- Email: [email protected]
Processing petrochemicals: Reverse osmosis in petrochemicals
16
Feature
Filtration+Separation October 2007
Processing petrochemicals:
Reverse osmosis in petrochemicals A
technique more commonly associated with desalination can also be applied in other liquid processes and may be particularly suited to oil. Anthony Bennett explains why. The process of reverse osmosis (RO) represents the finest, or tightest, level of aqueous filtration available today. Ordinary filters use a ‘screen’ or ‘depth filter’ medium to separate particles from the feed water streams. An RO system employs a semi-permeable membrane that separates a very high percentage of molecules based not only on their physical size but also their molecular size and ionic charge. There are three different types of filter media that are operated in a similar manner, ultrafiltration (UF), nanofiltration (NF) and RO. All three of these membrane types are permeable to water molecules but retain impurities based on their molecular ‘size’ – molecular weight and in the case of UF also their 3-D molecular configuration. RO and NF also retain impurities based on their ionic charge so can retain, or reject, charged materials with a low molecular size. The mechanism by which each of these membrane types reject contaminants is slightly different but to prevent the membrane from becoming fouled by retained solids and salts forced against it by the pressurised stream of feed water all three types have to be operated in a way which reduces the rate of membrane fouling by these contaminants. These membrane types are used in systems which employ cross-flow filtration. This allows some filtered water to pass through the membrane while a flow of water also sweeps the surface of the membrane taking rejected contaminants away from the membrane surface.
In the processing of crude oil and gas, there are important associated applications which depend on using membranes and which are critical to maintaining the commercial operation of the industry.
Operating a RO or NF system produces a continuous stream of water which contains the materials rejected by the membrane – this will typically be a waste stream but in some applications it may be the material in the concentrate stream which is required. The maximum conversion of feed water to permeate (or filtrate) is typically 70–80%.
Feature
Filtration+Separation October 2007
Table 1: General performance of different types of membranes UF NF RO
Molecular weight range (Daltons 1,000 – 1,000,000 > 200 > 200
The three types of membranes have general types of performance, as shown in Table 1, although individual membrane types from different membrane suppliers may have different profiles of performance and individual membranes usually have to be tested for each application. This article will consider two applications within the petrochem industry which use RO and/or NF membranes. Initially the petrochem industry would appear to be an unlikely place to use RO membranes which have been designed to operate on aqueous feed streams and remove dissolved ions, particles and bacteria from the feed water. Certainly there are probably limited opportunities to use RO membranes in the processing of crude oil and gas, however, there are some important associated applications which do depend on using membranes and which are just as critical to maintaining the commercial operation of the industry.
Using water to produce oil Deposits, or reservoirs, of crude oil lie at various depths below the ground. Oil can
Ionic Rejection of Monovalent Ions None 50 – 90% 98 – 99 %
be recovered from reservoirs just below the surface by means of open cast mining while from greater depths, it has to be recovered by drilling. At great depths, the crude oil is subjected to the pressure of the ground above it and of any natural gas which exists in the reservoir. When tapped by a drill the oil flows out of the borehole under pressure since it is lighter than water and the surrounding rocks. This so-called primary production is normally possible without further measures to aid the flow of oil. When the pressure in the reservoir drops, the oil has to be pumped out. The pressure within the reservoir can also be increased by pumping water or natural gas into the borehole. This is called secondary production. As the reservoir becomes exhausted, the amount of water in the output increases and, finally, more water than oil leaves the well. This signals the end of this production method. During oil production, reservoir pressure decays due to oil and gas abstraction, resulting in a decrease in production. Well pressure
