Abstract:
A filtration system suitable for recovering base stock from used lubricating oil and other applications pass feedstock over nano-filtration membranes assembled as a stack of membranes all experiencing parallel flow. On exiting a first stack of membranes the feedstock passes through an opening in a pressure-sustaining separator plate to flow in the reverse direction past a second stack of membranes and subsequently establish a serpentine flow of feedstock through multiple stacks of membranes. The stacks of membranes all share a common pressure containment vessel. Pressure boosters installed in the flow-through openings of separator plates separating consecutive stacks can serve to restore lost pressure of the feedstock and maintain effective permeation of permeate through the membranes. A pressure control valve at the outlet to the permeate-receiving cavities of a stack can be used to adjust the trans-membrane pressure.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to an apparatus for separating fluid mixtures by filtration membranes which are arranged into membrane stacks in a supporting frame. More specifically this invention describes equipment and procedures using nano-filtration membranes for cleaning used oil to bring it back to a starting base stock for possible reuse. The invention also has applications in other fields where a filtrate or permeate is to be extracted from a feedstock. This includes, for example, dewatering food-containing liquids to produce concentrates and the purification of gelatin to high standards. Other applications include separating lighter hydrocarbons from heavier hydrocarbons in the petroleum industry. 
       BACKGROUND TO THE INVENTION 
       [0002]    A useful technology for recovering usable base stock from used lube oil can employ nano-filtration membranes. Colloquially, a process based upon use of open osmosis membranes can be referred to as “nano-filtration”. However use of such membranes is distinguishable from “filtration” in the following respects: separation of fluids takes place at the membrane surface based on attractions and repulsions of specific dissolved chemical moieties; this is not a filtration of solid particles in the traditional sense. This is instead analogous to reverse osmosis. 
         [0003]    Accordingly, although the expressions “nano-filtration”, “micro-filtration”, ‘ultra-filtration”, “hyper-filtration”, “filtrate”, “permeate”, “filtering medium” may be used in the course of this disclosure, these expressions are actually intended to extend to the case where there is a separation of two a stream into a permeate and a concentrate by any analogous process. The invention is not limited to the use of a specific type of membrane. 
         [0004]    Lubricating (lube) oils consist of a starting base stock and an additive package. The inherent value of lube oil has led to many attempts at reclaiming the base stock from used lubricating oil with varying levels of success. One technique is to pass the used oil, appropriately pre-conditioned, over a nano-filtration membrane. 
         [0005]    Attempts at using commercially available membrane containment systems include the DDS (De Danske Sukkerfabrikker) plate and frame equipment described in U.S. Pat. No. 3,872,015. 
         [0006]    A previous patent to Kutowy et. al. U.S. Pat. No. 4,814,088 of Mar. 21, 1989 addresses a membrane-based ultrafiltration process to clean mildly used lube oil as well as crude oil and other chemicals. The contents of this and the following Kutowy US patents are adopted herein by reference. 
         [0007]    Other patents to Kutowy et. al., U.S. Pat. No. 5,002,667 Mar. 26, 1991, and U.S. Pat. No. 5,624,555 Apr. 29, 1997 describe using a metallic plate and frame for membrane support. In particular the latter patent describes a paired-membrane panel assembly which incorporates two membranes each overlying a respective perforated membrane support panel located adjacent to the individual membrane&#39;s permeate or low pressure side. Such paired membrane support panels are mounted in parallel exposing all parallel membranes to feedstock flowing in the same direction. 
         [0008]    Feedstock in a membrane system usually requires some pre-treatment. Used lube oil becomes unfit for its purpose due to physical contamination and chemical changes. Water and glycol exist in several forms in used crankcase oil. It is desirable for such contaminants to be reduced to a minimum before a feedstock is exposed to a nano-filtration membrane. 
         [0009]    The presence of water and glycol in particular poses a problem to base stock reclamation through small pored membranes such as nano-filtration membranes. This is because of the formation of emulsions that tend to stick and block pores in membranes. Water and glycol have to be virtually completely removed for a nano-filtration membrane-based process to be most effective. Thus the feedstock for a nano-membrane filter should be “membrane compatible” and “feedstock” as used herein is so intended. 
         [0010]    Use of nano-membrane filters gives rise to a number of structural requirements for the membrane support structure. 
