1. By: Maryam Tangestanifard Supervisor: Prof. H. S. Ghaziaskar
Supercritical Fluids as Solvent in
Chemical Synthesis
By: Maryam Tangestanifard Supervisor: Prof. H. S. Ghaziaskar

2. Definition of a Supercritical Fluid
Definition by IUPAC
A mixture or element:
Above its critical pressure (Pc)
Above its critical temperature (Tc)
Below its condensation pressure
The critical point represents the highest T and P at which the substance can exist as a vapour and liquid in equilibrium

3. Supercritical Fluids
Near-critical region: region extends all around the critical point, but nonsupercritical section only
subcritical liquids: Liquid phases at temperatures below but not too far below Tc
subcritical gases: subcritical gases are those at pressures below Pc

4. Characteristics of a Supercritical Fluid
Dense gas
Densities similar to liquids
Occupies entire volume available
Solubilities approaching liquid phase
Dissolve materials into their components
Completely miscible with gases (N2/ H2)
Diffusivities approaching gas phase
Viscosities nearer to gas
Diffusivity much higher than a liquid
Density, viscosity, diffusivity and solvent power dependent on temperature and pressure

5. Advantages of SCF
Energy cost due to elevated pressures and temperatures
More expensive than traditional solvent systems
Safety hazards related to high pressure and temperature
Using the fluids must have some real advantage
Advantages fall into four categories
Environmental benefits
Health and safety benefits
Process benefits
Chemical benefits

6. Health, Safety and Environment Benefits
Replaces “less green” liquid organic solvents
No acute toxicity (H2O and CO2)
No liquid wastes (except water)
Non-carcinogenic (except C6H6)
Non toxic (except NH3)
Non-flammable (CO2, H2O)

7. Chemical Benefits High reaction rate due to:
Dissolving capabilities
High concentration of reactant gases ( H2 / O2 )
Eliminating inter-phase transport limitations
Higher diffusivities than liquids
Better heat transfer than gases
Variable dielectric constant (polar SCF)
Adjustable solvent power
Enhanced catalytic activity due to anti-coking of scCO2
Higher solubilites than corresponding gases for heavy organics
Improved catalyst lifetime
High product selectivities
Increased pressure may favour desired product selectivity

8. Chemical Benefits High reaction rate due to:
Dissolving capabilities
High concentration of reactant gases ( H2 / O2 )
Eliminating inter-phase transport limitations
Higher diffusivities than liquids
Better heat transfer than gases
Variable dielectric constant (polar SCF)
Adjustable solvent power
Enhanced catalytic activity due to anti-coking of scCO2
Higher solubilites than corresponding gases for heavy organics
Improved catalyst lifetime
High product selectivities
Increased pressure may favour desired product selectivity

9. Applications Catalyzed reactions Alkylation Amination Cracking
Gasification
Esterification
Fischer-Tropsch Synthesis
Hydrogenation
Isomerization
Oxidation
Polymerization

10. High heat of combustion
Why Hydrogen?
High heat of combustion
per unit weight
No pollution
Recyclable
U.S. Energy Information Administration foresees a 56% increase in the world energy demand in the following 30 years.
CO2 emissions already increased from to billion metric tons from to 2014, and in 2040 it is expected to reach 45.5 billion metric tons

11. H2 Utilization
Fuel cell vehicle application

12. Electrolysis with renewable energy
Hydrogen Production
Bio-hydrogen
(Green algae)
Electrolysis with renewable energy
High-Temperature
water splitting

13. Natural Gas Gasification
Steam Reforming
CH4 + H2O → CO + 3H2 (Ni Catalyst)
H2/CO = 3
Endothermic
Favored for small scale operations
Partial Oxidation
CH4 + ½O2 → CO + 2H2
H2/CO ≈ 1.70
Exothermic
Favored for large scale applications
Autothermal Reforming
A combination of Steam Reforming and Partial Oxidation

14. Why Biomass to hydrogen?
Biomass has the potential to accelerate the realization of hydrogen as a major fuel of the future.
Biomass is renewable, consumes atmospheric CO2 during growth and is a CO2 neutral resource in life cycle.
It can have a small net CO2 impact compared to fossil fuels.