is therefore, in most cases, maintained by injection of seawater. However, when normal high sulphate seawater is injected into reservoirs which have formation water containing barium and strontium, mixing occurs forming a supersaturated barium and/or strontium sulphate solution. When the pressure decreases in and around the production wells, the supersaturated barium and/or strontium sulphate solution is no longer stable and precipitation occurs. The result is scale formation in the production tubing and/or plugging of reservoir rock around the production well and petroleum reserves are often lost. By removing sulphate from injected seawater, the potential for scaling is prevented. In deep water and other complex oil developments, sulphate removal, and the subsequent prevention of scale, provides significant cost advantages. This technology uses nanofiltration membranes which may have been specially developed or adapted to remove sulphates from seawater prior to injection into an oil producing reservoir. This technology may be used on oil platforms and Floating Production, Storage and Off-loading (FPSO) systems. Sulphate in sea water makes up 7.7% by weight of all the dissolved solids while chloride is typically 55% and sodium 30.6%. If RO technology was used the membrane system
17
18
Feature
Filtration+Separation October 2007
Table 2: Examples of impurities present in Produced Water Components
Initial Quality (mg/l)
Intermediate Quality (mg/l)
Final Quality (mg/l)
Sodium
8
23050
28174
Potassium
0.1
3106
1376
Calcium
98
2050
2410
Magnesium
1
737
272
Barium
1
0.4
3.25
Strontium
0.2
46.1
134.9
185
38.25
Chloride
100
41200
49365
Sulphate
6
4093
85.5
Boron
0.43
4
8.1
2
Aluminium
0.1
1.3
0.18
0.2
0.5
1.24
0.1
Dissolved iron
Chromium Manganese
0.5 1
3.1
5.13
0.3
Nickel
15.7
0.19
0.1
Copper
0.44
0.43
0.05
25
16.4
0.1
Arsenic
0.5
0.003
0.05
Selenium
0.1
0.001
0.02
Silver
0.1
0.0013
0.01
Cadmium
0.05
0.013
0.005
Mercury
1
0.00004
0.0001
Lead
0.5
1.51
0.005
0.03
0.01
76000
81900
500
500
400
Zinc
2.54
Treated Water Quality (mg/l)
3.83
Tin TDS
250
COD Suspended solids
320
320
320
Visually Neutral
pH
4.5
4.8 to 7.4
4.56
6 to 9
would reject the predominating monovalent ions as well as the sulphate ions and would desalinate the sea water (another useful RO process but one that will not be discussed in this article). Removing a large percentage of all the ions is not necessary so for this process nanofiltration membranes are used which can specifically reject the larger negatively charged sulphate ions. Other divalent ions such as calcium and magnesium are also rejected but to a lesser degree. Typically, a 98% rejection of sulphates can be expected. Other smaller monovalent ions such as sodium and chloride pass through into the permeate for final injection. The reject stream which contains the highly concentrated sulphate ions can be returned to the ocean. An additional benefit of reducing the sulphate in the injected seawater is the reduction in the source of sulphur that can be converted to hydrogen sulphide by thermophilic sulphate reducing bacteria. The generation of hydrogen sulphide is called ‘souring’. Although sulphate removal is ‘simply’ a nanofiltration system with its associated pretreatment and ancillaries, the system must be specially packaged to operate reliably and safely in the severe and hazardous conditions encountered on offshore installations and
period the quality will decrease and towards the end of the life of the well the contamination will increase further. In order for this Produced Water to be discharged to the environment it is necessary to remove the petrochemical byproducts from the water, and also sometimes the dissolved ions. A second source of contaminated water can be the surface water which is collected on site from run off rain water etc. The spectrum of contamination in this may be lower than that in the Produced Water but there will still be some ionic impurities and petrochemical residues so the treatment required for this contaminated stream will be similar to that for the Produced Water stream. A possible system design incorporating RO/ Nanofiltration that could remove oil and other contaminants from these streams and produce treated water which will be suitable for return to the sea or river is shown below.
Pretreatment
Membrane Separation Polishing Processes
Sludge Treatment
Treated Water
to meet the stringent oil and gas industry specifications.
