In counter-current flow, the feed flows on the shell-side. The feed thus contacts the outside of the fibers or flat membrane, and part of the solute permeates to the bor­­­­s of the fibers or flat membrane. The fibers are closed at one end, so permeate is discharged from one end only. Good distribution of flow on the shell-side is a design problem for large diameter units.
A counter-current flow membrane module for liquid separations consisting of a body comprising hollow fiber membranes wherein the hollow fibers membranes are arranged perpendicular to the longitudinal axis of the module and the channels in the hollow fibers are connected to a space present around the body, which is furthermore surrounded by a shell. The module comprises two or more transverse current flow (cross-flow) segments formed by seals between the body and the shell, wherein at least one fiber layer or fiber fabric is applied per segment.
Counter-current flow membrane systems in which feed and permeate are consequently arranged running counter to each other are known as "Membrane rectification column", "Counter current reverse osmosis membrane column" (CCRO) and "Continuous" or "Reflux membrane column". Such an arrangement is known for gas separation, pervaporation and liquid separations,



                  
whilst a number of other uses such as concentrating organic substances from aqueous solutions and purification of waste water are also known.
Furthermore, a liquid-liquid separation in a reflux membrane column is known, whereby the operation of such a principle is experimentally demonstrated for an ethanol/water system.
Although it has been clearly recognized that, in the case of liquid-liquid separations in which high osmotic pressure differentials can occur, a counter-current flow arrangement might be superior to the available single-stage embodiment, their implementation has hardly been started. The advantage of counter-current flow would appear in particular on concentrating diluted solutions such as ethanol/water mixtures which are obtained, for example, by fermentation. Economic evaluations show that, on concentrating a 5% ethanol solution by means of a counter-current flow process, capital expenditure and energy costs can be reduced by a factor of at least two as compared to a single-stage process.
Consequently, there is a requirement for a counter-current flow membrane module in which a local cross-flow or transverse current flow is combined in one module with a general counter-current flow, in order to achieve a higher mass transfer and less fouling.




                                            

The invention accordingly provides a counter-current flow membrane module for liquid separations consisting of a body comprising hollow fiber membranes in which the hollow fiber membranes are arranged perpendicularly to the longitudinal axis of the module and the channels in the hollow fibers are connected to a space present around the body which is furthermore surrounded by a shell, characterized in that the module comprises two or more transverse-current flow (cross-flow) segments formed by seals between the body and the shell, wherein at least one fiber layer or fiber fabric per segment is applied.
By making feed and permeate flow in a general counter-current flow and locally in a cross-flow or transverse current flow, an optimization of the motive force behind the separation process is achieved.
For reverse osmosis, for example, this means that the concentration difference across the membrane and, consequently, osmotic pressure differential, is kept to a minimum so that the effective pressure differential is a maximum. For dialysis, on the other hand, this achieves a maximum concentration gradient across the membrane and, consequently, a maximum motive force.
The invention is based in particular on mounting devices in the module for implementing cross-flow or transverse current flow as well as counter-current flow.