WO1997004857A1

WO1997004857A1 - Membrane filtration apparatus

Info

Publication number: WO1997004857A1
Application number: PCT/GB1996/001854
Authority: WO
Inventors: Zanfengh Cui
Original assignee: Isis Innovation Limited
Priority date: 1995-07-27
Filing date: 1996-07-29
Publication date: 1997-02-13
Also published as: GB9515438D0

Abstract

Membrane filtration apparatus suitable for use in microfiltration, ultrafiltration and nanofiltration has a filtration container (10) within which a plurality of membrane tubes (100) are aligned substantially vertically, four are shown in the figure. A downcomer (23) is also provided within the container (10) which has a gas-liquid separator (11) immediately above both the membrane tubes (100) and the downcomer (23). Pressurised gas is supplied to the pressurised liquid within the container (10) through a gas distributor (24). The membrane filtration apparatus relies on the gas lift effect of the bubbles rising through the container to cause circulation of the fluid and at the same time the gas bubbles improve permeate flux and reduce membrane fouling.

Description

MEMBRANE FILTRATION APPARATUS

The present invention relates to membrane filtration apparatus and in particular to cross-flow membrane separation apparatus. Cross-flow membrane separation processes such as microfiltration, ultrafiltration and nanofiltration play an important role in many industrial separation processes, for example protein product concentration, cell stripping and recovery in downstream processing, waste water treatment, etc. The principal limitation to the wider adoption of membrane processes lies in the flux decline associated with concentration polarisation and membrane fouling.

Conventionally, cross-flow separation involves use of a mechanical circulating pump which forces the media to be filtered in a tangential flow across a membrane surface under pressure. This has become usual practice to prevent cake formation. Also, to suppress the concentration polarisation layer, high cross-flow velocity is normally required to maintain a relatively high permeate flux. This approach to maintaining sustainable permeate flux results in high capital and operational costs. For example, in a laboratory scale bioseparation process, the membrane module, the centre of the process, typically contributes less than 50% of the total capital cost whereas the pump accounts for a large proportion of the total capital cost.

There is an additional problem specific to industrial waste water treatment that the operational energy cost owing to the high circulation velocity requirement is the main limitation on the membrane separation process being implemented.

The present invention seeks to overcome the disadvantages described above with respect to conventional membrane separation apparatus. In particular the present invention seeks to reduce both capital and operational costs and to resolve problems involving concentration polarisation and membrane fouling.

The present invention provides membrane filtration apparatus comprising a filtration container containing one or more membrane filter elements orientated substantially vertically; a source of liquid to be filtered fluidly connected to the filtration container; and a pressurised gas supply having an outlet in communication with a lower portion of the filtration container whereby circulation of the liquid within the filtration container is driven by the flow of gas. In this way the liquid to be filtered is driven to circulate tangentially across the membrane filter elements without the use of a mechanical circulating pump. The liquid flow driven by the gas lift effect in the filtration container is sufficient to ensure sustainable permeate flux. Moreover, the use of a two-phase, gas-liquid, cross-flow enables a high permeate flux to be maintained at low liquid cross-flow velocity and reduces membrane fouling. ideally the transmembrane pressure is controlled by adjusting the pressure and fiow rate of the gas supply.

Preferably a circulation channel which extends between a gas-liquid separator and an inlet of the filtration container is provided which enables filtered liquid to circulate back for refiltering within the filtration container. The circulation channel may be either within or external of the filtration container. Also, the source of liquid to be filtered may be fluidly connected to the outlet of the pressurised gas supply so that a two-phase mixture of liquid and gas is supplied to the lower portion of the filtration container. The outlet of the pressurised gas supply may be connected to a single nozzle, or a plurality of nozzle elements located in the lower portion of the filtration container and in a separate alternative the liquid to be filtered may be separately supplied to the filtration container. The membrane filter elements may be tubular, flat-sheet, spirai or hollow fibre. In addition, the pressurised gas supply may be air, nitrogen, argon, or carbon dioxide.

Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a first embodiment of membrane filtration apparatus in accordance with the invention;

Figure 2 is a schematic diagram of a second embodiment of membrane filtration apparatus in accordance with the invention; and

Figure 3 is a schematic diagram of a third embodiment of membrane filtration apparatus in accordance with the invention.