Treating water produced with oil or gas As well as being an important aid in the production of oil from a well, water from a well is also a significant waste product and is termed ‘Produced Water’. One figure quoted for the United States is that Produced Water coming from oil wells is 8 times the volume of the oil produced. International conventions have set a provisional, generally voluntary, target of 40 ppm for hydrocarbons in offshore operation discharges, anything above 100 ppm being considered an oil spill. For onshore operations the discharge limits must usually meet the quality of the local water sources in the area as a minimum, or the local municipal effluent discharge limits For example, as natural gas is taken from a well it contains some Produced Water. As production from the well progresses the water content may increase and the level of contamination in the water will also increase. The initial quality of this water is expected to be relatively clean and typically this may be available for up to the first 2–4 years of operation of the gas well. After this
To achieve complete treatment of all the waste water streams, and to fully treat the waste byproducts from these streams so that not only the water but all the other waste contaminants can be safely disposed off, separate treatment systems may be required. A system to treat the Produced Water may also treat water produced from other sources, such as filtrate from the filter press, which requires some further treatment prior to discharge. A system designed to treat the surface water may be simpler since the water will contain fewer heavy metals and organic compounds than the ‘Produced’ water. A system to process the waste streams from all the other treatment processes may be based around a sludge dewatering plant which can produce a solid filter cake of the concentrated solids in a form suitable for off site disposal. The plant will be designed to treat the different water qualities presented throughout the life of the well and produce a treated water stream which complies with the discharge specification for metals, aromatics and oil in water. Examples of the type of impurities that might be present in the three different qualities of Produced Water
Feature
Filtration+Separation October 2007
and a possible final discharge specification are shown in Table 2. The treatment plant will integrate a number of different technologies to achieve the level of purification required. Examples of these technologies are: • Tilted Plate Separator; • UF (including dosing); • NF (including antiscalant dosing) (RO membranes may be fitted initially and then changed to Nanofiltration membranes later when the more highly contaminated water is produced from the well.); • Activated carbon adsorption; • Selective ion exchange. The waste water from all the stages is sent to a sludge treatment plant prior to being recycled to the inlet to the Produced Water system. The reduce the level of heavy metals and organic materials in the Produced Water a selective absorption processes, such as ion exchange and activated carbon, can be used. However, the relatively high levels of these contaminants in the feed water would require large volumes of media and long residence times to be used and the ion exchange resin media would also have to be frequently regenerated and the activated carbon replaced frequently.
For this reason a combination of technologies are used to pre-treat the Produced Water. UF and RO or NF membrane separation is used to achieve up to 98% removal of the heavy metals and 40–90% of the organic molecules prior to the absorption processes. This significantly reduces the challenge to the ion exchange resin and activated carbon and makes them more of a ‘polishing’ step. This allows smaller bed volumes of media to be used to achieve the final water quality. However, the RO or NF membranes can be susceptible to fouling from suspended solids, oil and scaling by precipitation of salts like calcium carbonate, calcium sulphate, barium sulphate etc. so dosing prior to the membrane treatment is used to help minimise this. The feed water pH is controlled to < 6 to retain iron in soluble form and antiscalant may be used. An UF stage may also be included to reduce the free oil present in the feed stream and to reduce the suspended solids to below the limits of detection. The UF membrane process can remove suspended solids and oil but will not remove any of the heavy metals present. However, the level of oil and solids in the Produced Water may be too high for the UF membranes to remove without fouling so prior to the ultrafiltration stage a separate oil removal process, such as a tilted plate
separator, may be used to remove the bulk of the suspended solids and oil. Despite the pre-treatment protection, the UF, NF and RO membranes will still require to be cleaned so a CIP system consisting of a chemical mixing tank and a CIP tank and feed pump system will be required. Because of the environmental and economical importance of this application different methods are continually being sought to optimise the performance of the membrane systems, for example one company has developed a proprietary membrane filtration system. This unit uses a vibrating membrane mechanism to avoid membrane fouling and the supplier has completed several treatment facility installations using this vibrating membrane system for treating all kinds of petrochemical wastewater.
