New proposal of common template for PSA presentations in … · 2016-06-16 · Some industrial applications with supercritical fluids may be often more costly than the conventional

María Isabel Roldán, PhD Researcher Group of High-Concentration Solar Technology e-mail: [email protected]

Mario Biencinto Murga, Researcher Group of Medium-Concentration Solar Technology e-mail: [email protected]

Supercritical CO2

Summer School June 2016


1. Supercritical fluids 2. Supercritical CO2 (s-CO2) 3. Experiences with CO2 in parabolic troughs 4. Design of a central receiver for s-CO2 5. Technical challenges 6. Summary

Contents


1. Supercritical fluids (s-fluids) 1.1 Introduction 1.2 Definition 1.3 Advantages and disadvantages 1.4 General applications 1.5 s-fluids used in industrial applications

1. Supercritical fluids


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Selection of an appropriate HTF improves:

Receiver efficiency Overall efficiency of the solar thermal power plant

To reach higher temperatures in HTF is

required

Limitations of commercial HTFs:

Thermally stable up to 600°C

Molten salts

Upper temperature limit of 400°C

Synthetic oil

Limited storage capacity and complex controls (115 bar)

Water/Steam

(Vignarooban et al. 2015, Ma et al. 2011)

Study of alternative heat transfer fluids such

as supercritical ones

1.1 Introduction


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1.2 Definition

What is a supercritical fluid?

Pressure-temperature phase diagram

Substance for which both pressure and temperature are above the critical values, where distinct liquid and gas do not exist

Gas-like viscosity and diffusivity.

Gaseous phase Liquid phase

Liquid-like density and solvating properties.

It can effuse through solids like a gas, and dissolve materials like a liquid.

Gases Supercritical fluids Liquids Density, kg/m3 1 100-1000 1000

Dynamic viscosity, μPa·s 10 50-100 500-1000 Diffusivity, mm2/s 1-10 0.01-0.1 0.001


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1.3 Advantages and disadvantages

Advantages of supercritical fluids Excellent solvent for various applications Thermodynamically stable Excellent heat transfer properties

(Knez et al. 2014, Ho et al. 2014)

Disadvantages of supercritical fluids High operating pressures required by supercritical conditions No suitable for facilities with moving connections (fluid leakages) Requirement of specific equipment and safety demands Thermal storage of supercritical fluids is not a viable option (direct storage is not possible, indirect storage is required) Some industrial applications with supercritical fluids may be often more costly than the conventional methods because of the technical requirements


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1.4 General applications

New reaction media for chemical and biochemical processes (alkylation, polymerization, oxidation, metathesis, hydrogenation, ammonolysis …)

Applications of high pressure technologies (Knez et al. 2014)

Special separation techniques such as chromatography using supercritical fluids

Extraction of solids and liquids such as oil extraction from seeds, fruits, leaves and flowers

Dry cleaning (removing liquids from solids without altering the solid structure)

Solvents in a broad range of syntheses

Heat transfer media in refrigeration systems and power cycles


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1.5 s-fluids used in industrial applications

s-H2O (Pc=22.1 MPa, Tc=643 K)

(Knez et al. 2014)

Non-carcinogenic, nontoxic, non-mutagenic, non-fammable and thermodynamically stable Natural occurrence in underwater volcanoes through hydrothermal vents (Tfluid up to 673 K) Hydrothermal vents Applications: Oxidation of hazardous wastes, hydrolysis of biomass polysaccharides

s-CO2 (Pc=7.38 MPa, Tc=304.2 K) Non-carcinogenic, nontoxic, non-mutagenic, non-fammable, cheap, abundant and thermodynamically stable

Commercial supercritical fluids

Applications: Dry cleaning, extraction processes in fine chemistry/cosmetics/pharmaceuticals s-CO2 applications


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1.5 s-fluids used in industrial applications

s-Sulfure hexafluoride (SF6) (Pc= 3.80 MPa, Tc=318.57 K)

(Knez et al. 2014)

SF6 molecule

Applications: Mobile phase in supercritical-fluid chromatography and also used in extraction of low polar substances

s-Propane (Pc= 4.27 MPa, Tc=370.01 K)

Highly flammable

Unconventional supercritical fluids

Applications: Selective solvent s-Propane explosion

s-Argon (Pc= 4.86 MPa, Tc=150.7 K)

