TRANSMEMBRANE PRESSURE CONTROLLED FILTRATION SYSTEM

Abstract: A fluid flow control apparatus is described in which a removable sterilisable or disposable element is provided and in which the pressure of the fluid inlet and fluid outlet of the filter unit are monitored and the pressures are compared and used to control the pressure across the filter unit. This is particularly suitable for cross-flow filtration which is a procedure whereby the process fluid is recirculated over the membrane surface. Pressure is measured by bleeding the inlet conduit through an inlet bleed conduit (62) which includes a pressure switch (67) and pressure transducer (65). The outlet conduit containing the retentate also has a bleed line (78) and a post-membrane pressure transducer (84) is inserted in the second bleed off line. With this arrangement the inlet and outlet pressures can be monitored and the pressure values are averaged in use to control the pressure switch or valve so as to maintain optimal control of the transmembrane pressure and optimise filtration for the particular fluids and membranes used.

Description: TRANSMEMBRANE PRESSURE CONTROLLED FILTRATION SYSTEM The present invention relates to fluid flow control apparatus and particularly, but not exclusively, to fluid flow control apparatus for use in separation and filtration systems. Fluid flow control apparatus for use in separation and filtration systems and similar fluid handling systems such as a life support systems should satisfy a number of desirable criteria in addition to being efficient and relatively inexpensive. The constituent parts of the fluid control apparatus should be easily cleaned or sterilised or be disposable as this is important when using biological fluids such as blood. The apparatus should require minimal fluid volume for analysis and should accommodate sensors for monitoring various fluid parameters of the fluid. The apparatus should also permit control of fluid flow rate and pressure to suit specific separation and filtration requirements. Existing separation and filtration equipment does not fulfil one or more of the above mentioned requirements. In particular, existing equipment does not facilitate sterilisation requiring disassembling of components and such equipment and often requires substantial volumes of fluid for monitoring purposes. These drawbacks are particularly evident when dealing with the treatment of biological fluids, for example, blood in dialysis or life support systems. In addition, control of flow rate and pressure is often complex in existing equipment and further reduces the sterilisability of the equipment. Thus, most existing systems tend to be aseptic rather than sterile, and this is less desirable in a clinical environment. In many applications it is desirable to control the filtration pressure within the filter unit so as to optimise filtration and at present this is not achieved with existing systems. Furthermore in applications involving filtration of biological fluids it is highly desirable that there should be no contact between the fluid being treated and the monitoring system to avoid the possibility of contamination. This is particularly relevant in life support systems. In other applications such as the isolation of cells and products from fermentation broths and the high value liquids, it is desirable that there is minimal loss of biomass and cell viability. It is an object of the present invention to provide a fluid flow control apparatus which obviates and mitigates at least one of the aforementioned problems. This is achieved by providing a fluid flow control apparatus in which a removable sterilisable or disposable element is provided and in which the pressure of the fluid inlet and fluid outlet of the filter unit are monitored and the pressures are compared and used to control the pressure across the filter unit. This is particularly important in cross-flow filtration which is a procedure whereby the process fluid is recirculated over the membrane surface. In the present case the fluid flow through the filter creates a pressure differential between the inlet and the outlet and the driving force through the membrane is determined by the transmembrane pressure (TMP) . The transmembrane pressure is obtained by measuring the inlet and outlet fluid pressures and averaging these values. The pressure of the filtrate has to be substracted from the average value, however, the filtrate pressure is small and is generally taken as zero. Pressure is measured by bleeding the inlet conduit through an inlet bleed conduit which includes a pressure switch and pressure transducer. The outlet conduit containing the retentate also has a bleed line and a post-membrane pressure transducer is inserted in the second bleed off line. With this arrangement the inlet and outlet pressures can be monitored and the pressure values are averaged in use to control the pressure switch or valve so as to maintain optimal control of the transmembrane pressure and optimise filtration for the particular fluids and membranes used. In one .arrangement, the flow-through arrangement, the pressure transducers are located in the bleed lines which means that the sensors are in contact with the fluid bein treated. In another arrangement, the pressure transducer are of the diaphragm type as is the pressure switch so that there is no contact between the fluid being treated and the monitoring and controlling apparatus. In a preferred arrangement the pressure monitoring and control system is