CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 63/004,278 filed on April 2, 2020, the disclosure of which is incorporated herein, in its entirety, by this reference.

BACKGROUND

There is a demand in industries (e.g., chlor-alkali and zero-liquid-discharge) for systems that concentrate salt brine to concentrations near saturation while not requiring as much energy as evaporation.

Salt concentration by reverse osmosis is low energy, but is limited in the salinity achievable. A method for membrane concentration of brines to high salinities was described in the U.S. Published Patent Application No. 20150014248 by Herron et.al, which is incorporated herein, in its entirety, by this reference. This method uses high pressure nanofiltration membranes, which allows a limited amount of salt to permeate the membranes, creating a saline permeate. Retentate with osmotic pressures well above the applied pressures are achievable because water flows through the membrane to the permeate until the difference in osmotic pressures between the feed and the permeate equals the applied pressure. By having a saline permeate and utilizing multiple passes for the saline permeate, water can be extracted from the feed at total dissolved solids (TDS) levels approaching saturation. The permeate will have lower TDS than the feed and can be dewatered by reverse osmosis.

High pressure, thin film composite, membranes generally have operational temperature limits below 40 °C. The high pressure nanofiltration process, since it recycles permeate, can add more than 10 °C if is used to concentrate brines to 260,000 ppm TDS. It would be highly advantageous in hot climates such as the middle east, to have membranes, which had stable performance at temperatures of 50 °C or higher.

SUMMARY

Embodiments disclosed herein are related to a method for producing a nanofiltration (NF) element having relatively stable performance at high temperature and pressure. The method includes producing an initial NF element with a membrane having a first permeability. The method also includes precompacting the initial NF element by subjecting the initial NF element including the membrane to a predetermined pressure at a predetemiined temperature thereby forming a final NF element with the membrane having a second permeability that is less permeable than the first permeability.

In an embodiment, a NF element having relatively stable performance at high temperature and pressure is described. The NF element includes a membrane precompacted at a predetermined pressure of about 60 bar to about 80 bar and at a predetermined temperature of at least 60 °C. The membrane has a permeability that is less than a permeability before the membrane was precompacted and is configured to operate stably at a temperature of about 50 °C or more.

Features from any of the disclosed embodiments may be used in combination with one another, without limitation. In addition, other features and advantages of the present disclosure will become apparent to those of ordinary skill in the art through consideration of the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The drawings illustrate several embodiments of the present disclosure, wherein identical reference numerals refer to identical or similar elements or features in different views or embodiments shown in the drawings.

FIG. 1 is a plot of a B value decline of nanofiltration membranes, according to an embodiment.

FIG. 2 is a log/log plat of the B value decline of the nanofiltration membranes, according to an embodiment.

DETAIFED DESCRIPTION OF VARIOUS EMBODIMENTS Embodiments disclosed herein are directed to a method of operating a high-pressure nanofiltration (NF) membrane so that it may be used at temperatures of 50 °C or higher with relatively stable performance. The performance of NF membranes in the embodiments disclosed herein is primarily dependent on the intrinsic salt permeability of the membrane. The issue being addressed is that the intrinsic salt permeability of commercial membranes decreases rapidly at temperatures approaching 50 °C. The proposed solution is based on the observation that the decline in salt permeability at elevated temperature is rapid at first, but levels off at a value somewhere near a third of the original flux. The proposed solution is to manufacture one or more membranes with far higher permeabilities than desired, then compact (e.g. , pre-compact) the one or more membranes at pressures of 60-80 bar and temperatures exceeding 60 °C. The one or more membranes will then have the desired permeability and will have reasonably stable performance at operation temperatures approaching 50 °C or higher. FIGS. 1 and 2 below demonstrate the feasibility of this stable performance at higher temperatures from experimental testing.

In some embodiments, the one or more membranes are compacted at pressures of about 55 bar to about 85 bar, about 55 bar to about 60 bar, about 60 bar to about 65 bar, about 65 bar to about 70 bar, about 70 bar to about 75 bar, about 75 bar to about 80 bar, about 80 bar to about 85 bar, at least about 60 bar, at least about 65 bar, at least about 70 bar, at least about 75 bar, or at least about 80 bar. In some embodiments, the one or more membranes are compacted at a temperature of at least about 45 °C, at least about 50 °C, at least about 55 °C, at least about 60 °C, at least about 65 °C, at least about 70 °C, at least about 75 °C, at least about 80 °C, at least about 85 °C, at least about 90 °C, at least about 95 °C, about 55 °C to about 95 °C, about 45 °C to about 50 °C, about 50 °C to about 55 °C, about 55 °C to about 65 °C, about 60 °C to about 65 °C, about 65 °C to about 70 °C, about 70 °C to about 75 °C, about 75 °C to about 80 °C, about 75 °C to about 80 °C, about 80 °C to about 85 °C, about 85 °C to about 90 °C, or about 90 °C to about 95 °C.

