GB2188563A

GB2188563A - Membrane having flow disturbing means

Info

Publication number: GB2188563A
Application number: GB08608053A
Authority: GB
Inventors: Imre Guila Racz; Wassink Jan Groot
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1986-04-02
Filing date: 1986-04-02
Publication date: 1987-10-07
Also published as: GB8608053D0

Abstract

A membrane containing a membrane layer having protrusions (e.g. studs and/or ribs) serving as flow disturbing means. The invention further relates to a process for separating a fluid mixture therewith. The mass transfer to and from the membrane layer is improved by inducing turbulence primarily at the surface of the membrane layer.

Description

SPECIFICATION Membrane having flow disturbing means The invention relates to a membrane having flow disturbing means and to a process for separating a fluid mixture therewith.

It is known to induce turbulence in a liquid being contacted with a membrane surface in order to reduce fouling thereof by applying a layer of net-like material (e.g. mesh) on the membrane surface.

Itisfurtherknown to apply mesh as spacer between two membrane layers in spiral wound membrane units.

However, in such cases mesh applied on the membrane surface will reduce the area available for contact of liquid with the membrane surface and masstransfertherethrough.

Moreover, the mesh may act as a plurality of static mixing elements which induce turbulence in the bulk of a liquid flowing along the membrane surface, thus causing a substantial pressure drop overthe membrane unit.

It has now been found that mass transfer to and from a membrane layer (and thereby the flux through a membrane) can be substantially improved by inducing turbulence primarily at the surface of a membrane layer having protrusions serving as flow disturbing means instead of in the bulk of a fluid (liquid) which is contacted therewith.

The invention therefore relates to a membrane having flow disturbing means which contains a membrane layer having protrusions serving as flow disturbing means.

A main advantage of the membranes according to the invention is that due to the excellent mass transfer concentration polarization across said membranes will be substantially reduced compared with membranes which do not comprise a membrane layer having protrusions formed therein. Consequently, the flux of permeate through the present membranes will be improved at a given pressure drop over the membranes, thereby reducing the required membrane surface area. Alternatively, ata constant flux the required pumping energy will be reduced compared with separation processes in which membranes provided with mesh as flow disturbing means are employed.

Another distinct advantage of the present membranes is that due to the turbulence induced by the protrusions in the surface of a membrane layer they are less susceptible to fouling which is importantfor many applications from an operational point of view.

The protrusions are formed in the membrane layer(s) itself, thereby allowing for a simpler construction of the membranes according to the invention compared with membranes which require a separate layer of mesh serving as flow disturbing means. In order to be optimally effective as flow disturbing means, substantially all ofthe protrusions preferably have a height which is greaterthan the thickness ofthe membrane layer. Preferably the protrusions have a height (measured perpendicular to the plane of the membrane layer) from 0.05-4 mm, and most preferably from 0.1-2 mm.

The protrusions may have any form which is suitableforflowdisturbing means providedthatthe membrane layer can be formed accordingly, Preferably, the protrusions comprise ribs (e.g. in the form of half cylinders divided along their axis) and/or studs which are most preferably regularly spaced along the surface of the membrane layer e.g. in aligned or staggered configurations. The distance between the sides oftwo protrusions facing each other is preferably from 1-20, and most preferably from 4-1 times the height of a protrusion.

The maximum width of the protrusions depends, of course, on the width ofthe membrane unit; preferably the protrusions have a width from 0.1-30 mm and most preferably from 0.5-15 mm (said width representing a diameter in case the protrusions comprise substantially cylindrical studs).

The protrusions may extend outwardly from both surfaces of a membrane layer which is preferred in particular when a fluid flow is maintained along both sides of the membrane (layer), as in a spirallywound membrane. However, preferably a membrane layer is arranged on at least one surface of a porous support layer having protrusions, in which case the protrusions in the membrane layer resulting from said arrangement most preferably extend outwardly from the surface of the membrane layer which is not in contact with the support layer in order to attain optimal turbulence offluidsflowing along said surface.

Suitable non-selective porous support layers for the present membranes include cloth, sintered glass, glass fibre and polymeric material; preferably, non-woven support layers are applied which comprise e.g.

polyester and/or cellulose derivatives such as cellulose acetate or nitro cellulose.

Preferably, a dense, selective membrane layer having a thickness from e.g. 0.1-25 Fm, and preferablyfrom 0.1-3 Fm (in order to attain a relatively high permeateflux) is arranged on one surface of a support layer having protrusions e.g. by applying pressure at the membrane layer which may be thereby stretched around the protrusions and consequently be structurally altered. Consequently, the thickness ofthe membrane layer may vary at different locations in the membrane.