incorporated within a fluid handling system which includes a fixed portion and a sterilisable or disposable portion which is removably engageable with the fixed portion. The removable portion is connectable to a pump and to conduits in the fluid handling system. The fixed portion is shaped to receive the removable portion, known as the cassette, and the fixed portion has apertures therein for receiving the pressure sensors and the pressure switch. When the cassette is inserted into the fixed portion, the apertures in the cassette and fixed portion are aligned. In the flow-through system the fluid passes between the cassette and the fixed portion through aseptic parts. In contrast, fluid does not pass when the diaphragm sensors are used because the diaphragm pressure sensor in both the cassette and fixed portion are aligned and pressure is monitored via the diaphragms without contacting the fluid being treated. In the diaphragm sensor arrangement, both pressure transducers in the fixed part contain channels which can be filled with fluid, the pressure of which can be adjusted to calibrate the sensors for the system being monitored. According to one aspect of the present invention as provided, apparatus for controlling the transmembrane pressure in the filtration device, the apparatus comprising filter means having an inlet for receiving an inlet fluid to be filtered and an outlet for receiving the concentrate from said filter means, first pressure monitoring means associated with an inlet conduit for measuring the inlet pressure to said filter means, second pressure monitoring means associated with the outlet conduit for measuring the pressure of the outlet fluid, means for comparing the inlet and outlet pressures measured and flow control means coupled to said first and second pressure monitoring means to provide a comparison signal, the flow control means being responsive to the comparison signal to control the flow of fluid through said filter unit to optimise control of the transmembrane pressure and filtration. Preferably the fluid flow control means is a pressure switch located in the inlet conduit. Alternatively, the pressure switch may be located in the outlet conduit or may be a valve for restricting flow in the inlet or outlet conduit. Preferably the pressure sensors are disposed in a bleed line of the inlet and outlet conduits so that the fluid flows past the sensors. Alternatively the pressure sensors are diaphragm pressure sensors so that there is no contact between the fluid being treated and the pressure monitoring apparatus. The first and second pressure transducers are coupled to first and second solenoid valves respectively, the outputs of which are combined within a single conduit, the valves being actuatable to purge fluid from the measurement fluid circuit. The transmembrane pressure controlled filtration system is used with a fluid flow control apparatus which includes a fixed portion and a removable portion, the fixed portion has pressure sensors and pressure switch disposed therein and are coupled to internal conduits within the fixed portion to facilitate flow-through of the fluid being monitored. The removable portion contains internal conduits which terminate in ports which mate with the corresponding ports in the fixed portion so as to form aseptic ports when the removable portion is inserted into the fixed portion. Alternatively, the removable portion contains at least two diaphragms each of which is disposed in the surface of the removable portion, one diaphragm being arranged to receive, through internal conduits, fluid from the filter and the output of which is fed to the valve, the other diaphragm being arranged to receive fluid from the pump and to return this fluid to the filter. Disposed in the fixed portion are corresponding diaphragms such that when the removable portion is inserted into the fixed portion, the diaphragms register with each other, the diaphragms on the fixed portion being coupled to respective pressure gauges for monitoring the filter inlet and outlet pressures and coupled to internal conduits which are filled with fluid to calibrate the pressure sensors. These and other aspects of the invention will become apparent from the following description when taken in combination with the accompanying drawings in which:- Figure 1 is a perspective and partly exploded view of fluid control apparatus; Figure 2 is a view of the apparatus of Fig. l taken in the direction of arrow 2; Figure 3 is a diagrammatic representation of an embodiment of a transmembrane pressure monitoring system in accordance with the present invention; Figures 4 a to e are partly sectioned plan, end and elevational views of a removable cassette in accordance with an embodiment of the invention in which the cassette contains diaphragm pressure transducers; Figures 5 a - d are plan, end and elevational views o the fixed part of the flow control system for receiving the removable cassette and which is modified to receive corresponding diaphragm pressure sensors and a pressure switch: Figure 6 is an enlarged side view of a diaphragm sensor mounted in the removable cassette, and Figure 7 is a graph of pressure against time for a fitted back-wash procedure using the system of Figs. 1 to 6 incorporated in a downstream processing unit. The fluid flow control apparatus 10 consists of two parts; a first main body portion, generally indicated by reference numeral 12, which remains fixed in the BIO 2000 control unit and a removable cassette element 