•
Contact: Anthony Bennett, Clarity. [email protected]
Acknowledgements: Material from published, unpublished and internet sources was referenced in preparing this article. In particular: • Christ Kennicott Water Technologies Ltd • Dow Chemical Company • Esmil Process Systems • New Logic Research Inc
19
Feature
Filtration+Separation October 2007
Processing petrochemicals:
Reverse osmosis in petrochemicals A
technique more commonly associated with desalination can also be applied in other liquid processes and may be particularly suited to oil. Anthony Bennett explains why. The process of reverse osmosis (RO) represents the finest, or tightest, level of aqueous filtration available today. Ordinary filters use a ‘screen’ or ‘depth filter’ medium to separate particles from the feed water streams. An RO system employs a semi-permeable membrane that separates a very high percentage of molecules based not only on their physical size but also their molecular size and ionic charge. There are three different types of filter media that are operated in a similar manner, ultrafiltration (UF), nanofiltration (NF) and RO. All three of these membrane types are permeable to water molecules but retain impurities based on their molecular ‘size’ – molecular weight and in the case of UF also their 3-D molecular configuration. RO and NF also retain impurities based on their ionic charge so can retain, or reject, charged materials with a low molecular size. The mechanism by which each of these membrane types reject contaminants is slightly different but to prevent the membrane from becoming fouled by retained solids and salts forced against it by the pressurised stream of feed water all three types have to be operated in a way which reduces the rate of membrane fouling by these contaminants. These membrane types are used in systems which employ cross-flow filtration. This allows some filtered water to pass through the membrane while a flow of water also sweeps the surface of the membrane taking rejected contaminants away from the membrane surface.
In the processing of crude oil and gas, there are important associated applications which depend on using membranes and which are critical to maintaining the commercial operation of the industry.
Operating a RO or NF system produces a continuous stream of water which contains the materials rejected by the membrane – this will typically be a waste stream but in some applications it may be the material in the concentrate stream which is required. The maximum conversion of feed water to permeate (or filtrate) is typically 70–80%.
Feature
Filtration+Separation October 2007
Table 1: General performance of different types of membranes UF NF RO
Molecular weight range (Daltons 1,000 – 1,000,000 > 200 > 200
The three types of membranes have general types of performance, as shown in Table 1, although individual membrane types from different membrane suppliers may have different profiles of performance and individual membranes usually have to be tested for each application. This article will consider two applications within the petrochem industry which use RO and/or NF membranes. Initially the petrochem industry would appear to be an unlikely place to use RO membranes which have been designed to operate on aqueous feed streams and remove dissolved ions, particles and bacteria from the feed water. Certainly there are probably limited opportunities to use RO membranes in the processing of crude oil and gas, however, there are some important associated applications which do depend on using membranes and which are just as critical to maintaining the commercial operation of the industry.
Using water to produce oil Deposits, or reservoirs, of crude oil lie at various depths below the ground. Oil can
Ionic Rejection of Monovalent Ions None 50 – 90% 98 – 99 %
be recovered from reservoirs just below the surface by means of open cast mining while from greater depths, it has to be recovered by drilling. At great depths, the crude oil is subjected to the pressure of the ground above it and of any natural gas which exists in the reservoir. When tapped by a drill the oil flows out of the borehole under pressure since it is lighter than water and the surrounding rocks. This so-called primary production is normally possible without further measures to aid the flow of oil. When the pressure in the reservoir drops, the oil has to be pumped out. The pressure within the reservoir can also be increased by pumping water or natural gas into the borehole. This is called secondary production. As the reservoir becomes exhausted, the amount of water in the output increases and, finally, more water than oil leaves the well. This signals the end of this production method. During oil production, reservoir pressure decays due to oil and gas abstraction, resulting in a decrease in production. Well pressure
