Abstract:
The present invention provides for a method of recovering energy from water produced during heavy oil recovery, comprising the steps of:  
     p) providing a high pressure blowdown stream;  
     q) flashing the high pressure blowdown stream to form low pressure waste energy stream and concentrated blowdown;  
     r) vaporizing produced water with low pressure waste energy stream;  
     s) preheating the produced water feed stream with concentrate blowdown stream;  
     t) providing a fluid circulation circuit including a heated separator and a reboiler exchanger in fluid communication;  
     u) passing preheated produced water feed stream into the heated separator;  
     v) passing the waste energy into the reboiler for recovery of heat energy;  
     w) vaporizing the produced water stream with the waste energy in the reboiler exchanger to generate a vapor fraction and a concentrate liquid contaminant fraction;  
     x) circulating at least a portion of the concentrate liquid fraction through the reboiler exchanger and the heated separator to maintain a ratio of mass of concentrate to vapor fraction of between 300 and 2 to result in a vapor fraction of about 1% by mass to less than 50% by mass exiting the reboiler exchanger to prevent fouling and scaling in the reboiler;  
     y) condensing the vapor fraction with an external condensing means; and  
     z) collecting condensed vapor fraction and waste energy stream substantially devoid of contaminants.

Description:
[0001]    This is a Divisional application of U.S. patent application No. 09/577,822 filed May 25, 2000 which is a Continuation-In-Part of U.S. patent application No. 09/468,915 filed Dec. 22, 1999, which in turn is a Continuation-In-Part of U.S. patent application No. 09/006,499, filed Jan. 14, 1998, which claims the benefit of U.S. Provisional Patent Application No. 60/035,493 filed Jan. 14, 1997. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is directed to a highly efficient water distillation process and an apparatus thereof and more particularly, the present invention is directed to a highly efficient water distillation process used in the thermal recovery of heavy oil which minimizes fouling and scaling of operating equipment over long periods of operation.  
         BACKGROUND OF THE INVENTION  
         [0003]    Throughout the many regions in the world, heavy oil, a hydrocarbon material having much higher viscosity or lower API gravity (less than 20° API, typically 7° to 12° API) than conventional petroleum crude, is more difficult to recover and requires enhanced thermal stimulation techniques of the subsurface reservoir to produce. More particularly, in areas of Western Canada heavy oil producers use a technique of injecting high pressure steam into the reservoir at typical pressures of about 1,500 to 3,000 psig, and in some cases as low as 150 psig. The steam heat energy is generated by an apparatus known as a steam generator to a 60 to 80% steam quality and injected into vertical or horizontal well arrangements to reduce the heavy oil viscosity. The flowable heavy oil is collected in adjacent producing wells and a combination of heavy oil, oil/water emulsion, condensed steam and formation brackish water (known as produced water) is produced to the surface. Using surface facilities, heavy oil is separated from the production fluids and recovered for commercial sale. The produced water, typically recovered at water/oil ratios of 2 to 5, is currently disposed of in subsurface disposal wells. Makeup water from an authorized ground water source is used to makeup the steam generator feed water demand. Typically the makeup water receives minimum treatment to reduce hardness and silica compounds to avoid scaling of the steam generator heat exchange surfaces and prevent a safety hazard. In some facilities, the concentrated brine water from the steam generator discharge is separated from the reservoir injection steam and disposed of in suitable deep disposal wells. This concentrated brine water can also be referred to as high pressure blowdown. This prevents excess and unnecessary hot water from being injected into the reservoir during the steam stimulation operation. Typical current heavy oil recovery practices using the steam injection technique are referred to as Cyclic Steam Stimulation (CCS or Huff n&#39;Puff), Steam Assisted Gravity Drained (SAGD) and Steam Assisted Gas Pushed (SAGP) methods.  
           [0004]    Public and regulatory pressures require that heavy oil producers implement water recovery and reuse practices and in some facilities a zero effluent discharge is required. This means that 100% of the water used be recovered and reused and the elimination of offsite disposal of effluent streams. The produced water, recovered from the oil separation facility and the HP (high pressure) steam separators, contains hardness components, dissolved and suspended silica and colloidal compounds (clay) and dissolved solids such as sodium chloride. If this brackish water is recycled without treatment, the operation of the steam generators is at risk due to fouling and scaling.  
           [0005]    A further problem encountered with the current heavy oil recovery practices using steam injection, is that as the operating temperatures of producing reservoirs are increased from 230° F. to greater than 400° F. to enhance the heavy oil recovery, the temperature of the recovered production fluids (oil and water) increase. To facilitate the common practice of atmospheric oil and water separation, significant quantities of steam is created when the fluid pressure is reduced. This steam is typically condensed by an external means, such as an air cooler to recover the condensed water. The heat energy of the condensing steam is discharged to the atmosphere and wasted.  
           [0006]    Until the advent of the present invention combining the recovery of waste heat energy with a highly efficient and non-scaling water distillation process, the recycle of heavy oil produced water and concentrated brine disposal streams has been technically and commercially restricted.  
           [0007]    Generally speaking, water distillation is a highly effective method of vaporizing a pure water distillate and recovering a concentrated liquid or solid containing a large quantity of non-volatile components. This process method can be an effective means to recover clean pure water from contaminated sources. However, water distillation processes typically have several problems not the least of which can be fouling or scaling of the apparatus with minerals or other components from the fluid being distilled. Common scaling compounds consist of calcium, magnesium and silicon. Fouling, or to a greater extent, scaling of the heat transfer surfaces have a detrimental effect on the capacity of the heat transfer components, causing conventional distillation processes to become inoperable.  
           [0008]    Tsuruta, in U.S. Pat. No. 4,566,947, incorporated by reference herein, issued Jan. 28, 1986, taught a general distillation process for fractionating liquids having different boiling points in the range of about 15° C. Although, Tsurata specifically recognized the need for a process to fractionate a mixed liquid, scaling or fouling was only addressed in passing and then only to state that “. . . water which is collected at the tower bottom . . . does not contain substances which foul the inside of the compressor.” in col. 7,I. 55 et seq. Moreover, Tsurata teaches a way from recovering waste heat as he indicates it disadvantageous to use 2 heat exchangers as there is an increase in energy consumption. Thus, it is obvious Tsurata does not contemplate solids, scaling, contaminants or the recovery of waste heat in any way.  
