OLSSON, Nicholas (1 Gilarth Street, Highett, Victoria 3190, AU)
PETCH, Genevieve (4/31 Second Street, Black Rock, Victoria 3193, AU)
CUMMINS, Peter (58 Blessington Street, St Kilda, Victoria 3182, AU)
OLSSON, Nicholas (1 Gilarth Street, Highett, Victoria 3190, AU)
PETCH, Genevieve (4/31 Second Street, Black Rock, Victoria 3193, AU)
| THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A process for treating an aqueous stream from a polymerisation process comprising the steps of: a) subjecting a stream containing polymer and water from a polymerization reactor to solid liquid separation to recover a polymer product and an aqueous stream containing suspended solids and dissolved solids; b) treating the aqueous stream to reduce suspended solids content; and c) treating the aqueous stream to remove dissolved solids and remaining suspended solids content following reduction of the suspended solids content in preceding treatment step (b). 2. A process of claim 1 wherein said aqueous stream is an acidic stream, having pH less than 7. 3. A process of claim 2 wherein pH of said aqueous stream is in the range 2.5 to 3.5. 4. A process of claim 2 wherein pH of said aqueous stream is higher than 3.5 if buffered. 5. A process of any one of the preceding claims wherein step (c) involves membrane separation step(s). 6. A process of any one of the preceding claims wherein steps (b) and (c) are each conducted in a plurality of stages. 7. A process of claim 6 wherein, in step (b), provision is made for solids separation over a wide particle size range of suspended solids. 8. A process of claim 7 wherein filtration or straining is used to remove larger particles. 9. A process of claim 7 or 8 wherein density separators, such as cyclones or hydrocyclones, are used to remove smaller solid particles in subsequent solid/liquid separation stage(s). 10. A process of any one of claims 5 to 9 wherein said membrane separation includes microfiltration followed by reverse osmosis. 11. A process of any one of claims 5 to 10 wherein said aqueous stream is cooled prior to a membrane separation step. 12. A process of claim 10 wherin said aqueous stream is cooled prior to reverse osmosis. 13. A process of any one of the preceding claims wherein said polymerisation process is a suspension polymerisation process for producing polyvinyl chloride (PVC). 14. A process of claim 13 wherein said aqueous stream contains particles of PVC as well as inorganic species selected from the group consisting of sodium, potassium, magnesium, calcium, total iron, insoluble iron, zinc, aluminium, chloride, sulphate, nitrate, phosphate, fluoride, silica, total alkalinity, hydroxide, bicarbonate and carbonate. 15. A process of claim 14 wherein said aqueous stream contains polyvinyl alcohol (PVA). 16. A process of claim 15 wherein at least step (b) is conducted at a temperature at which PVA remains, essentially or at least predominantly, in solution. 17. A process of any one of claims 13 to 16 wherein a solid/liquid separation step to recover PVC polymer product is centrifugation and said aqueous stream is a centrate from the centrifugation step. 18. A process of any one of the preceding claims being conducted under sterile conditions. 19. A process of any one of the preceding claims wherein following treatment steps (b) and (c), said treated aqueous stream is treated water which is recycled for use in a polymer production or processing plant. 20. A process of claim 19 wherein said recycled treated water is used as process charge water in polymerisation reactors. 21.A process of claim 20 wherein said recycled treated water is blended with a make up water stream having required quality for use in the polymerisation process prior to being directed to polymerisation reactor(s). 22.A process of claim 21 wherein a ratio of treated water to make up water, is dependent on a grade of polymer to be produced in the polymerisation process. 23.A system for treating an aqueous stream from a polymerisation process comprising: a) a solid liquid separation stage for subjecting a stream containing polymer and water from a polymerization reactor to solid liquid separation to recover a polymer product and an aqueous stream containing suspended solids and dissolved solids; b) a suspended solids content reduction stage; and c) a dissolved solids content reduction stage, for removing dissolved solids and remaining suspended solids content following the suspended solids content reduction stage (b). 24.A system for treating an aqueous stream from a polymerization process of any one of claims 1 to 22. 25.A process for treating an aqueous stream arising from polymer production or processing comprising the steps of: a) subjecting a stream containing polymer and water to solid liquid separation to recover a polymer product and an aqueous stream containing suspended solids and dissolved solids; b) treating the aqueous stream to reduce suspended solids content; and c) treating the aqueous stream to remove dissolved solids and remaining suspended solids content following reduction of the suspended solids content in preceding treatment step (b). 26.A process of claim 25 wherein said treated aqueous stream from step c) is recycled or otherwise distributed for use in a polymer production plant or for other industrial uses. |
POLYMERISATION PROCESS
This invention relates to a process and system for treating an aqueous stream from a polymerisation process.
