Apparatus and method for adding carbon dioxide gas to ultra pure water

An apparatus for adding carbon dioxide to ultra pure water, wherein the resistivity of the ultra pure water is controlled to have a desired value by bringing the carbon dioxide gas into contact with the ultra pure water via a gas permeable membrane so as to supply the carbon dioxide gas to the ultra pure water, the apparatus comprising:

• an inlet in fluid communication with a source of a fluctuating flow of untreated ultra pure water,
• means for adding carbon dioxide gas to the ultra pure water, the means comprising a membrane module having a gas permeable membrane, the membrane module supplying the carbon dioxide gas to the ultra pure water up to the saturation concentration or up to an approximately constant concentration near the saturation concentration at any flow rate in an expected fluctuation range, whereby the ultra pure water to which the carbon dioxide gas is added has a constant resistivity at any flow rate of the ultra pure water supplied, and
• means for combining the ultra pure water to which the carbon dioxide gas has been added and untreated ultra pure water, to which carbon dioxide gas has not been added, and for uniformly mixing them,
• whereby the ultra pure water to which the carbon dioxide gas has been added is diluted with the untreated ultra pure water so as to produce, after mixing, resistivity-controlled ultra pure water, which has the predetermined resistivity.

A process for adding carbon dioxide gas to ultra pure water to produce a resistivity-controlled ultra pure water at a rate corresponding to a fluctuating rate of ultra pure water used in a subsequent process, the process for adding carbon dioxide to ultra pure water comprising the steps of:

• dividing a fluctuating flow of untreated ultra pure water, which is supplied at a rate corresponding to a rate of resistivity-controlled ultra pure water used in the subsequent process, into two flows, one of which is larger than the other, with constant proportions, by a distributing device,
• supplying the smaller flow to a hollow-fiber membrane module, in which the smaller flow and the flow of carbon dioxide gas are separated by a hollow-fiber membrane, so as to produce a carbon-dioxide-added ultra pure water, in which ultra pure water contains carbon dioxide at the saturation concentration or at an approximately constant concentration near the saturation concentration at any flow rate in an expected fluctuation range,
• combining the carbon-dioxide-added ultra pure water with the larger flow of the untreated ultra pure water, and
• uniformly mixing the combined water to produce ultra pure water which has a predetermined controlled resistivity.