         [0011]    In order to provide a useful quantity of permeate when exposing liquid feedstock to a membrane, the membrane is normally supported to carry a substantial trans-membrane pressure, e.g. on the order of 100 psig. Further, passing a flow of feedstock as a working fluid over a membrane surface under pressure is preferably done in a confined space, e.g., a depth that is preferably only a moderate multiple of the thickness of the membrane and/or the membrane and its supporting perforated panel. This confined space has a preferred depth to maximize the quantity of working fluid that comes into contact with the membrane surface and to maintain flow velocity. (“Fluid” as used herein refers to a liquid unless the context indicates otherwise.) Establishing the correct flow rate over a membrane helps keep the membrane surface clean. 
         [0012]    As a consequence of this narrow confinement the working fluid will suffer a pressure drop as it passes as a cross-flow along the length of a membrane. Over a distance of, say, 2 meters in length, the pressure drop could be on order of 10 psig for used lubricating oil, depending on the depth and viscosity of the flowing feedstock layer. 
         [0013]    If the working fluid is to be exposed to an extended surface area of membrane, e.g., past multiple supported membrane surfaces connected in series, this cross-flow pressure loss will accumulate. All along the membrane surfaces the pressure must be kept above a minimum pressure, for example 100 psig, to sustain effective permeation. Therefore the entry pressure of the working fluid as it is exposed to the first membrane must, according to one solution, be high enough to accommodate the subsequent pressure losses for the flowing working fluid to maintain the minimum, e.g. 100 psig, pressure needed to force permeate through the membrane at a reasonable rate. 
         [0014]    To contain high pressure fluid requires strong frames, sealing plates and seals. Typically these are made of steel. As the requirement for strength goes up (to accommodate higher pressures) the weight of such supporting assemblies increases. This places higher demands on the handling apparatus as well as imposing increased cost. 
         [0015]    It would therefore be desirable to provide a support assembly for filter membranes having minimized weight and strength requirements. Correspondingly, the input pressure of the working fluid should be limited to the extent practically possible. This invention addresses such objectives. 
         [0016]    The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification. 
       SUMMARY OF THE INVENTION 
       [0017]    According to one variant, the invention addresses a filtration system suitable for recovering base stock from used lubricating oil by passing such feedstock over a nano-filtration membrane surface. The invention may also be employed for processing other feedstocks. 
         [0018]    In order to produce permeate from a feedstock at least two, i.e. multiple, membrane supports carry respectively membranes, each support having a receiving space within to serve as a cavity for accepting permeate driven through the membranes by pressure applied to the feedstock, each support also having a permeate-receiving cavity outlet to drain-off permeate. The multiple membrane supports are mounted in a common pressure-containing vessel having feedstock inlets and concentrate outlets. The pressure vessel contains at least one pressure-sustaining separator plate positioned between at least two adjacent membrane supports, the separator plate having a flow-through opening at one end to allow fluid to flow from one membrane support to the next. 
         [0019]    The separator plates allow different pressures to develop in consecutive chambers defined by the separator plate(s) that contain the membrane supports, avoiding exposing the membrane supports to a pressure differential that would otherwise arise due to a drop in the pressure of the feedstock as it flows through the system. 
         [0020]    The support panels are preferably formed from two permeable panels mounted back-to-back with two respective membranes located on their outer-facing surfaces. The two panels define between them the receiving space to serve as the cavity for accepting permeate driven through the two membranes. Collectively these components constitute a “panel assembly”. In normal usage the feedstock flows in the same direction when passing over the two membranes carried on the respective outer sides of a membrane support panel assembly. 
         [0021]    Optionally and preferably the respective permeable panels are formed of thin material to reduce weight. Rolled steel sheeting that has been pressed into shape and has been perforated over the greater part of its surface to make it permeable has been found suitable. Use of lightly built panel assemblies is complemented by the structural integrity of the pressure-sustaining separator plates. 
         [0022]    While reference is made to the word “panel” this expression is intended to include any form of support, such as a braced mesh, that performs in a similar manner. 
         [0023]    Preferably the panel assemblies are themselves assembled in groups as a stack of panel assemblies, all membranes within the stack experiencing parallel flow within the chamber defined by an associated separator plate. On exiting a first stack of membranes, the feedstock passes through an opening in one end of the separator plate to flow past a second stack of membranes. In a preferred arrangement the flow through the second stack is in the reverse direction to the flow through the first stack, being located adjacent to the first stack but separated therefrom by the separator plate. 
         [0024]    The stacks of membranes can all share a common pressure containment vessel. A system can be arranged to rely upon the serpentine flow of feedstock through multiple stacks of membranes within that vessel. As a further feature of the invention pressure boosters installed in the flow-through openings of separator plates separating consecutive stacks can serve to restore lost pressure of the feedstock and maintain effective permeation of permeate through the membranes. 
