GB2465144A

GB2465144A - Process for the treatment of materials in a vessel with a compacting stage and an additional loading step

Info

Publication number: GB2465144A
Application number: GB0820165A
Authority: GB
Inventors: Duncan Grierson; James Crankshaw; Christopher Davis; John Bland
Original assignee: Sterecycle Ltd
Current assignee: Sterecycle Ltd
Priority date: 2008-11-04
Filing date: 2008-11-04
Publication date: 2010-05-12
Also published as: GB0820165D0

Abstract

A process to treat materials in a vessel at elevated temperature and pressure (for example, an autoclave), wherein the vessel is loaded with compactable material to be treated, wherein the material is subjected to at least one compacting stage and additional filling step, the material after the compacting and additional filling step(s) is subjected to the main cooking or treatment cycle, and the treated material from the vessel is obtained, which treated material preferably is classified into one or more useful fractions. The process is particularly suitable for treatment of waste such as municipal solid waste (MSW) or medical waste. Preferably, the compacting stage is performed with the vessel closed wherein the vessel is heated, and pressurized, while rotation and agitation are progressing, for sufficient time to reduce the volume of the material by 10vol%. This allows an additional 5vol% of waste to be added before the main processing step, increasing the batch load size and efficiency of the process and reducing the energy required. The vessel may be inclined and the material may be heated using steam.

Description

PROCESS FOR TREATMENT OF MATERIALS IN A VESSEL

The invention relates to a process for treatment of materials in a vessel at elevated pressure and temperature. Suitable materials are waste such as municipal solid waste or medical waste.

Waste, in particular municipal solid waste (MSW) and similar types of commercial waste, is an increasing burden on landfills and other forms of waste treatment. Part of the waste stream may be recycled through separated collection. Part of the landfill may be circumvented by incineration; however this is very costly and may have negative environmental impacts. One of the alternatives suggested is treating the waste in a vessel, which may be under pressure, with heat and/or steam while tumbling the content in order to break down the organic content and enable easier separation while the waste is sterilized. Several examples of this idea exists, such as disclosed in EP 0771237, US 4844351, US 6306248, US 4974781, WO 03/09299, WO 2004/018767, WO 03/025101, WO 2008/010970, WO 2008/112168 and WO 2008081028..

The technology has interesting potential: The organic fibres can be used as clean biomass fuel or compost; the glass/grit fraction can be used either by separating the glass for uses in recycled glass products, or it can be used as recycled aggregates. The municipal waste does not have to be pre-selected or pre-treated, and is reduced to about 30 volume% or more of its original size. Further, when fibres are separated, and other useful materials are recycled, landfill can be limited to less than volume% of the original waste.

The technology, however, has several challenges. One is that it is capital-intensive and it is important to maximise the amount of waste that can be processed by a given plant. Another is that it is useful to reduce the amount of energy used during the process and thus improve its economic returns. A third is that it can be useful to reduce the amount of water or steam consumed by the process, for environmental considerations and/or to improve economic returns. Finally it can be useful to reduce the amount of water in the final fibre product to better fit it for some final market uses (to improve the calorific value as a fuel, for example). All previous methods suggested for overcoming one or more of these four challenges have to date involved the purchase of additional equipment, the consumption of more primary energy or some other compromise.

It is an object of the invention to improve the overall economics of the process in one or more of the above mentioned aspects. The improvement to the basic technologies as described in this invention allows for all four of the above improvements.

According to the present invention, the process to treat materials in a vessel at elevated temperature and pressure comprises the following steps: the vessel is loaded with compactable material to be treated, and the material is subjected to at least one compacting stage and additional filling step, the material after the compacting and additional filling step(s) is subjected to the main cooking or treatment cycle, and the treated material from the vessel is obtained, which treated material preferably is classified into one or more useful fractions.

In a preferred embodiment, the compacting step is performed for sufficient time and at a sufficient temperature to have the material reduced by at least about 10 vol%, after which step the additional material occupies at least 5 vol%.

In a further preferred embodiment, the vessel is closed during the compacting step.

The compacting step is preferably performed in such a controlled way that little or no VOC's or steam is emitted into the environment.