15. What is Biomass Gasification?
Conversion of solid fuels into combustible gas mixture called producer gas (CO + H2 + CH4)

16. Gasification of Wet Biomass
Conventional thermal gasification
High temperature (900°C)
Drying required
Gasification
Anaerobic digestion
Slow reaction rates
fermentation sludge and wastewater from the reactors
supercritical water gasification
Supercritical conditions
No drying required

17. Hydrothermal Conversion of Biomass
The term hydrothermal refers to an aqueous system at temperatures and
pressures near or above the critical point of water.
Hydrothermal conversion of biomass can be classified as:
Carbonization
Oxidation
liquefaction
gasification

18. Comparison of SCWG with the Other Biomass Conversion Routes
Total efficiency of heat utilization processes versus biomass moisture content

19. SCW as a Solvent Solubility of limits of various salts at 25 MPa
Benzene solubility in high-pressure water

20. SCW as a Reactant Water as a reactant
Contributions in hydrolysis reaction
Resource of hydrogen
Resource of radicals

21. SCW as a Catalyst Acid catalyzed reactions of tert-BuOH in SCW
Disproportionation of benzaldehyde in SCW

22. Catalytic Supercritical Water Casification

23. Oxidation in scH2O (SCWO)
Oxidation reaction

24. Oxidation in scH2O (SCWO)
SCWO of organic wastes
Complete oxidation to CO2
Complete miscibility of nonpolar organic with scH2O
Single fluid phase
Faster reaction rates
With or without heterogeneous catalyst
Motivation for catalyst:
Reduce energy and processing costs
Target:
Complete conversion at low temperatures and short residence time
Supercritical water is much used for the total oxidation of organic wastes to co2.
This technologi provides an alternative to more established waste and wastewater treatment methods such as incineration and wet air oxidation.
The co2 yields and selectivitys from catalytic scwo were much higher than those from gas-phase catalytic oxidation.
The use of water gives a faster reaction rate compared to the conventional methods

25. Fischer-Tropsch SynthesisX G L
Coal
Biomass
Natural Gas
Gasification
Syngas
Processing
Fuel
&
Chemicals
Fischer-
Tropsch
Synthesis
Syncrude
Refining &
Upgrading

26. strong hydrogenating catalyst
Main Reactions
Alkane formation favored by H2/CO
nCO + (2n+1)H CnH2n+2 + nH2O
strong hydrogenating catalyst
Alkene formation
nCO + 2n H CnH2n + nH2O favored by H2/CO
Water-gas-shite reaction
CO + H2O CO2 + H2
WGS activity is high in Fe catalyst and low in Co or Ru catalyst
Helpful to adjust H2/CO ratio

27. Technology/ Reactors Fluidized Bed Reactors Fixed Bed Reactors
Better temperature control
High yields for Gasoline and light products
Fixed Bed Reactors
Originally used
Challenges associated with removal of heat

28. Technology/ Reactors Slurry Reactors
Small catalyst particles suspended in a liquid with low vapor pressure
Low Temperature
Flexible design
High yield for waxes

29. Comparison of Supercritical FTS with Conventional FTS
Catalytic activity
Increased catalyst pore accessibility in SC-FTS
Enhanced product desorption
In situ extraction of heavy products
Heat transfer
Enhance thermal conductivity leading to improved heat transfer
Maximum temperature difference along the reactor in the supercritical phase was around 5 °C compared to 15 °C in the gas phase
Catalyst stability
Fast removal of wax from catalyst pores
Temperature distribution in supercritical fixed bed reactor
Product selectivity
Hydrocarbons distribution
CH4 and CO2 selectivity
Olefin distribution

30. Product Selectivity in SC-FTS
Hydrocarbons distribution
Low H2/CO ratio leading to lower methane selectivity and higher chain propagation
More free and active sites for re-adsorption and enhanced chain growth
High residence time in the GP-FTS can lead to lower chain growth
CH4 and CO2 selectivity
In the SC-FTS, there is a decrease in methane selectivity even as syngas conversion increases
Local overheating of the catalyst surface in the GP-FTS
Diffusivity of hydrogen is higher than that of CO in the GP-FTS
Olefin distribution

