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some concept of Novel Separation Processes are Asymmetric Membrane, Centrifugal Separation Processes, Cloud Point Extraction, Colloidal Particles, Common Stationary Phase.Main points of this lecture are: Supercritical Fluid Extraction, Certain Temperature, Pressure Condition, Temperature History, Reduction, Surface Tension, Diffusivities Higher, Supercritical Solvents, Relatively Non-Toxic, Low Cost
Typology: Study notes
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Separation processes, membranes, electric field assisted separation, liquid membrane, cloud point extraction, electrophoretic separation, supercritical fluid extraction
At a certain temperature and pressure condition (critical condition), liquid and vapor phases of a substance become indistinguishable. A substance whose temperature and pressure are higher than its critical point is known as supercritical fluid (SCF). Fig. 10. shows typical pressure – temperature history of a substance.
Temperature
Pressure Solid Vapour
Liquid (^) Supercritical regime
Fig. 10.1: Typical pressure – temperature history of a substance Physical and thermal properties of SCFs are in between pure liquid and gas. Changes in properties are for a SCF are as follows: (i) Liquid like densities (ii) Reduction in surface tension (iii) Gas like viscosity (iv) Gas like compressibility properties (v) Diffusivities higher than liquids
It is good for removal or solubilize polar solutes, as it has a permanent dipole moment. N 2 O is better than CO 2 for extraction of polychlorinated dibenzodioxines from fly ash. Disadvantage: It is highly explosive
Disadvantage: (i) High Pc and Tc (Tc > 374^0 C, Pc > 221 bar) (ii) At these conditions, H 2 O is corrosive.
(i) Threshold pressure: Pressure at which miscibility of solute starts. Fig. 10. shows typical solubility curve of a material at a particular temperature.
Threshold Pressure
Pressure
Solubility(g/l) T = 45 (^0) C for CO 2
Fig. 10.2: Typical solubility curve of a material at a particular temperature. (ii) Pressure at which solute reaches its maximum solubility. (iii) Knowledge of physical properties of solutes (particularly, melting points). Solutes are dissolved better in liquid state.
Co-solvents are added to modify the polarity of the SCF, so that the power of SCF to solvate polar solutes increases. Ex: CO 2 should be mixed with 1-10% of methanol to solubilize more polar solutes.
(i) Simple expansion of SCF leads to lowering in solubility capacity of it. Thus, dissolved solutes are separated. (ii) They have liquid like density but superior mass transfer behaviour compared to liquids due to high diffusivity and low surface tension so that they can penetrate into the porous structure of solid matrix to release the solute.
There exists four mechanisms for solubilization of solutes in SCF: (i) If there is no interaction between solute and solid phase, the process is dissolution of solute in suitable solvent. (ii) If there is interaction between solute and solid, extraction is desorption. Adsorption isotherm of solute on the solid in presence of solvent determines the equilibrium. (iii) Swelling of solid phase by the solvent accompanied by extraction of entrapped solutes through the first two mechanisms. (iv) Reactive extraction.
Supercritical fluid (SF) is pumped through a pre heater into the vessel containing solid solute and the resultant solution is sent into a precipitation chamber by expansion through capillary or laser drilled nozzle. At precipitation chamber, pressure is much lower and solute solubility in SF is quite low and solute precipitates out of it. Size distribution and morphology of precipitated material is a function of pre-expansion concentration and its conditions. Pre-expansion concentration in term depends on SF, nature of solute, addition of co-solvent, operating pressure and temperature. Particle size is smaller and distribution is narrower if pre-expansion concentration is higher. Example: Naphthalene extraction process A typical naphthalene extractor is shown in Fig. 10.4.
Separator
Compressor
Pressure reduction valve
Fig. 10.4: A typical naphthalene extractor
The solubility diagram of naphthalene is shown in Fig. 10.5.
300 bar
120 bar 90 bar 70 ba r 20 30 40 Temperature (^0 C)
15
1.
15%
Concentrationof naphthalene(wt%) log scale
Fig. 10.5: Solubility diagram of naphthalene under supercritical condition
A naphthalene-chalk dust mixture is fed to the extraction vessel. Assume, extraction condition is 300 bar and 55^0 C. At this condition CO 2 SCF contains naphthalene dissolved at 15 wt%. Then it is expanded to 90 bar through pressure reduction valve (assuming isenthalpic expansion). After expansion its temperature is 36^0 C at 90 bar. Solubility of naphthalene is 2.5%. So, it falls out of solution. Precipitated naphthalene is collected and CO 2 is compressed to 55^0 C and 300 bar and recycled to extractor.
0.125 lb naphthalene extracted for 1lb CO 2 So, 0.125 lb naphthalene extracted for = 1000125 =8 lb CO 2 So, we require 8 lb CO 2 to recycle for extraction of 1 lb naphthalene. Energy required for compression from 90 to 300 bar is = 12.6 ×8 BTU = 100.8 BTU for extraction of 1 lb naphthalene