14. The main body portion 12 has two parts; a cassette guide 16 and a separate valve control part 18. The guide 16 and valve 18 are spaced apart to define a channel 20 for slidably receiving the removable cassette element 14 in a close fitting arrangement. The guide 16 has aseptic ports 22a, b disposed on its inner surface 24 for registering with like ports 23a, b disposed on the outer surface 25 of cassette element 14. In the assembled condition pressure sensors 67, 84 and pressure switch 65 are securely fastened in the top of the guide 14 to monitor these fluid parameters. Pressure sensitive sensors 67, 84 measure the inlet and outlet pressures as will be described. The cassette element 14 is generally L-shaped in plan and consists of a machined block of medical grade sterilisable stainless steel and is of sufficient thickness to accommodate fluid flow channels as will be fully explained later. The cassette element 14 contains the apparatus inlet 15 and various other inlets and outlets as will also be explained. The cassette also includes an element which is part of the control valve 18. This is a fixed cylindrical boss 30 around which is located a flexible conduit 32 which connects the conduit 13 tube with the unit outlet tube 17. The main part of control valve 18 includes a rotable cam 34 coupled to an electric motor 36 via gears 38 and a chain and sprocket drive 40. When the cassette element 14 is in an assembled position as in Fig. 1 or 2, the electric motor is operable by a central control unit, not shown within apparatus 10, to control the position of cam 34 so that the distance between the cam 34 and boss 30 is controlled to define a 'nip' on tube 32. This nip is used to create the back pressure in the separation element or filter to suit the filtration requirements of the blood. Reference is now made to Figures 1, 2 and 3 of the drawings which depicts a fluid circuit generally indicated by reference numeral 50 for use with a BIO 2000 filtration system (Bio-flow Ltd., U.K.). In the system shown in Figures 1, 2 and 3 a fluid conduit 52 has an inlet for receiving fluid to be treated. A three lobe roller pump 54 forces the fluid along conduit 52 and through an inlet 58 to a hollow fibre filter unit generally shown by reference numeral 60. A bleed line 62a is taken from the inlet conduit 64 so that the inlet fluid can be monitored by various sensors. In the example shown, the pressure transducer 65 and pressure switch 67 are located in the bleed line 62 for measuring the pressure parameters of the inlet fluid. The bleed line is fed to a solenoid valve 66, the output of which is connected to a conduit 70a which, in turn, is coupled via outlet 72 to the system being treated. The filter unit has a filter output 74 which feeds outlet conduit 76 which passes through a pressure control valve, not shown in the interest of clarity, and the output from the valve constitutes the return fluid which is fed back to the source, in this example the vascular system of a patient. The outlet conduit has a bleed line 78 which is connected to a second solenoid valve 80 which has an output 82 which feeds into input conduit 70 as described above. A pressure transducer 84 is located in the bleed line 78 for measuring the pressure of the filter output fluid. It will be understood that conduits 62a,78a and 70b are located in the removable portion 14 or cassette of the BIO 2000 filtration system when these conduits are coupled via aseptic ports 63, 77 and 81 to conduits 62b, 78b and 70a respectively which are located in the fixed portion 12. In the arrangement shown in Figure 3, the pressure switch 67 and pressure sensors 65 and 84 are located in line so that the fluid flows through and is in contact with the sensors. With this arrangement, transmembrane pressure measurement (TMP) is achieved by monitoring the inlet pressure with transducer 24 and the outlet pressure with transducer 84. The transmembrane pressure is calculated at the average of the inlet and outlet pressures (Po + Pi/2) less the filtrate pressure (Pf) . However, filtrate or permeate pressure is low and is generally taken as zero so that the average transmembrane pressure is deemed to be the average of the inlet and outlet pressures. The measured pressures are processed in a routine manner to provide the average pressure used to control pressure switch 67 to vary the flow through the solenoid or to affect the back pressure of the system or can be used controlling the valve to vary the pressure in the filter outlet conduit by varying the distance between the cam and boss and thus affect the transmembrane pressure. With this arrangement, control of the transmembrane pressure is optimised to minimise effects of gel polarisation and the like which is a process which is known to slow down filtration. Reference is now made to Figures 4 and 5 of the drawings which depicts the removable and fixed part respectively of the fluid flow control apparatus BIO 2000 filtration system. Figures 2a to e show various views of the removable cassette which is adapted to slide into the fixed portion shown in Figures 3a to d. In this embodiment the system is generally similar to that shown in Figure 1, 2 and 3 except that pressure is measured using diaphragm pressure system as opposed to a flow-through system as will be now explained. This means that the fluid being filtered does not contact the monitoring system which has important implications in maintaining the sterility and the integrity of the fluid being treated. As best seen in