Applications: Extraction of petroleum hydrocarbons from soil


2. Supercritical CO2

2.1 s-CO2 Properties 2.2 Industrial applications 2.3 Use of s-CO2 in Concentrating Solar Thermal Technologies

2. Supercritical CO2


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Supercritical fluid presents a homogeneous phase Critical point: 7.38 MPa (73.8 bar) 304.2 K (31.2° C) s-CO2 expands like a gas Liquid-like density and solvating properties

2.1 s-CO2 properties

Pressure-temperature phase diagram

Phases of Carbon Dioxide Gas

Liquid

Supercritical fluid Increasing temperature and pressure


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Liquid-like density at ambient temperature and greater density values with higher Pop At higher temperatures, the thermal conductivity increases from 500 K and its variation with the pressure gets lower

Thermo-physical properties depending on the operating pressure (Roldán, 2015) (http://webbook.nist.gov/chemistry/fluid/)


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Thermo-physical properties depending on the operating pressure

Viscosity slightly increases from 500 K and its variation with the pressure gets slower at higher temperatures Specific heat reaches practically a constant value from 600 K, regardless of pressure

(Roldán, 2015) (http://webbook.nist.gov/chemistry/fluid/)


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2.2 Industrial applications of s-CO2

Decaffeination of tea and coffee

Polymeric processing: foaming agent, solvent and antisolvent

Refrigerant in mobile air-conditioners and heat pump systems

Proposed for using in supercritical Rankine and Brayton cycles to reduce the electricity cost

Capturing and storage method of CO2 emissions

Extraction of hop constituents and spices Applications performed at industrial scale

Innovative applications


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2.3 Use of s-CO2 in Concentrating Solar Thermal Technologies

Why is s-CO2 interesting as working fluid in Concentrating Solar Thermal Technologies?

Good thermo-hydraulic properties in comparison with other pressurized gases: higher density decreases the pumping power consumption

Operating temperature is not limited by the working fluid

Environmentally friendly Abundant and cheap

Thermodynamically stable Used in power cycles with higher efficiency (s-CO2 Brayton cycles) Possibility of thermal energy storage with a separate medium

Critical temperature similar to ambient conditions


3. Experiences with CO2 in parabolic troughs 3.1 Parabolic trough technology 3.2 Innovative heat transfer fluids 3.3 Experiences with CO2/s-CO2

3.4 Conclusions

3. s-CO2 as HTF in parabolic troughs


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3.1 Parabolic trough technology

Parabolic troughs (PT): line-focus collectors with parabolic-trough reflector (mirrors) to concentrate solar radiation onto a receiver tube located at the focal line

Parabolic –trough facility


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Brackets Single-axis tracking Structure Thermal Insulation Foundations


Main components of the parabolic trough collector (PTC)

Absorber tube Reflectors


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Steel absorber Glass Envelope Vacuum annulus Glass-to-metal seal

Main components of the receiver tube

Receiver tube: metal tube with selective coating and a glass cover to reduce convection losses. The heat absorbed is transferred to a heat transfer fluid (HTF) circulating inside.


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Working temperature limited up to 673 K (400˚C) Environmental and economic issues: toxic, flammable, expensive Fluid degradation (need of periodical reposition) Requirement of auxiliary systems for fluid operation (anti-freeze system, expansion vessels)

Conventional HTFs (synthetic oils) present several problems:

3.2 Innovative heat transfer fluids

Alternative HTFs are proposed for parabolic troughs:

Water (Direct Steam Generation) Molten salts Pressurized gases: Helium, N2/Air, CO2/s-CO2


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Innovative heat transfer fluids proposed

Innovative HTFs Advantages Disadvantages

Water + Higher efficiency in the integration with heat storage (HTF-steam heat exchanger no required)

- Expensive and limited storage capacity - Complex controls of two-phase fluid (water/steam)

Molten salts + Higher efficiency in the integration with heat storage (used as storage medium, no HTF-salts heat exchanger)

- High freezing point increases energy consumption (heat tracing is required)

Pressurized gases + Clean + No phase change (ease of operation and control) + Possible integration with storage and power block

- Heat exchanger (not viable as storage medium) - Higher pumping power (lower density)


