DE2046055C - Process for concentrating solutions by reverse osmosis - Google Patents

Description

the one occurring on the low-pressure side of this membrane dilute solution is discarded or returned to the starting solution.

The working pressure in the first and second stage is preferably greater than R ² n \ where R is the rejection ratio of the respective membrane and π 'is the osmotic pressure of the solution fed to the membrane.

The invention is described in detail below. Among other things, the Reference is made to the drawing to explain the invention on the basis of an exemplary embodiment serves. It shows a flow diagram for a plant for carrying out the method according to the invention.

The method according to the invention can best be explained using the following equations:

The salt rejection or rejection R in reverse osmosis is given by the following equation:

C '- C " C ■ '■

■ -V --'- ■? ■ "'

in which C and C "are the solids concentrations (or concentrations of dissolved solids) on the high pressure and low pressure side of the membrane. The difference AC between these concentrations can therefore be represented by the following equation:

/ IC = C "- C = -RC- (2)

The osmotic pressure π of a solution is almost directly proportional to its concentration, and the difference between the osmotic pressures Απ can therefore be given by the equation:

An = -Απ 'can be reproduced; and further is

Jv = -Lp (AP- σ Απ),

whereby:

Jv = Vv assei.stromung, i.e. liters of solvent (e.g. water) that pass through 1 m ^{2 of} membrane per day,

AP = hydrostatic pressure difference between the low pressure and high pressure side of the membrane,

Lp = volume barrier neation coefficient , i.e. liters of liquid that pass through 1 m ² at a pressure of 1 kg / cm ² per day, Απ - osmotic pressure difference between the solution eggs on the low-pressure and high-pressure side of the membrane and σ = salt rejection or rejection at infinity large applied pressure difference.

If Jv is large, then ag becomes * R, and equation (3) then takes the form:

Jv = -Lp (AP + R * n ').

In order for inverse osmosis to take place, the hydrostatic pressure exerted on one side of the membrane must therefore always be greater than Λ ² π '.

According to the invention, a method for concentrating a feed solution is now provided, in which the feed solution exposed to a certain working pressure is first sent through a system with highly repellent membranes arranged in series. The (strongly) diluted solution passing through the membranes becomes wet ^; tigt, and the concentrated solution obtained as a (preliminary) product solution is still sent under a working pressure as a feed solution to a system of weakly repellent membranes.

The product solution of this second stage is collected, and that through the weakly repellent Diluted solution that has penetrated the membranes can be eliminated or, preferably, used as the starting feed

solution. The operating pressure of the

directed to the strongly repellent membranes

Solutions should be higher than the value of Α ² π '

(as defined above) for the strong

/ repellent membrane, and the operating pressure of the solution fed to the weakly repellent membranes should be greater than the value of R * n ' for the weakly repellent membrane.

The rejection of the strongly repellent membrane must be higher than that of the following membrane

group. Strongly repellent membranes are those membranes that have a maximum amount of the Dissolved solids, d. H. nearly 100 per cent of the products other than solvent, be rejected. It is only desired that

the almost pure solvent passes through the highly repellent membranes. In this case σ ^ R ^ 1 and the feed solution can be concentrated within this system until its osmotic pressure is almost equal to the operating pressure used.

■ Weakly repellent membranes allow the solvent (a certain proportion of the dissolved or) some dissolved solids to pass through the membrane. In this case ΊΛ R < 1, and the feed solution already enriched to a certain extent in the first stage, ie by the strongly repellent membranes, can be further concentrated until the new value of R ^z n of the weakly repellent membranes is again almost the same the work pressure is.

The permeation rate of the solids or the dissolved matter through the weakly repellent membrane can vary depending on the working conditions and the desired concentration of solution. The lower The concentration of dissolved matter required for the product solution is, for example, the higher it can be the rejection rate of the membranes because the product solution then has a low osmotic Pressure and a reasonable hydrostatic

so table pressure in the second stage is still acceptable Causes flow velocities.

On the other hand, the weakly repellent membranes should be the more permeable, ever higher the fü. the final product solution required concentration (and thus its osmotic pressure) is. Preferably, the amount of solute retained should be at least equal to the amount of besides Solids or dissolved solids passing through the membranes of the two-stage process in the solvent Be shares.

The type of membrane to be used is not critical in the process according to the invention as long as the membrane meets the special requirements of the concentration process to be carried out, ie has the desired volume permeation coefficient (Lp) and can withstand the required operating pressure. In addition, the membrane should of course be insoluble and above that

Solvent and all components of the solution be chemically inert. Cellulose acetate membranes, which are commercially available, are currently preferred.

The membranes can be of any suitable shape, such as tubular or flat being. A series of end-to-end tubular membranes forms a preferred one System of highly repellent membranes.

