Abstract:
A method and a system for recycling spent ethylene or propylene glycol recovered from aircraft deicing solutions is described. The recovered spent ethylene or propylene glycol contains water and other undesirable substances which require to be removed to produce a solution free of these substances which can be further processed to remove substantially all of the water content and to blend-in additives required to certify the final glycol solution for aircraft deicing. The method and system employs a computer controller for continuous automatic batch processing of the spent glycol, including, in combination, filtering, distillation, blending and testing in specific sequences and achieves an improved quality recycled glycol of a purity of at least 99.5% and preferably between 99.6% to 99.9% glycol concentration.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an improved method and a system for recycling spent ethylene or propylene glycol recovered from aircraft deicing solutions to produce virgin glycol having a concentration of at least 99.5% and up to 99.9%. 
       BACKGROUND ART 
       [0002]    It is a requisite to de-ice the accumulation of frost, ice and snow or combinations thereof on the wings of aircrafts to restore the natural shape of the wings by removing any frost, ice or snow build-up that distorts its shape and to permit proper functioning of the ailerons. Ice, frost or snow also adds to the weight of the aircraft, thereby impeding flight. The removal of frost, ice or snow is effected by spraying a de-icing glycol solution to cause the frost, ice or snow to melt away and protects the aircraft for sufficient time after de-icing against further precipitation. The concentration of glycol in the solution varies according to climatic conditions. Such solutions typically comprise glycol, water, and minor amounts of additives such as surfactants, and a corrosion inhibitor. 
         [0003]    After spraying the de-icing solution on the aircraft, the solution becomes diluted with water from melted frost, ice and snow and becomes contaminated with pollutants present on the tarmac area where aircraft propagate to be de-iced. These areas are usually concrete areas polluted with sand, abraded rubber from aircraft tires, oil, combustion residue, fumes from the aircraft, deicing salts, grit, traces of fuel, solid debris such as grass and leaves, and chemicals found in concrete. In the past, such waste glycol solutions where harnessed in reservoirs or vacuum trucks for disposal and treatment at remote sites for disposal. Such a practice proved to be a burden on the environment and resulted in the loss of glycol which is an expensive product. Typically, the airport assumes the cost of this disposal and by recovering the spent glycol substantive savings is passed on to the airport facility. 
         [0004]    Over the last decades, efforts have been made to recycle glycol residue from aircraft de-icing solutions and various recovery systems have been placed in operation using various methods such as aerobic digesters, cyclone separators, chemical cleaning techniques using absorption and ion exchangers, percolation towers, distillation and stripping columns or towers, etc. However, the operation of some of these have proven problematic and very costly and some have not achieved the desired sought after result of recovering glycol of sufficient purity comparable to purchased virgin glycol, for recycling for use in an aircraft deicing glycol solution. Some of the known techniques have claimed to recycle such glycol solutions to a purity of 99.5% but many have failed to produce this purity. Examples of known techniques can be found, for example, in the patent literature with reference to U.S. Pat. Nos. 5,904,321; 5,411,668; 7,713,319; 8,252,149; US Patent Application Publications 2011/0263909 and 2013/0190539, as well as Canadian Patents 2,116,827 and 2,223,922. Another publication on the topic of recycling glycol from airport tarmacs can be found in an article entitled “A Tour Of The Munich International Airport&#39;s Deicing Recycling Plant”, such article being accessible on the web at http;//www.aviationpros.com/article/10616425. 
       SUMMARY OF THE INVENTION 
       [0005]    There is a need to provide an improved method and a system, which is substantially automatic, for recycling spent ethylene or propylene glycol recovered from aircraft deicing solutions to produce virgin glycol having a concentration of at least 99.5% and typically in the order of from between 99.6% to 99.9%. 
         [0006]    It is a feature of the present invention to provide an improved method and system of recycling spent ethylene or propylene glycol recovered from aircraft deicing solutions and which meets the above mentioned need. 
         [0007]    It is a further feature of the present invention to provide an improved method of recycling spent ethylene or propylene glycol from aircraft deicing solutions and wherein the method comprises the combination of an improved sequence of steps for treating of the reclaimed glycol solution and which steps are substantially automatically controlled to produce an aircraft de-icing glycol solution with a glycol concentration of at least 99.5% and typically in the order of from between 99.6% to 99.9%. 
         [0008]    A further feature of the present invention is to provide an improved system for recycling spent ethylene or propylene glycol recovered from aircraft deicing solutions containing glycol, water and other impurities to produce a substantially virgin glycol having a concentration of at least 99.5% and typically in the order of from between 99.6% to 99.9%. 