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Pressurized gases

Fluid Density, kg/m3 Thermal conductivity, W/m-K

Viscosity, kg/m-s

Specific heat, J/kg-K

Helium 7.024 0.27840 35.07·10-6 5186

N2/Air 47.96 0.05074 32.58·10-6 1114

CO2/s-CO2 78.83 0.04968 31.48·10-6 1171

Thermal properties at 100 bar and 673 K (Muñoz-Anton et al. 2014)

Pressurized gases Advantages Disadvantages

Helium + High specific heat + High thermal conductivity

- Expensive - Severe leaking problems

N2/air + Wide availability from ambient air using a N2 generating device

- Lower thermal conductivity and specific heat

CO2/s-CO2 + Wide availability + Higher density, better controlled in operation

- Lower thermal conductivity and specific heat


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3.3 Experiences with CO2/s-CO2

PTC gas loop at PSA

Main equipment Two ET-50-m collectors (≈350 kWth) with receiver tubes for high P and T Hermetic blower to propel the gas Air-cooler to dissipate 400 kWth Shell and heat exchanger connected to a molten-salt storage system Gas storage and filling system

The facility is able to work with CO2/s-CO2/pressurized gases (N2) up to 525°C and 100 bar


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PTC gas loop at PSA

Successful operation with CO2 up to 500˚C at 70 bar (close to supercritical conditions, Pc=73.8 bar) - Test campaign 2008-2012

Collectors in operation with clouds

Operation graph for 500˚C at 45 bar in a sunny day (two transients due to defocusing of collectors)


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Main issues in the parabolic-trough operation with CO2

Low heat transfer coefficient between tube and gas (h≈600-1200 W/m2-K ΔT=50-70˚C) due to the low heat capacity and thermal conductivity of the gas

Leaks in mobile connections

Corrosion risk for carbon steel in case for water presence

Condensation/freezing under certain operating conditions

T, ˚C

Tfluid= 420 ˚C

Deformation of absorber tubes with low mass-flow rates


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Actions to prevent deformation of absorber tubes

Emergency procedure to guarantee a proper mass-flow rate under different operating conditions (collector cooling: defocusing and circulate a higher mass flow)

Proper insulation of absorber-tube supports to avoid undesired tube movement and its deformation

Definition of control strategies with feed-forward actions for mass-flow regulation:

» Risk of deformation depends on direct normal irradiance (DNI) Minimum mass-flow rate should be defined by solar radiation

» Fast measurements of solar radiation are required to taking actions in advance (instant response of PV pyranometers against conventional ones with a response of 18 s based on thermopile sensors)


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Prevention of leaks

Actions: Try to avoid manual valves, use of graphite joints and welded connections (instead of flanged/threaded connections)

Leaks: Produced because CO2 has lower density than liquids and operates at high pressures. It leaks out of joints and mobile parts (periodically refilling of CO2)

Possible solutions for mobile connections: » Graphite injectors: Addition of more graphite in ball joints to reduce leaks but it is not a definitive solution » Flexible hoses: Tested at hot conditions with no significant leaks, but with higher pressure drop (around 3 times) than through ball joints Increase of pumping power (20-30%)

Graphite injector Flexible hose

Moderador

Notas de la presentación


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Corrosion problems in circuits with CO2

Carbonic acid production in the presence of water: CO2 + H2O H2CO3

H2CO3 attacks carbon steel and other materials

Actions to prevent corrosion

Complete drying of the circuit after hydraulic test, including vacuum

Circuit protection against ambient air entry during refilling or maintenance procedures due to its water vapour content

Control of water content in the circuit by an hygrometer

Consider the initial water content in cylinders of the CO2 filling system


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Condensation/freezing of CO2 under certain operating conditions Condensation of CO2 before start-up

Before operation is started, CO2 pressure is around half of the nominal pressure (Pnom≈ 80-100 bar) risk of condensation in winter with static fluid under cold conditions (Tsaturation=0°C at 35 bar) Condensed CO2 may be dangerous for blowers start-up with low mass-flow rate until CO2 is heated above its saturation temperature The liquid CO2-water mixture might increase corrosion issues Control the circuit points, where CO2 may condense, by using sensors and heat tracing No reliable measurements under CO2 condensation avoid it by heat tracing of instrumentation circuits

Freezing of CO2 due to pressure reduction in the filling line High pressure gradient when CO2 circulates from the stock cylinders to the circuit gas expansion sudden temperature decrease freezing Freezing of CO2 prevented by heat tracing of the filling line


