the submerged combustion vessel 5. Thus, the inorganic components can be recovered in the form of a solution or slurry from a line 18.

Though the use of the submerged combustion vessel 5 is most advantageous with respect to thermal efficiency, a packed tower, spray tower, jet scrubber or venturi scrubber may be used as the contacting zone, if desired.

Figure 2 shows another embodiment of the apparatus of the present invention. In the Figure, components designated at 101 through 105 and 107 through 124 are corresponding to and the same as the components 1 through 5 and 7 through 24, respectively, both in functions and features. In this embodiment, the recycling liquid heated by direct heat exchange with the vapor laden exhaust combustion gas at a gas-liquid contacting towe 106, is stocked in a collecting zone preferably formed in the lower section of the tower 106. A portion of the heated recycling liquid collected in that section 126 of the tower 106 is directly introduced by means of a pump 107 into a heat exchanger 108 as the hot recycling liquid.

Thus, in this embodiment, heating the liquid waste in the heat exchanger 108 is effected by indirect heat exchange with the recycling liquid heated at the heat recovery tower 106 and introduced from the lower section of the tower 106. Because the total volume of the recycling liquid tends to increase by the condensation of the steam in the exhaust combustion gas at the gas-liquid contacting tower 106, a portion of the recycling liquid is discharged, preferably by overflowing, from the lower section 126 of the tower 106 and introduced into a submerged combustion vessel 105 through a line 125.

Figure 3 shows a modification of the apparatus of the Figure 2. In the Figure, components shown at 203 through 207, 209, 210, 212, 214, 218 through 220, 222 and 225 correspond to and are the same as the components 103 through 107, 109, 110, 112, 114, 118 through 120, 122 and 125, respectively in their functions and features. In this embodiment, a heat exchanger 208 is formed in the lower section of the gas-liquid contacting tower 206 and the heated liquid collected in that section is utilized as the said hot recycling liquid.

The following non-limiting example will further illustrate our process.

Example A liquid waste composed of 7 wt % of Na2SO4, 3 wt % of organic compounds (calorific value of 4.550 Kcal/Kg) and 90 wt % of water was subjected to a continuous combustion treatment using the apparatus as shown in Figure 1. The volume of the liquid waste was reduced, by concentration at evaporator 9, to one thirds.

3000 Kg/hour of the fresh liquid waste having a temperature of 25"C was continuously introduced into the heat exchanger 8 where it was heated to 610C by indirect heat exchange with a hot recycling liquid. The heated liquid waste was introduced into the evaporator 9 to effect the concentration thereof under a pressure of about 120 mmHg and at a temperature of about 58"C. 30500 Kg/hour of the concentrated liquid waste was returned to the heat exchanger 8. 1000 Kg/ hour of the concentrated liquid which had a composition of 21 wt % of Na2SO4, 9 wt % of the organic components and 70 wt % of water, was continuously fed to the atomization nozzle to effect the combustion thereof at a temperature of 950"C and an air ratio of 1.2.

170 Kg/hour of a heavy fuel oil was used as the auxiliary fuel. The combustion products were cooled in the submerged combustion vessel 5. The vapor laden gas emitted from the vessel 5 and having a temperature of about 90 C, was introduced into the heat recovery tower 6. From the top of the tower 6, a recycling liquid which had a tempeature of 76"C and which was introduced from the heat exchanger 8, was showered in an amount of 76000 Kg/hour, through which the waste combustion gas was counter-currently passed. The gas exhausted overhead from the tower 6 had a temperature of 80 C. The recycling liquid heated at the heat recovery tower 6 was introduced into the vessel 5. 76000 Kg/hour of the aqueous solution having a temperature of 91 C in the vessel was withdrawn therefrom and introduced into the heat exchanger 8 as the hot recycling liquid.