31. Supercritical Fischer- Tropsch synthesis
Different CO-conversions due to different rates of diffusion
DGASS > DSCF > Dliquid
Different Chain growth probabilities due to CO:H2 diffusion
Similar SCF and gas diffusion inside the catalyst pore
Effective molar diffusion in the supercritical phase
By looking at the results in the table one can se that the CO conversion in the scf is inbetween the conversion in gas and liquid phase, but it is much closer to the conversion in gas phase.
The product effluent is higher for scf than for both gas-phase and liquid phase reactions.
The chain growth possibility is higher for gas-phase and scf than for the liquid phase. This is probably due to the different CO:H2 diffusivities in the different phases.
Similar diffusion in scf and gas inside the catalyst pores

32. Distribution of Hydrocarbon Products
Carbon Number
The alkene content decreased with increased carbon number for all phases
Increase in hydrogenation rate relative to diffusion rate
Longer residence time on catalyst surface for high molecular weight hydrocarbons
Higher alkene content in SCF
Alkenes were quickly extracted and transported by SCF out of the catalyst
Minimizing re-adsorption and hydrogenation

33. Wax Production Addition of Heavy Alkene to the Supercritical Phase
synthesis of waxy hydrocarbons through FTS reaction
- catalyst bed and reactor blockage by wax
- Selective synthesis of waxy hydrocarbons not easy
Studied the effect of addition
of heavy alkenes
Addition: 4 mol% (based on CO)
1-tetradecene and 1-hexadecene
Catalyst: Co-La/SiO2
Temperature: 220°C
Pressure: 35 bar
Supercritical fluid:n-pentane (Tc=196.6°C, Pc=33.7 bar)
p(CO+H2) = 10 bar

34. Wax Production Addition of Heavy Alkene to the Supercritical Phase
Carbon chain growth accelerated by addition of alkenes
Alkenes diffuse inside the catalyst pores to reach the metal sites
Adsorb as alkyl radicals to initiate carbon chain growth
suppression of methane formation, high CO conversion and low C02 selectivity
The resulting chains are indistinguishable from chains formed from synthesis gas
Addition of heavy alkenes does not have any effect in gas phase reactions

35. Conclusions Control of phase behaviour
SCF (used as solvent or reactant) provides opportunities to enhance and control heterogeneous catalytic reactions:
Control of phase behaviour
Elimination of gas/liquid and liquid/liquid mass transfer resistance
Enhanced diffusion rate in reactions
Enhanced heat transfer
Easier product separation
Improved catalyst lifetime
Tunability of solvents by pressure and cosolvents
Pressure effect on rate constants
Control of selectivity by solvent- reactant interaction

36. Conclusions
Compared with the conventional methods SCWG process is characterized by:
its high reaction efficiency and H2 selectivity
and can be fed by high moisture content material .
If the good cost performance can be provided, SCWG process holds great potential to be large-scale commercialized in the future.
In the SC-FTS the overall product distribution shifts towards heavier products compared to GP-FTS.
The olefin content in supercritical media exceeds those in other
reaction phases
The CO2 yields and selectivitys from catalytic SCWO were much higher than those from gas-phase catalytic oxidation.

37. References
Yakaboylu, O., Harinck, J., Smit , K. G., Jong,W., Review of Supercritical Water Gasification of Biomass: A Literature and Technology Overview. Energies 2015, 8, 859.
Matsumura, Y., Minowa, T., Potic, B., Review of Biomass gasification in near- and super-critical water: Status and prospects. Biombioe. 2005, 29, 269.
Guo, Y., Wang, S. Z., Xu, D. H., Gong, Y. M., Ma, H. H., Tang, X.Y., Review of catalytic supercritical water gasification for hydrogen production from biomass. Renewable and Sustainable Energy Reviews 2010, 14, 334.
Mogalicherla, A. K ., Elbashir,N. O., Development of a Kinetic Model for Supercritical Fluids Fischer-Tropsch Synthesis. Energy Fuels 2011, 25, 878.
Anastas, P, T., Heine, L. G., Williamson, T. C., Green Chemical Syntheses and Processes. Eds.; ACS symposium series 767; American Chemical Society: Washington, DC, 2000, pp
Elmalik, E. E.,Tora, E., Mogalicherla, A. K ., Elbashir,N. O., Solvent selection for commercial supercritical Fischer–Tropsch synthesis process. Fuel Processing Technology 2011, 92, 1525

38. Thanks for Your Attention38