by Allan S. Myerson, Ph.D.

Introduction and Background

The Puraq Company, under the direction of the late Leon Lazare, developed a novel desalination technology which involves liquid-liquid extraction employing proprietary polymers with novel characteristics. Liquid-liquid extraction is a process in which two immiscible liquids are used to transfer a third component (in this case salt) from one liquid to another. In the case of the Puraq process, the two liquids are seawater (or any brackish water) and the polymer. The Puraq polymers have the property of forming homogenous solutions with water at particular temperatures. When the temperatures of these solutions are increased, they separate into two liquid phases. One phase is polymer rich and also contains some water, while the other is an aqueous phase which contains virtually no polymer. If the original solution contains salt, the salt partitions between the two phases with the polymer-rich phase containing significantly less salt then the aqueous phase.

The other novel characteristic of the Puraq polymers is their lack of solubility in water. This means that when the polymer-water solution phase separates, the water contains a very low concentration of polymer. The purpose of this work was to obtain samples of the proprietary Puraq polymer and verify the phase behavior of the polymer and the distribution coefficients (the ability to remove the salt from water) with those originally reported by Puraq.

Experimental Studies

The proprietary Puraq polymer, designated U-11755, was specially synthesized for this work. Data on the properties of this polymer in published and proprietary Puraq documents indicated that it has the properties necessary to achieve an efficient desalination process.

The phase behavior of the U-11755 polymer was measured by light scattering, using polymer concentrations from 0.1 to 90 weight percent polymer with pure water and from 1 to 90 weight percent polymer using water containing 3% sodium chloride. In these experiments, homogenous mixtures of water and polymer were made of given polymer-water mixtures. The temperature was slowly increased while a laser is used to probe the solution. When the mixture separated into two phases, a light scattering signal was detected and the temperature was recorded.

The results are given in "appendix 1" (attached Excel file) and demonstrate essentially identical behavior to that reported in the published and unpublished Puraq material. I will point out, however, that the published Puraq material has an error in Figure 2 of the 1992 paper in Desalination. The words "homogeneous" and "heterogeneous" are reversed; when correctly labeled, the figure shows that phase separation occurs as the temperature is increased.

The viscosity of the Puraq polymer U-11755 was also measured along with various compositions of Puraq polymer and water. These data are needed for process design and had not been previously obtained. The viscosity was obtained using capillary and parallel disk rheometers. The data show no surprises and are in the range expected. These findings are given in "appendix 2"(see second attached Excel file).

The last set of experiments performed was to measure the partitioning of salt between the polymer phase and water phase using Puraq polymer U-11755. This is known as the distribution coefficient and is defined as the concentration of salt in the water phase divided by the concentration of salt in the polymer phase. Published and unpublished Puraq data on the distribution coefficients obtained using Puraq polymer U-11755 exist. The purpose of this work is to verify these values.

The procedure used in these studies was to prepare a 3% salt solution in distilled water and then to mix this water with Polymer U-11755 in differing proportions making a homogeneous mixture. Each mixture was then slowly heated until phase separation occurred. A sample of the water phase and the polymer phase was then obtained from each and analyzed for sodium content using Atomic Absorption Spectroscopy.

The results obtained closely match the results reported by Puraq. For example, using a 50-50 mixture of water and polymer as the starting material with a concentration of 3% sodium chloride (30,000 PPM) results in salt concentration in the polymer rich phase after phase separation of 1.7% (17,000 PPM) and in the water phase of 4.3% (43,000 PPM) for a distribution coefficient of approximately 2.5. Starting with a mixture of 80% polymer and 20% water results in salt concentration in the polymer rich phase of approximately 0.2% (2000 PPM) and in the water phase of 3.7% (37,000 PPM) for a distribution coefficient of about 18, again very close to the value previously reported by Puraq.

Discussion

Currently, technology in water desalination employs either distillation or membrane technology. Distillation requires large energy input and is not economical unless very cheap sources of energy are available (for example in Saudi Arabia). Membrane technology is less energy intensive but is capital intensive and suffers from problems related to membrane fouling and breakage. It is usually used only for intermittent production of fresh water in emergencies. The Puraq process shows promise as a more economical method for the production of large amounts of fresh water with relatively low energy input. This report has demonstrated that the proprietary Puraq polymers have the properties previously reported and can be used in developing a commercial process. The next steps required are a small scale continuous process which incorporates the entire Puraq technology, followed by detailed process design and economic analysis.