           [0009]    The problem of scaling and the recognition of nucleate boiling and the importance of this physical phenomenon in maintaining a wetted surface in a circuit containing a heat exchanger known. The nucleate boiling regime for a pool of water at atmospheric pressure is a fairly specific area where individual bubbles form and which has been adequately discussed in the Principles of Heat Transfer, Third Edition, by Frank Kreith and in Heat Transfer, Seventh Edition by J. P. Holman.  
           [0010]    Another common problem with typical water distillation processes is high energy input requirements. Without a source of waste heat energy and a means to effectively recover input energy, the energy required is equivalent to the latent heat of vaporization of water at a given pressure/temperature. Water distillation, under this condition is not commercially viable for water remediation applications. Heavy oil producing facilities commonly consist of high energy related fluid streams suitable as sources for waste heat energy recovery.  
           [0011]    Several variables must be considered to overcome the problems with conventional distillation methods. The following three equations describe the basic heat transfer relationships within a water distillation system:  
           Q (total) =U*A*LMTD  (1) 
           Q (sensible heat) =m*CP*(T1−T2)  (2) 
           O (latent heat) =m*L  (3) 
           [0012]    where  
           [0013]    Q=quantity of heat transferred (BTU hr −1 )  
           [0014]    U=overall heat transfer coefficient or ability of system to transfer heat (BTU hr  −1  ft −2  F −1 )  
           [0015]    A=heat transfer surface area (ft 2 )  
           [0016]    LMTD=log mean temperature difference or the thermal drive of the system (F)  
           [0017]    m=mass flow of fluid in liquid or vapor state (lb hr −1 )  
           [0018]    Cp=fluid specific heat (BTU hr −1  F −1 )  
           [0019]    T1,T2=temperature of fluid entering or exiting system (F)  
           [0020]    L=latent heat of vaporization or condensation (BTU lb −1 )  
           [0021]    In order to have an efficient distillation system, the quantity of heat exchanged and recovered, Q, expressed by the above stated equations, must be maximized, while at the same time obeying the practical limits for the remaining variables and preventing scaling and fouling. For a given fluid and fluid dynamics within a given heat exchange apparatus, the variables, U, Cp and L are relatively non-variable. Therefore, careful consideration must be given to the variables A, Q/A, LMTD, m, and T1 &amp; T2 to overcome the problems associated with distillation of contaminated water.  
           [0022]    To fully overcome the problems related to distilling contaminated water from a heavy oil thermal recovery facility and eliminate scaling, other essential factors must be considered beyond the basic equations stated above:  
           [0023]    transforming effective sources of waste heat energy;  
           [0024]    the rate by which the heat is transferred within the distillation system, known as heat flux or QA −1  (Btu hr −1  ft −2 )  
           [0025]    the level of contaminates in the concentrate;  
           [0026]    the final boiling point of the concentrate relative to the saturation temperature of the vapor stream;  
           [0027]    the degree of supersaturation and level of precipitation of the concentrate; and  
           [0028]    level of vaporization of the evaporating stream.  
           [0029]    Until the advent of the present invention, effectively recovering waste heat energy from a heavy oil facility and maximizing the quantity of heat transferred and recovered with a water distillation process, without the tendency of fouling or scaling, could not be realized over a long term period.  
         SUMMARY OF THE INVENTION  
         [0030]    In one embodiment of the present invention there is provided a method of recovering energy from water produced during heavy oil recovery, comprising the steps of:  
           [0031]    a) providing a high pressure blowdown stream;  
           [0032]    b) flashing the high pressure blowdown stream to form low pressure waste energy stream and concentrated blowdown;  
           [0033]    c) vaporizing produced water with low pressure waste energy stream;  
           [0034]    d) preheating the produced water feed stream with concentrate blowdown stream;  
           [0035]    e) providing a fluid circulation circuit including a heated separator and a reboiler exchanger in fluid communication;  
           [0036]    f) passing preheated produced water feed stream into the heated separator;  
           [0037]    g) passing the waste energy into the reboiler for recovery of heat energy;  
           [0038]    h) vaporizing the produced water stream with the waste energy in the reboiler exchanger to generate a vapor fraction and a concentrate liquid contaminant fraction;  
           [0039]    i) circulating at least a portion of the concentrate liquid fraction through the reboiler exchanger and the heated separator to maintain a ratio of mass of concentrate to vapor fraction of between 300 and 2 to result in a vapor fraction of about 1% by mass to less than 50% by mass exiting the reboiler exchanger to prevent fouling and scaling in the reboiler;  
           [0040]    j) condensing the vapor fraction with an external condensing means; and  
           [0041]    k) collecting condensed vapor fraction and waste energy stream substantially devoid of contaminants.  
           [0042]    In another embodiment of the present invention there is provided a method of recovering energy from water produced during heavy oil recovery, comprising the steps of:  
           [0043]    a) providing a high pressure blowdown stream;  
           [0044]    b) flashing the high pressure blowdown stream to form a low pressure waste energy stream and concentrated blowdown stream;  
           [0045]    c) vaporizing at least a portion of the produced water with the low pressure waste energy stream;  
           [0046]    d) preheating the produced water with the concentrated blowdown stream;  
           [0047]    e) providing a fluid circuit including a heated separator and a reboiler exchanger in communication;  
           [0048]    f) providing a vapor circuit including the heated separator, compressor means and the reboiler exchanger in communication;  
           [0049]    g) passing preheated produced water into the heated separator;  
           [0050]    h) vaporizing the preheated produced water with the low pressure waste energy and a compressed vapor stream in the reboiler exchanger to generate a vapor fraction and concentrate liquid fraction;  
           [0051]    i) treating the vapor fraction formed by the low pressure waste energy with an external condenser means;  
           [0052]    j) recovering any remaining portion of the vapor fraction by the compressor means;  
           [0053]    k) circulating at least a portion of the concentrate liquid fraction through the reboiler exchanger and the heated separator to maintain a ratio of mass of concentrate to vapor fraction of 300 to near 2 to result in a vapor fraction of near 1% by mass to less than 50% by mass exiting the reboiler exchanger to prevent fouling and scaling in the reboiler exchanger; and  
           [0054]    l) collecting the condensed vapor fraction and the waste energy stream substantially devoid of contaminants.  