With increasing focus on cost reduction and environmental awareness, it is becoming imperative for water used in industrial plants to be recycled. Treatment processes vary, to a large extent with the particular processes carried out in industrial plants as well as the water needs of that plant. For example, some industrial plants may produce aqueous streams in the form of highly contaminated water but only require low quality water for use within them. Other plants may produce highly contaminated water and require a high quality of water for applications within those plants. For example, water may play an important role within a chemical process, an effluent water stream being recovered elsewhere in the plant as a by-product of the process. In that case, if the water is to be reused, it may need to meet a high quality or purity standard. Indicative of such plants are polymer processing and polymer production plants, for example those for producing polyvinyl chloride.
Polyvinyl chloride (PVC) is a polymer derived from the polymerisation of vinyl chloride monomer (VCM), a chlorinated hydrocarbon. PVC is a safe and very versatile polymer which can be used in both rigid and soft (flexible) applications depending on the additives that it is mixed with. PVC known as rigid or unplasticised PVC ("uPVC") is made from unplasticised resin. Flexible PVC includes plasticisers. Whatever the type of PVC, it is supplied as a resin, in various grades, for use in manufacture of building products such as pipes. Annual Australian PVC consumption is over 210000 tonnes annually.
PVC is produced by a three stage process involving:
(1 ) Reaction;
(2) Recovery; and
(3) Drying
following which the PVC product is packaged and despatched. While the process details may vary slightly with the grade of PVC to be produced, these three stages are present in all PVC production processes. In the reaction stage (1 ), PVC is produced by polymerising VCM in autoclave(s) or pressurised polymerisation reactor(s) where the PVC polymer is formed in the presence of water, suspending agents and initiators at controlled temperature. A significant volume of high quality water, typically sourced from a mains water supply is required for the polymerisation process. This represents a significant process cost as well as a demand on potentially limited high quality water supplies.
Heating of the initiators, whether organic peroxides or in situ initiators, generates free radicals which start the polymerisation reaction. The amount and type of suspending agents (together with the reaction temperature) determine the size of droplets of monomer dispersed in the water as well as the porosity of PVC grains produced.
The reaction temperature in the autoclave(s) fixes the average molecular weight as well as the size and structure of the polymer particles and it is desirably controlled within the range of 50-80 0 C, dependent on the average molecular weight of PVC polymer required using cooling jacket(s) for the autoclave(s). VCM initially disperses to a mist of fine droplets which then agglomerate to the final
PVC grains during polymerisation. Water also helps in removing the heat of reaction because the chemical reaction which creates PVC is exothermic, giving off heat. Approximately equal volumes of water and VCM are piped into the autoclave(s) creating the suspension which begins the process of polymerisation.
The VCM polymerises or converts to form a slurry of PVC grains in water.
After time for an acceptable degree of conversion (which typically takes 3-5 hours), the reaction is stopped chemically and then slurry is discharged from the autoclave(s) to degassing and removing unconverted VCM.
In the recovery stage (2), unconverted VCM is removed from the PVC slurry. VCM gas is vented from degassing to a gasholder and remaining VCM dissolved in the PVC polymer is removed by live steam in a stripper. The recovered VCM is compressed, cooled and liquefied; water is separated and the VCM liquid is returned to VCM storage for reuse. Almost all of the VCM that has not been used in the PVC batch is removed and reused.
In the drying stage, PVC is separated from slurry, containing approximately 30% w/w PVC) to complete the production process. Typically, the water is removed by centrifugation as slurry passes through centrifuge(s). However, other water separation or dewatering techniques may be employed. A damp 'cake' of polymer is obtained, this cake being conveyed to drying for example in fluid bed dryer(s). Here remaining water contained in porous PVC grains evaporates as a stream of heated air bubbles through the polymer powder.
