Abstract:
A system for recovering methylene chloride from an aqueous waste stream is disclosed. The system includes a distillation unit, at least one heat transfer unit, receptacles for methylene chloride and waste product, a temperature probe and a controller configured to selectively divert product flow from the distillation unit to either of the receptacles or to a recycle line back to the distillation unit depending upon the temperature measured by the probe. Also disclosed is a related process of recovering methylene chloride with such a system.

Description:
BACKGROUND 
     There is disclosed herein a process for recovering methylene chloride from an aqueous water solution and the recovered methylene chlorine produced thereby. More particularly, the exemplary embodiment disclosed herein relates to the use of a recycle stream in a batch distillation process for recovering methylene chloride from a waste stream. The process reduces hazardous waste and improves the reclaim efficiency of methylene chloride. The exemplary embodiment finds particular application in conjunction with recovering methylene chloride used as a solvent in photoreceptor fabrication, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications. 
     Photoreceptor fabrication requires reagent grade (i.e., 99.99% purity) methylene chloride which is used as a solvent. Typically, a recycling system captures the solvent as it vaporizes in a carbon bed. The captured solvent is removed from the carbon bed with steam and the resulting water/methylene chloride mixture is distilled to recover the solvent. Previous operations monitored the distillation process and diverted the recovered solvent once the quality or concentration level of the solvent fell below reagent grade. This technical grade material can be sold to entities who market it as paint remover, etc. However, this practice requires that the methylene chloride solvent is frequently replenished during the photoreceptor fabrication process. Furthermore, another disadvantage of this practice is that the low quality solvent shipped to other entities be designated as hazardous waste. 
     The present exemplary embodiment contemplates a new and improved process for recovering methylene chloride which overcomes the above-referenced problems and others. 
     BRIEF DESCRIPTION 
     The present exemplary embodiment provides, in one aspect, a system for recovering methylene chloride from an aqueous waste stream. The system comprises a distillation unit having an inlet adapted to receive an aqueous waste feed stream containing methylene chloride. The distillation unit is also adapted to produce a product stream at an outlet of the distillation unit. The system also comprises at least one heat transfer unit in communication with the product stream and adapted to transfer thermal energy from the product stream. The heat transfer unit has an inlet and an outlet for the product stream. The inlet of the heat transfer unit is in communication with the product stream of the distillation unit. The system also comprises a methylene chloride receptacle having a first selectively controllable inlet valve. The system also comprises a waste receptacle having a second selectively controllable inlet valve. The system further comprises a recycle flow line having a third selectively controllable inlet valve. The recycle line is in communication with the feed stream. Each of the methylene chloride receptacle, waste receptacle, and recycle flow line is in selective communication with the outlet of the heat transfer unit for receiving the product stream. The system additionally comprises a first temperature sensor adapted to measure the temperature of the product stream exiting the distillation unit and a second temperature sensor adapted to measure the temperature at the bottom of the distillation unit. A controller is also provided in communication with the first and second temperature sensors, the first selectively controllable inlet valve, the second selectively controllable inlet valve, and the third selectively controllable inlet valve. Upon the temperature sensors sensing the product stream and bottom portion of the distillation unit having particular temperatures, the controller independently actuates each of the first, second, and third inlet valves to selectively divert the product stream. 
     The exemplary embodiment also provides, in another aspect, a system for reclaiming methylene chloride from a mixture including methylene chloride and water. The system comprises a distillation unit having an inlet for receiving the mixture and an outlet for a product exiting the unit. The system also comprises a plurality of control valves having an inlet and at least a first, a second, and a third outlet. The inlet of the valves is in communication with the outlet of the distillation unit. The valves are adapted to divert flow from the inlet to one of the first, second, or third outlets upon actuation. The system also comprises a flow line providing communication between the third outlet and the inlet of the distillation unit. A plurality of temperature sensors are also provided. The temperature sensors are adapted to measure the temperature of the product exiting the distillation unit and the temperature of a lower region of the distillation unit to provide at least one output signal indicative of the measured temperature. The system further comprises a controller adapted to receive the output signal from the temperature sensors and actuate the control valves. Upon the temperature sensors measuring a particular temperature of the product exiting the distillation unit and a certain temperature of the lower region of the distillation unit, the controller actuates the control valves to divert flow from the inlet of the control valves to one of the first, second, or third outlets of the valves. 