           [0055]    In another embodiment of the present invention there is provided a method of recovering energy from water produced during heavy oil recovery, comprising the steps of:  
           [0056]    a) providing a high pressure blowdown stream;  
           [0057]    b) flashing the high pressure blowdown stream to form a low pressure waste energy stream and concentrated blowdown stream;  
           [0058]    c) vaporizing the produced water with the low pressure waste energy stream;  
           [0059]    d) preheating the produced water with the concentrated blowdown stream;  
           [0060]    e) providing a fluid circulation circuit including a heated separator and a reboiler exchanger in fluid communication;  
           [0061]    f) passing the produced water feed stream into the heated separator;  
           [0062]    g) passing the low pressure stream waste energy into the reboiler;  
           [0063]    h) vaporizing the produced water with the low pressure waste energy in the reboiler exchanger; to generate a first vapor fraction and a concentrate liquid contaminant fraction;  
           [0064]    i) circulating at least a portion of the concentrate liquid contaminant fraction through the reboiler exchanger and the heated separator to maintain a ratio of mass of concentrate to vapor fraction of 300 to near 2 to result in a vapor fraction of near 1% by mass to less than 50% by mass exiting the reboiler exchanger to prevent fouling and scaling in the reboiler;  
           [0065]    j) providing crystallization means and a reboiler exchanger in communication with the vapor fraction;  
           [0066]    k) removing a portion of the concentrate liquid contaminant fraction for feed to the crystallization means;  
           [0067]    l) passing the vapor fraction into the reboiler for providing heat energy for precipitating solids from the concentrate liquid contaminant fraction;  
           [0068]    m) creating a second vapor fraction from the crystallization means and a substantially solid fraction stream;  
           [0069]    n) condensing the second vapor fraction with condenser means; and  
           [0070]    o) collecting a condensed first vapor fraction, a condensed second vapor fraction and a condensed waste energy stream.  
           [0071]    As further advantages to this methodology, the input costs are lowered due to, if sufficient low grade waste energy is made available, a compressor is not required to treat the produced water. Further still, the method protocol facilitates water recovery and results in a zero waste water effluent solution since the contaminants are converted to solid waste.  
           [0072]    In terms of the breadth for this process, the same could be easily employed to any heavy oil recovery operation using steam for thermal stimulation, such as conventional steam flood, cyclic steam stimulation (CSS or Huff n&#39;Puff), steam assisted gravity drain (SAGD) and steam and gas pushed (SAGP).  
           [0073]    Having thus described the invention, reference will now be made to the accompanying drawings illustrating the preferred embodiments. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0074]    [0074]FIG. 1 is a schematic illustration of the overall process according to one embodiment of the described invention;  
         [0075]    [0075]FIG. 2 is a schematic illustration of the overall process according to a further embodiment of the described invention;  
         [0076]    [0076]FIG. 3 is a schematic illustration of the water treatment unit contained within the embodiment of the described invention;  
         [0077]    [0077]FIG. 4 is an alternate embodiment of FIG. 3;  
         [0078]    [0078]FIG. 5 is a further alternate embodiment of FIG. 3;  
         [0079]    [0079]FIG. 6 illustrates, in a schematic form, the typical pressure and temperature conditions around the evaporation components;  
         [0080]    [0080]FIG. 7 is a process condensing/evaporating curve for the system reboiler exchanger;  
         [0081]    [0081]FIG. 8 illustrates the schematic of the flow pattern for the reboiler plate/plate heat exchanger;  
         [0082]    [0082]FIG. 9 is a chart illustrating the level of vaporization in the reboiler, occurring in the circulating fluid with relation to the ratio of circulating fluid mass to vapor mass;  
         [0083]    [0083]FIG. 10 is a chart illustrating the resulting localized concentration effect in the reboiler with varying vapor fractions;  
         [0084]    [0084]FIG. 11 is a chart showing the test data obtained from a distillation unit pilot;  
         [0085]    [0085]FIG. 12 is a schematic illustration of the overall process according to a further embodiment of the described invention; and  
         [0086]    [0086]FIG. 13 is a schematic illustration of the overall process according to a further embodiment of the described invention. 
     
    
       [0087]    Similar numerals employed in the text denote similar elements.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0088]    Referring now to FIG. 1, shown is an example of one embodiment of the present invention. Water is sourced from a suitable ground water source  105 , recycled or makeup water from conventional treatment methods at  100 , such as lime and caustic softening, ion exchange softening or distillation. Primarily, the feed water must have the hardness components, such as calcium, magnesium and silica, removed to prevent scaling of the high pressure steam generators  125 . The dissolved solids are less than 8,000 ppm (w) producing the desired 80% quality high pressure steam. The total dissolved solids (TDS) primarily consists of sodium chloride. The volumes of water can range from as low as 10,000 barrels per day (BPD) for piloting heavy oil thermal facilities, to rates exceeding 100,000 BPD for commercial heavy oil thermal facilities.  
         [0089]    The conditioned water from  110  is pumped by a series of feed pumps  115  to the high pressure steam generator  125 . The typical steam generator  125 , generates a 60 to 80% steam quality at pressures ranging from 1,000 psig to 3,000 psig or greater depending on the nature of the oil reservoir. This type of oilfield steam generator, familiar to those skilled in the art, is limited to less than 100% steam quality due to the inherent design and restriction related to tube scaling. Other boilers, conventional steam boilers and cogeneration heat recovery boilers can be used to generate saturated steam or 100% quality steam or superheated steam if high quality water, such as distilled water could be commercially made available.  