More in particular, according to the present invention, the following steps are performed in a process to treat materials in a vessel at elevated temperature and pressure (1) preferably, the vessel is inclined to be loaded, at about 100 or higher, up to about 60° but more usually up to about 45° or more from the horizontal; (2) the vessel is loaded with the material to be treated to at least 50% full but preferably at least 75% and even more preferably at least about 95% full (3) preferably, the vessel is closed (4) optionally, a vacuum up to -0.9 barg is applied (5) the material is heated via the addition of steam, indirect heating or via any other method, generally to a temperature of 70°C to 150 °C but more preferably 95 °C to 110 °C. This can be done under pressure of about 0.1 barg or more up to 3barg but preferably below 0.Sbarg if desired. Heating can start at any time in the sequence, including before the material is charged to the vessel. In case steam is used to heat the material, it is preferred to start the addition of substantial amounts of steam only after closing of the door.

(6) the material inside the vessel preferably is agitated by rotation of the vessel, intermittently if required, as soon as loading has commenced (7) optionally, if the vessel can be inclined or declined along its axis, (in the art usually but not limited to +60° to -20° relative to the horizontal) it may be tipped to an incline optimum for this part of the process at any stage, but usually after loading has finished (8) the vessel is held, usually while rotation and agitation is progressing, for between 1 and 45 minutes, more preferably between 3 and 15 minutes and most preferably about 5 mm or more at elevated temperature; (9) the vessel is depressurised and may be cooled, and the door opened. The original material has now been pre-heated and compacted significantly generally about 25% or more, mostly about 30% or more, up to 50%.

(10) At least about 5% of further material (preferably about 20% or more, more preferably about 30% or more), which may be different to the first if desired, is further loaded into the vessel (11) the vessel is again closed and optionally steps 1 to 10 are then repeated one or more times; (1 2) when sufficient pre-heating and compaction cycles have taken place the main cooking or treatment cycle, for example as described in the prior art, is initiated According to the present invention, the to be treated material is pre-heated and compacted significantly generally about 20% or more, mostly about 30% or more, up to 50%, which allows further introduction of waste materials. The compacting step requires generally less than 20% of the time of a total cycle or less, preferably 15% or less; longer times generally allow for further reduction to for example 30-50%.

The further introduction of waste allows for a very significant increase of plant capacity.

Some prior art suggests to compact the waste somewhat during filling.

For example, US 5540391 describes to tilt and rotate the vessel during filling; US 6397492 describes to keep the vessel heated, and to introduce steam during loading; EP09081 90 describes to comminute the waste, while W0200808201 8 suggests to use a bag splitter or press. These methods are either hardly effective, require extensive wasted steam treatment and/or require additional investment. However, none of the prior art describes a compaction step and thereafter a refill, wherein the compaction step requires the treatment of the content of the vessel for several minutes at 70 °C or higher.

The advantage of this treatment and compaction stage is (1) it increases the charge of material that is possible to be treated in the vessel in any given batch, and thus can improve the throughput and economic returns of the technology at little or no additional Capital cost. Normally, a batch treatment takes between 150 and 350 minutes for a 20 tonne batch, with less or more time required for smaller and larger batches respectively; now it is possible, by adding for example about -45 mm to one cycle, to extend the capacity with about 10-30% or more; which is a significant advantage. Generally, the time needed for the pretreatment step is about 3- 15% of the period of the main process, and it can be about 5-10%.

(2) as a result of the increase of throughput, it reduces energy use per tonne of material treated which improves the technology's cost or environmental position (3) as a result of the reduced amount of energy per tonne of waste, if steam is used as one or more of the heating media, it reduces the quantity of moisture in the resultant products, especially the fibre product. This almost always improves its cost or performance characteristics, either because as a fuel it will have a higher calorific value and/or require less water to be extracted to obtain a target calorific value or because if a cost has to be paid to dispose of the fibre it will have less mass and therefore the disposal cost will be lower (4) as a result of the reduced amount of energy per tonne of waste, the amount of steam required per tonne of waste processed is reduced, with a consequent economic and environmental benefit (5) as a result of the reduced amount of steam necessary for treatment a tonne of waste, the amount of steam condensate arising from the vessels per tonne of waste processed is reduced, with a consequent economic and environmental benefit This method is applicable to any enclosed autoclave-based heat-or steam-treatment process for waste and other products where it is desired to process and separate more useful fractions of a mixture of substances and where the materials introduced in the vessel are compactable. Known methods for treatment of compactable materials include (i) methods using pressure from atmospheric up to steam pressures including and beyond 4 barg (ii) methods using one or more of direct steam, hot gas or water, hot jackets and internal heated features, like flutes or helices, as a method of heating (iii) vessels where the waste is charged and discharged from the same or different ends (iv) methods where the contents are changed by chemical transformation (i.e. by a cooking process) or just dried and compacted.