To compare the heat recovery efficiency, the liquid waste was also subjected to the treatment using the apparatus shown in Figure 2 and to the combustion treatment described with reference to Figure 2 of United States patent No. 3,912,577. In either case, the liquid waste was concentrated to one thirds. The results were as summarized in the Table below.

t( C) v a c Figure 1 24.7 8 1.2 2.8 Figure 2 22.9 12 1 3.6 Reference 27.5 0 2.8 1 t: Logarithmic mean temperature difference of the liquid waste between the outlet and inlet of the heat exchanger.

v: relative volume, in terms of flow rate, of recycling liquid, if the flow rate of the liquid waste circulating into the heat exchanger is 30.

a: relative area of heat transfer surface of the heat exchanger.

c: relative over-all heat transfer coefficient of the heat exchanger.

As is seen from the above table, specific embodiments of our apparatus may employ a heat transfer surface area of the heat exchanger which is as little as one half or one third of that required in the previously employed process. One might think that the prior process would be more advantageous in terms of energy consumption, because it requires no liquid transfer.

However, we have found that examples of the present process exhibit an overall advantage in energy consumption terms, because the previously proposed process requires considerable energy to pass the waste combustion gas through the gas-liquid indirect heat exchanger with its complicated structure.

The data in the above table further show that the flow rate of recycling liquid in the example of the apparatus illustrated in Figure 1 is smaller than for the Figure 2 example, since greater temperature difference is obtainable. However, since the recycling liquid in the system of Figure 1 contains a larger amount of scale-forming materials, it requires a relatively larger heat transfer surface area than the system of Figure 2.

WHAT WE CLAIM IS: 1. A process for the treatment of a liquid waste, comprising: heating the liquid waste by indirect heat exchange with a hot recycling liquid, thereby also cooling the- recycling liquid; concentrating the heated liquid waste under reduced pressure; combusting the liquid waste so concentrated, thereby producing combustion products; bringing the combustion products into contact with a liquid in a liquid contact zone; and bringing at least some of the resultant vapour laden exhaust combustion gas into direct contact with at least some of the cooled recycling liquid resulting from said indirect heat exchange, thereby heating said liquid; and at least a portion of said hot recycling liquid being derived from the hot liquid which results from said direct contact.

2. A process according to Claim 1, wherein the hot recycling liquid is obtained by abstracting from said liquid contact zone a portion of the liquid therein, the hot liquid from said direct contact being introduced into the liquid contact zone.

3. A process according to Claim 1, wherein the hot recycling liquid consists of at least a portion of the hot liquid resulting from said direct contact.

4. A process according to Claim 3, further comprising collecting hot liquid resulting from said contact in a collecting zone before introducing any of the said liquid into the indirect heat exchange step.

5. A process according to Claim 4, in which liquid is withdrawn from the collecting zone and introduced into the liquid contact zone to maintain the volume of liquid in the collecting zone within a predetermined level.

6. A process according to any preceding claim, wherein a portion of the liquid in the liquid contacting zone is withdrawn in order to recover inorganic components from the waste.

7. Apparatus for the treatment of a liquid waste, comprising a heat exchanger of the indirect contact type; an evaporator connected to the heat exchanger for concentrating heated liquid waste derived therefrom; means for combusting the thus concentrated liquid waste; a gas-liquid contactor connected to receive combustion products from the combustion means; and a heat exchanger connected to receive cooled liquid from said indirect heat exchanger and to heat the same by direct contact with vapour laden exhaust combustion gas emitted from the contactor; means being provided for introducing heated liquid from the direct contact heat exchanger into the contactor, and for introducing a portion of the liquid in the contactor into the indirect heat exchanger to heat said liquid waste.