         [0090]    Certain heavy oil reservoirs, for example, the heavy oil reservoirs using the SAGD technique mentioned earlier cannot transfer less than 100% quality steam to the reservoir without affecting oil recovery performance. For these operations, a high pressure steam separator, denoted as  130 , is provided to separate the saturated steam  135  from the HP liquid phase  140 , also referred to as the superheated brine. Some facilities use a portion of the energy available in  140  by exchanging at  120  with the steam generator feed water  115  before releasing the pressure. The amount of heat energy recovered varies on the level of steam pressure at  135 , but generally is limited to a small portion. Therefore, most SAGD heavy oil facilities have a significant quantity of waste heat energy available in stream  140 , which has limited use and is typically rejected to a cooling tower or in a cooler as waste heat. This waste energy stream can be delivered to a high efficiency water distillation unit  180  for treatment of the produced water  175  and have significant impact on lowering the commercial cost of water treatment and improving the production cost of heavy oil. The most significant impact, however, is the environmental benefits in that the makeup water and contaminated water for disposal can be eliminated and a significant portion of the waste energy can be recovered, resulting in a reduction of consumed fuel gas and overall air emissions.  
         [0091]    High pressure steam  135  is injected into the reservoir  145  through well bore  150 . Depending on the type of heavy oil recovery technique used, the well configuration will vary. FIG. 1 illustrates a typical SAGD arrangement where the steam is injected in a horizontal well bore and the heavy oil production fluid is recovered in an adjacent horizontal well bore  155 . The production fluids are received at the surface and transferred by production lines  160  to the oil recovery facility  165 . Heavy oil, typically less than 20° API and greater than 7° API is removed and commercially sold for petroleum refining.  
         [0092]    The produced water  175 , received at a typically water to oil ratio of 2 to 5, is transferred to the water treatment unit generally denoted as  180 . The produced water will contain sodium chloride, silica, dissolved organic hydrocarbons, calcium and magnesium, primarily originating from the oil reservoir and initial makeup water source.  
         [0093]    The concentrated brine wastewater or solids can be extracted from the water treatment unit  180  as stream  185 . This stream typically has no commercial value and requires disposal on site or offsite, depending on the heavy oil facility location.  
         [0094]    Generally, the high efficiency distillation water treatment unit noted as  180  will recover greater than 80% to 100% of the produced water in the form of pure distilled water at stream  100 .  
         [0095]    Referring now to FIG. 2, shown is an example of a further embodiment of the present invention. This example represents a heavy oil recovery facility where the thermal conditions of the production fluids after the producing well bore  155  and well head  160  are required to be greater than the conventional 230° F., approaching 400° F. to 500° F., in order to enhance the performance of the heavy oil recovery. The hot production fluids pass through the degassing separator  161  as the pressure is reduced at  162  to feed the oil/water separation unit. Steam  163  is created at typical pressures of 40 to 60 psig (generally less than 100 psig) from the degassing separator  161 . This low grade steam  163  is transferred to the high efficiency water distillation unit, commonly denoted as  180 , for use in evaporating distilled water from the produced water. This technique of heat recovery can be used if the quantity of associated and/or injected lift gas is low relative to the steam produced in the production fluids  160 .  
         [0096]    If there is fairly high associated gas content in the heavy oil, not typically encountered, and/or lift gas is artificially injected in the production well bore  155 , then an alternate heat energy recovery technique is required. The hot production fluids will transfer through any suitable heat exchange means to drop the temperature before entering the degassing separator  161 . The waste heat energy is extracted from  164  using a suitable heat transfer medium and transferred by  165  to the high efficiency water distillation unit  180  to produce water to distilled water.  
         [0097]    As shown in FIG. 2, both heat recovery methods using streams  140  and  165  can be used independently or combined depending on the operating conditions of the heavy oil reservoir  145  and the benefits gained by each method.  
         [0098]    Referring now to FIG. 3, shown is an example of one embodiment of the high efficiency distillation unit generally denoted as  180 .  
         [0099]    The produced water feed stream, generally denoted by numeral  175 , is introduced into a pretreatment step, generally denoted by  12 , for removing insolubles, volatiles and/or performing other pH or conditioning steps for preparing the feed stream  175 . Volatile components are vented from the feed stream at  14 , while less volatile components are discharged from the feed stream at  16 . The pretreated feed stream exiting  12  is then passed into a preheater  18  to elevate the temperature of the feed stream to enhance sensible heat recovery prior to introduction into a heated separator  20 . The feed stream may be split into multiple streams and passed through other secondary sensible heat secondary recovery preheaters to maximize the full recovery potential of the unit. Such arrangements will be appreciated by those skilled in the art. The multiple preheaters may be configured as a single multi-service preheater or separate units as denoted by  18  and  26 . The separate feed streams are recombined and heated to near heated separator conditions prior to entering the heated separator  20 . If desired, the feed stream can also be introduced into the forced circulation stream to create a local dilution effect in the reboiler. The heated separator may comprise a multiple separation unit, such as a cyclonic separator. The lower section, broadly denoted by numeral  22 , has cyclonic action to suspend solid material in the concentrate and discharge what is referred to as “blowdown” or concentrate as denoted by line  24 . The rate of blowdown  24 , continuous or batch, controls the concentration of components in the heated separator  20 , thereby regulating the degree of saturation of the concentrate, the degree of supersaturation, subsequent precipitation of solids and the boiling temperature in the heated separator  20 . The blowdown  24 , at heated separator  20  temperature and concentration is passed through the secondary preheater  26  for heat recovery to the feed stream via line  28 . The blowdown stream  24  is reduced to a temperature within about 3° F. to approach to the feed stream from  12  and released as stream  185 .  