Specific methods for which this invention is very suitable include processes as described in EP 0771237, US 4844351, US 6306248, US 4974781, WO 03/09299, WO 2004/018767, WO 03/025101, WO 2008/010970, WO 2008/112168 and W02008081028.

Materials that can be advantageously processed with the method of the present invention include wastes and other material from municipal, industrial or other sources such as medical facilities, agricultural and food waste or products, the products of kerbside recycling schemes or biomass from forestry and horticulture. It will be clear for the skilled person, that all sorts of compactable materials that need treatment in an autoclave are useful for the present invention. The invention will be further described for waste, but this is not to be construed as a limitation unless specifically stated.

In the usual embodiments of the autoclave technology, the process to treat these materials comprises some or all of the following steps: (i) pretreating material to improve its properties if required (ii) feeding waste material into a pressure vessel (iii) agitating the vessel while applying heat to the contents of vessel via one of the methods above, to change the properties of the contents (iv) cooling and/or depressurising the vessel (v) discharging the vessel (vi) classifying the treated waste to yield at least a fibrous fraction, further (optionally) yielding several fractions, such as shattered glass and grit, and larger parts such as plastics and metals (vii) further separation and/or improvement of the fractions The pretreatment step is optional, but may include selecting waste, such as for example taking out massive blocks of concrete, carpets and the like. Other pretreatment may include mixing several waste streams and/or pretreatment like adding water or chemicals to the waste.

Feeding of the waste material can be performed as is common in the art, like for example by a conveyer from a hopper. Preferably, the amount of waste is weighted such as to be able to adjust the process to the amount of waste in the vessel.

The vessel can be about horizontal during charging. In case of a horizontal vessel, rotation of the vessel having helical fins or paddles that causes the waste to be transported over the length of the vessel. However, it is preferred to have the vessel tilted above the horizontal, such that the opening of the vessel is at the higher point by for example about 5° or more, preferably about 100 or more, and even more preferably about 15° or more, and even more preferably about 30° or more.

Sometimes, in particular with smaller vessels, it may be possible to have the vessel in a vertical position with the opening upwards, but generally, the tilt will be about 70° or lower, and even more often about 60° or lower. An angle of between 45° and 70° is considered optimal, in particular if the vessel can be rotated.

Preferably, the vessel is rotated during loading, which causes the waste to flow more easily and quickly into the vessel, with less chance of sticking or bridging during filling. Bridging could give the premature appearance that the vessel may be fully loaded.

The vessel is loaded with the material to be treated to at least 50% full but preferably at least 75% and even more preferably at least about 95% full.

When the vessel is filled, preferably, the door of the vessel is closed as to allow vacuum and pressure. Although the invention will also work without applying vacuum or pressure, the cycle time can be shortened if the non-atmospheric pressures can be used. Another advantage of working with a closed door is, that VOC's, and/or waste steam can be treated to preclude contamination of the environment.

Optionally, a vacuum of about -0.1 barg to up to -0.9 barg, preferably about -0.4 to -0.8 barg is applied. This has the advantage of withdrawing VOC's, which can be treated to preclude emission in the environment. Applying vacuum has the further advantage that steam that is thereafter introduced is more effective because it is not diluted by air.

The material is heated in the compacting step, generally to a temperature of 70 °C or more, up to about 150 °C or less, but preferably 90 °C to 110 °C. The use of this temperature allows for fast compacting. Higher temperatures would be possible, but are less attractive from economic point of view. Heating can be suitably done, via the addition of steam (direct heating), indirect heating via heated coils and/or jacket, or via any other method. The heating step can be done at atmospheric pressure, or under pressure of about 0.1 barg or more up to 3barg but preferably below 0.Sbarg if desired. Working at such slightly increased pressure increases the speed of this step, but is not necessary. Heating can start at any time in the sequence, including before the material is charged to the vessel. In that case, preferably indirect heating is used. In case steam is used to heat the material, it is preferred to start the addition of substantial amounts of steam only after closing of the door. A closed door is also preferred in case the precompacting step is performed at about atmospheric pressure, because the gas stream emitted during the heating step can be easily captured, for example in a scrubber. However, at atmospheric pressure the door may also be a closed extraction hood or the like. In contrast, with an open door, all the emitted gas (comprising steam and VOC's) is emitted in the atmosphere.

Capturing this would require a large suction hood, which requires additional investments. Closing the vessel with the autoclave door is preferred, as that does not necessitates any further investment.

The material inside the vessel preferably is agitated by rotation of the vessel, intermittently if required, as soon as loading has commenced. It is also possible to mix the content with a stirrer.