         [0009]    According to the above features, from a broad aspect, the present invention provides a method of recycling spent ethylene or propylene glycol recovered from aircraft de-icing solutions containing glycol, water and other substances to produce substantially virgin glycol. The method comprises, in combination, the steps of recovering spent glycol from the deicing facilities of airports and storing the spent glycol in one or more storage tanks. Thereafter, the spent glycol having a predetermined low % concentration is removed to produce a working spent glycol and storing same in a storage means. The working spent glycol is filtered through at least two filtering stages to substantially remove all solids and other substances and stored in a holding tank. An evaporation step then removes water from a batch of filtered spent glycol by heating the batch to a temperature to evaporate water to bring the working spent glycol to a glycol concentration of about 50%. The glycol concentration solution is then stored in a buffer tank. The pH of the solution of glycol concentration of 50% is then adjusted to a desired value and then it is carbon filtered and fed to a holding tank which feeds a predetermined batch to an evaporator section of a distillation tower operating under vacuum where it is then heated to predetermined temperatures to evaporate the liquids in the glycol concentration of 50% in a stream of vapors. The temperature of the stream of vapor in a packing section of the distillation section of the tower is sensed to monitor the actual temperature of the stream of vapor and temperature signals representative thereof are sent to a computer controller. The stream of vapor at the top of the distillation tower is cooled as it exits from the packing section to condense into a liquid phase. The glycol concentration of the condensed liquid is monitored on a continuous basis to detect the glycol concentration of the condensed liquid and glycol concentration signals are sent to the computer controller which correlates these with the temperature signals to determine the appropriate time to operate valves to recover from the condensed liquid, in associated ones of reservoirs, water, water mixed with glycol below a concentration of 99.5%, and glycol solution of at least 99.5% having a concentration of at least 99.5%. The water, water mixed with glycol below a concentration of 99.5%, and the glycol solution of at least 99.5% is automatically directed to associated reservoirs. The glycol of at least 99.5% is tested and blended and directed to certified holding tanks for use by aircraft de-icing vehicles. 
         [0010]    According to a further broad aspect of the present invention there is provided a system for recycling spent ethylene or propylene glycol recovered from aircraft deicing solutions containing glycol, water and other substances to produce substantially virgin glycol. The system comprises, in combination, collection means for recovering spent glycol from the tarmac of an aircraft de-icing area. Storage means is provided for storing the recovered spent glycol. Means is further provided to remove spent glycol having a predetermined low % glycol concentration from the storage means. A working tank is also provided for maintaining a predetermined volume of the spent glycol having a glycol concentration above the predetermined low % glycol concentration. At least two filter stages is also provided to filter the spent glycol from the working tank to remove substantially all solid particles from the other substances and to feed it to a holding tank to feed an evaporator stage having one or more evaporators to evaporate water from the batch of spent glycol solution by boiling the batch at a temperature sufficient to evaporate water to produce a spent glycol having a glycol concentration of about 50% which is then transferred to a buffer tank. Means is provided to adjust the pH of the batch of glycol concentration from the buffer tank to a desired pH value and it is then carbon filtered and fed to a distillation tower holding tank to accumulate a predetermined volume of the spent glycol concentration above about 50%. A distillation tower is provided and operates under vacuum and has a lower evaporator section and an upper chimney section provided with steel packings to retain heat from a stream of vapors released from the lower evaporator section. Temperature sensors monitor the temperature of the chimney section along the packings to provide actual temperature signals to the computer controller representative of the actual temperature to the stream of vapors drawn through the packings and provides glycol concentration value signals to the computer controller. The lower evaporator section has a temperature controlled heater to evaporate the spent glycol in sequence to create the stream of vapors whose vapors are drawn through the chimney section and into a condensing coil downstream of the chimney section to produce condensed liquid which is fed through a cooling device. Measuring means is further provided to measure, on a continuous basis, the glycol concentration in the condensed liquid, and provide glycol concentration values to the computer controller to correlate with the temperature signals to determine the appropriate time to operate valves to separate, in associated reservoirs, water, water mixed with glycol having a glycol concentration of less than 99.5% purity, and substantially virgin glycol having a glycol concentration of at least 99.5% up to 99.9%. Means is then provided to certify and store the substantially virgin glycol in certified holding tanks ready for blending for use by aircraft de-icing vehicles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A preferred embodiment of the present invention will now be described with reference to the accompanying drawings in which; 
           [0012]      FIGS. 1A to 1C  are schematic and partly block diagrams, illustrating the improved batch processing method and system of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    Referring to the drawings there will now be described a preferred embodiment of the improved method and system of the present invention for recycling spent ethylene or propylene glycol recovered from aircraft deicing solutions. As shown in  FIG. 1A , the spent ethylene glycol herein of Type I or type IV is recovered from the tarmac  10  of an airport where aircrafts, such as the one schematically illustrated at  11 , are sprayed with deicing solution  12  from deicing vehicles  13 . The deicing solution is usually heated to a temperature of 60 to 80 degrees C. (centigrade). To recover spent glycol, the tarmac area is modified to be fitted with one or more storm drains  14  connected to underground conduits  15  to channel the spent glycol to a storage chamber  16  which is segmented. The spent glycol enters a first compartment  16 ′ which also communicates with a larger chamber  16 ″. The configuration of these chambers allows some sediments to precipitate at the bottom of chamber  16 ″. A sample of spent glycol is continuously flowing through a densimeter  19  driven by pump  17  located in chamber  16 ′ in order to assess the spent glycol concentration. If the concentration of the spent glycol is below 5%, the spent glycol is not suitable for recycling. Therefore, the computer controller  25  allows the level of spent glycol to gradually rise into chambers  16  until it reaches an overflow  15 ′ to city sewage. 