30


(www.ohlindustrial.com)

Other experiences in line-focusing systems: Linear Fresnel Concentrators with pressurized gas

Advantage: Fixed receiver with less mobile parts leakage reduction

R&D project: FUTURO SOLAR (OHL Industrial & UPM)

Fresnel facility (≈300 kWth) with pressurized gas Operation foreseen for mid-2016, no published results

(http://www.ohlindustrial.com/en/innovation/rdi/futuro-solar/)

http://www.ohlindustrial.com/en/innovation/rdi/futuro-solar/


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3.4 Conclusions

Pressurized CO2/s-CO2 for line-focus collectors shows several advantages over conventional HTFs

» Clean and safe fluid, the working temperature is not limited, suitable integration with the thermal storage system and power block

The technical feasibility of CO2 as HTF for parabolic troughs has been demonstrated in a test facility at the PSA

The main issues of CO2 can be addressed with proper design, operation and control procedure (heat transfer in receiver pipe, corrosion risk, condensation/freezing)

Some technical and economic challenges need further research (leaks in mobile connections, solar field optimization, cost reduction of the facility)


4. Design of a central receiver for s-CO2 4.1 Central receiver technology 4.2 Tubular receiver design for s-CO2 4.3 Receiver integration within the STE plant 4.4 Conclusions

4. s-CO2 as HTF in central receivers


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4.1 Central receiver technology

Central receiver: facility with a large number of computer-assisted mirrors (heliostats) which track the sun individually over two axes and concentrate the solar radiation onto a single receiver at the top of a central tower, where the heat is transferred to a fluid


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4.1 Central receiver technology (Ho et al. 2014)

Tubular central receiver: Design with an array of thin-walled metal tubes irradiated by concentrated sunlight used to transfer the heat received to a working fluid which is transported to storage system or to the power block

External-type receiver

Cavity-type receiver

Cavity receivers have a lower radiation heat loss than that for external ones (slightly lower thermal efficiencies for external-type receivers)

Tubular panel

Tube size and wall thickness selected to maximise heat transfer and minimise pumping losses Small tube diameters improve convection, but increase pumping losses and material costs

Central receiver design: tubular configuration

Wall thicknesses decrease with small diameters, at a given pressure

Key considerations in the design of a tubular receiver


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Design of a tubular central receiver for supercritical CO2

Commercial HTFs in tubular central receivers

Molten nitrate salt (Solar Two): The same fluid can be used as the storage medium, but its decomposition occurs above 600-630˚C

Water/steam (Solar One, PS10, PS20): Turbine inlet pressures of 9-13 MPa for conventional steam cycles (ηth-el≈35%). Higher pressures are required for the supercritical phase at temperatures above 650˚C.

Innovative HTFs in tubular central receivers

4.1 Central receiver technology

Fluoride salts: Stable in liquid form below 1000˚C. No degradation (simple anion) Carbonate salts: Less aggressive with respect to corrosion (protective barrier with stable oxide layers). Salt degradation at high temperatures. s-CO2: Proposed because small tube diameters may enable the high pressure required for the supercritical phase. Turbine inlet conditions for s-CO2 Brayton cycle: 15-24 MPa and 700˚C (ηth-el≈50%)


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4.2 Tubular receiver design for s-CO2

How could we design a central receiver for s-CO2? Selection of the receiver type: tubular design (small diameter to enable high pressures)

Simulation of previous designs tested for other HTF with CFD modelling to study the fluid behaviour according to the geometry

To develop the simulations considering s-CO2 as HTF in order to determine initial operating conditions

Design optimisation, regarding different configurations for the tube geometry

Model validation with experimental results obtained from the design tested

Analysis of a tubular receiver design for supercritical CO2


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Preliminary analysis to evaluate the behaviour of the s-CO2 in comparison with a commercial HTF (molten salts)

Definition of the minimum operating conditions required by s-CO2 Evaluation of the heat gained by both fluids at the same boundary conditions (Tin, Qv)

General procedure

Selection of a tubular receiver design tested at PSA with molten salts as HTF

Development of a simplified

CFD model

Model validation (molten salts)

Analysis of the s-CO2 behavior

Comparison between molten salts and s-CO2


Case study

Objectives

Design optimisation


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Description of the selected receiver

Consisting of 3 modules with 20 alloy tubes in each one Tubes are irradiated and transfer the heat to the fluid The fluid enters at the bottom of the central module and flows through the lateral ones

Selection of the solution domain

Simplified 2D model Symmetrical geometry of the central module Subdomains: Section of the tube thickness and fluid.