Optionally, if the vessel can be tipped along its axis, (in the art usually but not limited to +60° to -20° about the horizontal) it may be tipped to an incline or decline to an optimum position for this part of the process at any stage, but usually after loading has finished. Such a position may distribute the material uniformly or concentrate it in one place or position it closer or further away from a steam or other inlet, or position it for some other purpose. A man skilled in the art will be able to find the optimal angle, which may be the same as the loading angle, or different.

the vessel is held, usually while rotation and agitation is progressing, for between 1 and 45 minutes, more preferably between 3 and 15 minutes and most preferably about 5 mm or more at elevated temperature. However, the required time for the heating step will be different, depending on the size and efficiency of the heat transfer to the content of the vessel. For example, if only indirect heating is used, the heating time is longer. If direct heating with steam is used, the required time will be dependent for example on the size of the steam lines, and the capacity of the boiler and/or accumulator. The time needed for an efficient downsizing (or compacting) of the waste can be relatively short. By using such short time, the average capacity of a plant can be increased without investment in hardware, and without causing more wastewater to be produced.

If elevated pressure is used during the compacting stage, the vessel is depressurised and may be cooled before the door is opened. It is also possible to apply vacuum to capture even more VOC material, but this is not necessary: the content of the vessel is not very hot, and still relatively dry, so a relatively low amount of vapour is produced when opening the door. These vapours can be captured by a ventilation hood.

The originally fed material has now been pre-heated and compacted. It has been shown, that normal MSW can be significantly compacted, generally about 20 vol% or more, preferably about 25% or more. The compacting is dependent on the type of waste, and medical waste can sometimes be compacted up to 50% as that has more void volume. On average, the compaction was about 30%.

When the precompacting step is finished, and for example, the door is opened, a further amount of waste can be supplied, like for example about 5% or more, preferably about 20% or more, more preferably about 30% or more. Preferably, the vessel is filled till about 95% full.

The further charge(s) of waste may be the same, or different. A different type of waste may be useful, for example when a part needs to be recycled, or when very difficult to treat waste is used. By using the additional volume, the further supplied waste is for example not more than 30% of the total volume.

Generally, after the first additional supply step, it is economically feasible to start the total treatment cycle. However, it may be useful to have another compacting cycle before initiating the full treatment cycle. If the waste can be compacted 50%, it is economically very useful to de two or maybe even three compaction cycles. The vessel is again closed and the above described steps are then repeated one or more times.

When sufficient pre-heating and compaction cycles have taken place the main cooking or treatment cycle, for example as described in the prior art, is initiated In the main treatment cycle, preferably, the vessel is first drawn vacuum till about -0.5 to -0.9 barg, and steam is introduced. It may be useful to use indirect heating in stead or in combination with steam supply. The content of the vessel is agitated, preferably by rotating the vessel while applying heat to the contents of vessel via one of the methods above, to change the properties of the contents. The content generally is sterilized and cleaned. Wood and fibrous material are broken down to fibers; plastic is partly softened and balled, adhesives are broken down, such that a treated waste can be obtained that is relatively clean and easy to handle.

After treatment, the vessel is cooled and/or depressurised. Preferably, vacuum is drawn in the vessel, so relatively dry fibres are obtained.

The fully treated waste can be discharged. In case a tiltable vessel with one door is used, the vessel is tilted preferably to about -20° or lower. However, the method is also suitable in autoclaves with two doors and/or with fixed autoclaves.

Generally, the waste is further classified over screens, picking line, eddy current and magnet, destoner and/or other apparatus. Generally, several useful fractions are obtained from the treated waste. Examples of such fractions include, but are not limited to, a fibrous fraction, shattered glass and grit, and larger parts such as plastics (like PET bottles) and metals.

The fibrous fraction may consist for more than 95% of organic materials, and can be used as composting additive; fuel, biogasification or otherwise. The clean glass and grit fraction can be used as filling material for roads or otherwise.

The above description has been focussed on the treatment of municipal solid waste, but the process is equally applicable to other processes where compactable material is treated under elevated pressure and temperature. The process is in particular suitable for treatment of medical waste, because medical waste is often delivered in containers with relatively large free space. Yet, the pretreatment, in particular when using vacuum before and after compacting, precludes dangerous materials to come into the environment. With medical waste it may be useful to heat the waste to e.g. 140 °C in the pretreatment for safety purposes. Yet, the pretreatment can be much shorter than a full cycle, and a very useful increase in capacity can be obtained without any investment. It should be noted that it is not uncommon to comminute the medical waste before loading the vessel. This has however as large disadvantage that the communiting has to be done in a chamber that is isolated and can be sterilized.