         [0025]    The two panels of a panel assembly define between them the receiving space for accepting permeate driven through the two membranes by outside pressure, e.g., 100 psig. This permeate-receiving cavity, which serves as a permeate collection chamber, has an outlet to drain-off permeate ensuring that the membrane has a low or limited back-pressure. This cavity may contain spacer members that function as a strut support to minimize deflection of the panels. Collectively these components constitute the membrane panel assembly 
         [0026]    This structure can be further incorporated into the following useful configurations. 
         [0027]    Multiple Membrane Panel Assemblies 
         [0028]    Generally, a filtration assembly to produce a permeate from a feedstock in accordance with the invention may comprise the following features:
       a. multiple membrane panel assemblies are mounted in a common pressure-containing outside vessel with the panel assemblies arrayed in a parallel configuration. The feedstock flows in the same direction on both sides of the panel assemblies for the lengths of the multiple membrane panel assemblies. Collectively the multiple membrane panel assemblies constitute the “stack” of panel assemblies.   b. at one entry end of the stack all individual panel assemblies receive feedstock from an inlet mounted on the pressure vessel. The distribution of the flow of feedstock around individual panel assemblies is facilitated by passageways within the pressure vessel that ensure relatively equal distribution. These passageways may be in the form of sealed penetrations through the membrane panel assemblies at their ends. The sealing around such passageways confines permeate to the permeate-receiving cavity. At another exit end of the stack, feedstock exiting through similar openings after exposure to the membranes of all panel assemblies in the stack is ultimately delivered to an outlet mounted on the pressure vessel for transfer to the next stage of processing.       
 
         [0031]    This parallel arrangement reduces the net pressure drop between the inlet and the outlet portions of the stack. 
         [0032]    The permeate which penetrates through the membranes into the respective individual permeate collection chambers exits through a permeate outlet from each panel assembly into a manifold connected to all such collection chambers in the stack. This manifold collects and delivers the permeate from the filtration assembly to an external storage vessel. The manifold may be built onto the bordering portions of an assembly of frames into which individual membrane panel pairs are mounted. The manifold may terminate at a separator plate which provides an outlet to the external environment. 
         [0033]    To locate the panel assemblies within the pressure vessel, each panel assembly can be constructed so that it is bounded by an individual frame. The frames are then positioned side by side with the perimeters of their respective membranes pinched there between. The frames are then clamped tightly together by exterior bolts. This provides a portion of the outer wall of the pressure vessel. This assembly of the frames secures the membranes in place. The thickness of these peripheral frames also determines the inter-panel assembly spacing which defines the depth of feedstock passing over the membrane surfaces. 
         [0034]    Series Flow 
         [0035]    A filtration assembly may contain more than a single stack of parallel membrane panel assemblies. Such stacks can be arranged in series to form a bank of such stacks. 
         [0036]    Instead of each stack in a bank having its own pressure vessel, they may all share a common pressure vessel, each consecutive stack being separated from an adjacent stack of membrane panel assemblies within the pressure vessel by a pressure-supporting separator plate. Each separator plate has a flow-through opening at one end to allow fluid to flow from one stack of membrane panel assemblies to the next. This opening will be proximate to the exit end of a first stack and positioned next to the inlet end of the next stack. In this arrangement the direction of feedstock flow is reversed in consecutive stacks. 
         [0037]    By assembling a bank of at least two stacks of membrane panel assemblies in this manner, a series flow of feedstock over membrane surfaces in each stack may be achieved. 
         [0038]    The number of stacks of membrane assemblies so connected may be increased along with inclusion of further separator plates so long as the trans-membrane pressure drop is sufficient to support adequate filtration. Conveniently the feedstock may flow in a serpentine manner through three or more stacks in a bank so configured. 
         [0039]    Pressure Boosting 
         [0040]    In the configuration as described there will be a cumulative pressure loss for the working fluid as it passes along the length of consecutive stacks of membranes within a bank. This would normally require that a high pressure be maintained at the inlet to the bank of filters. Operating containers at elevated pressures have strength requirements and sealing problems that are inconvenient to address. 
         [0041]    Advantageously to address this problem, the separator plate flow-through opening(s) may be provided with an inter-stack pressure booster mechanism to restore lost pressure. This pressure booster can be in the form of propeller or turbine-like blades or other form of impeller that is mounted in the flow-through opening(s) in one or more separator plates. Such openings may be dimensioned to be close-fitting to the periphery of the impeller, i.e. being circular, to support the pressure differential being formed. The pressure boosters may be actuated by individual electric motors or they may be mounted on one or more rotating shafts that are driven from outside the pressure vessel. 