     The exemplary embodiment, in yet another aspect, also provides a process for recovering methylene chloride from a mixture including water and methylene chloride. The process comprises providing a system including (i) a distillation unit having an inlet and an outlet, (ii) a plurality of selectively positionable control valves each having an inlet and an outlet, one of the outlets being in communication with the inlet of the distillation unit, (iii) a plurality of temperature sensors and means for providing a control signal corresponding to sensed temperatures, and (iv) a controller in communication with the temperature sensors and the control valves. The process also comprises directing the mixture including water and methylene chloride to the inlet of the distillation unit. The process further comprises operating the distillation unit to produce a product stream at the outlet of the distillation unit. The process additionally comprises measuring the temperature of the product stream and a temperature at a lower region of the distillation unit with the temperature sensors. The process further comprises providing the control signal from the temperature sensors to the controller. The process also comprises operating the controller to selectively position the control valves based upon the control signal. 
     One advantage of the present exemplary embodiment is the provision of improved recovery of methylene chloride from an aqueous waste stream. 
     Another advantage of the present exemplary embodiment is that the amount of hazardous waste in the form of a stream of methylene chloride and water, is reduced. 
     Still further advantages and benefits of the present exemplary embodiment will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exemplary embodiment may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the exemplary embodiment. 
         FIG. 1  is a process schematic according to the exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Conventional processes for reclaiming methylene chloride often result in disposing below grade or off-specification methylene chloride of relatively high purity but not reagent grade (99.99%). The present exemplary embodiment utilizes a “recycle” immediately following the reagent grade methylene chloride cutoff point. This purge aspect redirects the below grade or off-specification methylene chloride back to the system feed tank rather than being disposed. By doing this, the off-specification methylene chloride is recycled in each batch thereby enabling increased reclaim efficiency of methylene chloride at reagent grade purity. 
     In a typical photoreceptor fabrication process, following an AMAT coating process, solvent laden air is purified through one or more carbon adsorbers. The absorber purifies the solvent laden air produced during the coating process. The absorber is then regenerated by steam produced by a steam generator, the steam is pumped through the carbon thereby capturing the solvents by the carbon particles. The steam/solvents mixture is then condensed. The condensed water/solvent mixture produced alone can be directed to a batch distillation process which boils the water/solvent mixture. The solvents boiling at a lower temperature than water are driven out of the boiler section and in the form of vapor rise up the distillation column. In this vapor form, they are refined and purified as they reach the upper levels of the column. These vapors finally exit the top of the column where they are condensed. The condensed liquid then flows through a reflux divider where the flow is split, with a portion being returned to the top of the column where it is revaporized and assists in refining the vapors at that point in the column. The remaining liquid is passed through a product cooler and either stored as pure methylene chloride or directed to a flammable waste tank for future disposal. 
     The present exemplary embodiment relates to the addition of a recycle operation. During the “product cut” or reclamation of pure methylene chloride, a control system monitors the top column temperature and the temperature of the “bottoms” or lower region of the distillation column. The term “bottoms” as used herein refers to the portion of the distillation column, generally located in a lower region of the column, at which the operating temperatures are typically the highest. When the top temperature reaches 101.5° F. and the bottom temperature reaches 104.5° F., the quality, i.e. concentration, of the methylene chloride cut begins to drop below the reagent grade level. At this point the recycle is initiated by causing the product valves to switch to the batch column feed tank rather than the flammable waste tank. The recycle remains active so long as the top temperature is greater than or equal to 102° F. and the bottom temperature is greater than or equal to 125° F. The recycle is terminated upon either or both of the top temperature being less than 102° F. and/or the bottom temperature being less than 125° F. At this point the solvent concentration of methylene chloride has dropped to less than 25%. The recycle is then terminated by causing the product valves to switch to the flammable waste tank. The distillation process continues until the top column temperature reaches 212° F. indicating that all the solvents have been removed with only water remaining. The water is then pumped to the steam generator feed tank and reused to make steam for adsorber regeneration. 
     The exemplary process schematic is illustrated in  FIG. 1 . The system  10  comprises a feed tank  20  which receives waste stream A. The waste stream A can be a waste stream from a photoreceptor fabrication process. The waste stream A contains water and methylene chloride, and potentially additional solvents. The system  10  also includes a distillation unit  30  and a heat source  40 . A steam generator  35  can be provided as the heat source  40 . The heat input to the unit  30  from the generator  35  may be governed by a control valve  37 . A temperature sensor  34  A is provided at the top region of the distillation unit  30 . A temperature sensor  34 B is provided at the bottom region of the distillation unit  30 . The system  10  also comprises heat exchangers  50  and  70  and one or more sources of a cooling medium such as cooling water designated in  FIG. 1  as CW 1  and CW 2 . The system  10  additionally comprises a reflux divider  60 . And, the system  10  comprises a product tank  80  for storing reclaimed methylene chloride, i.e., shown as stream C. The system  10  also comprises a waste tank  90  for collecting minor waste stream D. System  10  also includes valves  100 ,  110 , and  120  and associated flow lines as further described below. 