With reference to Figure 1 , the membrane filtration apparatus comprises a filtration container 10 which may be in the form of a shell and which contains a single substantially vertically orientated tubular membrane filter element or membrane tube 10' which is connected to an integral gas-liquid separator 11 and source or feed tank 12 of liquid to be filtered. The feed tank 12 is positioned so that its base is a predetermined height above the base of the filtration container 10. The upper region of the tank which is empty functions as the gas-liquid separator 11. The output of the source 12 of liquid is fluidly connected along supply line 13 with the outlet 14 of a pressurised gas supply 15. The outlet of the pressurised gas supply is also connected to the bottom of the membrane tube 10' so that a two-phase mixture of liquid and gas is fed to the inside of the membrane tube 10' and flows across the internal surface of the membrane tube. The filtration container 10 also has an outlet 9 through which the permeate or filter liquid is collected.

During operation liquid is batchwise fed to the source 12 via feed line 16. In Figure 1 liquid from the source, which is pressurised during operation, flows via supply line 13 and the pressurised gas supply outlet 14 into the filtration container 10. Within the filtration container the gas bubbles in the liquid rise causing the liquid within the membrane tube 10' to also rise and thereby flow tangentially across the internal surface of the membrane tube. At the upper region of the filtration container 10 the two- phase media emerges and is fed to the separator 11 by a conduit 17. Within the separator the mixture of liquid and gas is separated with the liquid being fed back to the source 12 for refiltering through an overflow arrangement. Gas is separated and flows out through a control valve.

Means for monitoring and controlling the pressure within the separator and more important within the membrane tube 10' is provided in the form of a pressure transducer 18 and a control valve 19. A product drainage line from the overflow tray in the separator 11 is also provided through a valve 20. The circulation of the liquid from the source 12 to the membrane tube 10' and back to the source 12 is repeated until the desired concentration of the liquid solution in source 12 is reached. The latter is decided by measuring the amount of permeate collected. This is done by monitoring the permeate flow rate with a flow meter 21 provided at the outlet 9 of the filtration container 10. Altematively the concentration of the solution in the feed tank may be directly measured. Similar flow meters 21 may also be provided on the supply line 13 from the source 12 and the outlet 14 of the pressurised gas supply 15. A one way valve 22 is also provided on the supply line 13 from the source 12 to prevent backflow. With the apparatus described continuous filtration may be performed, in which case the liquid in feed line 16 is pressurised and is supplied to the tank 12 continuously. Retentate can be withdrawn from the drainage line through the valve 20. Turning now to Figure 2, alternative membrane filtration apparatus is shown in which the separator 11 is located within the filtration container 10. In addition, in Figure 2 the output of the separator 11 is connected to the supply line 13 from the source 12 instead of being input into the source 12 as in Figure 1. The output of the separator which is in the form of a circulation channel 23 is also known as a downcomer. It will also be seen from Figure 2 that the supply line 13 from the source 12 is not connected to the outlet 14 of the pressurised gas supply and instead is separately fluidly connected to the lower portion of the filtration container 10. The outlet 14 of the pressurised gas supply 15 in Figure 2 is connected to a gas distributor 24 which is in the form of a plurality of nozzle elements within the filtration container 10. The individual nozzle elements of the gas distributor 24 are aligned with a plurality of vertically aligned membrane tubes 100. The separator 11 which is located immediately above the membrane tubes 100 has a further outlet 25 to enable the retentate to be removed.

In use liquid from the source 12 is fed along the supply line 13 into the lower portion of the filtration container 10. Separately, pressurised gas is fed through the outlet 14 into the gas distributor and emerges as Iines of bubbles from each of the individual nozzle elements. The bubbles rise up through the membrane tubes 100 drawing liquid with them and causing the liquid to flow across the surfaces of the membrane tubes. At the top of the membrane tubes 100 the liquid and gas is separated with the liquid being recirculated through the downcomer 23 back into the filtration container 10 at the bottom. The fluid within the downcomer does not include bubbles as these have been removed in the separator 11. The fluid therefore is drawn down through the downcomer partially by the effect of gravity as the fluid has a greater density than the two-phase mixture within the membrane tubes 100, but also as a result of the effect of the motion of gas bubbles upwards within the tubes. The retentate may also be removed from the separator through outlet 25. This apparatus can be operated either continuously or batchwise. For batchwise operation an increase in the liquid holdup volume in the separator 11 is required.