         [0042]    In a case where a bank of membrane stacks contains three or more stacks with the consecutive stacks separated by two or more separator plates, multiple pressure boosters may be installed in the flow-through openings in each of the respective separator plates. However, consecutive separator plates need not necessarily be so equipped. Optionally only every second separator plate may be provided with a pressure booster at one end. This arrangement facilitates mounting consecutive pressure boosters on a single, shared rotating shaft. 
         [0043]    In order for individual pressure boosters to be mounted on a common rotating shaft, the respective flow-through openings in such separator plates should be aligned. The penetrations of the shaft through the wall of the pressure vessel, and the consecutive intervening separator plates where such plates are penetrated, should all contain seals that will limit pressure leakage. 
         [0044]    In this manner an indefinite number of sets of membrane stacks may be arranged in series without the necessity of raising the inlet pressure to inconvenient levels. 
         [0045]    Permeate Back-Pressure Control 
         [0046]    An important consideration when assembling multiple stacks of membrane panels in respective chambers all connected in series within a common pressure containment vessel, is to control the pressure differential across the membranes. Typically membranes have a preferred range of trans-membrane pressure, e.g., about 100 psi. 
         [0047]    If, in order to accommodate progressive pressure loss as the feedstock passes through multiple stacks of membrane panels connected in series, it is elected to provide feedstock to the pressure vessel inlet at a moderately elevated pressure, e.g. 130 PSI, then it may be practical to have feedstock flow through a few, e.g., 2 or 3, stacks with the feedstock pressure dropping consecutively from stack to stack. The membranes in the initial stack will be exposed to an elevated trans-membrane pressure, but this may be at a level that is tolerable. However, when a larger number of stacks are employed in a series arrangement it is preferable to maintain the trans-membrane pressure at its preferred operating level. In cases where the inlet feedstock pressure is particularly elevated, it may be necessary to protect the membranes from exposure to an excessively elevated feedstock pressure. 
         [0048]    An arrangement with this objective is to control the back-pressure within the permeate collection chambers of at least some of the stacks of membrane support panels. 
         [0049]    In the proposed configuration, the permeate outlet from each membrane support panel in a stack delivers permeate to a stack manifold that collects the permeate drainage from the individual panels. Conveniently this collection system may deliver permeate to a separator plate at the end of the stack. This separator plate then provides a passageway for the permeate to exit the pressure vessel. The outlet from this separator plate can be provided with a back-pressure control valve having an associated pressure sensor and valve control system. This valve can adjust the back-pressure in the permeate collection chambers within the associated stack, placing the trans-membrane pressure for all panels within the stack within a desired range. 
         [0050]    The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow. 
         [0051]    Wherever ranges of values are referenced within this specification, sub-ranges therein are intended to be included within the scope of the invention unless otherwise indicated or are incompatible with such other variants. Where characteristics are attributed to one or another variant of the invention, unless otherwise indicated, such characteristics are intended to apply to all other variants of the invention where such characteristics are appropriate or compatible with such other variants. 
     
    
     
       SUMMARY OF THE FIGURES 
         [0052]      FIG. 1  is a schematic cross-sectional view through a nano-membrane over which is flowing in cross-flow a feedstock which provides a permeate that passes through the membrane. This figure is intended only as a conceptual introduction and is marked as “Prior Art”. 
           [0053]      FIG. 2  is a schematic cross-sectional depiction of the layout of a pressure vessel and external supporting components, indicating the flow of feedstock through multiple chambers divided by separator plates in the context of a used oil recycling operation. Membrane support panels in  FIG. 2  are depicted schematically as lines for clarity of depiction. 
           [0054]      FIG. 3  is a face view of a basic membrane panel with its individual frame assembly. 
           [0055]      FIG. 3A  is a cross-sectional side view through  FIG. 3 . 
           [0056]      FIG. 4  is a cross-sectional schematic view of a stack of membrane panel assemblies of the type as in  FIG. 3  in an expanded state before compression to form a pressure vessel. 
           [0057]      FIG. 5  shows a further schematic exploded cross-sectional view of a stack of membrane panel assemblies as in  FIGS. 3-4  showing the flow of feedstock and permeate. In this figure the feedstock follows a parallel path over the membrane surfaces of two membrane panel assemblies before being recirculated. Details of the permeate manifold and exit passageways are shown in  FIG. 8 . 
           [0058]      FIG. 6  is a schematic exploded cross-sectional view of a bank of four stacks of membrane panel assemblies as in  FIG. 3-5  with permeate manifold passageways top and bottom. 
