EWT Water Technology

Membrane Degasser

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Summary:

Membrane degassing or membrane gas transfer is a membrane separation process for removal of dissolved gases from liquids, in water treatment especially for removal of carbon dioxide(CO₂) and oxygen (O₂) from water.

A membrane contactor is a device for membrane degassing, mainly consisting of an outer pressure vessel with inside hollow fibre membranes. Liquids flows through the pressure vessel and around the outside of the hollow fibre membranes. Depending upon the operating mode, there is either sweep gas flowing through the inside of the hollow fibre membranes, or they are being evacuated by vacuum pumps. Both will result in the gas diffusing from the liquid and through the hollow fibre membranes, to be either removed with the sweep gas or evacuated by the vacuum pumps.

Membrane gas transfer a continuous process. During operation, a small amount of waste water will need to be periodically discharged from the gas side, in addition to any cooling water requirements for the vacuum pumps.

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Design Example:

Membrane gas transfer plant for removal of CO₂ from water, combined air sweep and vacuum mode, two lines, with two membrane contactors arranged in series for each line.

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Applications:

Removal of CO₂ from →reverse osmosis permeate, for example upstream of an →EDI plant for demineralisation.
Removal of CO₂ from dealkalised water, for example as an alternative for a tower-type CO₂-degasser located between the cation exchanger and the anion exchanger unit of an →ion exchange demineralisation plant.

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Technical Data:

degassed water volume flow	available from approx. 0.5 up to 150 m³/h for each line
amount of lines	usually 1x 100% or 2x 50%
degassed water quality	free carbon dioxide	< 5 ... 10 mg/L CO₂, depending upon the process conditions
	oxygen	depending upon the plant design and process conditions
differential pressure at nominal load	approx. 0.2 ... 0.5 bar
operating pressure, absolute	≤ 7 bar on the liquid side approx. 75 mbar on the gas side, for CO₂-removal
operating temperature	5 ... 30 °C
sweep gas	• ambient air or compressed air, for CO₂-removal • nitrogen, for oxygen-removal
sweep gas consumption	depending upon the plant design and the process conditions
waste water discharge	approx. 0.5 ... 2.5 L/day, plus any required cooling water for the vacuum pumps
recommended raw water quality	turbidity	< 0.5 NTU
	total suspended solids (TSS)	< 1 mg/L
	Silt Density Index (SDI_{15 min})	< 3 %/min
	total alkaline earths (hardness)	< 0.1 mmol/L
	iron	< 0.3 mg/L Fe
	free chlorine	< 0.5 mg/L Cl₂
	surfactants	not detectable
	total organic carbon (TOC)	< 1 ... 3 mg/L C
	oil, grease	< 0.5 mg/L
total service life of the membrane contactors	usually approx. 3 ... 5 years
material options	membrane contactor	• polypropylene (PP) • stainless steel (1.4404)
	vacuum pump	• stainless steel (e.g. 1.4301, 1.4408)
	pipelines	• polyvinyl chloride (PVC) • polypropylene (PP) • polyvinylidene fluoride (PVDF) • stainless steel (e.g. 1.4541, 1.4571)
	valves	• polyvinyl chloride (PVC) • polypropylene (PP) • polyvinylidene fluoride (PVDF) • stainless steel (e.g. 1.4541, 1.4571)
	gaskets	• etyhlene propylene diene monomer rubber (EPDM) • polytetrafluorethylene (PTFE)
control options	start-up, service, and shut-down	• fully automated, via control circuitry • fully automated, via PLC

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Downloads:

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2018-05-05 • water treatment made in Germany • Company Information • Privacy