METHOD FOR TREATMENT OF A HOT PYROLYSIS GAS

Abstract

The present invention relates to a method for treatment of a hot gas generated by a pyrolysis or a gasification process, wherein the hot gas is passed to a first unit for particle removal and cooling, whereby subsequently, a gaseous stream is removed from a first condensed stream thus obtained and passed to a second unit for particle removal and cooling, wherein a second condensed stream is obtained, said first condensed stream and second condensed stream being recycled to said first unit and said second unit, respectively.

Claims

1. A method for treatment of a hot gas generated by a pyrolysis or a gasification process, wherein the hot gas is passed to a first unit for particle removal and cooling, whereby subsequently, a gaseous stream is removed from a first condensed stream thus obtained and passed to a second unit for particle removal and cooling, wherein a second condensed stream is obtained, said first condensed stream and second condensed stream being recycled to said first unit and said second unit, respectively.

2. The method of claim 1, wherein said step of recycling comprises injecting in said first unit of said first condensed stream as small droplets in the stream of hot gas.

3. The method of claim 1, wherein said step of recycling comprises injecting in said second unit of said second condensed stream as small droplets in the gaseous stream originating from said first condensed stream.

4. The method of claim 2, wherein a liquid-to-gas ratio (L/G ratio) in said first unit is in a range of 10-40, preferably 15-30, even more preferably 20-25, said L/G ratio being expressed on mass basis (i.e. kg liquid per kg gas).

5. The method of claim 3, wherein the liquid-to-gas ratio (L/G ratio) in said second unit is in a range of 20-60, preferably 30-50, even more preferably 35-45, said L/G ratio being expressed on mass basis (i.e. kg liquid per kg gas).

6. The method of claim 1, wherein the effluent from said first unit for particle removal and cooling is passed to a first knock-out vessel, from which first knockout vessel said gaseous stream is removed from said first condensed stream.

7. The method of claim 6, wherein the temperature of said effluent from said first unit for particle removal and cooling is at most 90 C., preferably at most 80 C., even more preferably at most 70 C.

8. The method of claim 6, wherein the temperature of said gaseous stream from said first knockout vessel is at most 90 C., preferably at most 80 C., even more preferably at most 70 C.

9. The method of claim 1, wherein said first condensed stream is cooled before said step of recycling said first condensed stream to said first unit for particle removal and cooling.

10. The method of claim 9, wherein said step of cooling said first condensed stream results in a stream having a temperature in a range of 10-70 C.

11. The method of claim 1, wherein the effluent from said second unit for particle removal and cooling is passed to a second knock-out vessel, from which second knockout vessel a gaseous stream is removed from said second condensed stream.

12. The method of claim 11, wherein said second condensed stream is cooled before said step of recycling said second condensed stream to said second unit for particle removal and cooling.

13. The method of claim 1, wherein a first purge stream is withdrawn from said first condensed stream, said purge stream not being recycled to said first unit.

14. The method of claim 13, wherein a second purge stream is withdrawn from said second condensed stream, said purge stream not being recycled to said second unit.

15. The method of claim 14, wherein said first and second purge stream are combined and used for further processes.

16. The method of claim 1, wherein said first and second unit for particle removal and cooling are of the type venturi scrubber, said venturi scrubber having a flow channel along a longitudinal axis and defined in sequence by a converging section, a throat section, and a diverging section, wherein nozzles configured for injecting a scrubbing liquid into said venturi scrubber are present in a wall surrounding said throat section or at the entrance to the converging section, or a combination thereof.

17. The method of claim 16, wherein the width of the throat section of the first venturi scrubber is larger than the width of the throat section of the second venturi scrubber.

18. The method of claim 17, wherein the flow rate of the gas in the throat section is at least 40 m/s.

19. The method of claim 18, wherein the angle of the taperingly shaped converging section is in a range of 35-60 degrees.

20. The method of claim 19, wherein the angle of the nozzles is in a range of 10-20 degrees.

21. The method of claim 20, wherein the angle of the taperingly shaped diverging section is in a range of 10-20 degrees.

22. A system for implementing the method according to claim 1, wherein a first venturi scrubber follows the oven in series, in the flow path of the hot gas leaving said oven, followed by a first gas-liquid separation, whereby behind the first gas-liquid separation, a first condensed stream is recycled back to the first venturi scrubber, a gaseous stream is sent to a second venturi scrubber, followed by a second gas-liquid separation, whereby behind the second gas-liquid separation, a second condensed stream is recycled back to the second venturi scrubber.

Description

[0043] The present invention is explained in more detail below with reference to schematic exemplary figures relating to exemplary embodiment variants of the invention.

[0044] FIG. 1 shows a process flow diagram of the present method.

[0045] FIG. 2 is a general schematic diagram of a venturi scrubber.

[0046] In FIG. 1 the method for treatment of a hot gas generated by an entrained flow gasification system is indicated by reference number 100. Hot pyrolysis gas 1, e.g. above a temperature of 400 C., originating from a pyrolysis process for recycling a scrap rubber, in particular tyres, as disclosed in International application WO2013/095145, is sent to a first venturi scrubber 2. First venturi scrubber 2 is provided with nozzles (not shown here) for injecting a scrubbing liquid into the throat of first venturi scrubber 2. The scrubbing liquid entering first venturi scrubber 2 is here first condensed stream 13. The effluent 3 from first venturi scrubber 2 comprising a gas-liquid phase is sent to a first knock-out vessel 4. In first knock-out vessel 4 a separation between a gaseous phase 6 and a liquid phase 5 takes place by means of gravimetric separation. Liquid phase 5, i.e. the condensate fraction of effluent 3, is returned to the inlet of first venturi scrubber 2. In the process flow diagram shown here liquid phase 5 is cooled in heat exchanger 20 and the thus cooled first condensed stream 13 is injected via nozzles in first venturi scrubber 2. A purge stream 16 is withdrawn from first condensed stream 13. The inlet of the first venturi is vapor, the inlet of the second venturi is also a vapour and the outlet of the first venturi is a gas. One can say that a gas refers to a substance that has a single defined thermodynamic state at a certain temperature whereas a vapor refers to a substance that is a mixture of two phases at a certain temperature, for example room temperature, namely gaseous and liquid phase.

[0047] The gaseous phase 6 formed in first knock-out vessel 4 is sent to a second venturi scrubber 7. Second venturi scrubber 7 is provided with nozzles (not shown here) for injecting a scrubbing liquid into the throat of second venturi scrubber 7. The scrubbing liquid 14 entering second venturi scrubber 7 originates from second condensed stream 11. The effluent 8 from second venturi scrubber 7 comprising a gas-liquid phase is sent to a second knock-out vessel 9. In second knock-out vessel 9 a separation between a gaseous phase 10 and a liquid phase 11 takes place. Liquid phase 11, i.e. the condensed fraction of effluent 8, is returned to the inlet of second venturi scrubber 7. In the process flow diagram shown here liquid phase 11 is cooled in heat exchanger 20 and the thus cooled second condensed stream 14 is injected via nozzles in second venturi scrubber 7. A purge stream 15 is withdrawn from second condensed stream 14. In FIG. 1 all heat exchangers 20 are supplied with a heat exchanging medium. In addition, in some embodiments heat exchangers 20 are interconnected.

[0048] According to the process flow diagram shown here purge stream 15 and purge stream 16 are combined as stream 17 for further use in unit 12.

[0049] From the process flow diagram shown here it is clear that there is no exchange between the scrubbing liquids 13 and 14. In other words, second condensed stream 14 is recycled to second venturi scrubber 7, first condensed stream 13 is recycled to first venturi scrubber 2.

[0050] During the start-up phase of the method for treatment of a hot gas generated by a pyrolysis or gasification process a phase comprising a low boiling oil with a small fraction of high boiling oil is present in both first knock-out vessel 4 and second knock-out vessel 9. This composition is used as the scrubbing liquid in the venturi scrubbers 3, 7. After some time the composition will be withdrawn from the system as stream 17. Second knock-out vessel 9 is preferably provided with a demister as shown in FIG. 1.

[0051] In FIG. 2 a venturi scrubber 50 is shown. Venturi scrubber 50 comprises, in sequential order, a confuser 54, i.e. a converging section, a throat section 53 and a diffuser 55, i.e. a diverging section. The hot gas inlet 52 enters venturi scrubber 50 at the top where nozzles 51 are located. Nozzles 51 eject a scrubbing liquid into the hot gas entering the top portion of venturi scrubber 50. The inlet gas stream enters converging section 54 and, as the area decreases, gas velocity increases. The inlet gas, forced to move at extremely high velocities in the small throat section 53 may shear the liquid from its walls, producing an enormous number of very tiny droplets. Due to the contraction of the annular vapor space available mixing is forced and intensified. The present inventors assume that breakup of the droplets is created by the nozzle in conjunction with the gas flow. Particle and mist removal occur in the diverging section 55 as the inlet gas stream mixes with the fog of tiny liquid droplets. The inlet stream then exits through the diverging section 55, where it is forced to slow down. In FIG. 2 the diameter of throat is indicated as D2, the outlet diameter of the venturi scrubber as D4, the inlet diameter of the venturi scrubber as D1. In an embodiment the diameter of throat section 53 is constant over its length. For an optimal design of such venturi scrubber the following equations are preferably valid D4>D1 and D2<D1.