           [0059]      FIG. 7  is a further view as in  FIG. 6  having additionally present pressure boosters in the form of multiple turbine blades mounted on a common shaft within the respective flow-through openings of two of the separator plates. 
           [0060]      FIG. 8  is a face view of a separator plate showing the permeate collection structure. 
           [0061]      FIG. 8A  is cross-sectional edge view of  FIG. 8 . 
           [0062]      FIG. 8B  is a further cross-sectional view of  FIG. 8  showing a mirror image arrangement of the permeate collection structure of  FIG. 8 . 
           [0063]      FIG. 9  is a face view of a modified separator plate having a perforated membrane support panel on one side. 
           [0064]      FIG. 9A  is cross-sectional edge view of  FIG. 8 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0065]    In  FIG. 1  a pressure containment vessel  10  contains feedstock  11  flowing under pressure  12  from an inlet  13  to an outlet  14  where it exits as a concentrate  15  depleted of permeate  25 . Inside the vessel  10  a membrane  20  is carried by a permeable, e.g. perforated, support  22  shown schematically as wire mesh  22  but in a preferred variant is a perforated metal panel. The membrane  20  has a skin  23  and a spongy sub-layer  24 . Permeate  25  that has passed through the membrane  20  into a permeate collection cavity  26  exits through a permeate outlet  27 . The membrane  20  may be cast onto a supporting scrim or carrier sheet (not shown) to give it improved dimensional stability. 
         [0066]    The cavity  26  may contain a permeable cavity propping structure  61  (shown in  FIG. 5 ) to minimize deflection of the support  22 . This can optionally be in the form of a further wire mesh that occupies the cavity  26  and supports the membrane support  22 . 
         [0067]    Membranes suitable for use with the invention in a used lubricating oil application are believed to be available from: 
         [0068]    Koch Membrane Systems, Inc. 
       850 Main Street 
     Wilmington, Mass. 
       [0069]    01887-3388 
       USA 
       [0070]    EMD Millipore Corporation 
       290 Concord Road 
     Billerica 
     Mass. 01821 
     United States of America 
       [0071]    U.S. Pat. No. 4,818,088 also describes a nano-membrane for use with aliphatic hydrocarbon liquids suitable for incorporation into the invention described herein in such application. 
         [0072]    System Layout 
         [0073]    In  FIG. 2  a holding tank  30  contains a supply of appropriately pre-treated feedstock  11 . A heater  29  adjusts the temperature of the feedstock  11  in the tank  30  to preferably around 90° C., e.g. 80°-110° C. in the lube oil application. Feedstock  11  is then delivered by a feedstock delivery and pressurizing pump  32  to a loop system  33  that extends through a containment vessel  35  bounded by end plates  38 . The feedstock  11  within the loop system  33  is circulated and kept pressurized by a circulating pump  34  until the desired amount of permeate has been extracted. 
         [0074]    Feedstock  11  enters the containment vessel  35  bounded by end plates  38  at an inlet  13 . This inlet  13  is fitted with an inlet diffuser  42  to distribute the flow amongst the membrane panel assemblies  41  within the containment vessel  35 . Initially the hot feedstock  11  heats the apparatus while being circulated at low speed. Then the circulation rate and pressure within the loop  33  can be increased to process the feedstock  11  more rapidly. 
         [0075]    The containment vessel  35  includes a series of individual membrane panel assemblies  41  (depicted schematically as lines  41  in  FIG. 2 ) around which the feedstock  11  passes in a serpentine flow path  37 . In this schematic figure, four stacks  45  of membrane panel assemblies  41  are depicted as being exposed to liquid flow. Each stack  45  is separated from adjacent stacks  45  by a pressure-supporting separator plate  46 . Aligned with the passageways  50  (in  FIG. 3 ) in the membrane panel assemblies  41  are flow-through openings  68  (in  FIG. 3A ) in the separator plates  46  allowing the feedstock  11  to pass from stack  45  to stack  45 . 
         [0076]    At the outlet collector  42  partially concentrated feedstock  11 A exits from containment  35  to flow around the loop  33 . Eventually a loop outlet pump  43  extracts more fully depleted concentrate  15  from the loop  33  through a back-pressure control valve  43  for delivery to a processed-concentrate holding tank  44 . 