is therefore, in most cases, maintained by injection of seawater. However, when normal high sulphate seawater is injected into reservoirs which have formation water containing barium and strontium, mixing occurs forming a supersaturated barium and/or strontium sulphate solution. When the pressure decreases in and around the production wells, the supersaturated barium and/or strontium sulphate solution is no longer stable and precipitation occurs. The result is scale formation in the production tubing and/or plugging of reservoir rock around the production well and petroleum reserves are often lost. By removing sulphate from injected seawater, the potential for scaling is prevented. In deep water and other complex oil developments, sulphate removal, and the subsequent prevention of scale, provides significant cost advantages. This technology uses nanofiltration membranes which may have been specially developed or adapted to remove sulphates from seawater prior to injection into an oil producing reservoir. This technology may be used on oil platforms and Floating Production, Storage and Off-loading (FPSO) systems. Sulphate in sea water makes up 7.7% by weight of all the dissolved solids while chloride is typically 55% and sodium 30.6%. If RO technology was used the membrane system
17
18
Feature
Filtration+Separation October 2007
Table 2: Examples of impurities present in Produced Water Components
Initial Quality (mg/l)
Intermediate Quality (mg/l)
Final Quality (mg/l)
Sodium
8
23050
28174
Potassium
0.1
3106
1376
Calcium
98
2050
2410
Magnesium
1
737
272
Barium
1
0.4
3.25
Strontium
0.2
46.1
134.9
185
38.25
Chloride
100
41200
49365
Sulphate
6
4093
85.5
Boron
0.43
4
8.1
2
Aluminium
0.1
1.3
0.18
0.2
0.5
1.24
0.1
Dissolved iron
Chromium Manganese
0.5 1
3.1
5.13
0.3
Nickel
15.7
0.19
0.1
Copper
0.44
0.43
0.05
25
16.4
0.1
Arsenic
0.5
0.003
0.05
Selenium
0.1
0.001
0.02
Silver
0.1
0.0013
0.01
Cadmium
0.05
0.013
0.005
Mercury
1
0.00004
0.0001
Lead
0.5
1.51
0.005
0.03
0.01
76000
81900
500
500
400
Zinc
2.54
Treated Water Quality (mg/l)
3.83
Tin TDS
250
COD Suspended solids
320
320
320
Visually Neutral
pH
4.5
4.8 to 7.4
4.56
6 to 9
would reject the predominating monovalent ions as well as the sulphate ions and would desalinate the sea water (another useful RO process but one that will not be discussed in this article). Removing a large percentage of all the ions is not necessary so for this process nanofiltration membranes are used which can specifically reject the larger negatively charged sulphate ions. Other divalent ions such as calcium and magnesium are also rejected but to a lesser degree. Typically, a 98% rejection of sulphates can be expected. Other smaller monovalent ions such as sodium and chloride pass through into the permeate for final injection. The reject stream which contains the highly concentrated sulphate ions can be returned to the ocean. An additional benefit of reducing the sulphate in the injected seawater is the reduction in the source of sulphur that can be converted to hydrogen sulphide by thermophilic sulphate reducing bacteria. The generation of hydrogen sulphide is called ‘souring’. Although sulphate removal is ‘simply’ a nanofiltration system with its associated pretreatment and ancillaries, the system must be specially packaged to operate reliably and safely in the severe and hazardous conditions encountered on offshore installations and
period the quality will decrease and towards the end of the life of the well the contamination will increase further. In order for this Produced Water to be discharged to the environment it is necessary to remove the petrochemical byproducts from the water, and also sometimes the dissolved ions. A second source of contaminated water can be the surface water which is collected on site from run off rain water etc. The spectrum of contamination in this may be lower than that in the Produced Water but there will still be some ionic impurities and petrochemical residues so the treatment required for this contaminated stream will be similar to that for the Produced Water stream. A possible system design incorporating RO/ Nanofiltration that could remove oil and other contaminants from these streams and produce treated water which will be suitable for return to the sea or river is shown below.
Pretreatment
Membrane Separation Polishing Processes
Sludge Treatment
Treated Water
to meet the stringent oil and gas industry specifications.