The same arrangement will-also apply to the system of the weakly repellent membranes preferred when large volumes of solvent in the second Step of the process from which the solution must be separated. Usually the membranes supported from the outside so that they can withstand the operating pressure when working with reverse osmosis. The dimensions of the membranes are determined taking into account the initial volume and the for the special food solution to be concentrated, the desired degree of concentration is selected.

The inventive method can be used to concentrate any feed solutions used to form a product solution with an osmotic pressure greater than the maximum required working pressure. Typical feed solutions are, for example Fruit juices, such as orange juice, strawberry juice, or apple juice, and diluted sugar solutions.

The process according to the invention can be carried out batchwise or continuously, with the latter mode of operation is preferred. Because the together with the solvent by the weak Dissolved fractions or solids passing through repellent membranes are very often valuable constituents it is advantageous and desirable to use the one derived from the weakly repellent membranes diluted solution to be returned to the original feed solution. Since only a small one The proportion of what was originally fed in can be used to carry out the procedure required area of the membrane can be reduced to a minimum.

An embodiment of a system according to the invention is shown schematically in the drawing. The different parts of the system can be made of different materials, that are compatible with the various components of the solution and withstand the operating pressure be able. Stainless steel is usually preferred for processing food solutions.

The installation or device comprises a feed tank 10, a high pressure pump 12, a system 14 of highly repellent membranes, a system 16 of weakly repellent membranes, a product tank 18 and a water tank 20. The pressures within the membrane systems 14 and 16 are set by printing controls 22 and 24. for the return of the dilute solution from system 16 to feed tank 10 are means such as line 26, intended. Hamlets are also valves for controlling the flow of fluid in various places of the system is provided, and an air collector 27 is arranged directly downstream of the pump 12.

S During operation, the feed solution is pumped out by Pump2 is pumped from the tank 10 to the highly repellent membrane system 14, in which some of the solvent (such as water) passes through the membrane in practically pure form and to the

ίο Water tank 20 flows out while the restricted Solution flows to system 16 of the weakly repellent membranes. The through the membranes of the Dilute solution passing through system 16 is returned to feed tank 10 via line 26.

The product solutions occurring in the system 16 of weakly repellent membranes are in the tank 18 collected. When the process is carried out continuously, the feed solution is continuously passed through the line 28 is fed to the tank 10 and the pro-

Ao duct solution from the tank 18 and the solvent from the tank 20 are also continuously removed.

The following examples serve to illustrate

of the invention, which of course in none WfMe should be restricted. All below

The parts and percentages given are parts by weight or percentages by weight, unless expressly stated something else is stated.

Examples i to 4

In these examples, cellulose acetate membranes were as described with reference to the drawing System was used, and the pressure and concentration measurements were taken from those shown in the drawing marked positions.

The results of the measurement of concentration, pressure and flow are in the following Table, in which the symbols have the following meanings:

C = concentration measured at 30 C in degrees Brix [Br] (concentration determination by spindles with the saccharometer according to B r i x),

Q ~ flow velocities in liters per day, Jv - water permeation in liters of water per m ²

Membrane per day,
R = mean rejection rate of the membranes,

P = working pressure in kg / cm *.
50

In the system used specifically for these examples, thirty-six tubular membranes with an inner diameter of 1.27 cm VIt ') were used in a row as the high-pressure membrane group 14, and thirty-six tubular membranes with a diameter of 1.27 cm were also used for the low-pressure membranes H.

At- Food- Cr Concentrations
(Br *) (at 30 ⁰ Q ε * Γ, c-, Qr I Flow velocities
(Litei / day) ß, β, Q * 10.3 C ₀ j 0 15th 44.5 148 Q. 112 13th 36 (D Sugar solution 10.8 10.6 0 37 128 161 95 Ii 33 (2) orange juice 10.9 10.5 0 6th 36.5 161 139 115 13th 46 (3) orange juice 10.5 10.5 0 19.5 46.5 102 174 79 43 23 (4) orange juice 13.0 145

The effectiveness of the new process and de ^r plant will be readily apparent from the results of examples. The invention enables the concentration of feed solutions to form a product solution which has a higher osmotic pressure than the sum of the highest hydrostatic working pressure exerted on the solution and corresponds to the osmotic pressure of the solvent solution removed from the system.

