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Our number
P2000-027
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Date
August 2000
Oxygen Removal from Water by Two Innovative Membrane Techniques R. van der Vaart* (project leader, researcher), B. Hafkamp, P.J. Koele, M. Querreveld (technical assistants), A.E. Jansen (Head ofDepartment) TNO Institute of Environmental Sciences, Energy Research and Process Innovation, P.O. Box 342, 7300 AH Apeldoorn, The Netherlands Prof. V.V. Volkov (Head ofDepartment), V.l. Lebedeva (researcher) TopchiefInstitute ofPetrochemical Synthesis, Polymerie Membrane Laboratory, Leninski Prospect 29, Moscow 117912, Russia Prof. V.M. Gryaznov (Head ofDepartment) Russian University ofPeoples Friendship, Institute ofCatalysis and Ecology, MiklykhoMaklaya Str. 6, Moscow 117198, Russia *e-mail:
[email protected]
Abstract The present paper describes new membrane techniques to remove oxygen from water, e.g., in ultra-pure water or boiler feed water production. Preliminary work on the preparation of catalytic membrane hollow fibres is presented. Also, a technique that we called Trans-Membrane Chemo-Sorption, TMCS, is described. It turned out that a well-attached palladium active layer could be attached to the surface of hydrophobic porous hollow fibres. The resulting microporous membranes tumed out to be leak-proofto water and showed promising oxygen removal for the use in a hydrogen-water catalytic contactor module. The TMCS option has been compared to the known membrane stripping method. The mass transfer with the reductor that was tested was lower than that for stripping with nitrogen at low pressure, which resulted in higher investment costs. On the other hand, the operating costs for TMCS are lower. In principle, a costs advantage of about 45% should be possible. Currently, a pilot installation with a capacity of 50 litres/hour is running at a nuc1ear power plant. 1 Introduetion The removal of oxygen from water is important in several industrial sectors, e.g., in the microelectronics industry (rinse water) [1], in the electricity production sector (boiler feed water) [2, 3], in the brewing ofbeer (heavy brewing) [4, 5] and in the oil & gas industry (injection water) [6]. In the above examples, the oxygen needs to be removed to prevent oxidation of substrates, equipment or dissolved components or to prevent biological growth. Several methods are available for oxygen removal [7]. Classical methods involve the (thermal) vacuum stripping of water in a voluminous packed bed, often combined with the application of a stripping gas. Also, the addition of chemieals like hydrazine or sulphite to the water system is in use [8].
Our number
P2000-027
Page
3
Date
August 2000
Nederlandse Organisatie voor toegepast-natuurwetenschappelijk onderzoek I Netherlands Organisation for Applied Scientific Research
Drawbacks of these methods are high energy consumption, the harsh chemical nature ofhydrazine, the increase of chemieals concentration (conductivity) in the product-water in the case of sulphite addition, and the consumption of chemicals. More sophisticated methods are based on reduction of oxygen with hydrogen over a Palladium catalyst or stripping via a membrane contactor. These methods are intrinsically clean. A drawback ofthe hydrogen method is the difficult mixing/dissolving ofhydrogen in water, requiring voluminous equipment [3]. To improve the dissolution ofhydrogen, membrane contactors are in use. The catalytic phase may be present on ion exchange beads in between hollow membrane fibres [9] or in a separate subsequent contactor. The present work describes two new oxygen removal systems. One based on a catalytic reduetion with hydrogen (Catalytic Membrane Contactor) and the other based on reduction with a regenerative reducing liquid (Trans-Membrane Chemo-Sorption, TMCS). The Catalytic Membrane Contactor The CMC system is based on the integration ofthe catalyst phase with a hydrophobic microporous membrane that keeps the hydrogen and water separated, as shown in Figure 1. The catalyst is present at the interface where the reaction takes place on the membrane extemal surface. This system minimises mass transfer resistances, resulting in optimal performance and minimal required membrane surface area, amount of catalyst and amount ofhydrogen lost to the water. No pre-mixing ofhydrogen with the water stream is required, which results in a much more compact installation. Moreover, this type ofmembrane is based on cornmercially available and relatively low-cost hollow fibre membranes. Trans-Membrane Chemo-Sorption In TMCS, a reducing liquid is used to react with the oxygen present in the water. The reducing liquid is contacted with the water via a hydrophobic microporous membrane as shown in Figure 2. In this work complexed iron (U) was used as reductor. Subsequently, the oxidised component, iron (lIl), is regenerated to the reduced state in an electrochemical cello Ideally, in this way a closed chemieals circuit is obtained. No chemieals are lost to the water stream. Currently, this system is in a small-scale pilot test (50 litres/hour) in a nuclear power plant. 2 Experimental methods CMC The base membranes used, were Accurel polypropylene hollow fibers of Akzo-Nobel with a extemal fibre diameter of 1 mm, a wall thickness of 0.2 mm and a porosity of about 70% at a pore size of about 0.2 urn. The catalytic palladium phase was introduced to the fibres by a dipcoating technique with a precursor, followed by reduction with hydrogen.
Our number
P2000-027
Page
4
Date
August 2000
TMCS For the TMCS experiments, the set-up ofFigure 3 was used. The applied membrane module was the commercially available 2.5" LiquiCell module of Celgard with the X40 membrane type with a surface area of 1.4 m2. In the electrochemical regeneration unit, a Nafion membrane was used with a surface area of 100 cm". As anode liquid a 0.5 M NaN03 solution was used. The reductor liquid was a 0.1 M Fe(II) solution with a pH set to about 2.5 with sulfurie acid. The reaction of iron with oxygen is then: 4Fe2+ + O2 (aq) + 4H+ => 4Fe3+ + 2H20 The reaction at the cathode of the regenerator: Fe3+ + e" => Fe2+, possibly together with water splitting: 2H20 + 2e- => H2 + 20K At the anode water splitting occurs:
Since in the reductor liquid OK is produced and H+ is consumed, a pH-stat (50% H2S04) was used to prevent the pH from increasing and, consequently, precipitation ofFe(OH)z. Reference: stripping at low pressure The standard membrane degassing technique is stripping with oxygen-free gas (e.g., nitrogen or methane, depending on the application) at low pressure. To evaluate the TMCS option, stripping at low pressure is used as a reference. The laboratory set-up of Figure 4 was used. As a strip gas, nitrogen 4.5 was applied. One and the same membrane module was used for stripping and TMCS experiments. The pressure was set to 100 mbar during all experiments since the evaporation of water through the membrane at room temperature drastically increases at lower pressures. The strip gas was in counter current with the water flow.
3 Results and Discussion CMC Different loadings of a well-attached palladium layer could be produced on the porous polypropylene hollow fibres. After deposition ofthe palladium, the fibres remained strongly hydrophobic and were leak-proof and suitable for use as catalytic contactor between the hydrogen gas phase and water phase to be treated. The mass transfer for oxygen was measured to be in the order of 10-4 mis, which is promising. The amount of Palladium on this membrane was