Figs. 4a to e, the removable cassette 14 is generally L-shaped in plan (Fig. 4e) . The middle portion has a leading end 92 which is shaped to be inserted into the fixed portion 94 to be received by channel 96. The fluid connections to the removable cassette are therefore principally at the trailing end 94. In the embodiment shown in Fig. 3 fluid is pumped in through inlet 96 as best seen in Figs. 4a and 4b whereupon it enters the diaphragm chamber 98 fitted with a diaphragm (not shown in the interest of clarity) and then exits along internal conduit 100. The filter output is fed in through conduit 102 to diaphragm chamber 104 and out through internal conduit 106. Reference is now made to Fig. 5 of the drawings which depicts the fixed part 12 of the cassette unit which is machined to receive the removable portion 14 within channel 96 in a sliding manner so that the diaphragms 98,104 lie in register with diaphragms 108,110 of the fixed portion respectively. Diaphragm chamber 108 is connected to inlet and outlet conduits 112,114 and diaphragm chamber 110 is connected to conduits 116,118 as best seen in Figs. 5a and 5b. Diaphragm chambers 108,110 are dimensioned to receive a diaphragm similar to that in chambers 98 and 104 as best seen in Fig. 6 and chambers 108 and 110 communicate with openings 120a and 120b into which pressure transducers may be fitted. Internal conduit 112 communicates with opening 120 into which a pressure switch may be disposed so that the cassette system is arranged to facilitate transmembrane pressure monitoring as described in the first embodiment. Reference is made to Fig. 4 which depicts an enlarged view of a section through aperture 104 shown in Fig. 4b. It will be seen that a diaphragm sensor is fitted as shown and projects slightly proud of the surface of the cassette 14. The diaphragms have an impervious silastic membrane to prevent fluid leakage. When the removable cassette 14 is inserted into channel 96 the diaphragms are aligned and contact each other so as to transmit a force from one membrane to the other. When the system is set up it is important that it is calibrated so that changes in pressure are transmitted correctly through the membrane for accurate measurement. This is achieved using the arrangement shown in Figs. 5a and b in which the diaphragm chambers 108,110 are coupled to internal conduits 112,114,116,118 which can be filled with fluid and then closed with a slug or screw threaded member which is adjustable so as to vary the pressure in the chamber. With this arrangement the system can be calibrated at the beginning of each measurement procedure. In operation the system operates substantially identically to the above described embodiment. The inlet fluid is pumped through the filtering unit and the inlet conduit is fed through the removable cassette portion 14 and in particular through inlet 94. The inlet pressure is measured by pressure sensor 113. Similarly the filter output is coupled to the cassette via internal conduits 102 and 106 and the filter outlet pressure is measured by sensor 115 coupled to diaphragm 74. In this way the pressure across the membrane, the transmembrane pressure is measured and the measured pressure can be averaged and used to control the flow through the filter unit so to optimise the transmembrane pressure and hence filtration. The transmembrane pressure signal can be used to control the pressure switch or valve can indicated above. An advantage of this arrangement is that there is no requirement for solenoid valves purging because there is no contamination of the measurement line. The transmembrane pressure filtration and separation system hereinbefore described has particular relevance in Downstream Processing, which is the isolation of cells and products from fermentation broths and other high value liquids. The use of a cross-flow mechanism in combination with low shear faces and low transmembrane pressures, results in reduced stress on sensitive cells, genetically engineered or otherwise, avoiding or minimising cell lysis and therefore avoiding the release of enzymes that may have an inhibiting or destructive effect on the final products. The Bio-Flo Downstream processing unit offers the operator full control over filtration or separation processes by means of a built-in solid state flow and pressure control mechanism and the cassette and diaphragm arrangement allows the system to be used under full containment conditions. The downstream processing unit is similar to the arrangement hereinbefore described with reference to Figs. 1 to 5, and includes a microprocessor which monitors pressure within the fluidlines and which switches on a back-flush pumping system once the pressure within the fluidlines exceeds a preset maximum level. This is best explained with reference to Fig. 7 which is a graph of pressure against time and shows that the back-wash procedure occurs when the pressure reaches 1.2 bar, for example. The period of back flushing is dependent on the material being processed and can be programmed by the operator. Because only one filter, or a set of filters may be back-washed at any one time, the filtration process is an ongoing process avoiding cell settlement in the fluidlines and filters. Also, the use of autoclavable filters and fluidlines allow for aseptic conditions when processing, for example, sterile fluids. It will be appreciated that various modifications may be made to the apparatus as hereinbefore described without departing from the scope of the invention.

