As stated in the process train justifications in Section 1 of this project the pervaporator unit was chosen to comply with the requirements of this section of the project; that there are enough unit operations for each member of the team to design in greater detail.
Due to this the pervaporator was decided to be added at the end of the process simply as a polishing step to remove extra ethanol prior it is discharged as wastewater in order to comply with an IPPC licence.
However using the predicted stream conditions calculated in Section 2, the pervaporator would be exposed to impracticable conditions thus resulting in an unrealistic design.
The completed material balance in Section 2 concluded that the pervaporator would be functioning with the following stream conditions shown in Table #.
It is clear that the pervaporator will be dealing with a minute amount of ethanol. This would be an extremely uneconomical and impractical process which would undoubtedly result in an implausible design.
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It was considered to have the pervaporator installed in the unlikely event that the distillation column (C-102) would fail in operation resulting in a much higher intake of material and more so ethanol. The feed conditions that would result from this scenario are shown in Table #.
In this situation the mass fraction of ethanol has increased significantly as the unreacted ethanol from the reactor (RE-101) is now no longer been recycled via the distillate stream of C-101. However, due to this column failure a third component is added to the unit operation.
After further investigation into the commercially available membranes from Sulzer Chemtech in Germany a leading producer in pervaporation technology, it was found that this component Diethyl Ether exceeds the specifications limits for the membranes designed to separate ethanol from water under there aprotic solvent limitations. An aprotic solvent is a solvent that cannot accept hydrogen bonds as the solvents oxygen element is only bonded to carbon elements thus incapable of hydrogen bonding (Loudon, 2002).
Figure - The chemical structure of Diethyl Ether (Chemspider.com, 2013).
Figure - Specification sheet for PERVAP®4101 polymeric membrane (Meintjes, 2011).
This limitation was found for all of the Sulzer Chemtech ethanol water separating membranes investigated with the appropriate information available with the lowest limitation been <0.1% and the highest at <1%. Thus, scenario 2 is therefore unfeasible as the feed specification in Table # has a 5% mass fraction of an aprotic solvent diethyl ether.
It was then decided to manipulate the initial scenario to create a more realistic situation. It was settled that the distillation column C-102 dropped in temperature and efficiently resulting in the pervaporater to be operated with a low concentration feed of ethanol at 5% mass fraction with the remainder of water. The initial overall flowrate of 1079 kg/hr is kept.
The membrane is the most important factor in any membrane separation process. The material chosen will ultimately determine the efficiency of the process as it has a direct bearing on the separation potential that can be achieved. The following membrane characteristics are essential to ensure an efficient and economical separation process (Seader, 2006).
Chemical and mechanical compatibility with the processing environment.
Stability (Long Life).
Ability to withstand large pressure differences across the membrane.
The selection of membrane material is determined by the application for which the membrane will be used.
Some of the materials used for pervaporation membranes are summarised in Figure#.
Figure - Membrane materials for pervaporation (Susanto, 2009)
As the membrane selected for the process will essentially dictate the entire design, time taken to investigate into and compare pervaporation membranes is crucial.
Preliminary Membrane Research
As demonstrated in Figure# there is several types of membranes available to separate almost any mixture. This however was quickly narrowed down to only a number of polymer materials that may be used.
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In this design the membrane will be required to separate water and ethanol therefore a hydrophilic membrane could be used that will preferential permeate the water from the binary mixture (Nunes, 2006). Or conversely, hydrophobic membranes that will preferential permeate the ethanol from the water.
Table Literature data relating to preferential sorption (Mulder, 2003).
Water / Methanol
Water / Ethanol
PVA, CA, PAN, PMM, Selemion , PDMS
Water / Propanol
Water / Butanol
Ethanol / 1,2 -dichloroethylene
PTFE / PVP
Ethanol / Chloroform
PTFE / PVP
Acetic Acid / 1,2 - dichloroethylene
In order to evaluate the optimum pervaporation membrane for this process, it is necessary to first define standard parameters which will allow a direct comparison between the membranes. When selecting a membrane there are two main performance parameters that need to be considered: The separation factor (selectivity) and the permeate flux (Chapman, 2008).
Separation factor (Selectivity)
The separation factor (of a membrane is a measure of the degree of preferentiality which the membrane shows for one component over the components of the system. The separation factor of a membrane critically affects the overall separation obtained and depends on the membrane material.
The separation factor is as follows (Huang, 1994)
i denotes the preferentially permeating component
j denotes the less permeating component
The permeate flux of a membrane is the mass passing per unit of time through a unit of membrane surface area. The permeate flux is as follows (Koros, 1996)
A= Membrane Area (
t = Time
To permit a direct comparison between the pervaportation membrane performance results it is critical that the influence parameters that dictate the membrane flux and the separation factor are as similar as possible.
Temperature influences the transport in a membrane in two ways, i.e. firstly by modifying the sorption-diffusion step inside the membrane and secondly by changing the activity driving force across the membrane (Lipnizki, 1999). Flux is strongly dependent on the feed temperature and the flux usually increases with an increase in temperature (Weyd, 2008). This is due to an increase in mobility of the permeating molecules and the effect on permeate fluxes because of the strong influence on the vapour pressures of the feed.