Water from the dewatering operation, called centrate where centrifugation has been used as the dewatering technique, is typically the largest waste effluent stream in the PVC production process. It is acidic, having pH about 3, containing an elevated level of both inorganic and organic dissolved solids and significant quantities of PVC particles as suspended solids. Temperature is 6O 0 C and a range of contaminants will be present in centrate dependent on the product grade being manufactured. The contaminants originate from additives used in the reaction stage of the PVC production process. These additives vary from PVC product grade to product grade. For example, Total Dissolved Solids (TDS) will vary depending on the type of polymerisation reaction initiator that is used. Dissolved inorganic chemicals may comprise predominantly calcium and chloride ions, especially where an in situ initiator is used.
In addition to the dissolved inorganic ions, there are dissolved organic compounds. These dissolved organics are mainly derived from Polyvinyl Alcohol (PVA), itself a vinyl polymer. PVA arises from additives used as suspension agents in the PVC polymerisation process. Other dissolved organic compounds are from additives used to terminate the polymerisation reaction, for example to routinely control product properties. Further, though the effluent leaving the PVC production process is sterile, dissolved PVA and other organics are potential nutrients for algae and other microbes.
Suspended PVC particles may vary in concentration from 5-300 mg/litre depending on the upstream process conditions. The particle size distribution is in the range of 1 μm to 200μm. The aim of the production process is to make PVC with particles approximately 100μm in size. In the effluent, there will generally be a peak of particles this size. However, the particle size distribution will differ dependent on the grade of PVC being produced. On some grades, there is another peak at 20μm. The presence of these various species within water recovered from the drying process presents particular difficulties particularly if it is desired to recycle the effluent water to the polymer production plant, particularly the centrate from centrifugation.
A prior centrate treatment process has involved filtration of centrate using a leaf filter. The leaf filter relies on the formation of a filter cake as the medium for the removal of suspended solids. Filtered water still contained high levels of dissolved solids, which was suitable for applications requiring low quality water such as reactor rinsing but not otherwise.
While the leaf filter process was effective at removing suspended solids, it had a number of operating issues including:
a) The process was labour intensive. After cleaning, a filter cake needs to be re-established. This is achieved by feeding coarse powder into the wastewater and operating the filter on recycle. Filter cake formation could take several hours depending on operator judgment, filtrate turbidity and pressure drop to determine that a filter cake had been established. Cleaning was particularly time intensive.
b) The leaf filter did not always reliably provide water free of suspended solids. Seal failures in tears or other flaws in the leaf filter could result in unfiltered water bypassing the filtration process altogether.
Other processes for treating aqueous streams in the form of effluent water from polymer production plants, especially PVC production plants, have been experimented with. Such processes may utilise membrane technology in an effort to separate some of the contaminants that have been identified above. Difficulties may be encountered when using such processes due to the challenging nature of the contaminants. High suspended solid loadings and the presence of sticky PVA are challenges for treatment processes.
It is an object of the present invention to provide a process for treating an aqueous stream from a polymerisation process which addresses the difficulties encountered with prior art processes.
With this object in view, the present invention provides, in one aspect, a process for treating an aqueous stream from a polymerisation process comprising the steps of: a) subjecting a stream containing polymer and water from a polymerization reactor to solid liquid separation to recover a polymer product and an aqueous stream containing suspended solids and dissolved solids;
b) treating the aqueous stream to reduce suspended solids content; and
c) treating the aqueous stream to remove dissolved solids and remaining suspended solids content following reduction of the suspended solids content in preceding treatment step (b).
The aqueous stream may be an acidic stream, having pH less than 7. pH of the aqueous stream may be in the range 2.5 to 3.5 and higher, if buffered.
