GB2355979A - Heating sludge - Google Patents

Abstract

Sludge, e.g. sewage sludge, is heat treated to cause pasteurisation by injecting steam into the sludge. Sludge to be treated is fed continuously into thermal reactor 10 via inlet 12, and steam is injected into the reactor via line 16 in such a way as to cause turbulent sludge flow conditions. The heated sludge is then fed via outlet 18 to one of three insulated heat treatment tanks 22, 24, 26, the containers being filled, holding sludge during pasteurisation or emptying via heat exchangers 34, 36 in turn.

Description

2355979 Heating Sludge The present invention concerns heating sludge,

especially but not exclusively the heat treatment of sewage sludge.

The main aim of the heat treatment of sludge is to pasteurise the sludge.

According to the present invention there is provided a method of heating sludge by continuously passing sludge through a thermal reactor injecting steam into a flow of sludge, the rate of sludge flow and manner of steam injection being such that highly turbulent conditions are provided in the sludge, removing heated sludge from the vessel and replacing the removed sludge by untreated sludge.

Preferably the sludge is caused to flow in a continuous pipework loop from a sludge inlet to a sludge outlet spaced from the inlet, steam being injected into the sludge downstream of the sludge inlet and closer to the inlet than the outlet.

Preferably the volume of steam is of the order of one hundred times that of the sludge into which it is injected.

Preferably the steam is injected into the sludge in the form of small bubbles, that is bubbles of around 200 micron diameter.

Preferably the steam is injected at a pressure of about 2 bar absolute and a quantity of around 1000 kilograms per hour, the sludge input/output being around 3 litres per second, the pipe diameter around 100 mm. and the rate of flow through the pipe around 20 litres per second.

Preferably treated sludge is collected sequentially in at least three collection tanks. Preferably while one tank is filling another is emptying and a 2 third is storing heated sludge during its heat treatment cycle.

Preferably the tanks are maintained at atmospheric pressure.

Preferably sludge is fed to the tanks at or near the bottom of the tanks. Preferably the air discharging from a filling tank is fed to an emptying tank.

Preferably treated sludge discharged from a tank is cooled in a two stage process in separate heat exchangers.

Preferably heat is removed from treated sludge in a first heat exchanger and transfers to unprocessed untreated sludge to be fed to the treatment vessel.

Preferably heat is removed from treated sludge in the second stage heat exchanger by a cooling fluid.

Further according to the present invention there is provided apparatus for heating sludge comprising a thermal reactor, means for circulating sludge continuously around the reactor, means for injecting steam into the flow of sludge, means for removing treated sludge from the reactor and means for replacing removed heated sludge by untreated sludge.

Preferably the treatment vessel is a continuous pipework loop having an inlet and at least one outlet spaced from the inlet, a steam injector for providing heating steam to the sludge flow in the form of small bubbles being provided adjacent the inlet.

Preferably three collection tanks are provided for collecting heated sludge, each tank having inlet and outlet means and control means for said inlet and outlet means whereby as one tank is being filled with heated sludge, another is being emptied of heat treated sludge, the or each remaining tank storing the sludge for complete heat treatment during said filling/emptying 3 process.

Preferably said tanks are maintained at atmospheric pressure by providing a connection between atmosphere and the air space at the top of each tank.

Preferably the sludge inlets and outlets are located at or near the bottom of each tank.

Preferably heat exchange means are provided in the outlet line from said tanks. Preferably two heat exchangers are provided the first heat exchanger exchanging heat between heat treated sludge and untreated sludge and the second, downstream, heat exchanger exchanging heat from heat treated sludge an a cooling fluid.

Preferably the pipework loop is arranged alongside the tanks at or near the base thereof.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:- Fig. I shows a block diagram of a sludge heat treatment apparatus; and Fig. 2 shows a more specific circuit diagram of a sludge heat treatment apparatus.

Fig. 1 shows a treatment vessel or thermal reactor 10 to which raw sewage sludge to be treated is fed through an inlet 12 by way of an inlet line 14. A heating medium which in the embodiment is steam is introduced into the reactor by way of a line 16 to heat the sludge in the reactor.

The input of heating medium and the form of the thermal reactor will be 4 described in greater detail below but it should be realised that untreated sludge is fed continuously through the inlet 14 and heated sludge is removed continuously at effectively the same rate from the outlet 18 and through a distribution pipe 20 to one of three, essentially identical insulated heat treatment tanks 22, 24, 26. The block diagram of Fig. I does not show arrangement of pumps, valves and control assemblies which control the flow of sludge through the pipe 20 in such a way that heated sludge is supplied to a first tank, for example 22 while a second tank, for example 24 is discharging sludge at effectively the same rate. The remaining tank 26, during this filling/emptying process is holding heated sludge therein so that heat treatment can be completed, the sludge being discharged from tank 24 having already experienced a complete heat treatment.