         [0100]    The upper section of heated separator  20 , containing mostly saturated steam vapor, is dedicated to vapor/liquid separation and may contain such features as a mesh pad or vane pack (not shown) to coalesce liquid droplets from the vapor stream. Vapor exiting the heated separator  20  and generally indicated by line  30 , constitutes environmental quality distillate and depending on the components present in the feed stream, may comprise potable water or boiler quality feed water. A portion of the vapor is transferred into the compressor  32  to elevate the pressure and temperature of the vapor stream above that of the heated separator  20 . The vapor stream can be at any pressure leaving the heated separator, including vacuum. This vapor is primarily saturated at the heated separator  20  conditions, however, it can become supersaturated if the concentrate contains components at a sufficient concentration to increase the boiling point of the vapor. This concept is known as boiling point rise or BPR and shall be understood so that the compression can be appropriately compensated. The additional energy imparted to the vapor stream sets up the required LMTD or thermal drive necessary to effect heat transfer in the reboiler heat exchanger, generally denoted by numeral  34 . Any remaining portion of vapor, denoted as  46 , is transferred to any suitable external condensing device  58  to recover vapor as distilled water in  48 .  
         [0101]    The compressor or blower, denoted by numeral  32 , may be any device known to those skilled in the art, which can induce about 3 to 15 psi head into the vapor and flow the desired level of vapor mass. The actual head required from the compressor  32  is specifically determined for each unit by the evaporating conditions in the heated separator  20  and the required LMTD for the reboiler  34 . The vapor leaving the compressor  32  is primarily superheated steam. The degree of superheat is dependant on the discharge pressure and efficiency of the compressor device  32 . Waste energy in the form of low pressure saturated steam, typically less than 100 psig, more specifically less than 50 psig, can be added to the compressed vapor before entering the reboiler exchanger  34 . The combined stream will reduce the level of superheat imported by the compressor.  
         [0102]    The reboiler exchanger  34  functions to condense the combined vapor received from compressor  32  and waste energy source  50 , to distillate draining from the reboiler  34  to a condensate receiver, denoted by numeral  36 . This step captures the superheat and latent heat of the combined vapor stream and transfers it by means of thermal drive into the concentrate circulating stream denoted by numeral  38 . The distillate accumulated in the receiver  36  generally is saturated liquid at a specific temperature and pressure condition. The additional sensible heat contained in the distillate is recovered by passing hot distillate using pump  40  back through the preheater  18 , where the exiting stream is cooled to about 3° F. within the incoming feed stream from  12 . The distilled water from receiver  36  and  48  can be combined to recover sensible heat prior to entering preheater  18  and is released as stream  100 .  
         [0103]    It has been found that by using a concentrate circulation pump  42  to circulate a prescribed quantity of concentrate from the heated separator  20 , through the reboiler exchanger  34 , significant results can be realized without over concentrating the concentrate and without the risk of fouling or scaling of the exchanger surfaces. The ratio of circulating concentrate mass to vapor is specifically selected to be in a range of less than 300 to at near 2, thereby precisely generating a vapor fraction of near 1% to less than 50% in the stream  38  exiting the reboiler exchanger  34 . This mass flow can be varied and set at the desired parameter by using a control device generally denoted by numeral  44 . More specifically, the desired target for vapor fraction in the exiting circulating stream  38 , when considering most contaminated feed streams, is less than 10% vapor fraction. The vapor generated in the stream  38  is equivalent in mass to the amount recovered as distillate at  100 . The vapor created in the reboiler exchanger  34 , even though it is very small in mass fraction (about 1 to 10% of the circulating mass), absorbs the majority of heat transferred from the condensing side of the reboiler  34 . The selection of vapor fraction and concentrate circulation rate is an important factor in reducing fouling and scaling, and preventing over concentrating of the fluid in the exchanger. To a greater extent, this parameter is most important to establish a very low temperature rise on the concentrate circulating fluid to maintain an effective LMTD without a temperature cross in the reboiler exchanger  34 . Any temperature rise will quickly eliminate the LMTD and heat transfer will stop. For example, if the pressure of the circulating concentrate were increased in the reboiler such that the fluid could not create some vapor, the temperature would rise by sensible heat absorption until no LMTD or thermal drive would exist and thus the heat transfer would decline. The back-pressure of the concentrate circulating system, consisting of static and friction head losses, is designed to be minimal. In fact, the back pressure is primarily equal to the static head loss of the vertical exchanger, as the dynamic pressure drop of the exchanger is minimized. The circulating concentrate flow is then selected to achieve near 1% to 10% vapor fraction in the outlet line  38 . The resulting temperature rise is very low and the LMTD remains at its design value.  
         [0104]    [0104]FIG. 3 illustrates an example whereby saturated waste steam is combined with compressed steam to absorb the waste heat energy in a single reboiler  34 . The combined steam is condensed to form a condensed distillate. If the available waste steam pressure is not compatible or could not be made to be compatible, then separate concentrate circulation loops and reboiler exchangers are provided, specifically designed to suit each heat source. Furthermore, if waste heat was only available by means of a non-condensing heat transfer fluid, then the waste heat exchange is designed to extract the heat from the transfer fluid without a condensed distillate. The key design features of the reboiler will always be to maintain the preferred mass liquid to vapor mass ratio to create a vapor mass of 1% to 10% in the evaporating fluid.  
         [0105]    Referring now to FIG. 4, shown is an alternate process scheme which allows for blowdown  24  from the heated separator  20  to be adjusted until the overall concentration effect or concentration factor (CF) of the system creates a supersaturated concentrate with respect to one or many components to cause precipitation. As the solids form and build in the heated separator  20 , the blowdown  24  is passed through a solid/liquid separation device, generally denoted by numeral  50 , for removal of the solids or sludge. As an alternative, the solid/liquid separation device  50  can be located between the reboiler pump  42  and the exchanger  34 , in a slipstream or total flow arrangement. The recovered liquid is further recycled back to the heated separator  20  as indicated by  52  and a portion representing the blowdown quantity, is further passed through the preheater  26  for heat recovery and cooled to about 3° F. of stream  175 . The solid/liquid separation device  50  can be of any form such as filter, hydro cyclone, centrifugal settler, gravity settler, centrifuge, decanting separator, known to those skilled in the art. This process is particularly attractive when the major objective is to recover a compound as a solid or when the compound is of significant commercial value.  
