text: produced water is water that has been extracted from oil well production fluid . in embodiments of the invention this fluid is delivered in a deoiled state to the evaporator system which treats the produced water by generating a high purity distillate suitable for recycling as boiler feedwater . the produced water is fed to the evaporator system with or without make - up water ( either fresh or brackish ), boiler blowdown and other waste water streams . a block - flow diagram ( fig1 ) shows these streams entering the evaporator system and being processed by a vertical falling - film evaporator system and / or a forced - circulation evaporator system to achieve zero liquid discharge ( zld ). antifoam , antiscalant , dispersant , and strong bases can be added to the system . the system recycles greater than 98 % of the feed water as high purity distillate . dry solids , passing the environmental protection agency ( epa ) paint filter test (“ test methods for evaluating solids waste , physical / chemical methods ”: united states government publication sw846 : method 9095b ) are discharged from the system along with vent steam . the feed streams are combined in a reaction tank where sodium hydroxide ( a strong base ) is added to react with the hardness in the feedwater to soften the feedwater . reaction chamber ph is maintained near 10 . 5 at 25 ° c . the added benefit to high ph softening of the feedwater is that the brine concentrates within the evaporator system at an elevated ph ( typically above ph of 10 . 5 ). it is well established within the material science community that a deaerated , high chloride environment that exists at high ph eliminates corrosion potential and allows for the use of leaner metallurgies for the construction of the evaporator system . these leaner metallurgies would typically include the grades 316l ss , 2304 duplex and 2205 duplex . ( 316 ss is a common industrial austenitic stainless steel grade . duplex stainless steels are steels that are made with a combination of austenitic and ferritic phases . the combined phases cause duplex steels to have superior strength and corrosion resistance properties compared to simple austenitic grades .) this precludes the need to use higher metallurgies which reduces the capital cost of the system . the feed water is preheated in a heat exchanger ( plate and frame - type , shell and tube - type , spiral - type , etc .) and deaerated to remove oxygen and volatile compounds such as light organic hydrocarbons . steam that vents from the shell - side of the evaporator ( s ) is used as the stripping steam in the deaerator . the steam is vented from the deaerator , collected in a header with other steam vent sources and processed in a catalytic oxidizer , flare stack , or other disposal system to prevent harmful compounds such as hydrogen sulfide and mercaptains from escaping to the environment . an automatically controlled preheater bypass may continuously maximize the enthalpy of the distillate flowing to the boiler system . this system may also minimize the vent steam rate . this control action minimizes the capital and operating cost of the vent disposal system . the deaerated and preheated feed stream is delivered to the evaporator system where vertical - tube falling - film evaporator preconcentrates the produced water and minimizes the wastewater volume fed to the forced - circulation evaporator . the vtff evaporator works by circulating brine water at a high flow rate to the evaporator top - head where the brine is distributed evenly on the inside perimeter of the vertical tubes . as the brine flows down the inside of the tube , steam condenses on the outside of the tube and transfers heat to the falling brine film . this brine water is vaporized inside the tube and flows out of the evaporator as water vapor . the brine distribution in the top - head of the evaporator may be achieved using a dual distributor tray technology ( fig2 ). the brine is evenly cascaded through two horizontal trays . flow is directed onto the upper tube - sheet where it flows evenly to the inside perimeter of the tube wall . this design precludes the use of spiral tube distributors , which are conventionally inserted into the tubes causing plugging problems . these distribution trays use relatively wide holes to allow any suspended solids to pass through without plugging . additionally , a strainer can be utilized upstream of the distribution trays to capture larger solids ; this offers additional protection against plugging . the vertical - tube evaporator used in embodiments of the invention has a straight - sump design ( fig3 & amp ; 4 ) which means that the lower sump has an equivalent diameter to that of the vertical tube - bundle section . this is significant because the high volume of recirculating brine falling from the tubes maintains a turbulent and downward moving flow pattern in the sump that prevent oil and organics from accumulating in the sump . this is a superior concept compared to the wide - sump evaporator ( fig5 ) because the wide - sump has a quiescent volume where oil , grease and other organics typically