The permeation through a membrane is controlled by the amount of sorption into the membrane as well as the diffusion through the membrane. The amount of a component absorbed into a membrane decreases if the temperature increases, but the diffusion rate increases. This means that the flux may increase or decrease if the temperature is increased, depending on the importance of the absorption or diffusion as rate controlling process (Baker, 1995) The preferential species usually has a higher affinity for the membrane material than the other components and if the temperature is increased, the permeability of this specie will not increase as much because the decrease in absorption will have a larger influence than with the less permeable components. The diffusity of all the components will increase, however, thereby increasing the flux of the preferential permeating species as well as the non-preferential permeating species. The net effect of an increase in feed temperature will thus most likely be a decrease in membrane selectivity (Dubey, 2005).
The temperature dependence of permeate flux for pervaporation generally follows an
Arrhenius type relationship (Xie, 2011).
The apparent activation energy for permeation depends on both the activation energy for diffusion and heat of sorption (Xie, 2011).
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Figure - Temperature dependence of permeance flux (J) and selectivity () in the pervaporation of ethanol and water through a bacterial cellulose membrane (Dubey, 2005)
The feed composition affects the sorption of liquid into the membrane, membrane swelling, and diffusion of components through the membrane and therefore the flux and selectivity of the membrane. Permeation takes place when the permeating species is absorbed into and diffused through the membrane. As the feed concentration of the permeating species increases, the quantity of this component absorbed by the membrane also increases (as the affinity by the membrane for a certain component is more than for the other components) and the membrane takes on a swollen state (Baker, 1995)
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Figure Ethyl butyrate flux vs. ethyl butyrate feed concentration (She, 2006)
Concentration polymerisation occurs as a result of the less permeable component(s) of the mixture forming a boundary layer near the membrane surface area. The concentration of the faster permeating component in the feed mixture decreases, while the concentration of the slower permeating components increases on the membrane surface. This phenomenon lowers the selectivity of pervaporation and is most likely to occur in membranes with high permselectivity. In membranes with low permeation flux the concentration polymerization becomes insignificant (Feng and Huang, 1997; Smitha et al., 2004).
The thickness of a membrane in a specification document refers to its dry thickness. Since flux is inversely proportional to membrane thickness, thin membranes favours the overall flux but decrease selectivity (Villaluenga, 2005). Thin membranes are used for low swelling glassy membranes and thick membranes are used for high swelling elastomeric membranes to maintain the selectivity.
Figure - Inverse of the overall permeability of methanol and MTBE as a function of the reciprocal membrane thickness for PPO membranes (Villaluenga, 2005).
Permeate pressure is another important operating parameter as a high vacuum is directly related to a high energy cost. Theoretically, the maximum flux is achieved at zero absolute permeate pressure. Figure 6 shows the effect of permeate pressure on water flux. Generally, the water flux decreased as the permeate pressure is increased since there is a decrease of driving force for mass transport. For pervaporation processes, the driving force is provided by the vapour pressure difference between the feed and permeate side of the membrane. With increasing permeate pressure (i.e. decreasing vacuum), as the feed side pressure remains unchanged, the transmembrane vapour pressure difference is increased. This leads to a decreased driving force and consequently water flux.
The permeation rate of any feed component increases as its partial permeate pressure is lowered. The highest conceivable permeate pressure is the vapor pressure of the penetrant in the liquid feed.
The effect of this parameter on pervaporation performance is dictated by the magnitude of the vapor pressures encountered, and by the difference in vapor pressures between them
Available Experimental Results - Hydrophilic Membranes
Four commercially available hydrophilic membranes developed by Sulzer Chemtech in Germany PERVAP®2201, PERVAP®2211, PERVAP®4101 and PERVAP®4060 were screened for their efficiency in separating ethanol from water and ethanol mixtures (Meintjes, 2011).
The most suitable membrane for ethanol removal was selected on a basis of high flux and high ethanol selectivity. The result of this experiment is applicable to this design as the ethanol feed used is at a low concentration of 10wt% and the aim of the pervaportator unit to remove as much ethanol as possible from the wastewater stream.
All of the four membranes tested are crosslinked Polyvinylalcohol (PVA) membranes supported on a Polyacrylonitrile (PAN) support layer coated on a polymer fleece.
Figure - Comparison of the total flux at different ethanol concentrations for different pervaporation membranes (Meintjes, 2011).
As seen in Figure# the highest flux was achieved by PERVAP®2201 membrane when pure water was used while the flux of the PERVAP®4060 membrane was the highest when ethanol concentrations of 10 and 20wt% was used. It assumed that would also be the outcome at 5wt% ethanol.
The ethanol selectivity's of the membranes are shown in Figure# .
Figure - Comparison of the ethanol selectivity at different ethanol concentrations for different membranes (Meintjes, 2011).
It is clear that the ethanol selectivity of the PERVAP®4060 membrane is the highest. The ethanol selectivity of the other three membranes (PERVAP®2201, PERVAP®2211, and PERVAP®4101) are all very low and therefore are not a suitable option for this design.
All four membranes were also exposed to stability tests where the membranes all remained stable at different ethanol conditions, ranging from 30 to 100wt% ethanol submerged for a minimum of 48 hours (Meintjes, 2011).
it was decided to use Sulzer Chemtechs PERVAP®4060 membrane for the pervaporator unit as it proved to have the highest ethanol flux and selectivity of all the membranes tested at the approximate proposed ethanol concentration.