Dehydration of hydrazine hydrate by pervaporation using commercially available polymer membranes
Date
2018-03
Authors
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH SUMMARY: Hydrazine (N2H4) is a valuable, Commercial, inorganic compound that is characterised
as a small, reactive molecule with good reducing properties. In its anhydrous form, hydrazine is used in the medical field and in space applications for the adjustment of altitude in orbital satellites (Schliebs, 1985). Commercially produced hydrazine hydrate solutions can be partially dehydrated by fractional distillation to provide a constant boiling mixture or azeotrope of about 71.5 wt. % hydrazine (Sunitha et al.,
2011). A possible alternative dehydration technique is the use of fractional distillation in combination with pervaporation (Dutta et al., 1996). Ravindra et al. (1999a) and
Satyanarayana and Bhattacharya (2004) are among a limited number of authors that investigated pervaporation of hydrazine monohydrate systems. The main aim of these
initial studies was to characterise the system and develop a laboratory synthesised membrane with optimal water selectivity and acceptable mass flux. The main aim of this study was to investigate the dehydration of hydrazine
monohydrate (36 wt. % water) by pervaporation using commercially available polymeric membranes. Three objectives were set in this study;
1. Screening of commercially available polymeric membranes for the hydrazinewater.
2. Characterising the best performing membrane (Pervap™ 4101) in terms of
sorption and pervaporation performance at various concentration (36 to 100
wt. % water) and temperature ranging from 30 to 60 °C.
3. Modelling the separation performance of the best performing membrane
(Pervap™ 4101) and comparing the performance with data from two literature
sources namely: Ravindra et al. (1999c) and Sunitha et al. (2011).
A standard experimental procedure was used to select and screen eight commercial membranes (Pervap™ 4060, 4100, 4101, 4102, POL-AL-M2, POL-OL-M1 and PEBA and PDMS) through visual and mechanical stability tests, contact angle
characterisation and pervaporation performance screening tests. A stable membrane that showed the highest pervaporation screening results (Pervap™ 4101) was characterised in terms of sorption and pervaporation selectivity, membrane swelling and total flux. Visual screening tests using hydrazine monohydrate at room temperature (25 °C) showed that the Pervap™ 4060, 4101 and 4102 membranes had no visual interaction with hydrazine. The visual observations were confirmed by mechanical stability tests that showed that Pervap™ 4101 and 4102 membranes had the smallest deviation in tensile strength after exposure to hydrazine. Commercial Pervap™ 4101 does, however, not compare to literature results obtained by Ravindra et al. (1999c) and Sunitha et al. (2011). Satyanarayana and Bhattacharya (2004) state that any membrane with a water
selectivity higher than 1.4 would be able to break the water-hydrazine azeotrope. Further pervaporation screening tests were conducted at 50 °C and 36 wt. % water which resulted in water selectivities as high as 1.6 for Pervap™ 4101 and 4102 membranes, with the former having the higher flux rate of 0.5 kg·m-2·h-1. Both membranes are theoretically able to successfully break the azeotrope, but the Pervap™ 4101 membrane was selected for further characterisation due to the higher flux and pervaporation performance index (PSI) obtained. Sorption studies on Pervap™ 4101 membranes at 50 °C and feed concentrations between 36 and 100 wt. % water revealed that membrane swelling was as high as
80 %. Both the sorption and pervaporation mechanisms are water selective, but higher diffusion water selectivity’s indicate that the separation process is diffusion controlled.
Sorption tests further confirmed that the sorption process is independent of temperature. An increase in pervaporation water feed concentration, from 36 wt. % water, decreased the experimental flux from 0.48 to 0.1 kg·m-2·h-1 and increased the water selectivity from 1.6 to 20, while an increase in temperature increased the flux and
decreased the water selectivity. The membrane transport was modelled in terms of the solution diffusion model with
the sorption step described with the Flory-Huggins theory and the diffusion step with Fick’s first law. A water-hydrazine interaction parameter of -2.05 was calculated for hydrazine monohydrate (36 wt. % water) and 50 °C that suggests a strong affinity between water and hydrazine. A oncentration independent interaction parameter between hydrazine-polymer (3.68) that is lower than water-polymer (5.47) confirms the preferential hydrazine to water sorption results.
The concentration dependent interaction parameter as proposed by Long (1965) describe the experimental partial fluxes of this study well (R2 > 0.9896). It also describes the reference pervaporation experimental data by Ravindra et al. (1999c) and Sunitha et al. (2011).
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar
Description
Thesis (MEng)--Stellenbosch University, 2018.
Keywords
Perforation, UCTD, Hydrazine hydrate, Membrane separation, Distallation, fractional, Polymers, Dehydration, Azeotrope