Method of using new solvents for forward osmosis

Abstract

A method is provided for using forward osmosis to remove impurities dissolved in an aqueous-based feed solution, where the method includes directing a solvent past a first side of a forward osmosis membrane and the feed solution is directed past a second side of the forward osmosis membrane, the solvent having a higher osmotic pressure than the feed solution so as to draw water across the membrane thereby diluting the solvent and concentrating the impurities in the feed solution, where the solvent is an amine-terminated branched PEG, such as amine-terminated glycerol ethoxylate, amine-terminated trimethylolpropane ethoxylate, or amine-terminated pentaerithritol ethoxylate, for example. The method further includes regenerating the solvent by exposing the diluted solvent to a gas containing CO2, whereby the CO2 is absorbed by the solvent, facilitating substantial separation of the solvent from water.

Claims

1. A method using forward osmosis for removing impurities dissolved in an aqueous-based feed solution, the method comprising directing a solvent past a first side of a forward osmosis membrane while the feed solution is directed past a second side of the forward osmosis membrane, the solvent having a higher osmotic pressure than the feed solution so as to draw water across the membrane thereby diluting the solvent, the solvent comprising an amine-terminated branched PEG.

2. The method of claim 1, wherein the solvent comprises amine-terminated glycerol ethoxylate.

3. The method of claim 1, wherein the solvent comprises amine-terminated trimethylolpropane ethoxylate.

4. The method of claim 1, wherein the solvent comprises amine-terminated pentaerythritol ethoxylate.

5. The method of claim 1, further comprising regenerating the solvent by exposing the diluted solvent to a gas containing CO.sub.2, whereby the CO.sub.2 is absorbed by the solvent, facilitating substantial separation of the solvent from water.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The aforementioned objects and advantages of the present invention, as well as additional objects and advantages thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings in which:

(2) FIG. 1 shows a schematic of one embodiment of the present invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

(3) Branched PEGs can be synthesized from glycerol (3 arms), trimethylolpropane (4 arms, though one of the arms has a methyl group), pentaerythriol (4 arms) and other organic compounds. Some simple branched PEGs commercially available are glycerol ethoxylates (GE), trimethylolpropane ethoxylates (TMPE) and pentaerythriol ethoxylates (PEE) of various molecular weights, based on the number of EO monomers in the polymer. Glycerol ethoxylate, with a molecular weight of 1000, has approximately 20 EO groups, but is a liquid at room temperature, and less viscous than PEG 300 (EO=6). Trimethylolpropane ethoxylate, with a MW of 1014, also has 20 EO groups, is liquid at room temperatures, and also less viscous than PEG 300. Other liquid branched ethoxylates include pentaerythriol ethoxylate, MW 270 (EO=3) and pentaerythriol ethoxylate, MW 797 (EO=15). All of these ethoxylates have terminal [OH] groups, except for the TMP ethoxylates, which have one terminal methyl group replacing one [OH] group, out of the four available. Branched PEGs also have advantageous properties of steric hindrance, exhibit lower viscosity than comparable linear PEGs, have lower melting points, and, thus, enable better absorption of water in the FO process.

(4) All of these branched PEGs exhibited very high osmotic pressures, around 200-300 atms, and are very suitable as osmotic draw solutes. Further discussion of such branched PEGS, and applications thereof, can be found in co-pending U.S. Ser. No. 15/271,175, filed on Sep. 20, 2016, incorporated herein in its entirety by reference. Given the propensity for CO.sub.2 absorption of the EO monomers in the physical solvents described above, as well as the superior absorption characteristics of amine-based solvents for CO.sub.2 and H.sub.2S, a new class of solvents, based on aminated branched polyethylene glycols, is postulated therein. The synthesis of such amine-terminated branched ethoxylates is fairly straightforward. One methodology used is as follows: glycerol ethoxylate is reacted with diethylene triamine (DETA) in the presence of acid catalyst at 95??100? C. in an inert atmosphere. The DETA quantity can be varied depending on requirements, with the maximum amount being 3.3 moles to 1 mole of glycerol ethoxylate. Other amines can be used, instead of DETA. DETA is preferred as this gives greater stability to the amine functionality. Amine-terminated glycerol ethoxylate, trimethylolpropane ethoxylate and pentaerythritol ethoxylate were synthesized.

(5) An additional physical phenomenon was discovered during the absorption of CO.sub.2 by aqueous solutions of these amine-terminated branched polymers. Before the absorption of carbon dioxide gas was performed, these polymers were completely soluble in water. However, after absorption of CO.sub.2, the aqueous polymer solution formed a two-phase mixture, clearly separated from each otheran amine-rich phase and a water-rich phase, in roughly the same proportions used for the original water-polymer mixtures. Both the amine-terminated glycerol ethoxylate and the amine-terminated pentaerythritol ethoxylate exhibited the same phenomena for complete water solubility before CO.sub.2 absorption and complete insolubility with water after CO.sub.2 absorption.

(6) An important feature of the inventive solvents contemplated herein is their capacity to exhibit very high osmotic pressures. Thus, they can absorb large amounts of water, and can be used effectively as a draw solution in a forward osmosis system. Osmotic pressure tests were performed over 24 hours, by balancing various concentrations of the synthesized chemicals against varying concentrations of MgCl.sub.2 solutions, separated in a U-tube fixture by a HTI CTA FO (cellulose triacetate forward osmosis) semi-permeable membrane. An aqueous solution of 95% w/w of all these chemicals exhibited an osmotic potential of greater that 150 atms at 25? C., and drew water from both 20% and 18% MgCl.sub.2 solutions.