         [0014]    The computer controller  25  also operates a further pump  17 ′ in the compartment  16 ″ to pump solution of 5% or more concentration from the tank compartment  16 ″ through a first filter  18  to remove sediments and feed the spent glycol of 5% or more concentration into holding tanks, herein only two tanks  22  and  23  being illustrated which are fitted with level detectors (not shown, but obvious to a person skilled in the art) to permit the computer controller to operate valves  21  and  20  to direct the spent glycol solution of 5% or more concentration to other holding tanks when a tank is full. 
         [0015]    It is pointed out that the computer controller has various monitoring and control stations or different computers and such is identified throughout the drawings by the letter “C” inside a small square and reference numeral  25 . It is also pointed out that throughout the drawings, the sizes of the squares, representing different units, tanks, filters, etc, are not representative of their size in proportion to one another. 
         [0016]    From the storage tanks  22  and  23  the spent glycol solution of 5% or more concentration is treated substantially automatically by the computer controller  25 , as follows by operating a valves  24  and a pump  30  spent glycol solution is subjected to a two stage filtering process. Firstly, the spent glycol is fed through a bag filter  28  and into a working tank  27  where a pump  30 ′ circulates the filtered spent glycol concentration of 5% or more through an ultra fine filter  31 , herein a ceramic filter, and back to the working tank  27  on a continuous basis. Ultra filtered spent glycol permeating through the ceramic filter by pressure is fed into holding tank  32  which has a level sensor  32 ′ which when a predetermined volume of filtered spent glycol is reached, signals the computer controller to stop the pumps  30  and  30 ′. The bag filtering stage has tow bag filters  28  and  28 ′ to provide for longer uninterrupted operation by switching over to the other bag when one of the bags becomes clog with sediments. This permits longer operational time of the system. The ultra fine ceramic filter  31  has a pore size of 0.5 microns to remove very fine solid particles leaving substantially only liquids, often referred to as permeated liquid. 
         [0017]    The filtered solution from holding tank  32  is then pumped, by pumps  34  and pump  34 ′ into two evaporators  33  and  33 ′ of like constructions. Each evaporator  33 ,  33 ′ is provided with two electric resistive heating elements  35  and  35 ′, respectively, which are operated by the computer controller  25  to ensure continuous operation of the evaporators in the event of failure of one of the resistive heating elements. Although not shown, these known evaporators  33  and  33 ′ are each provided with a compressor (not shown) to extract heat from the vapors released in their flue  33 ″ and re-use the heat from the vapors as a principal source to heat the water in the reservoirs. The resistive heating elements  35  and  35 ′ are used to pre-heat the water at start-up, permitting the evaporation of water at lower temperature under vacuum conditions. 
