The manufacture and evaluation of ceramic membranes

Ven, Nelke C. Van de (Nelke Cornelia) (2001-12)

Thesis (MScEng)--University of Stellenbosch, 2001.

Thesis

ENGLISH ABSTRACT: Inorganic membranes offer numerous advantages, such as stability at high temperatures and a long lifetime. Two Russian professors invented and patented a method for manufacturing a tubular ceramic membrane [Linkov & Belyakov, 1996]. Their membranes were used in the water purification industry and in gas separation research at the University of Stellenbosch [Keuier, 2000], but the performance of the membranes have been reported to be inconsistent. This project investigates the manufacturing process for these membranes in an attempt to improve their inconsistent performance. It also provides useful insight into relevant methods of evaluation. Thirty-two cerarruc membranes were successfully manufactured according to the patent by Linkov and Belyakov [1996], with slight modifications to the original manufacturing process. It was found for example that, to obtain membranes with a thickness of 1 mm, the casting suspension should contain 1.85 grams of water per gram of oxides, instead of the 1.15 grams of water per gram of oxides according to Linkov and Belyakov [1996]. The quality of the gypsum mould and the drying of the green body were found to be the most difficult steps in the manufacturing process. Gas permeabilities for the manufactured membranes were typically lxIO-5 mol/misl'a for argon and nitrogen and 4.SxIO-5 mol/rrr'sl'a for hydrogen. Water permeabilities for the manufactured membranes were typically 600 Vm2hbar. Gas permeability coefficients for the manufactured membranes were typically SxIO-15 m2 for nitrogen and argon and 8xIO-15 m2 for hydrogen. The water permeability coefficients were typically 1.7xl 0-15 m2 . The gas and water permeabilities for the manufactured membranes were typically 5 and 10 times higher than the permeability values for membranes manufactured by Linkov. Gas and liquid permeability coefficients for the manufactured membranes, taking into account the thickness of the membranes, were 7 and 14 times higher than those achieved with Linkov's membranes. Linkov's membranes were on average thinner than the manufactured membranes, while the permeability of the manufactured membranes was higher, explaining the high permeability coefficients. The nitrogen and argon permeabilities, as well as their permeability coefficients were found to increase linearly with increasing pressure difference. However, the hydrogen permeability and permeability coefficients as well as the water permeability coefficients, were pressure independent. The gas permeability results also indicated that the permeability of the manufactured membranes increased with increasing sintering time and temperature. Combining the gas permeability results with the selectivity results, manufactured membranes with higher gas permeability had the same selectivity as Linkov's membrane. Therefore the manufactured membranes had a higher capacity than Linkov's membranes, with the same selectivity. For the manufactured membranes, however, a lower mechanical strength was reported, typically 9 N/mm2 , compared to the mechanical strength of Linkov's membrane which was roughly 10 times higher. To further improve the membranes, a number of options can be investigated: • The influence of a higher zirconia content on the ceramic membranes (between 29W'1o and 36.6W'1o for best mechanical strength). • Increasing of the sintering time and temperature for a more sintered and mechanically stronger membrane. • Examining the relationship between the permeability and mechanical strength of the membranes.

AFRIKAANSE OPSOMMING: Anorganiese membrane besit 'n aantal voordele, soos stabiliteit by hoë temperatuur toepassings en 'n langer leeftyd. Twee Russiese professore het 'n vervaardigingsmethode vir buisvormige keramiekmembrane uitgevind en gepatenteer [Linkovand Belyakov, 1996]. Hulle membrane was gebruik in die watersuiwerings industrie sowel as in 'n gasskeidings ondersoek by die Universiteit van Stellenbosch [Keuier, 2000], maar die prestasie van die membrane was wisselvallig. Hierdie projek ondersoek die vervaardigings metode vir hierdie kerarniekmembrane in 'n poging die wisselvallige prestasie van die membrane te verbeter. Die projek gee ook bruikbare insig in relevante toetsmetodes vir die vervaardigde membrane. Twee en dertig keramiekmembrane was suksesvol vervaardig volgens die patent van Linkoven Belyakov [1996], met klein veranderings. Dit was byvoorbeeld gevind dat, om membrane te vervaardig met 'n dikte van 1 mm, die gietsuspensie 1.85 gram water per gram oxide moet bevat in plaas van een gram water per een gram oxides soos volgens Linkoven Belyakov [1996]. Die kwaliteit van die gips vorm en die droog van die groen liggaam was die moeilikste beheerbare stappe in die vervaardigingsproses. Die gasdeurlaatbaarheid van die vervaardigde membrane was tipies Ix 10-5 mollm2sPa vir argon en stikstof en 4.5xl0-s mol/m'sl'a vir waterstof Waterdeurlaatbaarheid van die vervaardigde membrane was 600 l/rrr'hbar. Gasdeurlaatbaarheidskoeffisiente vir die vervaardigde membrane was tipies 5xl 0-15 m2 vir argon en stikstof en 8xl 0-15 m2 vir waterstof Die waterdeurdringbaarheidskoeffisiente was tipies 1.7xl 0-15 m2 . Die gas- en waterdeurlaatbaarheid vir die vervaardigde membrane was tipies 5 en 10 maal hoër as die deurlaatbaarheid waardes vir membrane wat deur Linkov vervaardig is. Gas- en waterdeurlaatbaarheidskoeffisiente vir die vervaardigde membrane, wat deur die dikte van die membrane beinvloed word, was 7 en 14 maal hoër as die bereik met Linkov se membrane. Linkov se membrane was gemiddeld dunner as die vervaardigde membrane, terwyl die deurlaatbaarheid van die vervaardigde membrane hoër was, wat die hoë deurlaatbaarheidskoeffisiente verklaar. Daar was gevind dat die argon- en stikstofdeurlaatbaarhede sowel as hulle deurlaatbaarheidskoeffisiente lineêr toeneem met toenemende drukverskil. Die waterstofdeurlaatbaarheid en -deurlaatbaarheidskoeffisiente sowel as die waterdeurlaatbaarheidskoeffisiente egter, was onafhanklik van die drukverskil. Die gasdeurlaatbaarheid resultate het ook aangedui dat die deurlaatbaarheid van die vervaardigde membrane toeneem met toenemende sintertyd en -ternperatuur. Wanneer die gasdeurlaatbaarheid resultate gekombineer word met die selektiwiteit resultate blyk dit dat, terwyl dat die vervaardigde membrane 'n hoër gasdeurlaatbaarheid het as Linkov se membrane, hulle nog steeds dieselfde selektiwiteit het. Dit beteken dat die kapasiteit van die vervaardigde membrane hoër is, terwyl die selektiwiteit van die skeidingsproses behou word. Vir die vervaardigde membrane was 'n laer meganiese sterkte gevind, tipies 9 N/mm2 , terwyl die meganiese sterkte van Linkov se membrane omtrend 10keer hoër was. Verskeie opsies kan ondersoek word om die membrane te verbeter: • Die invloed van 'n hoër zirkonia inhoud op die keramiekmembrane (tussen 29m% en 36.6m% vir beste meganiese sterkte). • Verlenging van die sintertyd en -temperatuur om 'n meer gesinterde en meganies sterker keramiekmembraan te verkry. • Ondersoek na die verhouding tussen deurlaatbaarheid en die sterkte van die membraan.

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