A wide variety of (pre)polymers can be applied for the membrane layers which are present in the membrane according to the invention. The applied polymers may either comprise thermoplastics, such as polyalkylenes orthermo-hardening compounds such as (silicone) rubbers, resins orfluor-elastomers (e.g.

polydimethyl siloxane, polyurethane, styrene-butadiene-styrene rubber orfluoro silicone elastomer). The applied polymers should preferably be compatible with the support layer in order to attain adequate bonding of said layers.

The membranes according to the invention can be used in many configurations e.g. in the form of substantially flat sheets, spiral-wound or tubular membranes. The tubes should have such a diameter(e.g.

from 0.5-5cm)thatthe protrusions in the membrane layer can extend inwardly, if required; the membrane layers can be arranged inside and/or outside such tubes.

The invention further relates to a process for separating a fluid mixture which comprises contacting said mixture with one side of a membrane as described hereinbefore and removing permeate from the other side ofthe membrane. The process is well suited for applications in which a fluid feed mixture comprising hydrocarbons dissolved in one or more solvents has to be separated, such as a mixture obtained by subjecting hydrocarbon oils to a solvent dewaxing treatment during which e.g. a toluene/methyl ethyl ketone solvent mixture is added to a wax-containing oil, followed by removing wax. The solvents are usually recovered from dewaxed- as well as from deasphalted-hydrocarbon oils by means of a conventional flashing process with relatively high energy consumption.With the process according to the invention it is possible to reduce said energy consumption considerably by recovering a major portion of the solvents via membranes and separating the remaining amount of solvents, if any, by conventional means.

The membrane and process according to the present invention is particularly suitable for reverse osmosis applications in which the pressure applied at the side ofthe membrane which is in contact with the feed mixture has to be higherthanthe pressure at the permeate side of the membrane. The pressure differential between both sides of the membrane is preferably from 2-100 barabs., and most preferably from 10-80 bar abs., depending on the osmotic pressure of the applied fluids.

The temperature at which the present membrane and process is applied mayvarywithin a wide range, provided that the membrane can withstand the operating conditions. Afeed temperature from - 25 "C to 200 0C is suitable in most cases whereas a temperature from -20 0C to 50 0C is preferred when solvents are to be recovered from dewaxed hydrocarbon oils.

The invention is illustrated by the following Example.

An aqueous solution containing 5.0 g sodium sulphate/l is passed with a velocity of 1 m/s along the surface oftwo cellulose acetate membranes A and B supported on a layer of non-woven polyester, while the pressure difference between said surface and the permeate-side of the membranes is maintained at 40 bar. Membrane A is a substantially flat membrane (not according to the invention) whereas membrane B contains protrusions essentially consisting of half cylinders (divided along their axis) having a radius of 1.5 mm and a width (the height of the cylinders) of 15 mm which half cylinders are arranged substantially parallel ata distance of 15 mm (measured from one axis to the next) in rows; the aqueous solution flows substantially perpendiculartotheaxisofthe half cylinders.

The results are given in thefollowing Table.

TABLE Membrane Flux {m31m2sJ A 8.33x10-6 B 15x10-6 The results clearly show that the membrane according to the invention has excellent mass transfer properties due to the protrusions in the membrane layer.

Claims

1. A membrane having flow disturbing means which contains a membrane layer having protrusions serving as flow disturbing means.

2. A membrane according to claim 1 wherein substantially all of the protrusions have a height greater than the thickness ofthe membrane layer.

3. A membrane according to claim 1 or 2 wherein the protrusions have a hightfrom 0.05-4 mm, and most preferably from 0.1-2 mm.

4. A membrane according to any ofthe preceding claims wherein the protrusions have a width from 0.1-30 mm, and most preferably from 0.5- 15 mm.

5. A membrane according to any of the preceding claims wherein the protrusions comprise studs and/or ribs.

6. A membrane according to any of the preceding claims wherein the protrusions are regularly spaced along the surface of the membrane layer.

7. A membrane according to any of the preceding claims wherein a membrane layer is arranged on at least one surface of a porous support layer having protrusions.

8. A membrane according to any of the preceding claims which is substantially flat ortubular.

9. A membrane according to any of the preceding claims which is spirally wound and along both sides of which a fluid flow can be maintained.

10. A membrane substantially as described hereinbefore.

11. A process for separating a fluid mixture which comprises contacting said mixture with one side of a membrane according to any of the preceding claims and removing permeate from the other side ofthe membrane.