         [0018]    Level sensors  37  transmit level indicating signals to the computer controller  25  to operate the pumps  34  and  34 ′ to provide sufficient spent solution in the evaporator whereby to switch one of the heating element on, the other being on stand-by and placed in service by a switch not shown, but obvious, in the event of failure of the other resistive heating element.. Temperature sensing probes  38  provide actual temperature values to the computer controller of the batch solution in the evaporators  33  and  33 ′. The heating elements  35  and  35 ′ are operated to control the temperature of the water to a boiling temperature of 100 degrees C. sufficient to evaporate water, but too low for ethylene or propylene glycol to evaporate. Refractometers  37 ″ detect the density of the spent solution at an appropriate level and feeds representative signals to the computer controller  25  to operate valves  39  and  39 ′ once the concentration of glycol attains 50% concentration. Valves  39  and  39 ′ are operated to transfer batches of the spent glycol of 50% concentration to a buffer tank  40  on a continuous basis. The evaporators  33  and  33 ′ operate on a continuous basis and as the volume of spent glycol solution in holding tank  32  drops to a predetermined level the pumps  30  and  30 ′ are again actuated to filter more spent solution to feed ultra filtered solution to the holding tank  32 . Any overflow from the buffer tank  40  is collected in reservoirs  40 ′ for future use. In this embodiment there are several reservoirs  40 ′ capable of holding 3 million liters of the solution of 50% glycol concentration. A pump  39 ″ supplies the solution to the buffer tank  40  when needed to maintain the process continuously operational. Also, the reservoirs  40  and  40 ′ are maintained inside a building to manage heat loss from the spent glycol of 50% concentration to save energy costs. In the preferred embodiment herein described, the buffer tank holds 40,000 liters of the solution. An overflow valve, not shown, recovers any solution which exceeds the capacity of the tank and feeds. 
         [0019]    After a desired volume, sensed by the signals from the level sensor  41 , is present in the buffer tank  40 , the computer controller  25  operates a transfer valve  42 ′ and a pump, not shown but obvious, to transfer progressively some of the spent glycol solution over to a pH adjustment tank  43  provided with an agitator  44  for analysis as the volume of spent glycol solution in tank  43  is known as detected by level sensing probe  48  having informed the computer controller. The computer controller  25  also operates a transfer valve  47  and a pump, not shown but obvious, when the pH sensor  48 ′, located in the tank  43 , measures any low pH by amending the spent glycol solution with sodium hydroxide (NaOH) solution from reservoir  46 . The pH is adjusted to a desired value of about 7.8 on the pH scale. The adjusted pH spent glycol solution of 50% concentration is then fed through a carbon filter  49  to remove color and odors from the spent glycol solution before being stored into a distillation tower feed tank  50  where a predetermined quantity of the pre-treated spent glycol is maintained and controlled by level sensors. 
         [0020]    As shown in  FIG. 1B , the process includes a distillation tower  51  which is essentially comprised of a bottom evaporator section  52  which includes a heating coil  53  in which is circulated, in a controlled manner, hot steam from a boiler  54  whereby to boil a predetermined batch of the spent glycol solution of 50% glycol concentration to evaporate substantially all of the liquid solution in the batch. Because the boiling temperature of water is far inferior to the boiling temperature of glycol, water will start evaporating as soon as the batch temperature reaches the boiling point of water. Accordingly, only water will evaporate at first, followed by water mixed with glycol, and lastly high concentration glycol. By controlling the flow rate of steam in the coil we can increase the temperature of the spent liquid in the evaporators section to evaporate the liquids in stages. Once all the liquids have evaporated, the sensed temperature in the packings will drop providing an indication that the entire batch is evaporated. 
         [0021]    A vacuum pump  55  connects to the distillation tower  51  at an appropriate downstream location outside the tower to place the tower under vacuum. The distillation tower has a chimney section  56  above the evaporator section  52  and in which there is provided metal packings  57 , herein stainless steel corrugated perforated sheets which accumulate heat from the hot stream of vapors released from the boiling liquid in the evaporator section. The temperature of the vapor passing through the packings is monitored by temperature sensors  63  feeding temperature signals  63 ′ to the computer controller  25  to provide an indication of the temperature of the stream of steam giving an indication of its content in relation to its temperature. These temperature signals also indicate the start and the end of the evaporation cycle. The temperature sensors may be in the form of thermistors retained biased and spaced apart vertically against the wall of the chimney section. The distillation tower is also thermally insulated by a shroud spaced about the inner casing with an appropriate insulation material disposed there between, much like the insulated tank of a water heater. The boiler is heated with natural gas and circulates sufficiently high temperature steam through the heating coil  53  whereby the boiling water temperature rises substantially quickly to the boiling point of glycol which is about 197 degrees C. which is far greater than the boiling point of water. The density of glycol is about 1.1132 g/cubic centimeter. It is pointed out that the vacuum conditions in the distillation tower decreases slightly the boiling points of each liquid to be evaporated, namely water and glycol, and this also provides an energy cost saving. 
