Membrane Technology Lowers Boiler Maintenance Costs
An alternative deaeration technology can reduce blowdown and chemical treatment expenses.
Highlights
- Membrane deaerators offer a compact, high-efficiency alternative to conventional steam deaerators and chemical treatment by removing dissolved oxygen inline under pressure with microporous hollow fiber membranes.
- They can deliver meaningful operating savings, including more than $8,500 annually in one example, driven by 94% lower chemical costs, 47% lower blowdown water costs, 47% energy savings, and payback in under two years.
- They also improve energy recovery opportunities, especially in refinery applications, because cooler feedwater entering the membrane deaerator can absorb more waste heat, increasing heat recovery and boosting steam production.
Boiler feedwater must be kept free of dissolved oxygen or else corrosion problems occur, leading to lower efficiency or even downtime. Conventional methods to remove these gases include high-temperature, forced-draft steam deaerators and chemical treatment.
An alternative to these methods is low-temperature membrane deaerators, which use microporous hollow fiber membranes with 10 times the surface area compared to mechanical technologies. Membrane deaerators are thus highly efficient, compact and can be used inline under pressure.
How it works
Membrane deaerators, sometimes called membrane contactors, are commonly used for gas transfer with water or with fluids having a similar surface tension to water. One technology utilizes a patented radial flow design and is available in a variety of sizes for flow rates ranging from 0.1 to 91 cubic meters per hour. Larger flow rates are easily handled by piping multiple deaerators in parallel.
The liquid and gas phases come in direct contact with each other but because the membrane is hydrophobic, boiler water won’t pass through it. Dissolved gases, however, can easily pass through the membrane. To obtain maximum gas transfer efficiency, a central baffle forces the liquid to flow radially over the membrane, increasing the surface contact. Dissolved oxygen in the water travels into the hollow membrane and is carried away.
What are the savings?
For a boiler operating at 10,000 lb/hr and 50 psig for 275 days per year with a feedwater temperature of 60°F, the membrane deaerator system can save more than $8,500 annually compared to a chemical treatment system with a blowdown of 10%. This includes:
- Chemical cost savings of 94%
- Blowdown water cost savings of 47% (blowdown is reduced to 5%)
- Energy savings of 47%
The payback for the upgrade in this situation would be less than two years.
Energy recovery
Lower-temperature boiler feed water generated by membrane deaerators can also be used for energy recovery at petroleum refineries. In a hydrocracking unit, hydrogen feed gas is produced by steam-methane reforming. The resulting hydrogen stream contains many impurities, including carbon monoxide, carbon dioxide and water vapor.
The process of removing these impurities releases a lot of heat, which can be used to preheat boiler feedwater. If boiler feedwater and the hot hydrogen steam are sent through a heat exchanger, the feedwater is preheated by the hydrogen stream and the hydrogen impurities are removed along with hydrogen stream condensate.
The temperature of the feedwater entering the membrane deaerator is much cooler (122°F versus 257°F) than in a steam deaerator, therefore it has a higher capacity to absorb heat. Significantly more heat is recovered from the much hotter hydrogen stream, which boosts steam production. So steam deaeration can be replaced with membrane deaeration. This same substitution can be used in combination with a gas turbine to achieve similar savings in other refinery applications.
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