         [0106]    Referring further to FIG. 5, shown is a further process variation by which the vapor stream may contain a portion of a particular contaminant from the feed stream. The heated separator  20  is equipped with a fractionation column  54  ahead of the compressor  32  and excess steam line  46 . The column  54  is used to fractionate and scrub out the contaminate using multiple stages in conjunction with clean cool water reflux, denoted by numeral  56 . The reflux can be drawn from either upstream or downstream of the preheater  18  or a combination, depending on the required reflux temperature. This process variation is attractive when the feed stream contains, for example, volatiles such as hydrocarbons, glycols, ammonia, amines, etc.  
         [0107]    [0107]FIG. 6 illustrates the typical pressure and temperature relationships of the various streams around the evaporation portion of the process. Numerical references are made from FIGS. 2 through 4 for this discussion. Although the specific process parameters are shown by way of example, they are modifiable to suit any specific distillation application. This schematically shows the conditions based on a fluid with no boiling point rise and the heated separator  20  operating at slightly above atmospheric pressure, 16 psia and 212.5° F. The circulating concentrate temperature rise is about 1° F. for a reboiler pressure drop of 2.5 psi. The vapor fraction of the circulating stream is about 10%. The conditions around the reboiler exchanger  34  can be represented on a evaporation/condensing curve as shown in FIG. 7. On the condensing side of the exchanger, superheated steam from the compressor at point C 1  at about 289° F. and 21.0 psia combines with the saturated waste heat stream source at C 2  and is condensed at the saturated pressure of the vapor at point C′, about 232° F. and 21.0 psia. This zone is commonly referred to as the desuperheating zone and consists of about 2% of the exchanger surface area, the remaining zone being the area by which the latent heat of condensation is released. The area required for desuperheating is reduced as the ratio of saturated waste heat to compressed steam increases. A slight drop in pressure and temperature will occur through the exchanger  34  due to the inherent pressure drop of the heat exchanger. The outlet conditions become about 231.8° F. and 20.9 psia. The surface temperature, on the condensing side, will be less than the saturation temperature of the incoming vapor, thereby forming a condensate film on the heat exchange surface. The heat transfer will therefore occur off the wet wall condition maintaining the effective temperature of the film at the vapor saturation temperature. The distillate will drain from the exchanger to the condenser receiver  36  at point D, keeping the reboiler free of liquid and exposing all of the heat exchanger surface to the condensing process.  
         [0108]    On the evaporating side, concentrate enters the exchanger counter currently from the bottom at point A at about 212.5° F. and 18.0 psia after the circulating pump  42 . The circulation rate is adjusted so that the ratio of concentrate mass is at least 10 times greater than the vapor rate. The temperature of the concentrate fluid begins to rise to point A′, and then levels off to about 213.2° F. as point B is reached, where the static head is overcome and the pressure reduces to 15.5 psia. While the concentrate rises up the exchanger  34 , vapor begins to form by forced convection, absorbing the latent heat transferred. By increasing the fluid mass on the evaporating side until the ratio of circulating mass to vapor mass falls within the desired range, the boiling effect is controlled within the forced convection and stable nucleate boiling regions. Because of the high mass flow of liquid, the heat transfer surface remains wetted at a temperature equivalent to the saturated temperature of the newly formed vapor. By further ensuring that the flux rate (QA −1 ) for a exchanger is below 6000 BTU hr −1  ft −2 , the temperature rise for the evaporating side can be maintained below 1° F. and the wetted film surface is maintained, thereby eliminating the risk of scaling. If the flux rate is too high, the instantaneous vapor acceleration pressure drop temporarily exceeds the available static head, resulting in unstable temporary backflow and possible breakdown of the wetted heat transfer surface. This may result in fouling of the heat transfer surface. Below heat fluxes of 6000 BTU hr −1  ft −2  and within the range of circulating concentrate mass to vapor mass less than 300, there exists a region where the liquid and vapor can co-exist in stable operation and maintain a fully wetted heat transfer surface on the evaporating side of the reboiler, without the risk of fouling or scaling.  
         [0109]    Reference to points A through D is also found on FIG. 8.  
         [0110]    [0110]FIG. 8 illustrates the elevation view of a highly efficient heat transfer exchanger  34 , known by those skilled in the art as a plate and frame heat exchanger, whereby rows of vertically stacked, gasketed plates  60  are arranged between two solid frames  62  and  64 . These devices are well known for their compact size and ability to have very high U values or overall heat transfer coefficients. This type of exchanger, arranged as a single pass, countercurrent flow configuration is well suited for the present invention and specifically offers the following benefits to perform the present invention:  
         [0111]    1. The plate type exchanger offers a low, fixed static head and a very low pressure drop on the concentrate circulating fluid or evaporating side, while providing a relatively high heat transfer coefficient;  
         [0112]    2. The heat flux can be easily adjusted by adding more surface area or plates in a given frame;  
         [0113]    3. The condensing side of a plate frame design is free draining and has low pressure drop, while maintaining a relatively high heat transfer coefficient;  
         [0114]    4. The highly effective heat transfer coefficient allows the surface temperatures to be very near to both fluid stream temperatures reducing the risk of fouling;  
         [0115]    5. The high turbulence and equivalent high fluid velocities results in low fouling and maintains the solids in homogeneous suspension as they pass through the exchanger.  
         [0116]    6. There are no hot or cold spots and no dead flow regions inherent to a plate frame design lowering the risk of fouling or scaling;  
         [0117]    7. Plates are smooth and well finished reducing the risk of fouling; and  
         [0118]    8. Low fluid residence time reduces the risk of precipitation, as there is insufficient time to reach equilibrium and generate scaling contaminants.  
         [0119]    More generically, the plate type heat exchanger is very compact and can be provided cost effectively with exotic alloy plates to resist fluid corrosion and stress corrosion cracking, common to desalination type applications. Other types of exchangers, shell and tube, double pipe, finned tube, spiral type, may also be considered by those skilled in the art, provided the specific requirements of the invention are maintained.  
         [0120]    [0120]FIG. 9 is a chart showing the preferred design range, globally denoted by 66, for the ratio of circulating concentrate mass flow in relation to the vapor mass flow. The desired range from about 10 to 100 results in a vapor fraction of less than 10% to near 1%.  