A duct 28 for discharging heat treated sludge has a pump assembly 30 incorporated therein and before treated sludge is discharged to digestors shown diagrammatically in the block diagram of Fig. I by the reference numerals 32 heat is recovered therefrom by passing the sludge through two heat exchangers 34, 36. In the first heat exchanger 34 hot treated sludge in the duct 28 is brought into heat exchange relationship with cold untreated sludge in the duct 14, raw sludge being supplied from a holding arrangement 40 by a pump 38 interposed in the inlet line 14.

In the second heat exchanger 36 further heat is removed from the heat treated sludge by passing it in heat exchange relationship with a cooling fluid, which is conveniently water, fed and discharged through a line 42.

The present invention realises that there are disadvantages in heating sludge by passing steam therethrough unless the steam and sludge are mixed in predetermined, controlled conditions. These conditions are achieved by the apparatus shown in the circuit diagram illustrated in Fig. 2 where the same reference numerals have been used for components similar to those described with reference to Fig. 1.

9 S The thermal reactor 10 comprises a pipe SO of approximately 100 mm diameter arranged in a closed loop and having an assembly of pumps 52 interposed therein, each pump having suitable valve and control assemblies which did not form part of the present invention and will not be described but the pumps are such that sludge is passed around a continuous loop at a relatively high velocity, for example around 20 litres. Sludge is introduced into the loop at inlet 12 by way of a supply line 14 having a pump assembly 38 interposed therein.

Heating steam is supplied to the pipework loop 50 from a steam boiler 54 through a supply line 16 and a steam injector 56 which provides steam bubbles in the flow of sludge, the bubbles being around 200 microns in diameter. This steam injected, coupled with the high sludge flow rate through the pipe 50 gives rise to highly turbulent conditions whereby transfer of heat from the condensing steam bubble to the sludge is efficiently and evenly achieved without mechanical or sonic disruption as the bubbles collapse. Heat transfers is good as a result of the high surface and of the plurality of small diameter bubbles.

The heated sludge must be maintained at or around this temperature of around 700C for a predetermined period of time to allow the pasteurisation process to be concluded. Three insulated non-pressurised holding tanks 24,26,28 are provided each connected at outlets 18 by pipes 20 to the pipework loop 50 the pipes 20 supplying heated sludge to or near to the base of each tank. Valves 58 and appropriate control means are provided in each supply line 20 and are activated such that at any one time one valve is opened so that only one tank is being filled at any particular time. As this tank is being filled one of the remaining two tanks is being emptied at the same rate through a discharge valve 60 into an outlet pipe 28 through which heat treated sludge is pumped by pump assembly 30. During a filling/emptying cycle the remaining tank is neither filled nor emptied so that heated sludge is held therein while the pasteurisation process is completed.

6 In practice the pipe loop SO is located alongside the tanks near to their bases and are of the loop serving as an outlet manifold.

The air space at the top of each tank is connected to that in the other two tanks by means air lines 62 such that on filling the removed air is transferred to the emptying tank thereby containing odours etc. within a closed system. The line 60 has a discharge vent 64 fitted with an odour control (not shown).

Means are provided for removing heat from the treated sludge passing along outlet duct 28 to digestors 32. The means comprise a first heat exchanger 34 through which passes a sludge loop 62 which taps off and returns sludge from and to the outlet line 28, the loop 62 having a pump 64 therein to encourage flow of sludge. The coolant for the heat exchanger 34 is untreated sludge tapped off and returned to the sludge supply line 14 between the pump assembly 38 and the sludge inlet 12.

The second heat exchanger 36 similarly taps off sludge from the outlet pipe 28 through a sludge loop 64 having a pump assembly 66 therein, the coolant being a suitable liquid which is passed through a line 42 from a supply 7 and is passed to waste 72.

It will be realised that the process and apparatus described above, particularly with reference to the circuit illustrated by Fig. 2 provides for the efficient pasteurisation of sewage sludge. The high flow rate of sludge through the pipework loop 30 caused by the pumping assembly 32 coupled with the injection of a plurality of small diameter bubbles by the injector 56 creates a Reynolds number flow condition which results in very high turbulence and consequently efficient transfer of heat from steam to the sludge. As the bubbles are small they condense rapidly but, by the same token, because they are small a bubble collapse gives minimal effects, for example, the noise problems created in other low flow/high bubble sized systems are mitigated. The arrangement is compact and can be located alongside the three storage tanks thereby reducing the space required and as the return run of the 7 pipework loop contains all three outlets 18 it serves as an outlet manifold thereby decreasing the pipework involved. Efficient recycling is achieved by use of, especially, the first heat exchanger 34 where heat from the heat treated sludge is transferred to untreated sludge. The final cooling of the heat treated sludge can be achieved using any suitable cooling medium, for example treated effluent. As the system is effectively a closed system, dispersal of undesirable odours into the ambient air is reduced to a minimum, the linking of the air in the storage tanks above the sludge level with the other neighbouring tanks enhancing this advantage. It will be further realised that at least the storage operation is carried out at atmospheric pressure so that there is no need to provide pressurised storage tanks thereby introducing the difficulties and relatively high expense following from the use of pressure vessels.