Other types of compacting materials are waste from slaughterhouses, supermarkets, and waste from landfills that need to be sanitized.

The invention will be elucidated by the following examples, without being limited thereto.

Example and comparative experiment In October 2008 at the Sterecycle plant at Sheffield Road, Rotherham UK, the following results comparing a standard treatment cycle with an improved cycle using the method above were obtained.

Date Process Description Total Total cycle

amount time processed 15-27 Comparative experiment (as described in 18t (yielding 310 minutes Oct US5540391) 9.9t fibre 2008 Standard cycle: open door of vessel at loading product) position; start rotation; charge 18t waste; close door and lower to horizontal; apply vac to -0.7 bar; apply steam to 145 °C; hold while rotating at temp for 60 minutes; vent and apply a vac to -0.7 barg; break vacuum, open door; discharge vessel; return to loading position 27-30 Example according the invention 25t (yielding 340 minutes Oct Improved cycle: open door of vessel at loading 14.5 t fibre 2008 position; start rotation; charge 18t waste; close door product) and lower to horizontal; apply vac to -0.7 barg; apply steam to 101 °C; hold while rotating at temp for 5 minutes; release pressure; return to loading position; open door; charge further 7t waste: close door and lower to horizontal; apply vac to -0.7 barg; apply steam to 145 °C; hold while rotating at temp for 60 minutes; apply vac to -0.7 barg; open door; discharge vessel; return to loading position From the experiments, it appears that the time necessary to treat one tonne of waste is decreased from 17.2 min/tonne of waste to 13.6 min/tonne. In this way, in an increase of capacity of the plant is obtained of more than 20 percent.

Claims

Claims 1. Process to treat materials in a vessel at elevated temperature and pressure, wherein the vessel is loaded with compactable material to be treated, and wherein the material is subjected to at least one compacting stage and additional filling step, the material after the compacting and additional filling step(s) is subjected to the main cooking or treatment cycle, and the treated material from the vessel is obtained, which treated material preferably is classified into one or more useful fractions.
2. Process according to claim 1, wherein the compacting stage is performed with the vessel closed.
3. Process according to any one of claims 1-2, wherein the compacting step is performed for sufficient time and at a sufficient temperature to have the material reduced with at least about 10 vol%, after which step the additional material occupies at least 5 vol%.
4. Process according to any one of the preceding claims, wherein the process to treat materials in a vessel at elevated temperature and pressure comprises the following steps: (1) the vessel is loaded with compactable material to be treated to at least 50% full but preferably at least 75% and even more preferably at least about 95% full; the process further comprises as compacting and additional filling step that: (2) the material is heated via the addition of steam, indirect heating or via another method, generally to a temperature of 70 °C to 150 °C but more preferably 95 °C to 110 °C, (3) the vessel is held, usually while rotation and agitation is progressing, for between 1 and 45 minutes, more preferably between 3 and 15 minutes and most preferably about 5 mm or more at elevated temperature; (4) and at least about 5% of further material (preferably about 20% or more, preferably about 30% or more), which may be different to the first if desired, is further loaded into the vessel (5) optionally, steps 2 to 4 are repeated one or more times; after which step(s), the material after the compacting and additional filling step(s) (6) is subjected to the main cooking or treatment cycle, (7) and the treated material from the vessel is obtained, (8) which treated material preferably is classified into one or more useful fractions.
5. Process according to any one of the preceding claims, wherein the material inside the vessel is agitated by rotation of the vessel, intermittently if required, as soon as loading has commenced
6. Process according to any one of the preceding claims, wherein the vessel is inclined to be loaded, at about 10° or higher, up to 60° but more usually up to about 45° or higher with respect to the horizontal.
7. Process according to any one of the preceding claims, wherein the compacting step is performed under pressure of 0.1 barg or more, and up to 3barg but preferably of below 0.5barg.
8. Process according claim 7, wherein the vessel is depressurised and may be cooled, and the door opened for further process steps
9. Process according to any one of the preceding claims, wherein a vacuum up to -0.9 barg is applied between steps after loading of the vessel.
10. Process according to any one of the preceding claims, wherein heating is started at any time before or during loading of the vessel, or after closing of the vessel.
11. Process according to any one of claims 2-10, wherein in case steam is used to heat the material, the addition of substantial amounts of steam is started after closing of the door.
12. Process according to any one of the preceding claims, wherein the vessel is tipped along its axis in the art usually but not limited to +60° to -20° with respect to the horizontal during the compacting stage.