Treating water produced with oil or gas As well as being an important aid in the production of oil from a well, water from a well is also a significant waste product and is termed ‘Produced Water’. One figure quoted for the United States is that Produced Water coming from oil wells is 8 times the volume of the oil produced. International conventions have set a provisional, generally voluntary, target of 40 ppm for hydrocarbons in offshore operation discharges, anything above 100 ppm being considered an oil spill. For onshore operations the discharge limits must usually meet the quality of the local water sources in the area as a minimum, or the local municipal effluent discharge limits For example, as natural gas is taken from a well it contains some Produced Water. As production from the well progresses the water content may increase and the level of contamination in the water will also increase. The initial quality of this water is expected to be relatively clean and typically this may be available for up to the first 2–4 years of operation of the gas well. After this
To achieve complete treatment of all the waste water streams, and to fully treat the waste byproducts from these streams so that not only the water but all the other waste contaminants can be safely disposed off, separate treatment systems may be required. A system to treat the Produced Water may also treat water produced from other sources, such as filtrate from the filter press, which requires some further treatment prior to discharge. A system designed to treat the surface water may be simpler since the water will contain fewer heavy metals and organic compounds than the ‘Produced’ water. A system to process the waste streams from all the other treatment processes may be based around a sludge dewatering plant which can produce a solid filter cake of the concentrated solids in a form suitable for off site disposal. The plant will be designed to treat the different water qualities presented throughout the life of the well and produce a treated water stream which complies with the discharge specification for metals, aromatics and oil in water. Examples of the type of impurities that might be present in the three different qualities of Produced Water
Feature
Filtration+Separation October 2007
and a possible final discharge specification are shown in Table 2. The treatment plant will integrate a number of different technologies to achieve the level of purification required. Examples of these technologies are: • Tilted Plate Separator; • UF (including dosing); • NF (including antiscalant dosing) (RO membranes may be fitted initially and then changed to Nanofiltration membranes later when the more highly contaminated water is produced from the well.); • Activated carbon adsorption; • Selective ion exchange. The waste water from all the stages is sent to a sludge treatment plant prior to being recycled to the inlet to the Produced Water system. The reduce the level of heavy metals and organic materials in the Produced Water a selective absorption processes, such as ion exchange and activated carbon, can be used. However, the relatively high levels of these contaminants in the feed water would require large volumes of media and long residence times to be used and the ion exchange resin media would also have to be frequently regenerated and the activated carbon replaced frequently.
For this reason a combination of technologies are used to pre-treat the Produced Water. UF and RO or NF membrane separation is used to achieve up to 98% removal of the heavy metals and 40–90% of the organic molecules prior to the absorption processes. This significantly reduces the challenge to the ion exchange resin and activated carbon and makes them more of a ‘polishing’ step. This allows smaller bed volumes of media to be used to achieve the final water quality. However, the RO or NF membranes can be susceptible to fouling from suspended solids, oil and scaling by precipitation of salts like calcium carbonate, calcium sulphate, barium sulphate etc. so dosing prior to the membrane treatment is used to help minimise this. The feed water pH is controlled to < 6 to retain iron in soluble form and antiscalant may be used. An UF stage may also be included to reduce the free oil present in the feed stream and to reduce the suspended solids to below the limits of detection. The UF membrane process can remove suspended solids and oil but will not remove any of the heavy metals present. However, the level of oil and solids in the Produced Water may be too high for the UF membranes to remove without fouling so prior to the ultrafiltration stage a separate oil removal process, such as a tilted plate
separator, may be used to remove the bulk of the suspended solids and oil. Despite the pre-treatment protection, the UF, NF and RO membranes will still require to be cleaned so a CIP system consisting of a chemical mixing tank and a CIP tank and feed pump system will be required. Because of the environmental and economical importance of this application different methods are continually being sought to optimise the performance of the membrane systems, for example one company has developed a proprietary membrane filtration system. This unit uses a vibrating membrane mechanism to avoid membrane fouling and the supplier has completed several treatment facility installations using this vibrating membrane system for treating all kinds of petrochemical wastewater.
•
Contact: Anthony Bennett, Clarity. [email protected]
Acknowledgements: Material from published, unpublished and internet sources was referenced in preparing this article. In particular: • Christ Kennicott Water Technologies Ltd • Dow Chemical Company • Esmil Process Systems • New Logic Research Inc
19