GB2168907A - Filtration - Google Patents

Abstract

To remove solids from a liquid such as sea water, the liquid is pumped into a membrane filter module, which may be of the cross-flow type, a pulsating flow being imposed on the liquid upstream of the filter by means of a rotating ball valve in series with a centrifugal pump, thereby reducing clogging of the filter membrane. The effect may be enhanced by providing the membrane with a grooved upstream surface, inducing local vortex mixing. Sea water so filtered may be deaerated and then pumped into oil-bearing rocks to displace the oil. <IMAGE>

Description

SPECIFICATION Separation process The present invention relates to a separation process and more particularly relates to the filtration of liquids by use of a membrane.

Filtration is a well known process for the separation of solids from fluids by passing the two-phase medium through a porous medium. The pores of the porous medium are in general smaller than the solids to be separated. Examples of this process are water filtration by passage through packed beds of sand or through a fibrous material. The filtered solids tend to collect on the upstream surface of the porous medium. The filtered solids are then removed, for example by back washing or by replacing the porous medium.

More recently membrane filtration has developed and is smaller in principle to filtration but the filtering medium is a sheet of material containing prepared pores or holes. In general polymeric membranes are available which have high strength, high flow rate capability, chemical inertness and are of relatively low cost.

However filtration processes require backwashing facilities or porous medium replacement ability which usually requires manual intervention by an operator in charge of the plant. The present invention relates to an improved membrane separation process which may alleviate the above disadvantages.

Thus according to the present invention there is provided a process for separating suspended solid material from a fluid comprising the steps of (a) passing the fluid containing the suspended solid material through a module, the fluid flow being of a pulsing (or oscillatory) form, (b) passing the fluid containing the suspended solid material over the surface of a membrane in the module preferably having a surface capable of producing vortex mixing in the fluid and (c) withdrawing the fluid passing through the membrane.

The term "the fluid flow being arranged to be of a pulsing form" is intended to include a fluid flow having an average flow of greater than zero which has a varying (e.g. sinusoidal) flow rate superimposed on it, the total fluid rate always being greater than zero.

The flow path may be closed and at a finite pressure so as to assist fluid flow through the membrane. A fraction of the flow may be recycled or disposed of, the bulk of the fluid flow passing through the membrane.

The suspended solid material may be for example particulate solids, marine organisms, inorganic detritus or bacteria.

According to a further aspect of the invention there is provided a filtration apparatus comprising (a) means for passing a fluid containing solid suspended material through a module, the means being capable of producing a pulsing or oscillatory form on the fluid flow (b) the module having a membrane the surface of which is capable of producing vortex mixing in the fluid when using pulsing flow and (c) means for withdrawing the fluid passing through the membrane.

The pulsing or oscillatory fluid flow may be unidirectional or reciprocatory. Generally liquid is pumped continuously e.g. by a centrifugal pump and uni-directional or reciprocatory pulsations are superimposed by use of suitable pumps, valve or other mechanisms.

The surface of the membrane may be modified in a variety of ways but it is preferred to modify it so as to produce vortex mixing under turbulent fluid flow conditions. A suitable form of surface modification is the use of membrane sheets into the surface of which grooves or furrows are machined.

The invention will now be described by way of example only and with reference to Figures 1 to 5 of the accompanying drawings.

In order to study vortex-mixing in turbulent flows a flat channel 1 of cross-section 3mm x 36mm and 275mm long was constructed in a "Perspex" frame. The walls consisted of pre-machined "Vyon filter" sheets 2 (by Porvair) into which approximately semi-circular grooves 3 of 75mm depth and 1.25mm separation were machined. Two layers of hydrophobic polypropylene ("Celgard") membranes were laid upon this structure, thus rendering the channel walls permeable to gases but not to liquids. An oxygen gas chamber 4 behind one wall was held at 1 bar gauge and the opposite wall 5 was at atmospheric pressure. This difference in pressure leads to a convectively enhanced diffusive transport of dissolved oxygen between the walls at a rate which is determined by the intensity of mixing in the liquid. This arrangement is illustrated in Figure 1.

A steady component of flow was provided by a centrifugal pump 6 and pulsations were imposed by placing a rotating ball-valve mechanism 7 between the pump 6 and the channel 8 (see Figure 2). The mechanism consisted of two branches: the main line (bore 20mm) contained a Saunder 2" ball valve which was continuously rotated so as to intermittently block the line, the second branch was a flexible tube 9 (bore 10mm) which could be clamped to varying degrees so as to effectively vary the stroke of the pulsations. The steady flow bulk velocity component was measured using a rotameter 10 and the pulsation amplitudes were measured with an electromagnetic flowmeter (not shown). This arrangement was used to impose a peak-to-peak velocity pulsation which was 50- of the steady flow component and approximately sinusoidal in form. The gas or oxygen supply was made from cylinder 11.The channel was enclosed in a water bath 12.

The mass transfer data is presented as a Sherwood number (Sh), being the ratio of oxygen gas flux at a particular Reynolds number (Re) and pulsation divided by the purely diffusive flux which is obtained when no velocities are imposed.