         [0077]    As shown in  FIG. 3  a membrane panel assembly  41  has two perforated panels  47  for supporting respective membranes  20  (not shown in this figure) on their outside surfaces. The perforations  48  optionally terminate before reaching the ends of the assembly  41 . Circular passageways  50 , shown as an exemplary three at each end, penetrate the two panels  47  near their respective ends where the panels  47  are preferentially pressed into contact with each other. Clamping circular sealing rings  54  bound the passageways  50  ensuring the integrity of the collection cavity  26  (in  FIG. 3A ) between the two panels  47 . Permeate conduits  58  along the panel perimeter at the collapsed ends allow permeate  25  to flow from the collection cavity  26  along the periphery of the panel-pair  47  (in  FIG. 3A ) to exit through permeate outlets  27  at one or more of the ends of the panels  47  and into permeate manifold  27 A. 
         [0078]    As best shown in  FIG. 3A , pinched between the two panels  47  along their outer peripheries is a stiffening frame  52 , preferably of welded steel and of rectangular cross-section. This frame  52  stiffens the panels  47 . The frame  52  also acts as a spacer between panels  47  and provides part of the wall of the containment vessel  35 . The outer edges of a membrane  20  (not shown in  FIG. 3  but shown as a line in  FIG. 3A ) on each panel&#39;s  47  outer boundary is also pinched between panels  47  and frames  52  under the compressive force of exterior bolts  56  when everything is assembled. Such bolts  56  (in  FIG. 4 ) extend between the end plates  38  around the periphery of the containment vessel  35 . 
         [0079]    In  FIG. 3A  the membrane  20  is pinched around the passageway  50  by the sealing rings  54 . The inside cavity  26  receives permeate from the feedstock  11 . This pinching seal may be enhanced by the use of a gasket (not shown) which will not only isolate the inner permeate collection cavity  26  from the feedstock flow  11  but will also help pinch the membrane  20  in place under the sealing ring  54 . 
         [0080]    Permeate conduits  58  can run adjacent to the inner portion of the frame  52  to carry permeate  25  to the ends of the membrane panel assemblies  41 . 
         [0081]    In  FIG. 4  a single stack  45  of individual membrane panel assemblies  41  is located within the containment of the pair of end plates  38  held together by bolts  56 . Collectively, these end plates  38  and the peripheries of the membrane panel assemblies  41  define the containment vessel  35 . 
         [0082]    Individual panel assemblies  41  have passageway openings  50 , also shown in plan view in  FIG. 3 , to allow parallel flow of feedstock  11  to be distributed in the spaces or gaps  53  between panel assemblies  41 . These gaps  53  provide a “headspace” for feedstock over the membrane  20 . Conveniently, in  FIGS. 4-7  these passageway openings  50  are shown as aligned openings in the panel assemblies  41  to accommodate a feature described further below. 
         [0083]    The height of the headspace provided by the gaps  53  has an important effect on the operation of the system. As this headspace  53  gets narrower, the pressure drop along a given length of membrane  20  will increase. If higher feedstock pressures are used, then, for a given gap height  53 , the feedstock  11  flow rate will be higher. This flow rate will help “scrub” non-passing feedstock matter off the surface of the membrane  20 , reducing membrane blockage. At the same time, such over-pressure can affect “concentration polarization” on the surface of the membrane. This has the consequence of thickening the boundary layer of fluid flow over the membrane, which will reduce permeate flow. For this reason trans-membrane pressure should not be allowed to become excessive. 
         [0084]      FIG. 5  shows the path of flow of feedstock  11  and permeate  25  in between and around a pair of panel assemblies  41 . Also as shown in  FIG. 5 , the cavity  26  contains a permeable cavity propping structure  61  to minimize deflection of the panel  47 . 
         [0085]    In  FIG. 5  permeate  25  is shown as flowing through the permeate outlet  27  penetrating the frame  52  at the upper end of the individual panel assemblies  41 . The permeate  25  is gathered through tabs  57  into a manifold  27 A of tubes for eventual further disposal as shown in  FIG. 8 . Permeate  25  exiting from each stack  45  of panels eventually passes through a back-pressure control valve  71  that is adjusted to maintain the pressure drop across the membrane  20  in the associated stack  45  of panel assemblies  41 . 
         [0086]    Serpentine Flow 
         [0087]    In  FIG. 6  multiple sets or “stacks”  45  of panel assemblies  41  are assembled to permit direction-reversing flow of feedstock  11  through consecutive stacks  45 . As in  FIGS. 4-6 , end plates  38  of the containment vessel  35  are shown but, for convenience of depiction, the membrane panel assemblies  41  are shown as being separated before the bolts  56  apply a compacting force. In actual use, the bolts  56  are tightened with the frames  52  dimensioned at the boundaries of the panel assemblies  41  to allow the bolts  56  to draw the panel assembly ends together. This action also secures the membrane  20  in position on the pair of associated panel assemblies  41 , pinching these components together while providing the spacing between panels that establishes the inter-panel gap and headspace  53 . 