According to the invention, an operation at a desired pressure level to prevent a Impairment of the membranes and a desired low ratio of volume of solvent, which passes through the membranes, reaches volume of recovered product. The latter feature helps to increase the required area of the membranes and to obtain a device with a certain product capacity.

example P, Pa pressure
(kg / cm) Pb Pt Salt rejection
of the membranes
(V.) Rb Wajse (permeation
(l / m day) Λ- * 58.4 57.6 Λ 55.5 53.4 Ra 63 4.1 (D 62.6 59.8 57 52 49.2 100 77 34.6 4.1 (2) 63.3 60.5 57.6 52.7 47.1 100 88 29.6 4.1 (3) 59.8 56.2 57 47.8 42.2 100 50 35.8 2.7 (4) 53.4 100 24.4

1 sheet of drawings

Claims (2)

1 2 containing solution to be concentrated as “feed solution” and claims:, the concentrated solution obtained after the concentration process is referred to as “product solution”. 1. Method for concentrating solutions by concentrating solutions by inverse osinverse osmosis, thereby marked ich- 5 mose, the Spe.selösung is with a semipermeable net that the starting solution is brought into contact in a first membrane, ie with a meme stage using a membrane that is permeable to water, but for practically only the pure solvent which allows dissolved constituents such as sugar and organic to pass through, is partially concentrated, and from which esters and electrolytes are practically impermeable and obtained partially concentrated solution further solution a pressure means is then applied to the feed solution in a second stage, which is higher than the osmotic pressure. When another membrane is separated, this way relatively pure water is separated from the solution through which the membrane apart from the solvent a certain amount of the membrane lets through, and that in which the solution enriched in the solution remains on the second stage on the high pressure side of the membrane 15 back feed side of the membrane. (The resulting solution, collected as a product solution, can of course also be separated from the water by means of a concentration process, while the diluted solution arising on the low-pressure side of these different solvents through reverse osmosis in the membrane can be discarded.)
or is returned to the starting solution. In the industrial application of the inverse 2. The method according to claim 1, characterized in that the working pressure in the first and of concentrates is greater than Rhi , in each case greater than Rhi, in each case greater than Rhi, with difficulties. A concentrated product solution R is the rejection ratio of the respective mem- brane usually has a very high osmotic branch and π is the osmotic pressure of the solution used to produce the mem- brane pressure and to achieve concentrated products. 35 must therefore be very high, above the osmotic pressure hydrostatic pressures lying in the product solution can be applied. If the currently available membranes are exposed to such high pressures, they are quickly subject to compression and theirs The invention relates to a method for concentration 3 ° permeability decreases. This compression of the mem- of solutions through inverse Os lose. Burning reduces their lifespan and affects For narrowing down solutions, the cost of the process if a different is considerable Process known, such as the evaporation of the solution, more frequent replacement of the membranes is necessary, by means of in the heat or in the vacuum or its. Another problem is the investment costs for the Freezing out and separation in crystallized form. 35 entire system that for hollow. Pressures designed However, such processes are not entirely satisfactory. gend, since to remove the solvent, an attempt has therefore been made to use one on the Phase change must be brought about, which the principles of inverse osmosis-based process The systems required for this are often complicated and wise to find the narrow solutions Makes procedure expensive. Further, at these 40 decreased hydrostatic pressures became economical Process sometimes valuable components of which can be generated, which is therefore a successful one concentrating solutions lost and / or decomposed using commercially available membranes and puts. less expensive systems. An improvement has recently been sought. According to the invention, there is now a method for by using reverse osmosis. 4ί Concentration of a feed solution by reverse osmosis The principles of reverse osmosis can be briefly provided in which a product solution is obtained can be summarized as follows: If one becomes whose osmotic pressure is higher than that Solution and pure water through a permeable for the water sum from the highest to the feed solution Barrier (membrane) from each other practiced hydrostatic working pressure and the osmotrnt an energy gradient occurs, the 50 table pressure of that for reverse osmosis causes the water to pass through the membrane used membranes, wanders through to the solution. The hydraulic one that is eliminated or removed. Through the The flow of water can be an effective pressure which is proportional to the concentration reduced to a minimum, so that titration of the solution and the osmotic pressure of the 55 is the generation of a desired product volume Solution «is called. The hydraulic flow can only require a relatively small membrane area per day therefore by increasing the on the solution is derlich. applied pressure can be slowed down. If the The invention consists in that the output hydrostatic Pressure of the solution equal to the osmotic solution in a first stage using a table pressure, no liquid flow occurs 60 membrane, which is practically only the pure solvent on and when the hydrostatic pressure is higher lets pass, is partially concentrated, and that of the than the osmotic pressure, the flow of the partially concentrated solution obtained is further reversed direction and it now passes water from the solution solvent in a second stage using the separating membrane on the fresh water side is separated from another membrane, which over. This latter process is called inverse osmosis 65 besides the solvent a certain amount of it designated. Lets dissolved solute through, and that those in the second In the description below and the on stage start on the high pressure side of the membrane the solvent and solute resulting solution is collected as a product solution while