High purity water production

Details High purity water production High purity water production

1999-06-11

2001-07-10

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

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

C210S651000, C210S638000, C210S900000, C210S641000, C210S085000, C210S746000, C210S096100

Reexamination Certificate

active

06258278

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to water purification and more particularly it relates to an improved process for producing high purity water.
In using reverse osmosis, one problem is the scaling of the membrane resulting from deposition of materials such as calcium and magnesium compounds, for example, as well as other compounds and elements. Fouling of the membranes interferes with flux, greatly reducing the efficiency of the water purification system.
Another problem in producing high purity product water in a double pass reverse osmosis system is the difficulty in rejecting gases such as carbon dioxide and/or ammonia. Such gases pass through the membrane and re-establish an equilibrium in the permeate, adversely affecting product water resistivity.
The pH of feedwater to a double pass reverse osmosis is often controlled to a particular value to provide high resistivity water. However, a pH range of feedwater that produces high resistivity product water in one instance may not always produce high resistivity product water in another instance. That is, pH of feedwater to the first pass reverse osmosis has the problem that it does not always provide a control that produces high resistivity product water.
The presence of total alkalinity due mainly to bicarbonate, smaller amounts of carbonate, with small contributions by other ions and of carbon dioxide in the feedwater is responsible for significant changes in apparent rejection of salts and thus in the conductivity of product water from a double pass reverse osmosis system. As noted, reverse osmosis membranes are transparent to dissolved gases. Thus, CO
2
present in the feed side of the first pass membrane passes through the membrane to the interpass while bicarbonate and carbonate comprising total alkalinity is mostly rejected along with other anions and cations. This results in a change in the total alkalinity:CO
2
ratio, a loss of buffering capacity and causes a drop in pH from feed to interpass or permeate from the first membrane in a double pass reverse osmosis system. The same process is repeated from the interpass to the second pass product. The resulting change in the interpass pH can have the result of moving the interpass pH away from the pH which results in high resistivity product water from the second pass reverse osmosis unit. Thus, as noted, setting the pH of the feedwater to a fixed value for a double pass reverse osmosis does not always result in high resistivity product water.
In addition, when a particular pH is chosen for producing high resistivity water from a particular feedwater, changes in the feedwater composition, e.g., alkalinity, can render the chosen pH not optimum. Thus, lower quality product water results even though the feedwater has been maintained within a narrow pH range which was, at one time, thought to be optimum. Further, it will be appreciated that different membranes have the capacity to reject different ions to a lesser or greater extent. That is, some membranes reject anions better than cations and vice versa. When there is preferential rejection, there can be leakage of the other or opposite ion. pH of the feedwater has a large impact on the capacity of the particular membrane to reject the particular anion or cation. However, any membrane's performance can vary in a systematic way with pH to reach a peak value for rejection, and thereafter its performance declines on either side of an optimum pH.
This concept is illustrated in
FIG. 2
where A and B denote the highest resistivity for a given pH value. On either side of these points, resistivity declines.
FIG. 3
also illustrates that different pH values can result in the same quality product water. However, on either side of the pH value, product quality declines. “A” can represent TDS and “B” high alkalinity. Further, the process is complicated by membrane selection. The negatively charged membrane of Fluid Systems Inc., referred to by the tradename HRRX membrane, operates in a pH range of 6.5 to 8 with a 99.4% rejection, while Toray's positively charged membrane, having the designation SU910S, operates at a pH of 9 to 9.5 with a 99.5% rejection. The lower pH is better for removing ammonia and the higher pH is better for removing carbon dioxide. Thus, it will be seen that there is a great need for a process which can be operated in a way which avoids these problems.
Attempts at removing carbon dioxide to provide high resistivity water in the past have only been partially successful and often end up further contaminating the water. For example, U.S. Pat. No. 4,574,049 discloses a process for removing carbon dioxide and other impurities from a water supply using double pass reverse osmosis membranes. The process includes providing a first reverse osmosis until having an inlet, a product outlet and a brine outlet; providing a second reverse osmosis unit having an inlet, a product outlet and a brine outlet; locating the second reverse osmosis unit downstream of the first reverse osmosis unit with the product outlet of the first reverse osmosis unit being coupled to the inlet of second reverse osmosis unit; providing water to be purified to the inlet of first reverse osmosis unit; treating the product from the reverse osmosis unit at a location upstream of second reverse osmosis unit with a chemical treatment agent comprising a solution having a pH that exceeds 7 to reduce carbon dioxide concentration of the product by chemical conversion and to ionize certain otherwise difficult to remove chemicals; and directing the product from second reverse osmosis unit toward a point of use or storage for purified water.
However, this process which normally uses sodium hydroxide for increasing the pH results in the addition of sodium which, because of its small ionic radius, is difficult to remove by subsequent membranes. Further, the addition of sodium hydroxide has another disadvantage in that the series of reactions removing carbon dioxide are relatively slow when compared to reverse osmosis unit contact time. Thus, the effectiveness of the operation is limited by the sodium hydroxide reactions, and further, this process does not remove ammonia.
U.S. Pat. No. 5,338,456 discloses a water purification process for removing dissolved solids of the type that are normally present in a municipal or similar water supply. The process uses a forced draft decarbonator having an inlet and a product outlet, a vacuum degasifier having an inlet, a product outlet and a water level sensor, and a reverse osmosis unit having an inlet, a product outlet and a brine outlet. The vacuum degasifier is located downstream of the forced draft decarbonator with the product outlet of the forced draft decarbonator being coupled to the inlet of the vacuum degasifier. The reverse osmosis unit is located downstream of the vacuum degasifier with the product outlet of the vacuum degasifier being coupled to the inlet of the reverse osmosis unit. Water to be purified is provided to the inlet of the forced draft decarbonator at a predetermined rate. According to the invention, the rate at which water to be purified is a provided to the inlet of the forced draft decarbonator is a function of a predetermined water level in the vacuum degasifier.
Japanese Patent 4-22490 discloses a pre-stage reverse osmosis membrane module, a post-stage reverse osmosis membrane module and a hydrophobic porous membrane module, to which an aqueous alkali solution circulating line is attached in the permeate side. That is, Japanese Patent 4-22490 utilizes an alkali solution in the permeate side to remove dissolved carbon dioxide by chemical reaction. The hydrophobic porous membrane module is placed between the pre-stage module and the post-stage module and has pores capable of permeating only gases. An inert gas blowing pipe is installed to the alkali aqueous solution circulating line.
Japanese Patent 2-2802 discloses reverse osmosis separator membrane module and degassing membrane module arranged in treating water line in series. The degassing membrane is formed by a po

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