accumulate requiring an intermittent oil removal system . accumulation of oil and organics worsen the distillate purity , cause damage to the system , and increase downtime . computational fluid dynamic analysis shows that there is very little mixing between the high velocity inner - core and low velocity annular volume ( example 3 ). the water vapor generated in the evaporator tubes flows downward , exits the evaporator and flows into a mist eliminator vessel ( fig6 ) that is external to the evaporator . the mist eliminator vessel makes a significant improvement in distillate purity by removing even very fine brine droplets ( diameter less than 5 micrometers ) from the vapor stream . the high - efficiency mist eliminator utilizes a multiple - stage design to guarantee the high purity of the distillate . the mist eliminators have at least two stages of mist elimination . stage one consists of chevrons . industrial chevrons mist eliminators are an array of thin gauge metal arranged in a zig - zag pattern such that mist droplets will contact the metal surfaces and be removed from the vapor stream . these chevrons may remove more than 95 % of the entrained brine droplets . this first stage is referred to as the coarse removal stage and is responsible for capturing the droplets of small , medium and large size . a second stage of chevrons is included to provide finer removal of the remaining entrained brine droplets . additional stages may be added for removal of increasingly finer droplets . it is the capture of these very fine droplets that guarantees that the invention will produce distillate of the highest purity . the integrated second and third stages of mist elimination are mesh agglomerator pads and polishing chevrons , respectively . mesh agglomeration pads are stationary mist elimination elements that consist of very fine diameter wire knitted together to form a pad . the agglomeration pads force the very small droplets to impact and coalesce to form medium and large size droplets . the very small droplets that have now been enlarged into medium and large size droplets are reentrained in the vapor and exit the second stage and enter the third stage . the reentrained droplets are then removed from the vapor by the third stage polishing chevron . the conventional technology utilizes mist eliminators designed for vertical flow that are installed in the inner annulus of the evaporator itself . this design is susceptible to solids accumulation because the solids - containing brine droplets drain at a low velocity because they are being opposed by the upwards flow of vapor . the hybrid mist eliminator design overcomes this deficiency by utilizing a mist eliminator system that is designed for horizontal flow and that is located externally to the vessel . horizontal flow design allows for superior brine - droplet removal because the vapor flow ( horizontal ) does not oppose the drainage path of the captured droplets ( vertically downwards ). thus , the horizontal mist eliminator design naturally alleviates solids plugging issues that are experienced by the conventional technology . furthermore , the first stage chevron can be provided with a washing system to intermittently cleanse the chevrons of any solids that do accumulate . the washing solution used is typically a high ph scrubbing solution prepared with sodium hydroxide which is designed to target the removal of silicate solids . the first stage chevrons are all washed at a regular frequency to ensure that solids do not accumulate to a high degree . washing of the chevrons can be performed all - at - once or in a sequence . the second stage is provided with a scrubbing system that continuously sprays a washing solution into the vapor stream . this absorbs volatile components and further enhances the distillate purity . the distillate purity can also be enhanced by arraying the evaporators in a multiple - effect configuration ( fig7 ) such that the vapor generated from one effect is the steam that drives evaporation in the next evaporator effect . typically two or more effects are used , though more could be included if desired . this design offers at least two key advantages over the conventional technology . first , there is a significant reduction in power consumption ( for mvc design only ) because less than half of the total vapor generated must be processed by the vapor compressor . this power reduction significantly reduces the operating cost of the evaporator system . second , more than half of the vapor produced by the total system is produced from brine at lower total dissolved solids ( tds ) concentrations . this is important because the fine droplets entrained in the vapor will have a lower tds concentration ( compared to the final brine concentration ) which results in an even higher purity distillate . the brine that is preconcentrated in the vtff evaporator ( s ) is blowndown to the forced - circulation evaporator which completes the concentration and allows the entire system to recycle better than 98 % of the total feed water . zld is achieved through the application of a drying technology , centrifugal