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Selection of the experimental steady state

Inlet pressure, Pa

Outlet pressure, Pa

Inlet temperature of the molten salts, K

Total power received, kW

589000 347000 715 (442°C) 720

Definition of the volumetric heat source

Measurement of the heat-flux distribution Assumption of three zones with constant irradiance Volumetric heat source evaluated according to the volume of each zone and considering the material absorptance

Qv zone A, W/m3 Qv zone B, W/m3 Qv zone C, W/m3

404651 1540465 845581


Boundary conditions for validation (molten salts)


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Boundary conditions for s-CO2 evaluation

Definition of the main simulation parameters

Outlet pressure set to 7.50 MPa Critical pressure (7.38 MPa) (http://webbook.nist.gov/chemistry/fluid/)

Head loss evaluation in central module Initial value for inlet pressure (11 MPa)

Inlet pressure, Pa Outlet pressure, Pa Inlet temperature of s-CO2, K Total power received, kW

11000000 7500000 800, 715, 600, 500 (527°C, 442°C, 327°C) 720

Each tube is divided in three sections to define the volumetric heat source Mesh quality was checked (equiangle skew = 0.44 < 0.5, mesh independency test) Viscosity model selected for turbulent flow κ-ε renormalization group (RNG) to consider areas with low Reynolds number (manifolds)

CFD simulation



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Validation: Comparison between experimental and numerical data (molten salts)

Experimental inlet pressure, Pa

Numerical inlet pressure, Pa

Deviation, %

Experimental outlet temperature, K

Numerical outlet temperature, K

Deviation, %

589000 550023 6.62 716.6 715.6 0.14

Simulation results are in agreement with experimental data (deviation < 7% for the inlet pressure), considering measurement uncertainties of around 1%.

Cases: Tin=800, 715, 600, 500K The heat gained by s-CO2 increases linearly with its inlet temperature (BC: free convection)

Experimental evaluation of heat losses proposed to check this trend

s-CO2 as HTF in solar tower receivers


Simulation results


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Non-homogeneous thermal and pressure

profiles

Optimisation of the receiver

design for s-CO2

Study of new tube geometries

Detailed analysis of the manifold geometry (3D model)

s-CO2 as HTF in solar tower receivers



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Comparison between molten salts and s-CO2

Simulation results (Tin=715K):

Fluid Inlet pressure, Pa

Outlet pressure, Pa Mass flow, kg/s Maximum fluid

temperature, K Heat gained by

the fluid, W

Molten salts 550023 347000 45.67 721 69370

S-CO2 9695240 7500000 24.90 903 321180

s-CO2 as HTF in solar tower receivers requires high operating pressures to ensure the supercritical condition (Pinlet≈10MPa) Power consumption and corrosion effect should be evaluated

Heat gained by s-CO2 is around 75% greater than the one gained by molten salts with the use of around half of the mass flow.


(Roldán et al. 2015)


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Optimisation: simulations for the complete absorber and greater tube length

Simulation results considering three tubular panels (Tin=750K):

Fluid Inlet pressure, Pa

Outlet pressure, Pa Mass flow, kg/s Maximum fluid

temperature, K Heat gained by

the fluid, W

Molten salts 551709 550973 95.3 895 22060754

s-CO2 24216635 24201324 95.3 963 25216567

Tubular central receivers require high operating pressures (Pinlet≈24MPa) to ensure the inlet condition for s-CO2 Brayton power cycles Power consumption and corrosion effect should be evaluated

For the same mass flow and inlet temperature, the heat gained by s-CO2 is around 12.5% greater than the one gained by molten salts Improvement of the power-cycle efficiency


(Roldán 2016)


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4.3 Receiver integration within the STE plant

For s-CO2 Brayton cycles, the pressure range at the turbine inlet is 15-25 MPa tubular receiver requires high operating pressures

Without storage system, the inlet conditions (P, T) of the power cycle define the operating conditions at the receiver outlet

Tubular central receiver has a fixed design (mobile connections avoided) leakage issues can be better controlled