Steps (b) and (c) are discrete or separate, though co-operating - steps, step (b) involving suspended solids removal step(s) and step (c) involving membrane separation step(s) for dissolved solids removal. Steps (b) and (c) may each be conducted in a plurality of stages. Therefore, in step (b), provision may be made for solids separation over a typically wide particle size range of suspended solids in the aqueous stream. That wide particle size range may include particles with sizes in the order of millimetres or larger to micron or sub- micron sized particles. These particles also vary in shape, porosity, density and chemical nature. Particle shape may be irregular making efficient separation challenging. Nevertheless, filtration (pre-filtration) or straining may be used to remove some larger particles (having average particle size of millimetres rather than microns) also. However, and most advantageously, other solid/liquid separation devices, such as cyclones or hydrocyclones which separate solids on the basis of relative density, may be used to remove, especially smaller solid particles, in subsequent solid/liquid separation stage(s).
In step (c), the membrane separation step(s) may include use of membranes with pore size selected to remove remaining particularly small suspended solids particles, to the extent these remain after step (b), and, particularly, dissolved solids. Such remaining suspended solid particles typically have micron or sub-micron size. Advantageously, the membrane separation steps may include microfiltration followed by reverse osmosis. Microfilitration is particularly targeted at removal of remaining especially small suspended solids particles having sub-micron particle size down to about 0.2 μm. Reverse osmosis is primarily targeted at removal of dissolved solids. While removal of substantially all suspended solids is an object of the process, it is not so necessary for all dissolved solids to be removed. However, the chemical composition of the treated water, the desired final product of the process, is relevant to the application(s) to which treated water may be put.
The polymerisation process may be a process, such as a suspension polymerisation process, for producing polyvinyl chloride (PVC) and the aqueous stream may contain particles of PVC as well as inorganic species selected from the group consisting of sodium, potassium, magnesium, calcium, total iron, insoluble iron, zinc, aluminium, chloride, sulphate, nitrate, phosphate, fluoride, silica, total alkalinity, hydroxide, bicarbonate and carbonate. These species may originate from the additives used in the PVC polymerisation process. The aqueous stream may contain polyvinyl alcohol (PVA) and, in this case, it is desirable for at least step (b) to be conducted at a temperature at which PVA remains, essentially or at least predominantly, in solution.
The solid/liquid separation step to recover polymer product, particularly PVC product, may be centrifugation in which case the aqueous stream is a centrate from the centrifugation step. The temperature of the centrate is a function of the stripping process (where unreacted VCM monomer is removed from the PVC particles produced in the autoclave(s)) which may be in the range 50 to 80 0 C. At these temperatures, the centrate is expected to be biologically inactive or substantially sterile. Desirably, the water treatment process is conducted under sterile conditions so that dissolved PVA and other organics do not promote algal and other microbial growth. Maintenance of a low or acidic pH is also desirable from the point of view of promoting sterility. Exposure to light and other atmospheric contaminants is also desirably to be prevented.
Other aqueous streams could be treated. One possibility is for an aqueous stream, including centrate, to be subjected to treatment. However, in such cases, suspended solids loadings (to which the process is sensitive), may be sufficiently high to impact on efficient operation of the process. In such cases, additional suspended solids removal steps may be needed to reduce suspended solids to a manageable level. Following treatment steps (b) and (c), treated aqueous stream is treated water which may be recycled, following any requisite adjustments, for use in the polymer production or processing plant. For example, pH adjustment or neutralisation of treated water may be necessary prior to delivering treated water to the polymerisation process. The content of certain chemical species, which correlate with additives (such as initiators, surfactants, suspension agents) typically added to the polymerisation process, dependent on the desired properties of the product polymer, may also impact quantities of additives to be introduced to the polymerisation reactor(s). Advantageously, the water quality is such that it may be used as process charge water in the polymerisation process and, in that case, treated water may be recycled to polymerisation reactor(s). Alternatively, treated water may be blended with a water stream having the required quality for use in the polymerisation process prior to being directed to polymerisation reactor(s). The ratio of treated water to make up water, which may be high quality town water or demineralised water, may be dependent on a grade of polymer, particularly PVC, to be produced in the polymerisation process.
Treated water may also or alternatively be directed to other applications, such as boiler feed and cooling water applications, within the polymer processing plant.
The aqueous stream may be cooled in a cooling step prior to a membrane treatment step. In particular, cooling is advantageously practiced prior to the reverse osmosis step for protection of the reverse osmosis membrane(s) where such are temperature sensitive. That is, water should not be delivered to the reverse osmosis membrane(s) at a temperature above the maximum recommended operating temperature for the reverse osmosis membrane(s).
In a further aspect, the present invention provides a system for treating an aqueous stream from a polymerisation process comprising:
a) a solid liquid separation stage for subjecting a stream containing polymer and water from a polymerization reactor to solid liquid separation to recover a polymer product and an aqueous stream containing suspended solids and dissolved solids;
b) a suspended solids content reduction stage; and c) a dissolved solids content reduction stage, for removing dissolved solids content and remaining suspended solids content, following the suspended solids content reduction stage (b).
In a still further aspect, the present invention provides a process for treating an aqueous stream arising from polymer production or processing comprising the steps of:
a) subjecting a stream containing polymer and water to solid liquid separation to recover a polymer product and an aqueous stream containing suspended solids and dissolved solids;
b) treating the aqueous stream to reduce suspended solids content; and
c) treating the aqueous stream to remove dissolved solids and remaining suspended solids content following reduction of the suspended solids content in preceding treatment step (b). Water treated by this process may be recycled or otherwise distributed for use in, for example, a polymer production plant or other industrial uses.
The treatment processes and systems of the present invention allow reuse of a significant volume of water from a polymer production and/or processing plant with potential benefits for process economics and the environment. At the same time, trade waste discharge can also be reduced because of water recycling achievable using the treatment processes and systems.
The treatment process and system of the present invention may be more fully understood from the following description of a preferred non-limiting embodiment thereof made with reference to the accompanying drawings in which:
Fig. 1 is a process flowsheet for a PVC production plant in which the water treatment process of one embodiment of the present invention is conducted;
Fig. 2 is a process flowsheet illustrating a water treatment plant for conducting a water treatment process in accordance with one embodiment of the present invention;
Fig. 3 is a detail of a hydrocyclone used for suspended solids removal in the water treatment plant shown in Fig. 2. Referring now to Fig. 1 , polyvinyl chloride (PVC) is produced in a batch suspension polymerisation process conducted within PVC production plant 10. During suspension polymerisation, vinyl chloride monomer (VCM) from VCM stores 9 is dispersed in water, from water supply 11a, in a reactor 12 (autoclave) together with an initiator and a surfactant or suspension agent from additives supply 11. The autoclave 12, of which only one is shown for purposes of illustration, is stainless steel lined and designed to withstand pressures of up to about 2000 kPa. As the PVC polymerisation process is exothermic, and temperature control is important to PVC product grade being produced, autoclave 12 is provided with a water cooling jacket (not shown) to cool temperature to the required range of 50 to 80 2 C. Water cooling towers are also employed within PVC production plant 10 to reject the heat of PVC polymerisation.
In line with conventional practice, the suspension polymerisation process is stopped well short of 100% conversion to PVC polymer. The polymerisation process may be between about 80 and 95% complete when stopped. An aqueous slurry 14 containing porous PVC grains and other components (suspended and dissolved solids) is recovered to degasser 16 where unconverted VCM is volatilised and recovered for return through VCM recovery units 13 to autoclave 12 for use in a further batch polymerisation when required. Some unconverted VCM remains absorbed in the PVC grains and this unconverted VCM may be recovered by steam stripping in stripper 18.
The aqueous stream or slurry 20 containing PVC polymer and water is subjected to solid/liquid separation in centrifuge(s) 22 to enable recovery of PVC product and an aqueous stream or centrate 24. Slurry tank 21 provides intermediate storage for slurry 20 prior to delivery to centrifuge(s) 22. PVC polymer product is subjected to further drying in fluidised bed dryer(s) 26 before storage in silo 29 and packaging and despatch stages 30.
The centrate 24 contains a mixture of smaller particles of PVC polymer, suspended solids and dissolved solids. The suspended solids may have a wide particle size range comprising particles having size in the order of millimetres and micron and sub-micron sized particles. The centrate 24 may also contain polyvinyl alcohol (PVA) in solution as the centrate 24 is warm having temperature in excess of about 50 9 C. Centrate 24 is also acidic, having low pH of about 3. The centrate 24 is treated to recover water for recycle to PVC production plant 10.