Various modifications can be made without departing from the scope of the invention, for example the pipework assembly of pumps control arrangement can be modified in any suitable manner. Additionally more than three storage tanks can be employed and heat exchange arrangements can be modified.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

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Claims (24)

1. A method of heating sludge by continuously passing sludge through a thermal reactor injecting steam into the flow of sludge, the rate of sludge flow and manner of steam injection being such that highly turbulent conditions are provided in the vessel, removing heated sludge from the vessel and replacing the removed sludge by untreated sludge.

2. A method as claimed in claim 1, in which the sludge is caused to flow in a continuous pipework loop from a sludge inlet to a sludge outlet spaced from the inlet, steam being injected into the sludge downstream of the sludge inlet and closer to the inlet than the outlet.

I A method as claimed in claim 1 or claim 2, in which the volume of steam is of the order of one hundred times that of the sludge into which it is injected.

4. A method as claimed in any of the preceding claims, in which the steam is injected into the sludge in the form of small bubbles, that is bubbles of around 200 micron diameter.

S. A method as claimed in any of the preceding claims, in which the steam is injected at a pressure of about 2 bar absolute and a quantity of around 1000 kilograms per hour, the sludge input/output being around 3 litres per second, the pipe diameter around 100 micron and the rate of flow through the pipe around 20 litres per second.

6. A method as claimed in any of the preceding claims, in which treated sludge is collected sequentially in at least three collection tanks.

7. A method as claimed in claim 6, in which while one tank is filling another is emptying and a third is storing heated sludge during its heat treatment cycle.

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8. A method as claimed in claim 6 or claim 7, in which the tanks are maintained at atmospheric pressure.

9. A method as claimed in any of claims 6 to 8, in which sludge is fed to the tanks at or near the bottom of the tanks.

10. A method as claimed in any of claims 7 to 9, in which the air discharging from a filling tank is fed to an emptying tank.

11. A method as claimed in any of claims 6 to 10, in which treated sludge discharged from a tank is cooled in a two stage process in separate heat exchangers.

12. A method as claimed in claim 11, in which heat is removed from treated sludge in a first heat exchanger and transfers to unprocessed untreated sludge to be fed to the treatment vessel.

13. A method as claimed in claim 11 or claim 12, in which heat is removed from treated sludge in the second stage heat exchanger by a cooling fluid.

14. Apparatus for heating sludge comprising a thermal reactor, means for circulating sludge continuously around the reactor, means for injecting steam into the flow of sludge, means for removing treated sludge from the reactor and means for replacing removed heated sludge by untreated sludge.

IS. Apparatus as claimed in claim 14 in which the thermal reactor is a continuous pipework loop having an inlet and at least one outlet spaced from the inlet, a steam injector for providing heating steam to the sludge flow in the form of small bubbles being provided adjacent the inlet.

16. Apparatus as claimed in claims 14 or 13, in which three collection tanks are provided for collecting heated sludge, each tank having inlet and outlet means and control means for said inlet and outlet means whereby as one tank is being filled with heated sludge, another is being emptied of heat treated sludge, the or each remaining tank storing the sludge for complete heat treatment during said filling/emptying process.

17. Apparatus as claimed in claim 16, in which said tanks are maintained at atmospheric pressure by providing a connection between atmosphere and the air space at the top of each tank.

18. Apparatus as claimed in claim 16 or claim 17, in which the sludge inlets and outlets are located at or near the bottom of each tank.

19. Apparatus as claimed in any of claims 14 to 18, in which heat exchange means are provided in the outlet line from said tanks.

20. Apparatus as claimed in claim 19, in which two heat exchangers are provided, the first heat exchanger exchanging heat between heat treated sludge and untreated sludge and the second, downstream, heat exchanger exchanging heat from heat treated sludge an a cooling fluid.

21. Apparatus as claimed in any of claims 14 to 20, in which the pipework loop is arranged alongside the tanks at or near the base thereof.

22. A method of heat treating sludge substantially as hereinbefore describc with reference to the accompanying drawings.

23. Apparatus for 'heat treating sludge substantially as hereinb described with reference to the accompanying drawings

24. Any novel subject matter or combination including novel subje( disclosed herein, whether or not within the scope of or relating to the! invention as any of the preceding claims.

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