         [0121]    [0121]FIG. 10 is a chart showing the resulting impact on the local concentration factor CF EXCHANGER  in relation to the risk of further supersaturation and precipitation within the heat exchanger. Generally, the system concentration factor can be expressed as follows:  
         CF TOTAL =CF BLOWDOWN ·CF EXCHANGER   
         [0122]    The concentration which reaches steady state in the heated separator results from the steady removal of vapor in balance with a continuous blowdown from the heated separator. The value of the CF TOTAL  is typically in the order of less than 5 to about 20 times, depending on the level and type of contaminates in the feed stream. Also depending on the level of vapor mass leaving the reboiler, the resulting CF EXCHANGER  is determined (between 1.0 and 1.1) and the blowdown rate adjusted so that the desired concentration levels are not exceeded in the reboiler. A typical example can be shown as follows:  
         [0123]    Feed stream contains 20,000 TDS, and it is desired not to exceed 100,000 TDS in the concentrate.  
         [0124]    It is determined that the most effective mass ratio will be 20, resulting in a vapor fraction of 5%, from FIG. 7.  
         [0125]    The CF EXCHANGER  is located from FIG. 8 to be about 1.07. The CF TOTAL  is calculated to be (100,000/20,000)=5.  
         [0126]    The CF BLOWDOWN  is calculated to be (5/1.07)=4.7.  
         [0127]    Therefore the corrected blowdown rate shall be (1/4.7)=21% of the inlet feed stream.  
         [0128]    Accordingly, by making use of a vapor recompression and waste heat recovery process in combination with a forced convection heat transfer system, and by following the steps of carefully selecting the ratio of mass flow of the circulating system to the vapor stream mass flow to be less than 300 to about 2, more specifically a ratio of about 10 to 100, selecting a heat flux of less than 6000 BTU hr −1  ft −2  and managing a blowdown stream to achieve the desired concentration effect (CF), the result is a very efficient water distillation unit which is not susceptible to fouling or scaling over long periods of operation. By combining the two known process schemes and incorporating the waste heat recovery scheme with a unique heat exchange configuration, and more particularly, designed with a specified concentrate circulation ratio not previously taught by prior art, allows the present invention to provide an effective method to distill water free of contaminants, without the risk of fouling and scaling.  
         [0129]    The following examples serve to illustrate the invention.  
       EXAMPLE 1  
       [0130]    This example calculation is a means of demonstrating the heat balance around the reboiler exchanger. This example represents a design basis of a distillation unit designed to recover 53,000 USGPD of clean distillate from a contaminated source.  
                                                                                         Exchanger Information                Surface Area   3,200 ft 2             Type   Gasketed Plate-Frame           U   542 BTU hr −1  ft −2  F −1             Corrected LMTD   10.40 F           Calculated Service Duty   (3,200)*(542)*(10.40)               18,041,224 BTU hr −1             Calculated Heat Flux   (18,041,224)/(3200)               5638 BTU hr −1  ft −2              Condensing Side                Inlet Conditions   289° F. @ 21.0 psia (superheated)           Outlet Conditions   231.8° F. @ 20.9 psia           Saturated Condensing   232.0° F. @ 21.0 psia           Temperature           Latent Heat of Condensing   957.4 BTU lb −1  @ 21.0 psia           Steam Flow   36.7 USgpm = 18,352 lb hr −1             Q DESUPERHEAT     (18,352)*(0.45)*(289-232)               471,131 BTU hr −1             Q CONDENSE     (18,041,224-471,131)               17,570,093 BTU hr −1             Calculated Flow   (17,570,093)/(957.4)               18,352 lb hr −1              Evaporating Side                Inlet Conditions   212.2° F. @ 18.0 psia           Outlet Conditions   213.6° F. @ 15.5 psia           Latent Heat of Vaporization   968.9 BTU hr −1  @ 15.5 psia           Ratio of Circulated Mass to   10           Vapor Mass           Concentrate Circulation Rate   370 USgpm               184,926 lb hr −1             Vapor Flow   18,352 lb hr −1             Percent Vapor   (18,352/184,926) = 10%           Q EVAPORATE     (18,352)*(968.9)               17,782,328 BTU hr −1             Q SENSIBLE     (184,926)*(1.0)*(213.6-212.2)               258,896 BTU hr −1             Q TOTAL     (17,782,328) + (258,896)               18,041,224 BTU hr −1                        
 
         [0131]    This example illustrates that the 10% vapor fraction created in the circulating fluid will capture 99% of the heat transferred from the condensing side and increase the circulating fluid temperature about 1° F., even though there is 10 times the mass of circulating liquid.  
       EXAMPLE 2  
       [0132]    A prototype unit was fabricated designed to recover 10,000 USgpd of clean distillate from a landfill leachate lagoon. The unit was tested over an extended period and detailed performance test data was collected during this period. The pilot operated successfully for an extended four month period and upon inspection fouling was negligible in the reboiler and heated separator. The equipment used at the pilot test included a Spencer™ Model GF36204E Blower Compressor providing a 3.0 psi differential pressure. Standard single pass, plate-frame heat exchangers were used during the test.  