         [0022]    At the top of the chimney section  56  there is mounted a condenser  58  which condenses the stream of vapor as it exits the distillation tower  51  to turn the vapor into its liquid phase. The condensation is outside the chimney whereby the condensate does not fall back into the chimney section. As mentioned, because the water evaporates earlier than the glycol, at the beginning of the evaporation process, mostly water is condensed and the condensate with some vapor is channeled by a long conduit  59  where further condensation takes place and into a further cooler  60  where any residual vapor is condensed. Nothing is released into the atmosphere to prevent the loss of glycol. Secured to an outlet pipe  61  of the cooler  60  is a refractometer  62  which measure the glycol concentration of glycol which may be contained in the condensed liquid and feeds its readings to the computer controller  25 , which in correlation with temperature signals  63  received from the chimney section, determines the proper time to operate valves  64  associated with a respective one of three reservoirs or tanks  65 ,  65 ′ and  65 ″. 
         [0023]    When only water is detected, valve  64  is open and the other two valves  64 ′ and  64 ″ are closed thereby to channel the water into reservoir  65  for disposal or re-use. When computer controller  25  determines that the condensate liquid contains glycol mixed with water, the computer controller  25  closes valves  64  and  64 ″ and opens valve  64 ′ to channel the mixture into reservoir  65 ′ until the computer controller determines from its correlated signals that the glycol concentration about 99.5%. The computer controller then closes the valves  64  and  64 ′ and opens valve  64 ″ to channel the remaining glycol which is classified as virgin glycol into reservoir  65 ″. Preferably, the condensed liquid is switched over to reservoir  65 ″ when the computer controller  25  receives condensate signals indicating 99.6% to 99.9% glycol concentration in the solution from refractometer  63  which is classified as virgin glycol. Level sensors  66 ,  66 ′ and  66 ″ provide signals to the computer controller indicating the volume of liquid in its associated reservoirs whereby to evacuate its contents to appropriate locations. Valve  67  dispenses the recovered water for appropriate storage in tank  68  for disposal or re-use. Valve  67 ′ permits its content to be transferred back into the holding tank  50  to be re-introduced into the distillation tower. Finally, residual material consisting of high boiling point compounds collect in the sump lower section of the evaporator section and is pumped into a waste tank  62  at the end of the evaporation cycle and another batch is introduced for evaporation and separation. 
         [0024]    Referring now to  FIG. 1C , it can be seen that the glycol solution batch from reservoir  65 ″ is now subjected to a further quality control testing in a further reservoir  70  to certify its glycol concentration by measuring the Fisher methodology, the Brix and testing of the pH. Sampling valves (not shown) are provided at various levels of the reservoir  70  for the extraction of samples for lab analysis. After certification, the computer controller  25  operates valves  71  to transfer the certified glycol into certified reservoirs  72 . The contents of the reservoir  72  is monitored by level sensors  72 ′ feeding information signals to the computer controller  25  whereby the content can be safely transferred and to indicate that the valves  71  need to be operated to switch the transfer of solutions to other reservoirs  72 ,  72 ′ and  72 ″. With the batch process of the present invention the concentration of the recycled glycol in the solution can be brought up to 99.9% whereby it achieves a very high level of purity which is very important to provide a final solution of near perfect concentration, the requisite for virgin glycol of the highest purity to assure for a perfect blended deicing solution for re-use. 
         [0025]    At this point in the process, the certified glycol solution  74  in one or several reservoirs  72  need to receive further final additives and this is effected in a blending tank  75 . The solution  74  is transferred into the blending tank by the computer controller  25  operating selected ones of transfer valves  76  until the blending tank is filled with a predetermined weight as sensed by a scale  78  mounted under the blending tank  75 . Once the scale  78  indicates that the blending tank has received its volume of glycol solution  74 , the computer controller  25  shuts off the valve(s)  76 . At this point ADF blending is done by the addition of additives  85 , also sensed by a scale  73 , which are introduced into the solution content  81  by operating pump  84  to bring the fluid solution to a desired concentration, for example 88%. An agitator  79  is operated by switching on its motor  80  to mix the glycol solution content  81 . Sampling valves  82  permit sampling the content  81  after the agitating cycle to effect tests, testing the pH, and the refracting index to assure that the content  81  can be certified as a type I de-icing glycol. Valves  86  and  87 , controlled by the computer controller  25  direct the final adjusted certified solution to additional reservoirs  88  equipped with valves  89  for dispensation to farm tank  90  at a location accessible to aircraft deicing vehicles. These vehicles effect on-line blending of the deicing glycol solution by adding water which may have been recovered in the process, depending on the outside temperature, to adjust the glycol concentration. 
         [0026]    It is within the ambit of the present invention to cover any obvious modifications of the preferred embodiment described, provided such modifications fall within the scope of the appended claims.