         [0088]    In  FIG. 6  separator plates  46  are present between consecutive stacks  45  of membrane panel assemblies  41 . As shown in  FIG. 8  the perimeter  72  of a separator plate  46  is shaped and dimensioned similarly to that of the membrane panel support assemblies  41  to ensure the integrity of the pressure containment volume  35 . Within this perimeter  72  the face surfaces  73  of the separator plates  46 , as with the end plates  35 , may be slightly inwardly displaced to provide headspace  53  for the membrane  20  on adjacent panel assemblies  41 . 
         [0089]    Pressure Boost 
         [0090]    In  FIG. 7  the flow-through openings  68  in the separator plates  46  are penetrated by a rotating shaft  64  passing there through. Mounted on such shaft  64  in the flow-through opening  68  in every second separator plate  46  is a pressure booster  65  in the form of a fluid impeller. The seal  69  where the shaft  64  pierces the intermediate separator plate  46  is intended to be pressure-tight. 
         [0091]    The shaft  64  is turned through a transmission  67  by an external electric motor  66 . Thus, as the feedstock  11  passes from stack  45  to stack  45  in the bank of stacks, its pressure is boosted, making-up for the pressure loss incurred by flowing in a cross-flow over the surface of the membranes  20 . The motor  66  may be a variable speed motor to control the amount of the pressure boost. Although a common shaft  64  is shown as actuating the pressure boosters  65 , each pressure booster  65  could have its own individual electric motor. 
         [0092]    As depicted in some of the Figures so far for the individual membrane support panel assemblies  41  and separator plates  46 , reference has been made to an opening, (in the form of a passageway  50  (in  FIG. 4 ) or flow-through opening  68  (in  FIG. 7 )), respectively formed therein near their ends. In fact multiple such openings  50 ,  68  may be present side by side to support a high flow rate through such openings  50 ,  68 . Singly or collectively such openings qualify as a passageway  50  or a flow-through opening  68 . In the case of multiple openings, multiple pressure boosters  65  should occupy the openings to maintain the pressure boost. 
         [0093]    In  FIG. 7  the multiple impellers  65  are positioned at the bottom of the first and third, and in expanded variants, in all odd numbered separator plates  46 . The second separator plate and all even numbered separator plates  46  each have a penetration with a pressure-tight bearing  69  for the shaft  64 , or multiple shafts  64  in the case of multiple openings  50 ,  68 . 
         [0094]    In configurations where the pressure drop within the flow of feedstock  11  is significant, e.g. the length of cross-flow along the membranes  20  in one or more stacks  45  is considerably extended or the feedstock  11  is viscous as in the case of heavy oil, a second set of pressure boosters  65  may be installed at the other end of the separator plate  46 . Thus further multiple impellers  65  may be positioned at the top of the second, fourth and all even numbered separator plates  46 . In this separate array of pressure boosters  65 , all odd numbered separator plates  46  would have appropriately aligned pressure-tight bearings  69 . This second shaft, or set of shafts, would have its own drive mechanism  66 ,  67  and speed control. For such long panels, the unit could beneficially be positioned on its side. 
         [0095]    Trans-Membrane Pressure Control 
         [0096]    To dispose of permeate  25  each stack  45  is provided with a first permeate outlet manifold  27 A (in  FIG. 5 ) that delivers permeate  25  to a proximate separator plate  46 . As shown in  FIG. 8  such plates have aligned permeate reception tabs  90 ,  91  corresponding to tabs  57  in  FIGS. 3 and 5  and blind recesses  92  (in  FIGS. 8A and 8B ) that receive the permeate manifold  27 A and divert permeate  25  out of the pressure containment vessel  35  through permeate pressure control valves  71 . Thereafter permeate  25  flows at near atmospheric pressure for accumulation outside the pressure vessel  10 . Only one permeate reception tab  90  is needed for a separator plate  46  but by providing two such tabs  90 ,  91  as mirror arrangements the separator plates  46  can be more versatile, avoiding the need to have “left” and “right” plates  46  on assembly. Each plate  46  can thereby receive permeate  25  from the stacks  45  on both or either side. 
