GB2036595A - A semi-permeable membrane separation apparatus - Google Patents

Abstract

A semi-permeable membrane separation apparatus comprising an elongate helical tubular pressure vessel 11 and a single semi-permeable membrane assembly 14 extending coaxially along the length of the bore of the pressure vessel. The membrane assembly defines a gap between the walls of the vessel through which a liquid feedstock may flow. <IMAGE>

Description

SPECIFICATION A semi-permeable membrane separation apparatus This invention relates to apparatus for separating constituents from a fluid by causing a constituent of the fluid to flow through a semi-permeable membrane. Examples of such separation apparatus are those used to separate solvents from solutions by an ultrafiltration process or a reverse osmosis process.

In the ultra-filtration process the fluid to be treated is fed to a surface of the semi-permeable membrane under pressures of between 30 to 100 Ibslsquare inch (0.2-0.7 MN/m2) and the semi-permeable membrane acts as a sieve which allows only molecules below a predetermined threshold size to pass through the membrane.

In the natural phenomena of osmosis, if a semipermeable membrane is positioned between a solution and a solvent of the solution, diffusion of the solvent through the membrane into the solution occurs and the solution becomes more dilute. The term "osmotic pressure" is used to denote that pressure which if exerted on the solution, will just prevent the flow of the solvent through the membrane into the solution by osmosis. In the reverse osmosis process, a pressure in excess of the "osmotic pressure is exerted on the solution to suppress the osmosis process and cause the solvent in the solution to flow back through the semi-permeable membrane in the reverse direction to that in which it would have flowed due to osmosis. In this way, the solution is depleted of its solvent and becomes more highly concentrated.

To limit fouling and concentration polarisation it is preferable for the feed solution to be fed at high velocity over the semi-permeable membrane. This is not always practical. For example, a reverse osmosis apparatus may be designed to produce 1,000 gallons per day of product but a typical feed rate would be of the order of 10 gallons per minute, therefore the maximum possible water recovery would be about 7%.

Ways which have been suggested for obtaining higher recovery rates from small units (typically 100 gallons per day) have included rotation of the membrane assembly relative to the feed solution (see for example a British Patent No. 1,498,449), or recirculation of the feed solution.

According to the present invention there is provided a semi-permeable membrane separation apparatus comprising a pressure vessel having an elongate chamber therein, a semi-permeable membrane assembly extending along the chamber and defining a gap extending longitudinally between the membrane assembly and that adjacent surface of the pressure vessel so that a liquid feedstock can flowtherethrough along the chamber, and an inlet and an outlet each communicating with the gap and in spaced relationship to each other along the chamber.

Preferably, the chamber defines a helical path, and desirably the pressure vessel is of tubular form and is wound around a spool.

The membrane assembly may comprise a rod having a plurality of longitudinally extending grooves therein, a permeable substrate in the form of a sleeve covering the rod, and a semi-permeable membrane disposed on the substrate.

Although metal rods may be used, rods of a plastics material and preferably of polypropylene or an unplasticised grade of polyvinyl chloride are preferred, a convenient rod diameter being 3.2 mm.

Such a rod can be made by continuous extrusion using conventional extrusion materials and machinery, and the grooves can be made either during the extrusion process or by machining subsequent to extrusion. The grooves do not need to be straight although this is convenient and indeed preferable, and a preferred size of groove for an arrangement as above described is a groove of square section 0.5 mm deep and 0.5 mm wide. Four such grooves are conveniently provided on each rod and calculations show that they will permit the flow of all the permeate that is expected to flow through such a membrane assembly. However, if desired for example, larger grooves, and an alternative number of grooves may be used.

The most convenient form of substrate is a winding of thread and this thead does not of itself need to be permeable, the permeability of the substrate being provided by the gaps between the threads. Threads of glass fibre, polyester fibre, nylon or polypropylene are all possible but polyester fibre is preferred. The threads may be applied to the rod by winding or braiding either immediately after the rod has been formed or at a later stage whichever is the more convenient, but are conveniently applied by a braiding machine operating directly following the output of the extrusion machine.

Reverse osmosis membranes are well known in the art, and essentially the preferred membranes are of the type which comprises a mixture of cellulose acetate, a pore-forming substance, and a solvent.

Various ways of mounting the membrane assembly in the pressure vessel are possible but it is preferred to use a membrane assembly as above described of about 3.7 mm outside diameter contained within its own pressure vessel, and arranged to project beyond the ends of the pressure vessel.

The mounting itself desirably comprises a manifold member one at each end of the pressure vessel, each manifold member being provided with a first seal to engage the end of the pressure vessel in a fluid tight manner and a second seal for effecting a seal between the outside of the membrane assembly and the manifold member, thereby to define a space between the first and the second seals.

The invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows a perspective representation of one form of semi-permeable separation apparatus for use in the treatment of solutions by reverse osmosis; Figure 2 shows to an enlarged scale a side view in medial section of part of one end of the apparatus of Figure 1; and Figure 3 shows to an enlarged scale a sectional view on the line Ill-Ill of Figure 2.

Referring to Figures 1 to 3, the apparatus shown comprises an 8 metre length of a tubular pressure vessel 11 of nylon and of 4.2 to 4.3 mm inside diameter and 6.4 mm outside diameter, the pressure vessel 11 being wound around a spool 12 of about 230 mm diameter, and provided at each end with a manifold 13.

A membrane assembly 14 a little longer than 8 metres, extends along the chamber provided in the bore of the tubular pressure vessel 11 and projects from each end of the pressure vessel 11. The membrane assembly 14 comprises a central supporting grooved rod 15 of plastics material (polypropylene) typically 3.2 mm outside diameter and having four equi-spaced longitudinally extending grooves 22 each 0.5 mm deep x 0.5 mm wide (shown exaggerated for clarity). Over the grooved rod 15 is a permeable substrate 16 which is in fact a braiding of polyester fibres, and over the substrate 16 is a semi-permeable membrane 17 derived from cellulose acetate, thus making an assembly of about 3.7 mm diameter.

The membrane assembly 14 is mounted in the pressure vessel 11 by the manifolds 13 each of which comprises a brass body 18 having a bore to receive theendofthepressurevessel 11 andafirstseal 19in the form of a metal ferrule which is urged into sealing engagement with the outside of the pressure vessel 11 and the body 18 by a clamping nut 20.

Spaced from the first seal 19 is a second seal 21 in the form of a rubber grommet for effecting a seal between the outside of the membrane assembly 14 and the body 18 and urged into sealing contact therewith by a clamping nut 23. The space between the seals 19 and 21 constitutes a distribution passage which communicates with an opening 24, the opening 24 in one of the manifolds 13 providing an inlet for a liquid feedstock into the pressure vessel 11 and the opening 24 in the other manifold 13 providing an outlet for the liquid feedstock from the pressure vessel 11.

In operation, in one experiment, a liquid feedstock to be treated by reverse osmosis, for example brine, was introduced to the pressure vessel 11 at a pressure of 4.2 MN/m2 and at a flow rate of 100 mis/min, this flow rate being sufficient to control fouling and concentration polarisation. The permeate (potable water) which passed through the semipermeable membrane 17 flowed along the grooves 22 in the grooved rod 15 and was discharged at each end of the membrane assembly 14 and collected in a collection tank 25. It was found that product recovery rates as high as 75% of the liquid feedstock could be obtained.

Although circular cross-section pressure vessels and membrane assemblies are preferred because of their superior stress characteristics, alternative cross-sections might be used, and the vessels wound about spools preferably of between 150-300 mm diameter. The manifold 13 may be made of alternative materials such as stainless steel or of a glass reinforced plastics material.

A plurality of the pressure vessels may be wound around a common spool one above the other and arranged to discharge the permeate therefrom into a common duct.

It will also be understood that the invention may be incorporated in apparatus intended for operation at different liquid feedstock pressures from that aforedescribed.

Claims (8)

1. A semi-permeable membrane separation apparatus comprising a pressure vessel having an elongate chamber therein, a semi-permeable membrane assembly extending along the chamber and defining a gap extending longitudinally between the membrane assembly and the adjacent surface of the pressure vessel so that a liquid feedstock can flow therethrough along the chamber, and an inlet and an outlet each communicating with the gap and in spaced relationship to each other along the chamber.

2. Apparatus as claimed in Claim 1, wherein the chamber defines a helical path.

3. Apparatus as claimed in Claim 1 or Claim 2, wherein the chamber is of circular cross-section and has a ratio of Iengthldiameter of at least about 54:1.

4. Apparatus as claimed in any one of the preceding Claims, wherein the pressure vessel is of tubular form.

5. Apparatus as claimed in Claim 4, including a spool, the tubular pressure vessel being wound around the spool.

6. Apparatus as claimed in any one of the preceding Claims, wherein the membrane assembly comprises a rod having a plurality of longitudinally extending grooves therein, a permeable substrate in the form of a sleeve covering the rod, and a semi-permeable membrane disposed on the substrate.

7. A plurality of apparatus as claimed in any one of the preceding Claims, the apparatus being connected together in parallel relationship so as to discharge into a common duct.

8. A semi-permeable membrane separation apparatus substantially as hereinbefore described with reference to Figures 1 to 3 of the accompanying drawings.