Turning now to Figure 3, a further membrane filtration device is shown similar to the apparatus of Figure 2, however, the downcomer 23 in Figure 3 is located within the filtration container 10 rather than extemally of the filtration container as in Figure 2. Otherwise the apparatus of Figures 2 and 3 are identical. During operation of the apparatus of Figure 3 the pressurised gas supply is fed through the gas distributor 24 so as to pump the liquid up through the membrane tubes 100. It will be noted that there are no nozzles adjacent to the bottom of the downcomer 23 in Figure 3. This is to ensure that the downflow of liquid through the downcomer 23 is not interrupted by rising gas bubbles. This apparatus can be operated continuously or batchwise and is particularly convenient as the circulation of the liquid to be filtered is contained within the filtration container 10. Although in Figures 1, 2 and 3 tubular membrane filter elements are shown it will be immediately apparent that alternative forms of membrane filters may be utilised such as flat-sheet, spiral or hollow fibre membranes. However, in the case of spiral or hollow fibre membranes the need for a gas distributor is less acute.

In addition, for most applications, air may be used as the working gas to enable circulation of the liquid over the membrane filter elements. The main advantage of using air is the easy availability of compressed air. In certain circumstances, however, the oxidation effect of oxygen in air needs to be avoided in which case altemative gasses for example nitrogen and argon in the separation of enzymes of bioproducts or carbon dioxide in brewery processes may be used. Reference was made above to the permeate flux being monitored by means of a flow meter or a weighing balance. Altematively, operation of the apparatus may be controlled by direct monitoring of the concentration of the retentate.

The pressure of the gas supply may vary in dependence on the type of filtration required. For example, for microfiltration a pressure less than 3 bar is preferred whereas for ultrafiltration pressures up to 5 bar may be employed and in the case of nanofiltration pressures up to 20 or even 40 bar may be employed. Also, the flow rates of the gas may vary from below 1 ms^"1 up to around 20 ms^'1 with respect to superficial velocity. For example, using a solution of dextran at a concentration of 1.9 g/l with a superficial air velocity of around 0.01 ms^"1 and a fluid velocity of around 0.12 ms^'1 permeate fluxes 30% higher than for single phase filtration have been achieved experimentally.

It will be immediately apparent that altemative arrangements ofthe apparatus may be employed while still enabling membrane filtration to be performed in the absence of a mechanical pump through the utilisation of the gas-lift effect of the pressurised gas supply.

Claims

1. Membrane filtration apparatus comprising a filtration container containing one or more membrane filter elements orientated substantially vertically; a source of liquid to be filtered fluidly connected to the filtration container; and a pressurised gas supply having an outlet in communication with a lower portion of the filtration container whereby circulation of the liquid within the filtration container is driven by the flow of gas.

2. Membrane filtration apparatus as claimed in claim 1 , further including a separator for separating the gas and liquid.

3. Membrane filtration apparatus as claimed in claim 2, wherein the separator is located within the filtration container.

4. Membrane filtration apparatus as claimed in claim 2 or 3, further comprising means for monitoring and controlling the pressure within the apparatus.

5. Membrane filtration apparatus as claimed in any one of claims 2 to 4, further comprising a circulation channel extending between the separator and the lower portion of the filtration container which enables filtered liquid to circulate back for refiltering.

6. Membrane filtration apparatus as claimed in claim 5, wherein the circulation channel is located substantially vertically within the filtration tank.

7. Membrane filtration apparatus as claimed in claim 5, wherein the circulation channel is located substantially vertically outside of the filtration container.

8. Membrane filtration apparatus as claimed in any one of the preceding claims wherein the source of liquid to be filtered is fluidly connected to the outlet of the pressurised gas supply whereby a two-phase mixture of liquid and gas is supplied to the lower portion of the filtration container.

9. Membrane filtration apparatus as claimed in claim 8, wherein the filtration container contains one tubular membrane.

10. Membrane filtration apparatus as claimed in any one of claims 1 to 7, wherein the outlet of the pressurised gas supply is connected to a gas distributor having a plurality of outlets located in the lower portion of the filtration container.

11. Membrane filtration apparatus as claimed in ciaim 10, wherein the filtration container contains a plurality of membrane tubes.

12. Membrane filtration apparatus as claimed in any one of the preceding claims wherein a one-way valve is provided between the source of liquid and the filtration container.

13. Membrane filtration apparatus as claimed in any one of claims 1 to 11 , wherein the source of liquid is pressurised.

14. Membrane filtration apparatus as claimed in any one of the preceding claims wherein the filtration container includes a permeate outlet having a flow meter to monitor the flow of permeate from the container.

15. Membrane filtration apparatus as claimed in any one of the preceding claims, wherein the pressurised gas supply is taken from air, nitrogen, argon, or carbon dioxide.

16. Membrane filtration apparatus as claimed in any one of claims 1 to 8 or 10, wherein the membrane filter elements are tubular, flat- sheet, spiral or hollow fibre.