(7) The above phenomena of phase separation from water after gas absorption has important implications for practical use of these chemicals in both seawater desalination and CO.sub.2 absorption, and major advantages in energy consumption for regeneration of these solvents. Since the amine-terminated branched polymers phase separate from water after gas absorption, the water-rich portion can be removed by filtration, and only the polymer-rich portion needs to be heated up to desorb the absorbed acid gas. Typical temperatures for desorption of CO.sub.2 from these amine-terminated branched PEGs is around 60? C., thus efficiently regenerating the polymers for use in the next cycle of forward osmosis.

(8) Osmotic pressures were computed for several synthesized amine-terminated branched ethoxylates against various concentrations of MgCl.sub.2 solutions. The results are shown in Table 1 below.

(9) TABLE-US-00001 TABLE 1 Observed osmotic pressures of various synthesized chemicals Chemical name GE1000-3NH.sub.2, GE1200-3NH.sub.2, TMP470-3NH.sub.2, PET797-4NH.sub.2, Glycerol ethoxylate, Glycerol ethoxylate, Trimethylolpropane pentaerythritol, amine-terminated amine-terminated amine-terminated amine-terminated Osmotic pressure 270 atms 285 atms 175 atms 245 atms of 95% aqueous solution

(10) The high osmotic pressures exhibited by the synthesized amine-terminated branched ethoxylates can be advantageously used as FO draw solutes, and the additional property of high CO.sub.2 absorption (and phase separation from water on CO.sub.2 absorption) can be used for both water treatment and flue gas treatment at power generation facilities, especially coal-fired power-plants and steam-assisted power generation, as well as in chemical plants where treated water is needed, along with CO.sub.2 removal from process gas streams. An example of a process flow diagram, utilizing both forward osmosis and CO.sub.2 sequestration, is shown in FIG. 1, described further below.

(11) Referring to embodiment 10 of FIG. 1, for example, fluid 12 containing a relatively high amount of salts, such as raw seawater or saline water needed for steam generation or cooling purposes for power-plants, is pretreated by either flocculation or ultra-filtration 14 to remove suspended solids and organic material. The pretreated water 16 is now sent to a forward Osmosis (FO) system 18, wherein pure water permeates through a semi-permeable membrane in the FO system, due to the large difference in osmotic pressure of the incoming water (typically 15-30 atms), as compared to the high osmotic pressure (around 150-275 atms) of a draw solution on the other side of the semi-permeable membrane in the FO system. The retentate from the FO system 20 is a hypersaline solution of salts and other dissolved materials in water, and can be sent to a salt farm for recovery of salt, or disposed of accordingly in an environmentally acceptable manner. The draw solution 24 can be one of many known draw solutions, including those described in U.S. Pat. No. 8,021,553 to Iyer, which is incorporated in its entirety herein by reference. Embodiments of the present invention, however, comprise preferably a 95% solution of amine-terminated branched ethoxylate polymers in water, which exhibit very high osmotic pressures.

(12) The concentrated draw solution 24 enters as the feed to the FO system 18, and due to extraction of the water from the raw/saline water 16, exits as a diluted draw solution 26 from the FO system. The diluted draw solution is now exposed to a flue gas stream 28 containing CO.sub.2 as a major constituent in a gas-liquid mixer 30. The absorption of CO.sub.2 by the amine-terminated branched ethoxylate polymers in the diluted solution causes phase separation of the polymer from water, enabling separation of the polymer, in a concentrated form, from its water mixture in a downstream liquid-liquid separator. The mixed amine-terminated branched ethoxylate polymers with absorbed CO.sub.2 is sent to the liquid-liquid separator 36 where a water-rich mixture 38 is separated from the concentrated amine-terminated branched ethoxylate polymers with absorbed CO.sub.2 40. The water rich mixture presumably contains some small amount of solvent, so is preferably directed to a reverse osmosis or nano-filtration module 42 to separate the water from any remaining solvent 44, which is then directed as concentrated solvent 24 to the FO module 18.

(13) At the same time, concentrated amine-terminated branched ethoxylate polymers with absorbed CO.sub.2 40 is sent to a heat exchanger 46, wherein hot flue gas 48 transfers heat to the incoming polymeric solution, thereby desorbing/liberating CO.sub.2 from the polymer. The desorbed CO.sub.2 50 liberated from the amine-terminated branched ethoxylate polymers can be collected or sequestered for various industrial applications. The regenerated (now CO.sub.2-free) concentrated polymer 52 is then used for the next cycle of water purification in the FO system 18 and further absorption of CO.sub.2 from flue gas exhaust. The fresh water 54 generated from the system 10 can be used for steam generation or for industrial cooling needs at the power-plant facility. The cleaned flue gas 58, now substantially free of carbon dioxide, can be vented from the mixer 30 to the atmosphere in an environmentally friendly manner.

(14) Persons of ordinary skill in the art may appreciate that numerous design configurations may be possible to enjoy the functional benefits of the inventive systems. Thus, given the wide variety of configurations and arrangements of embodiments of the present invention the scope of the invention is reflected by the breadth of the claims below rather than narrowed by the embodiments described above.