separation technology or filtration to dewater the solids such that they will pass the epa paint filter test . example 1 describes performance of an evaporation system treating produced water by generating high quality distillate . a pilot scale test was run on a representative sample of produced water from a sagd process to ascertain the effectiveness of several design enhancements . the objectives were 1 ) to prove the effectiveness of softening the produced water upstream of the evaporator and 2 ) to prove that straight - sump falling film evaporator design could process a high amount of oil and grease . of course , the satisfaction of these test objectives should not be viewed as a requirement for an embodiment to be within the scope of the claims . the total run time of the pilot testing was 972 hrs . the evaporator system was operated at a concentration factor of 26 for a net recovery of 96 %. the process stages included produced water softening reactor , produced water preheater , deaerator , vertical - tube falling film evaporator ( straight - sump ), multiple - stage mist elimination , and a final vapor condenser . evaporation on the pilot scale was driven by a steam generator such that the process models a multiple - effect falling film evaporation system . the produced water contained 15 . 0 ppm of oil and grease . this oil and grease level was acceptable for the operation of the evaporator system with the straight sump evaporator . there was no visible evidence of an oil or hydrocarbon phase accumulating in the sump above the water phase . this was due to the continuous turbulence provided by the straight sump design . the oil did not accumulate in the evaporator system and was continuously blowndown with the brine concentrate leaving the evaporator ( concentration factor = 26 ). the produced water was added to a softening reaction vessel prior to the preheating , stage . in the softening reaction vessel , sodium hydroxide was added to the stream at concentrations in the range of 500 ppm to 1 , 000 ppm . the water was softened as salts of calcium and magnesium precipitated from the solution . an antiscalant chemical was also added to the feedwater in the amount of 16 mg / l . the salts that precipitated were suspended in solution with a mixer and moved through the rest of the system and were eventually blowndown from the evaporator with the concentrated brine . the average solids size produced in the softening reaction step was 5 . 8 micrometers which is a size suitable for movement through the evaporator without significant accumulation . example 2 reports effectiveness of the hybrid mist eliminator . quantitatively , the effectiveness of the hybrid mist eliminator can be seen by comparing the droplet capture efficiency at various droplet sizes . the droplet capture of the hybrid external mist eliminator is superior to the conventional internal mist eliminator through the entire spectrum but is more pronounced in the capture efficiency of the fine droplets . for droplets with a diameter of 5 . 0 micrometers , the hybrid mist eliminator captures 6 % more droplets than the internal mist eliminator . for droplets with a diameter of 4 . 0 microns , the hybrid mist eliminator captures 34 % more ; at 3 . 0 micrometers , the hybrid mist eliminator captures a staggering 80 % more droplets . the improved droplet removal efficiencies is physically observed in this process by producing water with a superior distillate quality because there are substantially fewer brine droplets that remain entrained in the vapor after the mist eliminator . example 3 reports the deficiency of the wide - sump design in that a quiescent volume is generated that leads to stagnation and accumulation of oil and hydrocarbons . a computation fluid dynamic study ( fig9 ) was undertaken to determine the effectiveness of an evaporator wide - sump to provide good mixing and prevent the creation of stagnant pockets . the results of the study show that the wide - sump does , in fact , create a quiescent volume in the annular region of the sump . the fluid velocities in this annular region range from approximately 0 feet per second to 1 feet per second . in the chamber , the fluid circulates counterclockwise from the bottom to the top . the recirculating velocities within the side chamber are very small and indicate very little mixing and very little new flow of liquid into this section of the tank . in contrast to the wide - sump , the straight - sump evaporator simulates fluid flow in a pipe and has a relatively uniform velocity profile that precludes the creation of stagnant side chambers . patents , patent applications , publications , scientific articles , books , web sites , and other documents and materials referenced or mentioned herein are indicative of the levels of skill of those skilled in the art to which the inventions pertain , as of the date each publication was written , and all are incorporated by reference as if fully rewritten herein . inclusion of a document in this specification is not an admission that the document represents prior invention or is prior art for any purpose .