The experimental results obtained from the apparatus when disilled water was used as the working fluid are shown in Figure 3 as a plot of Sherwood number (Sh) versus Reynolds number (Re) with and without 8Hz pulsations. These data indicate that application of pulsations increased the Sherwood number by about 20%. When milk was used as the working fluid, the membrane was rapidly fouled and lead to a reduction in the measured Sherwood number. Such effects prevented increase in the Reynolds number of the uninterrupted flow from significantly increasing the Sherwood number above a value of about 40 (see Figure 3). However, this phenomenon could be in part reversed by the application of pulsations. For the larger flows the Sherwood number could be enchanced by a factor of about 2 after imposition of pulsations.A similar effect was observed when the working fluid was a saline solution of waxy lipidic ester (concentration = c = 1 part in 7,500 15,000). In Figure 4 the effect of alternatively imposing and removing pulsations for various Reynolds numbers is shown. Application of flow pulsations leads to an enhancement in mass transfer which is gradually lost once the pulsations have been removed. This is believed to be caused by stripping and subsequent reforming of a fouling layer.

Measurements of mass transfer have been made in turbulent flows at Reynolds numbers of up to 14,000. Grooved walls were used and 8Hz pulsations were imposed onto the flow. In fouling liquids this arrangement improved mass transfer fluxes by as much as a factor of two. It is believed that the nature and concentration of the foulant which is present in a system determines the extent to which membrane deposition impedes transfer through the walls. However, in the present study of two different fouling systems in turbulent flow, it is evident that significant increases in mass transfer fluxes may be achieved by the imposition of flow pulsations. The flow circuit has not been optimized and it is likely that with modifications, operating regions of larger improvements could be identified.

A petroleum reservoir is formed by a suitably shaped porous stratum of rock which is capped with an impervious rock. The nature of the reservoir rock is important as the oil is stored in the small spaces or pores which separates the individual rock grains. Sandstones and limestones are generally porous and in the main these are the most common types of reservoir rocks.

To assist in the recovery of crude oil from the reservoir, it is common practice to inject water under pressure into the reservoir rock by injection wells to drive the oil through the rock into nearby producing wells.

With offshore or coastal oil reservoirs, i.e. reservoirs having an abundant, readily available supply of sea water, it is clearly an advantage to make use of this. However, raw sea water is not suitable for direct injection for numerous reasons. It contains significant quantities of dissolved oxygen, about 10 ppm at 10 C, which are sufficient to induce corrosion and encourage bacterial growth. In addition, of course, sea water contains dissolved salts, cheifly chlorides of sodium and magnesium, but calcium, sulphates, carbonates and bicarbonates are also present It also contains inorganic detritus, marine organisms and bacteria. Because of this, sea water must be subjected to an exhaustive treatment procedure before injection including sterilisation, deoxygenation and the addition of various inhibitors.

It is necessary to filter the water before injection, but the filtration step gives rise to particular problems.

Because of prior treatment the sea water and its contents are generally subjected to severe shearing before filtration, e.g. by pumps. This can give rise to a proteinaceous and lipidic pulpy dispersion, resulting from the marine organisms together with fine particles of inorganic material such as sand.

This material is intractable, difficult to remove by filtration and quickly clogs conventional filter cloths. It is a gelatinous material of indeterminate composition of both organic and inorganic origin which adheres to conventional filters and is difficult to remove by conventional back washing techniques.

Sand filters or diatomaceous earth have been suggested for the removal of this material from sea water because of their filtering efficiency and ease of regeneration, but these are bulky and heavy and are undesirable for use on offshore platforms where space and weight are at a premium.

It is envisaged that the above technique for alleviating fouling of membrane filters can be extended to a filtration apparatus suitable for the filtration of sea water prior to injection into a petroleum reservoir.

Figure 5 is a schematic diagram of a water injection system for injecting filtered sea water into a petroleum reservoir, the system having a filtration apparatus according to the invention.

Coarse (about 80 ,am) filtered sea water is passed to the filter module by means of a pumping system which causes pulsating or oscillating flow through the module. This may be achieved by use of a rotating ball valve upstream of the membrane filter module. The filter module has a membrane filter e.g. of an acrylic polymer, the upper surface of which has grooves, furrows or corrugations so as to produce vortex mixing in the sea water flow.

The pressure in the line is maintained at a positive value so as to encourage flow through the membrane. A by-pass may be used directing the cross flow component of the feed to disposal or to utilities. Also backwashing facilities may be provided.

The filtered sea water is then passed through a number of stages including deoxygenation, scale inhibition, addition of oxygen scavenging material and biocides and analysis before being passed into the petroleum reservoir by means of injection pumps.

Claims (9)

1. Process for separating suspended solid material from a fluid comprising the steps of (a) passing the fluid containing the suspended fluid material into a module, the fluid flow being of a pulsing (or oscillatory) form, (b) passing the fluid over the surface of a membrane in the module and (c) withdrawing the fluid passing through the membrane.

2. Process according to claim 1 in which the surface of the membrane is adapted to produce vortex mixing in the fluid.

3. Process according to claim 2 in which the membrane has grooves or furrows in its surface.

4. Process according to any of claims 1 to 3 in which the pulsing or oscillatory fluid flow is uni-directional or reciprocatory.

5. Process according to any of the preceding claims in which the fluid flow path is at a finite pressure so as to assist fluid flow through the membrane.

6. Process for separating suspended solid material from a fluid as hereinbefore described and with reference to the accompanying drawings.

7. Filtration apparatus for carrying out the process as claimed in any of claims 1 to 6 comprising means for passing a fluid containing solid suspended material into a module, the means being capable of producing a pulsing or oscillatory form as the fluid flow, the module having a membrane the surface of which is adapted to produce vortex mixing in the fluid when using pulsing or oscillatory flow and means for withdrawing the fluid passing through the membrane.

8. Filtration apparatus according to claim 7 in which the means for passing the fluid through the module is a centrifugal pump and a rotating ball valve mechanism.

9. Filtration apparatus as hereinbefore described and with reference to the accompanying drawings.