Referring now to Fig. 2, the centrate 24 is fed to water treatment plant 40 to reduce suspended solids and dissolved solids content. Dissolved species include inorganic species such as sodium, potassium, magnesium, calcium, total iron, insoluble iron, zinc, aluminium, chloride, sulphate, nitrate, phosphate, fluoride, silica, total alkalinity, hydroxide, bicarbonate and carbonate. In addition to the dissolved inorganic ions, there are dissolved organic compounds including polyvinyl alcohol (PVA) derived from PVA additives used as suspension agents in the polymerisation process. The PVA molecules tend to adhere to the surface of PVC particles making them sticky. Other dissolved organic compounds are from additives used to terminate the PVC polymerisation reaction. Centrate 24, having the above noted pH and temperature, is sterile. Suspended solids are mainly PVC particles that vary in concentration from 5-300 mg/litre dependent on conditions in PVC polymer production plant 10. The particle size distribution of these PVC particles is in the range of 1 μm to 200 μm. The actual particle size distribution will differ depending on the grade of PVC produced.
Water treatment plant 40 is set up as a three stage process:
(1 ) Initial solids removal - hydrocyclones
(2) Suspended solids removal - microfiltration (MF)
(3) Dissolved solids removal - Reverse Osmosis (RO)
which stages are described below.
Centrate 24 is first directed to an agitated receiver tank 42 and then stream 43 is pumped by pump 42a onto a two stage hydrocyclone assembly 44 having two hydrocyclones 45, one of which is shown in schematic in Fig. 3. Hydrocyclone assembly 44 separates solids based on their relative higher densities than water, the operating principle being based on creation of a vortex that forces the denser particles down and out of the underflow nozzle 45a as conveniently illustrated in Fig. 3. The specific gravity of PVC is 1.4 whereas water has specific gravity about 1. However, because the PVC particles present in centrate 24 are porous and irregular in shape the actual specific gravity of the particles is approximately 1.3 (after taking into account water inside the pores of the particles). Hydrocyclone 44 is effective in removing PVC particles having particle size greater than 40 μm, more specifically being capable of removing 80%, or better, of solids having particle size of greater than 50 μm. This significantly reduces suspended solids loading to subsequent membrane separation stages of the water treatment plant 40 and is therefore important to the efficient operation of those MF and RO stages 50 and 60 to be described further below.
The hydrocyclone assembly 44 could be supplemented by a filter upstream from assembly 44 if required. Such a filter would advantageously be of self- cleaning type.
Hydrocyclone overflow 46, which may be passed through a fine strainer in case lower density contaminants should pass through hydrocyclone assembly 44, is directed to microfiltration (MF) feed tank 48 prior to pumping, through pump 49 to microfiltration stage 50. The MF membrane is a hollow fibre polyethersulfone (PES) membrane, having 2.2 mm inner diameter fibres, designed to remove all suspended solids, in an inside to outside microfiltration process, down to a size of 0.2 μm. Fibre internal diameter of 2.2 mm is selected to minimise risk of blockage. The hollow fibre membrane process operates as an "inside-out" filtration process. Stream 46 is passed down the inside of the hollow fibres with permeate (clean water) 52 passing through the fibre wall. Crossflow down the membrane fibre is used to remove solids that deposit on the membrane wall. This is achieved with a recycle pump (not shown) to achieve a high flow. The MF stage 50 has a 10% concentrate bleed 54 which allows contaminants to be bled from the system. Backwashing is regularly practised to prevent fouling. During a backwash, permeate 52 is forced back through the membrane wall (outside-in) removing build up on the membrane wall while the concentrate 54 is being purged from the MF stage 50. The flow in the hollow membrane fibres is also reversed to ensure complete flushing of the membrane.
The membrane is chemically cleaned to remove organics and other membrane deposits that cannot be dislodged by the backwash process. Caustic soda, in combination with sodium hypochlorite, is used as the main cleaning agent. The water recovery efficiency for the MF stage 50 is about 90%.
