Apparatus and method for multistage reverse osmosis separation

Details Apparatus and method for multistage reverse osmosis separation Apparatus and method for multistage reverse osmosis separation

1995-10-02

2001-02-13

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S641000

Reexamination Certificate

active

06187200

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to novel reverse osmosis separation apparatus and method designed, particularly, for high-concentration solutions. This invention makes it possible to obtain a low-concentration solution from a high-concentration solution with high recovery ratio and with small energy, while it also makes it possible to produce higher-concentration concentrate with smaller energy as compared to conventional reverse osmosis separation. In particular, the apparatus and method of this invention can be used for the desalination of brackish water and sea water, treatment of waste water, recovery of valuable material, etc. The invention is particularly effective for desalination or concentration of high salinity solution.
BACKGROUND OF THE INVENTION
Many techniques are available for solution separation. In recent years, membrane separation has been in wider use because it requires smaller energy and smaller resources. Microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO) are among the membrane separation techniques. More recently, loose RO or nanofiltration (NF), whose functions are between those of reverse osmosis and ultrafiltration, has come in use. Reverse osmosis, for example, is currently used for desalination of sea water or brakish water to provide water for industrial, agricultural and household uses. With reverse osmosis, a pressure higher than the osmotic pressure is exerted on salt water to allow it to permeate reverse osmosis membrane to obtain desalted water. This technique can produce drinking water from sea water, brine, or a water which contains harmful substances, and has been used for the preparation of ultra-pure water for industrial use, treatment of waste water, and recovery of useful materials.
The production of fresh water from sea water by reverse osmosis has the advantage that it involves no phase transition such as found in evaporation. In addition, it requires less energy and less operation maintenance, resulting in its wider use in recent years.
For separation of a solution by reverse osmosis, it is necessary to supply a solution to the reverse osmosis membrane with a pressure larger than the chemical potential (which can be expressed in terms of osmotic pressure) of the solution which depends on the content of the solute in the solution. When a reverse osmosis membrane module is used for separation from sea water, for example, a pressure above 30 atm, or more practically a pressure above 50 atm, is required. Sufficient reverse osmosis separation performance cannot be obtained at pressures lower than this.
Concerning sea water desalination through reverse osmosis membrane, for example, permeable sea water recovery of conventional sea water desalination is not more than 40%. The concentration of sea water in the reverse osmosis membrane module increases from 3.5% to about 6% as a volume of fresh water equal to 40% of the supplied sea water is obtained through the membrane. A pressure larger than the osmotic pressure corresponding to the concentration of the concentrate (45 atm for 6% sea water concentrate) is required to achieve permeate water recovery ratio of 40%. Practically, a pressure about 20 atm larger than the osmotic pressure that corresponds to the concentrate concentration (which is called the effective pressure) is necessary to produce a sufficient fresh water that can be used as drinking water. Thus, reverse osmosis membrane separation for desalination of sea water have been conventionally operated under a pressure of 60-65 atm to achieve a recovery ratio of 40%.
A higher permeate water recovery (recovery ratio) is more desirable since the recovery ratio directly affects the required cost. Conventionally, however, there have been limits to recovery ratio improvement. That means, an increased recovery ratio may require a very high pressure. In addition, as the concentration of sea water components increases and in higher recovery ratio operating conditions, the contents of scale components such as calcium carbonate, calcium sulfate, strontium sulfate and other salts deposits on the reverse osmosis membrane as scale to cause clogging.
At the recovery ratio of about 40% (which is now widely recognized as the practically maximum recovery ratio), it is unlikely that such scale may be formed in significant amount and therefore no special means are required against them. If an attempt is to be made to operate reverse osmosis separation at a higher recovery ratio, a scale prevention agent that increases the solubility of salts should be added in order to control the deposition of these scale components. Despite the addition of such a scale prevention agent, however, the control of the deposition of said scale components is effective only to increase the concentrate concentration by 10-11 percentage points. For the desalination of sea water of a salt concentration of 3.5%, a mass balance analysis indicates a limit recovery ratio of 65-68%.
Taking into account the effects of various other components of sea water, the practical limit of recovery ratio at which a reverse osmosis sea water desalination plant can be operated stably would be about 60%.
In a practical sea water desalination process, a pressure about 20 atm higher than the concentrate's osmotic pressure should be applied on the reverse osmosis membrane, as stated above. When the salt concentration in sea water is assumed to be 3.5% and a recovery ratio of 60%, the concentration of salt becomes 8.8%, which corresponds to an osmotic pressure of about 70 atm. Thus, a pressure of about 90 atm has to be applied to the reverse osmosis membrane.
For practical uses, several reverse osmosis elements connected in series are loaded in a pressure vessel, which is called a module, and many modules are installed in parallel in a practical plant. The recovery ratio of a sea water desalination plant is defined as the ratio of the total water permeation to the total sea water supplied to the reverse osmosis modules. In an ordinary plant, since modules are installed in parallel, the recovery ratio is equal to the ratio of the desalted water obtained from a module to the sea water supply to that module. In case that one module contains six reverse osmosis membrane elements and that 198 m
3
/day of sea water is supplied to the module to produce 78 m
3
/day of desalted water (40% recovery ratio), a simulation shows that 18-19 m
3
/day and 15-17 m
3
/day of desalted water comes from the first and second elements, respectively, followed by decreasing amounts from the remaining elements to produce a total 78 m
3
/day of desalted water. Thus, in total, desalted water is obtained from the entire module at 40% recovery ratio despite a small desalted water recovery ratio for each element.
Prevention of fouling and concentration polarization (localization of solute) is an important factor to be considered in establishing operation conditions of a reverse osmosis membrane separation process. To prevent fouling, the rate of desalted water production from one reverse osmosis membrane element should be controlled below a certain limit (fouling-resistant permissible flux). If the rate exceeds the limit, the fouling on the membrane will be accelerated to cause trouble. The fouling-resistant permissible flux for high-performance reverse osmosis membrane is generally in the vicinity of 0.75 m
3
/m
2
eday, which corresponds to an yield of 20 m
3
/day for a reverse osmosis membrane element with a membrane area of 26.5 m
2
(the membrane area of an element is assumed to be 26.5 m
2
in all calculations hereinafter). Thus, to prevent fouling, the desalted water production rate of an element should be controlled below 20 m
3
/day.
The rate of water supply to elements in module decreases as water flows from upstream elements to downstream ones. Concentration polarization referred to above is caused due to a decrease in the flow rate of supplied water through the membrane in the final element. Concentration polarization not only reduces the membrane performance but also accelerate

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