         [0097]    By providing each back-pressure valve  71  (in  FIGS. 8, 8A, and 8B ) with a pressure sensor  84  and individual valve controller (not shown), the controller can receive signals from the sensor  84  and deliver signals to control the valve  71 . This allows different back pressures to be established for various stacks  45  through which the feedstock  11  is passing at progressively decreasing feedstock pressures  12  if there is no inter-stack pressure boost. The pressure of the feedstock  11  around each stack  45  can be interpolated by knowing the inlet  13  and outlet  14  pressures in order set back-pressure valves  71  to create the preferred trans-membrane pressure differential. 
         [0098]    Drain tabs  93  (in  FIGS. 8, 8A, and 8B ) at the other end of the separator plate  46  can be fitted with manual valves  82  for use when permeate  25  is to be drained from the panel assemblies  41  on disassembly. 
         [0099]    The permeate back-pressure control system as described is suitable for providing a preferred trans-membrane pressure when feedstock  11  is delivered to the containment vessel inlet  13  at a significantly elevated inlet pressure level  12 . The consecutive pressure-boosting provisions for the individual consecutive stacks  45  described previously as part of this invention can obviate the need to deliver feedstock  11  to the container inlet  13  at an elevated inlet pressure  12 . Nevertheless, in order to maintain trans-membrane pressures at reasonable values in either such cases, the permeate back-pressure control system as described can be used to set or fine-tune the trans-membrane pressure for individual stacks by adjusting the pressure of the associated membrane collection cavities  26 . 
         [0100]    Hybrid Separator Plate 
         [0101]    The separator plate  46  need not be an independent component.  FIGS. 9, 9A  show a hybrid separator plate  46 A and single membrane support panel  47 . A perforated metal panel  47  is mounted on a modified separator plate  46 A. Permeate  25  flows directly to the blind recess  92  through the permeate conduit pathway  58  in the modified separator plate  46 A. The hole  50  in panel  47  is ringed by a modified sealing ring  54 A that engages flow-through opening  68  in the modified separator plate  46 A. This modified ring  54 A and a shaped portion  52 A of the plate  46 A configured as a frame  52  position the membrane  20  in place. The modified separator plate  46 A has a perimeter on one side, shaft penetration  61  and pressure seal  69  as before. 
         [0102]    In this variant the lightly built perforated metal panel  47  is supported and stiffened by the pressure-sustaining modified separation plate  46 A providing effectively a stiffened membrane panel support assembly  41  with a separator plate  46  embedded therein. If desired the modified separator plate  46 A may also be perforated although this may prove costly for a thickened plate. 
         [0103]    Number of Panels in Each Stack 
         [0104]    As the feedstock  11  passes through a series of stacks  45 , its pressure will be progressively reduced. At the same time, a portion of its volume will be carried-away in the permeate  25  that passes through the membranes  20 . This loss of volume, after a number of stacks  45  have been passed-through will reduce the rate of feedstock  11  flow across membrane  20  surfaces. 
         [0105]    To maintain the cross-flow fluid velocity at a desired level, the number of membrane support panels  41  in later stacks  45  in the series can be reduced. Thus, for example, where the initial stack count includes twenty membrane panels, then after, say, ten stacks in the series, the twenty first stack may have its panel count reduced to nineteen. This process can be repeated if the number of stacks in the series is extended substantially. The values in the example given will vary with the viscosity of the feedstock  11 , the length of panel assemblies  41 , the number of stacks in the system and other parameters. 
         [0106]    Mounting of Membrane Support Panels 
         [0107]    When finally assembled, the membrane support panels  41  and separator plates  46  which provide a portion of the boundaries of the pressure containment vessel  35  are held rigidly in place by the compressive force of the end plates  38  that are drawn towards each other by tightening the peripheral arrangement of bolts  56 . This compressive force is high and the integrity of the arrangement once assembled is secure. 
         [0108]    During initial assembly, temporary rails may be provided between the two end plates  38  to align individual panels being positioned there between in respect of their vertical position. Spacers located alongside side bolts  56  can ensure proper alignment in the horizontal direction. 
         [0109]    In most applications where a pure base stock is required for producing fresh lubricating oil, the permeate  25  may be subject to a final treatment by passing it through a commercially available Polishing Unit that relies on activated clays. It is not represented that the output from the filtration system as describe is absolutely ready for use as a base stock for preparing lubricating oil. 
         [0110]    While the above description has focused on an apparatus for recovering base lube oil stock from used lubricating oil, the invention and the apparatus hereinafter claimed is equally applicable to any suitable liquid filtration process that relies on a membrane as the filtering medium. 
       CONCLUSION 
       [0111]    The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects, is further described and defined in the claims which now follow. 
         [0112]    These claims, and the language used therein, are to be understood in terms of the variants of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.