     The operation of system  10  is as follows. Waste stream A containing water, methylene chloride and potentially additional solvents is directed through a flow line  22  to the feed tank  20 . The contents of the feed tank  20  are fed to the distillation unit  30  through feed line  24 . As will be understood by those skilled in the art, heat from the heater  40 , or steam from the generator  35 , vaporizes components in the feed into a vapor component which exits the distillation unit through flow line  32 . The sensor  34 A measures the temperature of the exiting vapor. That temperature information is designated as T TOP . The sensor  34 B measures the temperature of the column bottoms, designated as T BOT . Flowing vapor from line  32  enters the heat exchanger  50  which induces full or partial condensation of the vapor. A supply of cooling water CW 2  is used. The cooled methylene chloride rich stream flows through line  52  into the reflux divider  60 , which forms two exiting streams. A first stream is returned to the distillation unit  30  through the line  36 , and the second stream is directed through flow line  62  to the second heat exchanger  70 . Cooling water CW 1  is utilized to reduce the temperature of the methylene chloride rich stream. Flow lines  72 ,  74 ,  76 ,  78 ,  92 ,  94 , and  96  provide flow communication from the outlet of the heat exchanger  70  to one or more of the product tank  80 , waste tank  90 , and the valve  120  as shown. Flow lines  26  and  122  provide flow communication between valve  120  and the feed tank  20 . 
     Depending upon the temperature T TOP  and T BOT  detected by the temperature sensors  34 A and  34 B, a controller  200  positions each of the valves  100 ,  110  and  120  as described herein. The controller  200  is in communication with the temperature sensors  34 A and  34 B via lines W and V, and in further communication with each of valves  100 ,  110  and  120  via lines Z, Y and X, respectively. It will be appreciated that each of lines V, W, X, Y, and Z are electrical communication or signal lines. And the respective valves can be positioned by actuating assemblies known in the art. 
     During operation of the system  10 , the methylene chloride-rich product stream is directed to the product tank  80  if the temperature measured by sensor  34 A, i.e., T TOP , is less than or equal to 101.5° F. and/or if the temperature measured by sensor  34 B, i.e., T BOT , is less than 104.5° F. During this phase of the process, the controller  200  receives control signals from the temperature sensors  34 A and  34 B and opens valve  100  and closes, or confirms closure of valves  110  and  120 . 
     When the temperature measured by sensor  34 A, i.e. T TOP , is greater than 101.5±0.3° F., preferably 101.5° F., and the temperature measured by sensor  34 B, i.e., T BOT , is greater than or equal to 104.5±0.5° F., preferably 104.5° F., controller  200  opens valve  120  and closes, or confirms closure of, valves  100  and  110 . This initiates the recycle operation. It will be appreciated that if a different temperature value is used than the preferred values, e.g., 101.3° F. for T TOP , appropriate modification of the operating cycle is made. In such an example, the product stream would be directed to the product tank  80  so long as the temperature measured by sensor  34 A is less than or equal to 101.3° F. 
     The recycle operation is continued so long as the temperature measured by sensor  34 A, i.e., T TOP , is greater than or equal to 102±0.3° F., preferably 102° F., and the temperature measured by sensor  34 B, i.e. T BOT , is greater than or equal to 125±5.0° F., preferably 125° F. If either of these conditions is not met, or if both of these conditions are not met, then the controller  200  terminates the recycle operation and closes valve  120  and opens valve  110  so that the product stream is directed to the waste tank  90 . Valve  100  remains closed. 
     It will be appreciated that the present exemplary embodiment system can utilize a wide array of process components. For example, instead of using multiple control valves, a single valving unit that provides a plurality of outputs may be employed. And, it will be appreciated that various sensor and signal configurations can be used. For example, a single control signal could be utilized for providing communication between the two temperature sensors  34 A and  34 B, and the controller  200 . Additionally, it may be desired in certain embodiments to limit the maximum thermal energy input to the distillation unit  30  to about 55% during the recycle operation. A further safety limit may be implemented by configuring the controller  200  to close the steam valve  37  upon sensing that the pressure of the entering steam exceeds 75% of its maximum pressure. Appropriate pressure sensors on the distillation unit  30  would be used, in communication with the controller  200 . A further safety measure that may be implemented is to terminate operation of the distillation unit  30  if a critical minimal level is reached within the distillation unit, such as, for example about 30% of its capacity. Furthermore, the various process units, components, and hardware and sources therefore will be understood by those skilled in the art. 
     The advantages of the present exemplary embodiment are financial and environmental. By increasing the efficiency of methylene chloride reclamation, the purchase and thus cost of methylene chloride is significantly reduced. Furthermore, the amount of hazardous waste generated is also significantly reduced. 
     In this regard, batch column testing was conducted to estimate savings from use of the present discovery exemplary embodiment. The following quantities of reclaimed methylene chloride were produced as shown in Table 2: 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Amounts of Methylene Chloride Typically Reclaimed 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Month 1 
                  2436 gal 
               