Permeate 52 leaving the MF stage 50 is essentially free of suspended solids of size greater than 0.2 μm but still contains dissolved solids that need to be removed, particularly if treated water is to be used as charge water for autoclave 12. To that end, the MF permeate 52 is passed through a three stage reverse osmosis (RO) stage 60 to reduce the concentration of dissolved solids.
In the initial solids removal (hydrocyclone assembly 44) and MF stage 50; it was desirable to keep water temperature as high as possible, the object being to keep PVA in solution to the greatest possible extent. As previously observed, PVA molecules tend to adhere to the surface of PVC particles making them sticky. This would increase the likelihood of the PVC particles adhering to the MF membrane. Desirably, then, centrate 24 and hydrocyclone overflow 46 are maintained at a temperature at which PVA remains in solution. If PVA comes out of solution, fine PVC particles adhere to the precipitated PVA droplets resulting in a PVC/PVA agglomeration. If this agglomeration is trapped or forms inside an MF membrane fibre it can result in irreversible damage. Such an agglomerate might be broken down with caustic soda but this would require a vigorous high shear cleaning process. If this agglomeration blocks the inside of an MF fibre, it is unlikely that it would be removed since it is not possible for cleaning chemical to enter a blocked membrane. To that end, water streams being treated for solids removal are maintained at as high a temperature as possible. Insulation and heaters, as well as removal of stages that allow water to cool before treatment, could be included to achieve this objective. This precaution also minimises microbial activity.
However, water fed to the RO stage 60 could still be sufficiently warm to cause damage to RO membranes. To that end, cooling of MF permeate 52 is provided for. For example, the permeate 52 may be air cooled, to below the maximum recommended membrane operating temperature (40 to 45 S C) prior to delivery to RO stage 60. A shell and tube heat exchanger could be used as an alternative.
The RO stage 60 removes dissolved salts and dissolved organic compounds from the water. This RO stage 60 is a conventional three stage process with a recovery efficiency of 90%. Species of particular concern here include chloride, calcium and sodium ions, which may vary in content dependent on the grade of PVC being produced in PVC production plant 10. The RO membranes are spiral wound polyamide membranes supplied from Koch Membrane Systems under the trade name Koch 2540-HR. However, other membranes having similar specifications would be available from other manufacturers. Products of the RO stage 60 are made up of 90% permeates and 10% concentrates. Permeate 62 from RO stage 60 may be recycled for reuse, following any necessary adjustment, in PVC production plant 10. For example, RO permeate 62 has low pH and requires neutralisation prior to recycle for use within PVC production plant 10. Additionally, RO permeate 62 still contains moderate levels of dissolved organic compounds (about 10 mg/l). These dissolved organic compounds may impact on the end use applications of the recycled water. For example, the dissolved organic compounds are nutrients for algae and microbes. Biocides may be added to minimise algal and microbial growth. Concentrate 64 is discharged to trade waste sewer.
Recycled water may be used for cooling tower make-up, rinse water and boiler feed water make-up. Advantageously, the treated and recycled water would be used as process charge water for the PVC polymerisation process itself. Prior to such use, various adjustments need to be made to the temperature and pH of the recycled water. For instance, pH must be near neutral value and water temperature needs to be cooled below 20 3 C. Additions and nature of additives (for example initiators) for use in the polymerisation process may also need to be adjusted according to the chemical analysis of recycled water. The recycled water may also be blended with other sources of high quality water especially when used for PVC process charge water. The actual ratio of recycle water to make up water may be made dependent on the grade of polymer to be produced in the polymerisation process.
Water loadings to water treatment plant 40 may vary with the production schedule of the polymer production plant 10. In particular, the water flow may depend on the grade of PVC being produced and the number of batches of PVC being produced at any one time. Suspended and dissolved solids loadings may also vary. Process settings may be controlled to take such variability into account.
Use of the above described process and system achieved reduction in high quality water, typically sourced from a municipal water system, of approximately 50%. At the same time, due to the ability to recycle water effectively, a 73% trade waste discharge was also achievable.
Modifications and variations to the process and system for treating an aqueous stream from a polymer processing plant described here may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention. For example, whilst the process and system of the present invention are particularly suitable and advantageous for treating water from PVC polymerisation, water streams from other polymerisation processes may be treatable using them.
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