         [0133]    The leachate feed, concentrated blowdown, and treated effluent characteristics were as follows:  
                                                                                 Blowdown                   Leachate   approx   Treated       Parameter   Units   Feed (2)     10% (2)     Effluent (2)                                  BOD   mg l −1     26   88   &lt;10       COD   mg l −1     277   1,207   11       TOC   mg l −1     59   549   6       TSS   mg l −1     33   145   &lt;2       VSS   mg l −1     15   29   &lt;2       TDS   mg l −1     5,473   53,000    &lt;50       Calcium   mg l −1     96   435   &lt;0.05       Magnesium   mg l −1     228   1,990   &lt;0.05       Sodium   mg l −1     550   4,650   &lt;2       Iron   mg l −1     5   469   .6       Total P   mg l −1     1.5   1.5   &lt;0.01       Ammonia as N   mg l −1     53   124   0.38 (1)         Total Alkalinity   mg l −1     2,353   2,930   1       as CaCO 3         Chlorides   mg l −1     217   784   0.2       Sulfates   mg l −1     350   20,000    &lt;2       Total Phenols   mg l −1     0.08   0.45   .017       Total Coliform   Col/100 cc   673   &lt;3   0       Colour   TCU   166   800   &lt;5       Turbidity   NTU   131   220   0.1                                  
 
         [0134]    The effluent is of such quality that it can be discharged to the surface water bodies exceeding virtually all regulatory guidelines. The compressor power consumption was measured and recorded for various performance points, including compressor turndown and recycle conditions. The measured power consumption was piotted on FIG. 10 as power consumption per 1,000 USgal for the various distillate flows. The test data curve was corrected for the compressor inefficiencies over the range of flows and a uniform power consumption value of 50 KW-hr/1000 USgal was derived. Allowing for standard compressor efficiencies of about 77%, the required power consumption for the high efficiency distillation unit is about 65 KW-hr/1000 USgal. The blowdown stream averaged about 10% of the feed stream throughout the test period, resulting in an average concentration factor (CF) of 10. A visual inspection was completed after the testing, showing no signs of scaling in the heated separator and reboiler equipment.  
         [0135]    Turning to FIG. 12, shown is another embodiment of the present invention wherein superheated brine blowdown  140  is received from the high pressure steam separator  130  in FIG. 1 and transferred to the high efficiency water distillation unit, generally denoted as  180 .  
         [0136]    The blowdown stream  140  is flashed into a low pressure separation  200  to create a low pressure (typically 10 to 50 psig) waste energy stream  203  and a low pressure concentrated blowdown stream  235 . The waste energy stream  203 , is passed through the reboiler exchanger  205 , condensed to distilled water and collected in the surge tank  215 .  
         [0137]    The HP concentrated blowdown stream  235  is heat exchanged with  240  to preheat the produced water  175  to  245 . The cooled concentrated blowdown is released for disposal as stream  185 . The waste energy from stream  203  is transferred to the circulating concentrate from the heated separator where, by way of controlling the circulating mass to vapor stream to a range of less than 300 to near 2, less than 50% vapor or more precisely less than 10% vapor, is generated in the circulating concentrate stream exiting the reboiler at  230 . The vapour formed in the circulating stream absorbs the latent heat of vaporization, while at the same time not allowing the temperature rise on the circulating concentrate to increase greater than about 1° F. and maintain an effective LMTD without a temperature cross in the reboilers exchanger  205 .  
         [0138]    The circulating concentrate is removed at a controlled rate at  265  by a pump  270  and exchanged at  243  with a portion of the produced feed water  241 . The preheated portion of produced water  244  is recombined with the main preheated produced water  245  prior to entering the heated separator  250 .  
         [0139]    If the quantity of waste energy  203  for a specific heavy oil site is less than the energy required to distill the desired produced water stream  245 , then a separate compressor  305  and reboiler  315  circuit is provided. The same circulating mass to vapour stream of less than 300 to near 2, less than 50% vapor or more precisely less than 10% vapor, is generated in the circulating concentrate stream exiting the reboiler at  350 .  
         [0140]    The excess steam  255  produced from the heated separator  250  is typically condensed using an external condenser  355 . The energy can be transformed into building heat or other process heating where suitable. Condensed water streams  320  and  360  are collected in condensate surge vessels  325  and  365  and further combined using pumps  220 ,  330 , and  375  to form the distilled water recycle feed water for the steam generators  125 . By using the above described method, water recovery rates of greater than 85% can be realized.  
         [0141]    If non-condensible volatiles are formed in the condensing process taking place in streams  210 ,  320  and  360 , then the vapour can be vented automatically by means of  217 ,  335  and  370  respectively.  
         [0142]    The operating pressure and corresponding temperature of the heated separator can be selected to operate over a wide range of full vacuum to less than 50 psig, more typically the pressure may be selected at slightly above or below atmospheric, 12 psia vacuum to 2 psig pressure.  
         [0143]    Turning to FIG. 13, shown is an even further variation of the present invention. In this embodiment, a portion of the waste heat energy from  200 , denoted as  202  and/or the excess steam  255  is used as the energy source for a crystallizer  405  by means of the exchanger  400  and pumping circuit  415  and  420 . The crystallizer operates at a boiling condition of at least 10° F. and most desirably between 20° F. and 30° F. less than the temperature of the steam from the heated separator  250 . The crystallizer may operate at pressures at, above or below atmospheric pressure. In the event that the energy in the waste energy stream  202  and/or excess steam  255  exceeds the required level for the crystallizer operation, then condensing may be conducted by an external condensing means.  
         [0144]    The concentrated blowdown stream  275  and heated separate concentrate stream  265  are fed to the crystallizer feed tank  280 . The near saturated brine is transferred by a feed pump  425  to the circulating loop of the crystallizer  410 . A slip stream of the circulating slurry at  410  is removed by pump  435  and passed through a solid/liquid separation device  440  or transferred directly to an evaporation pond. A typical solid/liquid separation device, familiar to those skilled in the art, may consist of a filter, filter press, gravity settler, clarifier, cyclone, decanter tank and centrifuge.  
         [0145]    The filtrate  450  is recycled to the crystallizer feed tank  280 . Solid material  445 , exiting the solid/liquid separator  440 , is collected in suitable storage facilities and transported for disposal. Any excess contaminated water collected in the storage areas  455  or evaporation pond can be recycled by pump  460  back to the crystallizer feed tank  280 .  
         [0146]    Condensed water streams  210 ,  405  and  490  can be collected and combined to form the distilled water recycle feed water for the steam generators  125 . By using the above described method, water recovery rates of 100% and zero discharge criteria can be achieved.  
         [0147]    In terms of the apparatus employable in the system it will be readily appreciated by those skilled in the art as to what examples of the heated separators, preheaters, reboiler, pumps, compressors/blowers, crystallizers, etc., will be most desirable. Other modifications will be readily appreciated without departing from the scope of the invention.