               
                   
                 Month 2 
                  2154 gal 
               
               
                   
                 Month 3 
                  1429 gal 
               
               
                   
                   
                  6019 Gals/qtr 
               
               
                   
                   
                  68617 Lbs/qtr 
               
               
                   
                   
                  68564 Reclaimed Lbs/qtr 
               
               
                   
                   
                  24076 Est Annual Gallons 
               
               
                   
                   
                 274254 Est Annual Lbs 
               
               
                   
                   
               
             
          
         
       
     
     These amounts of reclaimed methylene chloride result in a substantial cost savings. Additionally, the purity of the reclaimed methylene chloride was exceedingly high. The following data set forth in Table 3 was obtained illustrating the batch column operating characteristics using the recycle as described herein: 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Batch Column Testing 
               
             
          
           
               
                   
                   
                   
                 Temp 
                 Temp 
                   
                   
                 Vinyl 
                   
                   
                   
                   
                   
                 CYCLE 
               
               
                 Time 
                 Temp Top 
                 Temp Mid 
                 Bottom 
                 Tank 
                 Vis % H 2 O 
                 Ethanol 
                 Chloride 
                 CLORO 
                 THF 
                 CYCLENE 
                 HEP 
                 TOL 
                 ONE 
               
               
                   
               
             
          
           
               
                 13:43 
                 100.3 
                 101.1 
                 111.5 
                 150.2 
                 0% 
                 424 
                 0 
                 0 
                 57 
                 2 
                 98 
                 36 
                 2 
               
               
                 13:45 
                 100.2 
                 101.1 
                 113.5 
                 155.0 
                 0% 
                 509 
                 0 
                 0 
                 6 
                 3 
                 134 
                 1 
                 0 
               
               
                 13:49 
                 100.3 
                 101.1 
                 114.7 
                 160.0 
                 0% 
                 518 
                 0 
                 0 
                 3 
                 3 
                 145 
                 0 
                 0 
               
               
                 13:53 
                 100.3 
                 100.9 
                 118.3 
                 165.0 
                 0% 
                 541 
                 0 
                 0 
                 6 
                 4 
                 205 
                 0 
                 0 
               
               
                 13:57 
                 100.2 
                 101.2 
                 122.8 
                 170.0 
                 0% 
                 634 
                 0 
                 0 
                 11 
                 10 
                 465 
                 0 
                 0 
               
               
                   
               
             
          
         
       
     
     The present exemplary embodiment diverts technical grade solvent back into feed tank or feed stream where it undergoes a subsequent pass through the distillation unit, capturing a significant fraction of the previously unclaimed methylene chloride. In a typical photoreceptor fabrication process, the exemplary embodiment will save a substantial amount of methylene chloride per year. EPA regulations require businesses to install the most efficient solvent recovery system available so this process has potential commercial value. 
     The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.