This document is a guide for water treatment engineers to better understand crossflow membrane applications for glycol-based aqueous solutions. It includes glycol applications we have sold equipment for, applications we have successfully tested, and applications we have not tested but are expected to be technically feasible.
Glycol-based solutions are commonly used by a number of industries. Glycol-based solutions are inexpensive, relatively easy to manufacture and modify, and have a number of uses. Glycerine (glycerol) has the chemical formula C3H5(OH)3 (MW = 92) and ethylene glycol (EG) has the chemical formula CH2(OH)CH2(OH) (MW = 62). A number of glycol-based chemicals are used that have the same root chemical formula with one or more different functional groups to create products with alternate properties and uses. EG polymers have a wide range in their degree of polymerization (DP) and typically maintain their DP well during use (i.e., they do not readily break down to monomer-EG, even at elevated temperatures).
The most common use for EG is as a heat exchange fluid, because it has excellent thermal conductivity properties and EG/water solutions have high boiling points and low freezing points. EG and EG derivatives are also used during manufacture of resins, pharmaceuticals, foods, surfactants, lubricants, inks, solvents, polymers (plasticizers), and other products. EG is poisonous if ingested in sufficient quantity.
Antifreeze is used as a coolant for automobiles and other vehicles. Antifreeze in a vehicle cooling system will eventually become contaminated with salts, iron, oils, and particles. A small portion of the EG will also break down into acetic acid, aldehydes, and other by-products. Re-use of antifreeze requires removal of some or all of these contaminants.
Disposal costs for antifreeze vary from $0.25-$5.00/gallon, although this will certainly vary.
A number of potential applications for antifreeze recycling exist:
This is the least expensive treatment for antifreeze, and is also the least effective method of producing re-usable fluid.
Depth cartridge filters are currently used by some garages for point-of-use recycling. This application typically involves pH adjustment, bag filtration, depth cartridge filters, and addition of anticorrosives and other chemicals. Some companies sell units to auto garages that do these steps during antifreeze system flush-outs. Antifreeze treated by this system is typically put back into the same car the fluid originally came from, to avoid contamination between vehicles. After a period of time, a volume of antifreeze recycled in this manner builds up too many dissolved solids and will require change-out.
UF of used antifreeze is the second least expensive treatment option, and produces a fair quality reusable permeate.
Application tests have demonstrated that Osmonics® proprietary ANO9 membrane (MWCO 15,000-25,000) can process oily, turbid used antifreeze and generate semitransparent permeate while experiencing little irreversible membrane fouling. During operation, a simple hot 43°C (110°F) water flush of an ANO9 membrane element typically returns a majority of the permeate flow (Qp), which is good recovery for a UF application of this type. Essentially, this UF application involves removing suspended solids and oils, and hydrophobic ANO9 UF membranes are typically very appropriate for processing oily wastes. Reusing UF permeate in vehicles will require pH adjustment and will usually also require re-addition of anti-corrosives, anti-foam agents, or other chemical additives greater than 15,000 MW that are removed by the UF membrane. UF permeate could potentially be re-used as a low-grade product for other industries: chemical manufacturing, lubricants, etc.
The UF application has the highest potential for antifreeze processing, including the following customers:
- Automotive/other vehicle maintenance companies who transport antifreeze to central processing sites.
- Government, for maintenance of government/military vehicles.
- Regional waste disposal companies.
- Vehicle fleet maintenance companies (taxis, delivery services, etc.).
- Individual garages.
UF permeate, from 2.2 above, can be further processed with NF. NF membranes with an appropriate MWCO can be used to pass EG while retaining color, a high portion of multivalent ions (hardness, etc.), and other contaminants. An appropriate NF membrane will retain a portion of ions contributing to conductivity and generate permeate that is water white. This application will typically require UF as pretreatment; NF membranes are not expected to withstand the fouling effects of waste antifreeze as feed.
This application has scaling and fouling potential under some circumstances, thus pilot testing of limited scale initial processing is recommended. The market for water white NF EG permeate is not developed, but is expected to be relatively significant. Industries that could potentially use water white NF EG permeate include cosmetics, ink manufacturing, chemical manufacturing, etc.
Reverse Osmosis (RO)
RO can be used to concentrate EG. Field installations demonstrate that EG passage with appropriate PA RO membranes is 5-10%, dependent on the application specifics. Due to the inherent osmotic pressure of EG, it can only be concentrated with RO to 10-12% at POP of 1,000 psig (69.0 bar). Because vehicle antifreeze typically contains >30% EG which is higher than the 10-12% maximum EG concentration, it is not feasible to concentrate EG in antifreeze for most applications.
The applications for RO for the vehicle antifreeze market typically will only be practical when the initial EG feed concentration is <6%, and it is useful to concentrate the dilute EG to the maximum achievable concentration of 10-12%. Such streams may include rinse water run-off that is diluted with water or similar streams.
Like the NF application, the RO application may have scaling and fouling potential that has to be dealt with on a case-by-case basis.
Airplane deicing fluid is sprayed on airplanes during cold seasons. Deicing fluid typically contains 30-50% EG (or monopropylene glycol), surfactants, and other chemical additives. These large volumes of fluid can be a considerable waste concern. Some airports deice their planes at each gate with a mobile deicing unit, which makes deicing fluid collection difficult. Other airports deice their planes at one central location immediately before plane takeoff, which makes deicing fluid collection much easier to manage.
A number of firms have investigated RO for reclamation of airplane deicing fluid. These reclamation systems typically involve:
- UF pretreatment (optional).
- Evaporation or distillation to dewater the fluid. Water and low concentrations of EG are distilled off, while
- surfactants and other contaminants remain behind.
- RO to concentrate the dilute EG in the distillation condensate 'tops.'
Metal Finishing Quenchant
Certain large-scale metal finishing operations require 'quenching' hot metal parts in thermally conductive fluid to remove heat. Quenchant fluids most commonly contain approximately 50% polyethylene glycol, polyalkylene glycol, or 'oxy' polyglycols as well as ppm levels of corrosion inhibitors. These polymers are typically >10,000 MW, but the solutions often contain smaller glycol by-products.
Osmonics has significant experience with RO machines that concentrate drag-out polyglycols for quenchant applications. These solutions have much lower osmotic pressure than mono EG solutions due to their high average MW; therefore, RO can be used to concentrate the drag-out polyglycols to ~50% concentration. This allows the concentrated polyglycols in the drag-out fluid to be directly reused.
Testing has indicated that 'oxy' polyglycols sometimes have detrimental effects on PA RO membranes, thus cellulosic RO membranes are recommended for concentrating 'oxy' polyglycols.
Other polyglycol concentration applications exist, but quenchant applications have proven to be technically feasible and have short capital equipment payback periods.
Equipment Lubricant and Heat Exchanger Fluid
Triethylene glycol, EG, and other glycols are used as lubricants and heat exchanger fluids. If equipment is stored outdoors in an area that can collect rain, or indoors where run-off is centrally collected, glycol from run-off can be collected, concentrated, and reused. As municipalities become stricter on storm run-off discharges, this will become a growing process application market.
As mentioned in 1.0, glycols are also used for manufacture of resins, pharmaceuticals, foods, surfactants, lubricants, inks, solvents, polymers (plasticizers), as well as many other products. Numerous applications appropriate for crossflow membranes exist, and only need to be preliminarily investigated to determine their technical and economic viability.