Claims: CLAIMS 1. Apparatus for controlling the transmembrane pressure in a filtration device, the apparatus comprising filter means having an inlet for receiving an inlet fluid to be filtered and an outlet for receiving the concentrate from said filter means, first pressure monitoring means associated with an inlet conduit for measuring the inlet pressure to said filter means, second pressure monitoring means associated with the outlet conduit for measuring the pressure of the outlet fluid, means for comparing the inlet and outlet pressures measured and flow control means coupled to said first and second pressure monitoring means to provide a comparison signal, the flow control means being responsive to the comparison signal to control the flow of fluid through said filter unit to optimise control of the transmembrane pressure and filtration 2. Apparatus as claimed in claim 1 the fluid flow control means is a pressure switch located in the inlet conduit. 3. Apparatus as claimed in claim 1 wherein the pressure switch is located in the outlet conduit or is a valve for restricting flow in the inlet or outlet conduit. 4. Apparatus as claimed in any preceding claim wherein the pressure sensors are disposed in a bleed line of the inlet and outlet conduits so that the fluid flows past the sensors. 5. Apparatus in any one of claims 1 to 3 wherein the pressure sensors are diaphragm pressure sensors so that there is no contact between the fluid being treated and the pressure monitoring apparatus. 6. Apparatus as claimed in any preceding claim wherein the first and second pressure transducers are coupled to first and second solenoid valves respectively, the outputs of which are combined within a single conduit, the valves being actuatable to purge fluid from the measurement fluid circuit. 7. Apparatus as claimed in any preceding claim wherein the transmembrane pressure controlled filtration system is used with a fluid flow control apparatus which includes a fixed portion and a removable portion, the fixed portion having pressure sensors and a pressure switch disposed therein and which are coupled to internal conduits within the fixed portion to facilitate flow-through of the fluid being monitored. 8. Apparatus as claimed in claim 7 wherein the removable portion contains internal conduits which terminate in ports which mate with the corresponding ports in the fixed portion so as to form aseptic ports when the removable portion is inserted into the fixed portion. 9. Apparatus as claimed in claim 7 wherein the removable portion contains at least two diaphragms each of which is disposed in the surface of the removable portion, one diaphragm being arranged to receive, through internal conduits, fluid from the filter and the output of which is fed to the valve, the other diaphragm being arranged to receive fluid from the pump and to return this fluid to the filter. 10. Apparatus as claimed in claim 9 wherein disposed in the fixed portion are corresponding diaphragms such that when the removable portion is inserted into the fixed portion, the diaphragms register with each other, the diaphragms on the fixed portion being coupled to respective pressure gauges for monitoring the filter inlet and outlet pressures and coupled to internal conduits which are filled with fluid to calibrate the pressure sensors. 11. Apparatus as claimed in claim 1 including processing means coupled to the fluidlines for measuring pressure therein and back-flush pumping means coupled to said processing means which is actuatable via said processing means to back-flush the system when the measured pressure within said fluidlines exceeds a preset level.

Inventors: HOOD ROBERT GORDON, HOOD, ROBERT, GORDON
Applicants: BIO FLO LTD, BIO-FLO LIMITED
Application year: 1990
Main classification: A61M1
IPC codes: A61M1, B01D61