Relevant aspects in the receiver integration within the STE plant

The direct storage of s-CO2 is not a viable option because of the high operating pressures of s-CO2 receivers Indirect storage with a separate medium


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4.3 Receiver integration within the STE plant

Commercial STE plant (HTF: water/steam) s-CO2 receiver integration within the STE plant

s-CO

2

s-CO

2

s-CO2 Brayton cycle Hybridisation

(biomass)


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4.4 Conclusions

A simplified 2D CFD model for a molten salt central receiver has been validated (maximum deviation of 6.6 %). For one-panel simulation and same inlet temperature (715 K), the heat gained by s-CO2 is around 75% greater than the one gained by molten salts with the use of around half of the mass flow.

Conclusions: case study

For complete-receiver simulation, greater tube length and same mass flow and inlet temperature (750 K), the heat gained by s-CO2 is around 12.5% greater than the one gained by molten salts Tubular central receivers require high operating pressures (Pinlet≈24MPa) to ensure the inlet condition for the supercritical power cycle.

Supercritical CO2 appears to be a promising HTF in tubular central receivers, but some other aspects should be further studied such as the storage capacity of molten salts in comparison with the direct use of s-CO2 in a more efficient power block together with its hybridisation.

General conclusion


5. Technical challenges

5. Technical challenges 5.1 s-CO2 as HTF in parabolic troughs 5.2 s-CO2 as HTF in central receivers


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5.1 s-CO2 as HTF in parabolic troughs

Technical challenges of using s-CO2 in parabolic troughs

Improvement of mobile connections to reduce leaks and limit the pressure drop

» Hybrid design for receiver tubes: 2D rotary joint + flexible hose

To enhance the heat transfer between receiver tube and gas » Thermal analysis of internally finned tubes, considering a limited pressure drop and low manufacturing costs

Alternative design for gas/salt heat exchangers with lower weight, thermal inertia and cost, together with better maintenance

» Helical design: 2-3 times lighter in weight and requires specialised manufacturers

» Printed-circuit design: 5-7 times lighter in weight with small orifices (risk of salt freezing), but further investigation is needed


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5.1 s-CO2 as HTF in parabolic troughs

Further techno-economic analyses are needed to evaluate the viability of using CO2/s-CO2 in line-focusing collectors:

» STE plants with s-CO2 show similar expected annual yields to the one of conventional plants, but costs are still higher Requirements for cost reduction » s-CO2 is a promising HTF for co-generation (combined heat & power) and hybridisation (clean fluid, storage at higher T increases solar contribution, significant fuel savings expected, high efficiency compared to STE plants)

» Integration with high-efficiency power cycles (s-CO2 and combined cycles)

Further solar field optimisation is required depending on the application: » New designs of parabolic troughs according to the operating temperature achieved (larger aperture and higher efficiency)

» Cost reduction and minimisation of maintenance issues (reducing molten salts circuits, heat exchangers and blowers)


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5.2 s-CO2 as HTF in central receivers

The operating conditions required (high pressure and temperature) may cause corrosion and leakages issues

Selection of suitable materials to avoid corrosion and to withstand thermal and mechanical shocks

Optimisation of the tube geometry (small diameters are required to obtain high pressures)

Technical challenges of using s-CO2 in central receivers

Integration of s-CO2 receiver with storage:

» Direct storage of supercritical fluids is not viable (high operating pressures) receiver directly connected with power block + hybridisation (biomass/fossil fuels)

» Intermediate heat exchange with a separate storage medium


1. Supercritical fluids 2. Supercritical CO2 (s-CO2) 3. Experiences with CO2 in parabolic troughs 4. Design of a central receiver for s-CO2 5. Technical challenges 6. Summary

Contents


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6. Summary

What have we learnt about supercritical CO2?

s-CO2

Supercritical fluids

HTF in solar receivers

Parabolic troughs

Central receivers

Technical challenges

Properties

Experiences in a test facility

Industrial applications

Theoretical design

Definition Applications


Thank you for your

attention Mario Biencinto Murga, Researcher Group of Medium-Concentration Solar Technology e-mail: [email protected]

M.Isabel Roldán Serrano, PhD Researcher Group of High-Concentration Solar Technology e-mail: [email protected]

Documents

New proposal of common template for PSA presentations in … · 2016-06-16 · Some industrial applications with supercritical fluids may be often more costly than the conventional