In some embodiments, the one or more membranes are compacted for a predetermined time of about 0.1 hour to about 75 hours, about 0.1 hour to about 10 hours, about 0.1 hour to about 5 hours, about 5 hours to about 10 hours, about 10 hours to about 15 hours, about 0.1 hour to about 2.5 hours, about 2.5 hours to about 5 hours, about 5 hours to about 7.5 hours, about 7.5 hours to about 10 hours, about 10 hours to about 12.5 hours, about 12.5 hours to about 15 hours, about 15 hours to about 25 hours, about 25 hours to about 35 hours, about 35 hours to about 45 hours, about 45 hours to about 55 hours, about 55 hours to about 65 hours, about 65 hours to about 75 hours, at least about 0.1 hours, at least about 1 hour, at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, at least about 6 hours, at least about 7 hours, at least about 8 hours, at least about 9 hours, at least about 10 hours, at least about 11 hours, at least about 12 hours, at least about 13 hours, at least about 14 hours, at least about 15 hours, at least about 20 hours, at least about 30 hours, at least about 40 hours, at least about 50 hours, at least about 60 hours, or at least about 70 hours. For example, in some embodiments, the membrane(s) may be compacted at 50 °C for as many as 50 hours.

Compacting of the one or more membranes may include positioning the one or more membranes in a housing and feeding a salt into the housing at a high pressure. The salt may be pumped enough to keep the salt from crystalizing and/or keeping the pressure constant. A large salt such as MgSC may be used to keep the flux low and the minimizing the pumping.

The performance of reverse osmosis (RO) and NF membranes are measured by two parameters: “A,” which is a measure of solution permeability of the membrane; and “B,” which is a measure of salt permeability. At high temperatures and pressures, the polymers in the membrane, particularly those in the thin rejection layer, begin to compact and degrade both the A and B value. The decay is typically logarithmic.

The A value of seawater RO membranes may permanently decline by as much as 80% when operated at temperatures over 45 °C and 60 bar. Reverse osmosis B values decline as well, however more slowly, which leads to a loss in rejection. This behavior makes the control of temperature critical for RO membranes.

Performance of high-pressure NF membranes used for concentration of salt brines is far more dependent on the membrane B value than the A value. NF membranes typically have far higher A values than RO membranes so that, during salt brine concentration, the flux of the membrane is primarily controlled by how quickly salt permeates the membrane. For example, if the feed concentration applied to the NF membrane is 120,000 TDS, sodium chloride and the applied pressure is 65 bar, the permeate concentration will be around 67,000 TDS. The membrane flux is determined by the rate salt permeates the membrane. Any salt permeating the membrane will bring water with it to keep the permeate at 67,000 TDS. As a result, the concentration of the feed increases, and by passing the feed through a series of membrane elements, high salinities can be achieved.

At high temperature and pressure, the logarithmic decline of the B value in NF membranes is rapid in the first weeks and decreases to a very gradual decline after several months. A plot of the B value decline for NF membranes is shown in FIG. 1 and a log/log plot of the decline is shown in FIG. 2. The declines for membranes with different initial B values are proportional, that is, at equal temperature and pressure. The salt permeability of nanofiltration membranes will decline by roughly similar percentages regardless of the initial permeability.

Systems and methods described herein may produce an initial membrane with a higher permeability than desired for the final membrane. Systems and methods described herein also may include a pre-compacting of the membrane before the final membrane is produced and offered for sale for use. Membranes produced according to this disclosure may have far more stable permeability over time. As an example, if the performance of elements 2 and 3 are compared in FIG. 2, and element 2 is pre-compacted for 10 hours while element 3 is pre-compacted for 0.1 hour, elements 2 and 3 will have substantially the same permeability. If they are both installed, the permeate flow of element 2 will decline from 0.4 gpm to 0.3 gpm after 100 hours, while the permeate flow of element 3 will decline from 0.4 gpm to 0.3 gpm in 1 hour. Further increasing both the initial permeability and the pre-compaction time will produce elements with even more stable performance.

As used herein, the term “about” or “substantially” refers to an allowable variance of the term modified by “about” or “substantially” by ±10% or ±5%. Further, the terms “less than,” “or less,” “greater than,” “more than,” or “or more” include, as an endpoint, the value that is modified by the terms “less than,” “or less,” “greater than,” “more than,” or “or more.”

While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting.