WASTE WATER TREATMENT BY EVAPORATION AND PREVENTION OF FOULING WITH CLEANING PARTICLES

Abstract

A process for the evaporation of a waste water stream having a mixture of inorganic and organic materials is provided. The process includes passing a waste water stream together with cleaning particles through a shell and tube heat exchanger of a forced circulation evaporator to mitigate scaling on the internal surface of the heat exchanger through which the stream of waste water flows. In another aspect, the invention concerns a forced circulation evaporator having a fluidized bed heat exchanger suitable for carrying out the method. The evaporator is located after a chemical process plant that produces a waste water containing a mixture of inorganic and organic materials, preferably after a dyestuff manufacturing plant.

Claims

1. A multi-effect evaporation process for the evaporation of a waste water stream comprising a mixture of inorganic and organic materials, said process comprising more than one forced circulation evaporator effects in series of which one or more and less than all effects comprise passing a waste water stream together with cleaning particles through a shell and tube heat exchanger of a forced circulation evaporator to mitigate fouling on the internal surface of the heat exchanger through which the waste water stream flows.

2. The process according to claim 1, wherein the one or more and less than all effects with the most extreme working temperatures relative to all effects of the multi-effect evaporation process comprise said passing the waste stream together with the cleaning particles through the shell and tube heat exchanger of the forced circulation evaporator.

3. The process according to claim 1, wherein the one or more and less than all effects with the highest concentration of said mixture in the waste water stream relative to all effects of the multi-effect evaporation process comprise said passing the waste stream together with the cleaning particles through the shell and tube heat exchanger of the forced circulation evaporator.

4. The process according to claim 1, wherein only one effect, and optionally the first effect of the multi-effect evaporation process comprise said passing the waste stream together with the cleaning particles through the shell and tube heat exchanger of the forced circulation evaporator.

5. A process for the evaporation of a waste water stream comprising a mixture of inorganic and organic materials, said process comprising passing said waste water stream comprising said mixture of inorganic and organic materials together with cleaning particles through a shell and tube heat exchanger of a forced circulation evaporator to mitigate fouling on the internal surface of the heat exchanger through which the waste water stream flows.

6. The process according to claim 1, wherein the waste water stream is passed through the heat exchanger with the particles from the start of the process to remove the fouling on the surface of the tubes as it starts to build up.

7. The process according to claim 1, wherein said organic materials comprise thermo-reactive organic compounds.

8. The process according to claim 1, wherein said waste stream at the start of the process has a chemical oxygen demand (COD) in the range of 5,000 mg/L to 100,000 mg/L.

9. The process according to claim 1, wherein said inorganic materials comprise one or more salts selected from the group consisting of ammonium salts, potassium salts, sodium salts, magnesium salts and calcium salts, and optionally from the group consisting of calcium carbonate, calcium sulfate, sodium chloride, sodium sulfonate, magnesium sulfonate and magnesium carbonate.

10. Process according to claim 1, wherein said waste water comprises waste water from a dyestuff production process.

11. The process according to claim 1, wherein said cleaning particles comprise stainless steel particles, optionally having a size in the range of 1 to 5 mm.

12. The process according to claim 1, wherein said waste water exiting said heat exchanger is led into a solid liquid separator, optionally a gravity separator, to yield a heated waste water liquid and a concentrated cleaning particles stream.

13. The process according to claim 1, wherein a heated waste water stream exiting said heat exchanger is led into a solid liquid separator to yield a heated liquid stream and a concentrated particles stream, said heated liquid stream is led into a flash vessel to produce a vapor discharge stream and a liquid discharge stream and wherein at least part of said vapor discharge stream and/or liquid discharge stream is led into a further forced circulation evaporator step as part of a multi-effect evaporation process, optionally wherein the further steps of the multi-effect evaporation process are carried out without fluidized bed heat exchangers.

14. The process according to claim 1, wherein the waste water stream flows through the heat exchanger with a flow velocity of about 0.5 to about 0.7 m/s.

15. The process according to claim 1, wherein the waste water stream with particles has a porosity in the range of 90 to 98%, wherein porosity is defined as the volume fraction of fluid per volume of the tube of the heat exchanger through which the waste water with particles flows.

16. The process according to claim 1, wherein said heat exchanger is heated by providing steam having a temperature in the range of about 110 to about 160° C.

17. A multi-effect evaporator (MEE) plant suitable for carrying out the method in accordance with claim 1, said plant comprising more than one forced circulation evaporator effect in series of which one or more and less than all effects comprise a shell and tube heat exchanger with a fluidized bed of cleaning particles that are arranged to pass through the tubes with a stream of waste water.

18. The multi-effect evaporator (MEE) plant according to claim 17, wherein the one or more and less than all effects that are adapted to carry out the process with the most extreme working temperatures relative to all effects of the multi-effect evaporation plant comprise said fluidized bed of cleaning particles.

19. The multi-effect evaporator (MEE) plant according claim 17, wherein the one or more and less than all effects that are adapted to receive the highest concentration of the mixture of inorganic and organic materials in the waste water stream relative to all effects of the multi-effect evaporation plant comprise said fluidized bed of cleaning particles.

20. The multi-effect evaporator (MEE) plant according to claim 17, wherein the first effect of the multi-effect evaporation plant comprise said fluidized bed of cleaning particles.

21. A forced circulation evaporator comprising an exchanger suitable for carrying out the method in accordance with claim 17, in particular including a shell and tube heat exchanger with a fluidized bed of cleaning particles that are arranged to pass through the tubes with a stream of waste water, said evaporator being located after a chemical process plant that produces a waste water comprising mixture of inorganic and organic materials, optionally after a dyestuff manufacturing plant.

22. The multi-effect evaporator (MEE) plant according to claim 17, wherein said one or more evaporators comprises: said fluidized bed heat exchanger is configured to heat a stream of waste water (WW) together with cleaning particles to form a stream of heated waste water containing said cleaning particles (HW), a solid liquid separator configured to receive the stream of heated waste water containing said cleaning particles (HW) and to separate a stream of concentrated cleaning particles (CP) from the heated waste water liquid (HL); a flash vessel configured to receive the heated waste water liquid (HL) and to discharge water vapor (DV) and a stream of concentrated waste water (CW) into a liquid circuit; liquid circuit that is configured to discharge concentrated waste water (CW), and configured to lead fresh waste water (FW) into the circuit to form a waste water stream (WW); wherein said solid liquid separator comprises a gravity separator.

23. The multi-effect evaporator (MEE) plant comprising the forced circulation evaporator according to claim 21, as a first effect, wherein subsequent effects comprise conventional heat exchangers without cleaning particles.

24. An evaporator plant comprising a single forced circulation evaporator according to claim 21, further comprising a vapor compressor between a vapor outlet of the flash vessel and a steam inlet of the fluidized bed heat exchanger.

25. (canceled)

Description

EXAMPLE 1

[0048] An existing, conventional full-scale evaporator for the treatment of waste water from a dyestuff producing company was retrofitted with a fluidized bed arrangement in accordance with the invention. The retrofit included addition of some specific components for the operation of a fluidized bed like an inlet channel for particle distribution and a gravity separator for separation of particles from the circulating fluid. A setup as illustrated in FIG. 2 was obtained as a first effect of a multi effect evaporator (MEE) plant. On average the steam temperature into the 1st effect was 115° C. The maximum evaporation capacity reached with this plant was around 4.8 m.sup.3/h.

[0049] The waste water may at the infeed e.g. have the following composition;

[0050] COD level of at least 20,000 mg/L

[0051] BOD level of at least 9,000 mg/L

[0052] TDS level of at least 40,000 mg/L

[0053] and elements or K, Na, CaCO.sub.3 (hardness), Cl, SO4 all above 1000 mg/L.

[0054] Typical waste water was found to have the following composition:

TABLE-US-00001 NH.sub.4 + NH.sub.3 350 mg/L K 1400 ppm Na 9600 ppm Mg 170 ppm Ca 300 ppm CaCO.sub.3 (hardness) 4300 ml/L Cl 13700 mg/L SO.sub.4 7700 mg/L CO.sub.2 70 mg/L chemical oxygen demand (COD) 18500 mg/L biological oxygen demand (BOD) 9000 mg/L Total dissolved solids (TDS) 43000 mg/L Total suspended solids (TSS) 180 mg/L pH 6.4

[0055] The plant performance was monitored by a daily balance of the feed entered, the condensate as collected and the product taken from the MEE. From the balance the daily average evaporation rate in m.sup.3/h is calculated. FIG. 6 shows the evaporation capacity as function of the hours of operation of the plant with the original configuration and after the retrofit of the 1st effect to the self-cleaning fluidized bed heat exchanger. The data shown below is as received from the operations team of the plant.

[0056] During the first 400 hours of operation with the fluidized bed the evaporation capacity has an increasing slope as compared to the operation with the conventional heat exchanger. Although the heat exchanger tubes were cleaned before the retrofit using high pressure water jet cleaning, not all tubes were 100% clean. Therefore, over the duration of 400 hours, likely some part of the existing fouling layer was removed due to the scouring action of fluidized bed. This hypothesis, that the tubes were not 100% clean was later confirmed during an inspection where tubes with some fouling layer were discovered. This existing fouling layer also had a negative effect because, parts of the fouling layer that came loose resulted in clogging of some tubes that subsequently were filled with particles.

[0057] During the inspection also the other effects were inspected. It was found that the tubes of effect 2, 3 and 4 were cleaner than before the retrofit of the first effect. A possible explanation for the observation that the fouling in the other effects that were not retrofitted had reduced as well could be that the application of fluidized bed has reduced the size of solids in the system because of the grinding effect by the particle motion. The reduction of the solids size has helped in reducing the particulate fouling in the other effects of the MEE plant.

[0058] Considering the observations during the inspection, the heat exchanger tubes were again cleaned to remove old layers and the tubes clogged with particles were cleared. Subsequent to the inspection and the cleaning of the tubes the heat exchanger was restarted. FIG. 5 shows the variation of evaporation capacity with further operation of the plant.

[0059] FIG. 7 confirms that with longer term operation, the evaporation capacity has remained stable for the plant. The curve fit shown is only for operation after 400 hours when the tubes were cleaned, and the curve proves constant evaporation.