Catalytic Tests I: Flow systems and product analysis Rolf Jentoft, November 15, 2002 Modern Methods in Heterogeneous Catalysis Research

Gas flow control Gas supply Product analysis Reactor Flow Control Flow Measurement Pressure Measurement Pressure Control Flow systems and product analysis

Important parameters for reactor feed flow Gas flow measurement and control Valves Mass Flow controllers Pressure measurement and control Saturators and vaporizers Product analysis Gas chromatography Mass spectroscopy Infra Red spectroscopy Outline

Parameters for reactor feed flow Flow rate: ml / minute or micromoles / minute PV = nRT use it! Volumetric flow calibration must specify conditions STP (0 °C, 760 mm Hg) Non-deal gas mixtures Correct for pressure: 1 ml in Berlin = 1.12 mlDenver Colorado

Pressure Do you need to control the pressure Pressure drop in reactor Is your flow meter independent of pressure Quartz reactor body: safety Condensation (products or reactants) Uncertainty of measurement During the design of a gas flow system consider the precision that you will need Calculate uncertainty before you build (Partial derivative analysis)

Laboratory reactors Flow: sccm/min 5 mg to 5 grams catalyst Pressures bar Minimize volume of tubing Minimize “dead volume”

Dry piston flow meters Not for in-line use Pressure and temperature correction necessary Solubility of gas in bubble liquid Inexpensive Often used for calibration standard Digital versions available Bubble flow meter Flow measurement

Rotameter (variable area flow meters) Can be installed in line Pressure and temperature correction necessary Gas specific calibration Flow measurement

Measures temperature increase for a set energy input Usually no temperature or pressure correction needed Thermal mass flow meter (Electronic mass flow meters)

Thermal mass flow meter (continued) Calibrated for specific gas and flow range Conversion factor, K, can be used for measurement of other gases Where flow at standard conditions density at standard conditions heat capacity at standard conditions Flow measurement

Thermal mass flow meter (continued) Flow measurement More recently, flow meters which have polynomial conversion factors are available Expensive Gas must be filtered

Flow Control Valve constant: C V, K V, f used to select valves Control Valves If Where specific gravity (Kg/dm 3 ) absolute temperature (K) absolute pressure (Kg/cm 2 )

Mass Flow Controller Combines measurement of mass flow with control valve

Minimize tube volume Check valves Reactor He O2O2 C3H6C3H6 Add check valves to stop back diffusion No “dead” volume (gasses are not flushed away)

Pressure measurement and control Bourden-tube gauge Pressure transducers Back pressure regulators Tend to pulse at low flows Specification should include minimum K V

Multiple control valves Reactor Pressure reduction Mass flow control Back pressure regulator Check valve Danger of interaction between control valves Oscillations due to interaction between pressure reducer and mass flow controller

Multiple control valves Reactor Pressure reduction Mass flow control Back pressure regulator Check valve De-couple regulators by increasing volume if possible Change control parameters of valves Add restrictions to change response of the system to valves

Saturators and vaporizers Reactants that are liquids at room temperature and atmospheric pressure If design is correct, gas is in equilibrium with liquid reagent and contains Saturators: carrier gas is bubbled through a volume (height) of the liquid to be evaporated Gas in Gas out Vapor content of outgoing gas function of temperature and pressure Need to control temperature and measure pressure

Analyze to determine saturation and stability Saturators and Vaporizers Carrier gas inert to liquid, (saturates liquid at very low concentration) Bubbles should be small (frit on bottom of inlet) Height of liquid must be controlled No foaming or formation of spray Tubing down stream of saturator must be at higher temperature than liquid

What is gas concentration in exit stream? saturation of methanol in N 2 at 21.2 °C and 1 bar Vapor pressure methanol = = bar Flow N 2 = 20 sccm/min bar 23.0 sccm/min Saturators and Vaporizers

Vaporizors: liquid flow is controlled and liquid is mixed with carrier gas and evaporated Liquid Pump Reactor Mass flow controller Heated Liquid flow are very low (5 microliters methanol / minute) The vaporization process tends to oscillate at low flows ? Configuration of vaporizer is not trivial Commercial products are available

Heating (tracing) tubes and valves Reactor Pressure reduction Mass flow control Product analysis With saturator or vaporizer, tubes must be heated Products tubes should also be heated Tubes need only be heated to stop condensation

Reactor Bypass Reactor Pressure reduction Mass flow control Check valve Pressure reduction Mass flow control Check valve Reactor Check for reactant stream purity / stability Particularly useful for systems with saturators

Product analysis Products are usually not unknowns Quantification is necessary Rapid analysis may be necessary Describe in more detail: Gas chromatography Mass spectroscopy Infrared spectroscopy

Product analysis: GC Best for quantification and stability Slower than MS or IR Not as flexible as MS GLC and GSC

Product analysis: GC

Injector system: Split / splitless Direct on-column Paralyzing Liquid injector usually included Useful for product identification and troubleshooting Not good for calibration

Gas sampling valves Sample loop adjusted to sample concentration and detection limit Disadvantage: can increase pressure upstream Product analysis: GC Injector system:

Product analysis: GC Columns: Capillary vs. Packed Separation of complex mixtures may require more than one column Vendors can be sent a product list and asked for solutions

Product analysis: GC Detectors:

Product analysis: GC Detectors: High sensitivity for hydrocarbons Destructive Does not detect water or many permanent gases Flame Ionization Detector

Product analysis: GC Detectors: Thermionic Emission Detector High sensitivity for nitrogen and phosphorus

Product analysis: GC Detectors: Thermal Conductivity Detector Changes in conductivity due to product in the carrier stream Non-destructive, often combined with FID

Product analysis: MS Fast scanning speeds Flexible and portable Less stable and more difficult to quantify Not suitable for complex mixtures Difficult or impossible to separate isomers Inlet Ionization Separation Detection

Product analysis: MS Inlet: Usually a capillary in the product stream takes continuously samples products Heating is necessary About 2 ml / min required Capillary leads to a pinhole or to a concentration device

Product analysis: MS Ionization: Electron or hard Ionization Fragments molecules Positive ions are directed to analyzer

Product analysis: MS Ionization: Electron or hard Ionization

Product analysis: MS Ionization: Soft ionization (Chemical ionization) Reagent gases are ionized (Hard ionization) Ionized reagents (CH 4 ) are mixed with product molecules Products MH + for amines or ethers Saturated hydrocarbons often give (M-1) +

Product analysis: MS Ionization:

Product analysis: MS Ionization:

Product analysis: MS Seperation: Quadrupol mass spectrometer

Product analysis: MS Ionization: Time of flight

Product analysis: MS Detectors: Faraday cup Stabile and reliable

Product analysis: MS Detectors: Multichannel electron multiplier Signal gain of 10 5

Product analysis: Analysis: After identification of products: Selected ions are monitored with time For simple mixtures a calibration matrix can be measured and concentrations can be calculated

Product analysis: IR spectroscopy A more specific IR analyzer Detector contains analyte, responds only to radiation absorbed by analyte

References: F.W. McLafferty and F. Turecek, Interpretation of Mass Spectra, Fourth Edition, University Science Books, 1993 H. H. Willard, L. L. Merritt, Jr., J. A. Dean, and F. A. Settle, Jr., Instrumental Methods of Analysis, Seventh Edition, Wadsworth, 1988