8. Apparatus for the treatment of a liquid waste, comprising a heat exchanger of the indirect contact type; an evaporator connected to the heat exchanger for concentrating heated liquid waste derived therefrom; means for combusting the thus concentrated liquid waste; a gas-liquid contactor connected to receive combustion products from the combustion means; and a heat exchanger connected to receive cooled liquid from said indirect heat exchanger and to heat the same by direct contact with vapor laden exhaust combustion gas emitted from the contactor; means being provided for collecting liquid heated by said direct contact; and liquid for heating said liquid waste in the indirect heat exchanger being derived from liquid so collected.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. t( C) v a c Figure 1 24.7 8 1.2 2.8 Figure 2 22.9 12 1 3.6 Reference 27.5 0 2.8 1 t: Logarithmic mean temperature difference of the liquid waste between the outlet and inlet of the heat exchanger. v: relative volume, in terms of flow rate, of recycling liquid, if the flow rate of the liquid waste circulating into the heat exchanger is 30. a: relative area of heat transfer surface of the heat exchanger. c: relative over-all heat transfer coefficient of the heat exchanger. As is seen from the above table, specific embodiments of our apparatus may employ a heat transfer surface area of the heat exchanger which is as little as one half or one third of that required in the previously employed process. One might think that the prior process would be more advantageous in terms of energy consumption, because it requires no liquid transfer. However, we have found that examples of the present process exhibit an overall advantage in energy consumption terms, because the previously proposed process requires considerable energy to pass the waste combustion gas through the gas-liquid indirect heat exchanger with its complicated structure. The data in the above table further show that the flow rate of recycling liquid in the example of the apparatus illustrated in Figure 1 is smaller than for the Figure 2 example, since greater temperature difference is obtainable. However, since the recycling liquid in the system of Figure 1 contains a larger amount of scale-forming materials, it requires a relatively larger heat transfer surface area than the system of Figure 2. WHAT WE CLAIM IS:

1. A process for the treatment of a liquid waste, comprising: heating the liquid waste by indirect heat exchange with a hot recycling liquid, thereby also cooling the- recycling liquid; concentrating the heated liquid waste under reduced pressure; combusting the liquid waste so concentrated, thereby producing combustion products; bringing the combustion products into contact with a liquid in a liquid contact zone; and bringing at least some of the resultant vapour laden exhaust combustion gas into direct contact with at least some of the cooled recycling liquid resulting from said indirect heat exchange, thereby heating said liquid; and at least a portion of said hot recycling liquid being derived from the hot liquid which results from said direct contact.

9. Apparatus according to Claim 8, wherein said indirect heat exchanger is formed in said

collecting means.

10. Apparatus according to Claim 8, wherein a conduit extends between the collecting means and the indirect heat exchanger for introducing the collected liquid into the indirect heat exchanger for introducing the collected liquid into the indirect heat exchanger.

11. Apparatus according to any one of Claims 8 to 10, wherein an overflow pipe extends between the collecting means and the contactor so that a portion of the collected liquid may overflow from the collecting means into the contactor so as to maintain the level of the collected liquid below a predetermined level.

12. Apparatus for the treatment of a liquid waste substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

13. A process for the treatment of aqueous waste, in which: the waste is concentrated and the resulting concentrate combusted; gases resulting from combustion and, if produced, any ashes being brought into direct contact with cooling water; moisture laden exhaust combustion gases resulting from such direct contact being introduced into a heat recovery tower in which the exhaust combustion gases are brought into counter-current contact with recycling water previously cooled by indirect heat exchange with incoming aqueous waste to concentrate the waste; and heat derived from recycling water heated by the exhaust combustion gases being utilised for concentrating fresh aqueous waste.

14. A process according to Claim 13, in which the said direct contact is performed in a submerged combustion vessel.

15. A process according to Claim 14, wherein the heated recycling water is introduced from the heat recovery tower into the submerged combustion vessel, and the liquid in the submerged combustion vessel is used as recycling water for heating, and thereby concentrating, fresh aqueous waste.

16. A process according to Claim 13 or 14, wherein the heated recycling water is directly employed for heating, and thereby concentrating, said fresh aqueous waste.

17. A process for the treatment of a liquid waste substantially as hereinbefore described with reference to the Example and/or the accompanying drawings.