Patent Application
20120309101
The invention relates to a reactor comprising a reactor chamber and a sensor situated inside the reactor chamber. Further, the invention relates to a method for analyzing a reactor state.
Reactor measurements are central to heterogeneous catalysis research. Apart from reactors which feature a number of discrete sampling points, typically less than ten, all reactor designs have in common that reaction products are analyzed in the reactor effluent stream be it gaseous or liquid. The reaction pathway, i.e. how the reactants are transformed into products remains hidden and only a single kinetic data point is obtained for a set of reaction variables. In addition to kinetic information, heterogeneous catalysts are often characterized spectroscopically to investigate their geometric and electronic structure, surface species and active sites. As a catalyst is a dynamic system and adapts to its chemical environment, in situ spectroscopic techniques become more and more important in heterogeneous catalysis research. These techniques aim at bridging the material and pressure gap by studying polycrystalline powders or supported catalysts under working reaction conditions.
U.S. Pat. No. 4,676,955 discloses an instrument to measure catalytic reaction rates. The reactor is built on the recycle reactor principle and has an internal recycle blower that is floating on a feed fluid. The recycle blower creates a large internal recycle flow and thereby eliminates all internal temperature and concentration fluctuations. During operation of the reaction vessel the catalyst is inserted into a catalytic basket situated inside the reaction vessel. After placing the catalytic basket inside the reaction vessel, the vessel is closed and then flushed with an inert gas, such as nitrogen, to remove all air and undesirable reactive gases contained therein. A feed gas is then fed into the reaction vessel until a desirable pressure is obtained. Further, the reaction vessel is heated until a temperature reaches a desirable range for the specific reaction to be analyzed. Once a desirable pressure and temperature has been reached, the reaction is allowed to continue for a suitable amount of time. During the reaction period, the fluid in the upper portion of the vessel cavity is drawn downwardly through the catalyst bed by impeller vanes and then outwardly to the side of the bottom portion of the cavity. The fluid is then forced upwardly and once again is drawn downwardly into the catalyst bed. Such recycling continues throughout the reaction with a small amount of product being removed through an outlet port for analysis. The mean cover of the reaction vessel is equipped with a desirable number of ports to accommodate various desired functions. Thus, a thermocouple may extend into the catalytic bed portion of the reaction vessel to measure the temperature therein.
M. Bosco, F. Vogel, Catalysis Today, 116 (2006) 348-353, describe an optically accessible channel reactor for the kinetic investigation of hydrocarbon reforming reactions. The reactor allows surface temperature measurements along the reactor through a quartz window using IR thermography. The catalyst is coated as a thin layer onto a metal plate placed at the bottom of the reactor channel. The massive metal body of the channel reactor may be heated by heating cartridges such that a uniform temperature distribution along the channel can be maintained. A small stream of gas can be withdrawn with a moveable capillary to measure the concentration profile in the reactor. The catalyst surface temperature is measured using an infrared camera. By using a quartz glass window the spectral range of the camera has to be in the near infrared region. To take into account the emissivity of the catalyst surface a calibration is performed. A thermocouple is placed in close contact to the catalyst surface and the reactor is heated to different temperatures until the reactor is well thermally equilibrated. A power law is fitted to the data points. After an infrared picture of the catalyst surface is taken, the measured intensities are read out along the center line of the reactor in flow direction. The temperature difference is calculated between the measured temperature of the catalyst surface during the reaction and during flushing the reactor with pure argon at constant setting of the heating system. The disadvantage of the described reactor lies in the fact that radiation heat losses are induced by measuring the temperature from outside through a window affecting indirectly flow and surface chemistry compared to an insulated reactor.
R. Horn, K. A. Williams, N. J. Degenstein, L. D. Schmidt, Journal of Catalysis 242 (2006) 92-102 investigated the mechanism of catalytic partial oxidation of CH4 on Rh-coated α-Al2O3 foam monoliths by measuring species and temperature profiles along the catalyst axis and comparing them with numerical simulations. A thin quartz capillary connected to a quadrupole mass spectrometer is moved through the catalyst with a spatial resolution of about 0.3 mm. The reaction is carried out in a quartz tube. High-purity reactants CH4, O2 and the internal standard Ar are fed through calibrated mass flow controllers through a side port at the bottom of the tube and leave the reactor from the top for incineration. An injection needle inserted through a septum at an end port at the bottom of the tube enables guided movement of the capillary without noticeable gas losses. α-Al2O3 foams loaded with Rh are used as catalysts. To avoid axial radiative heat losses in the reaction tube, two uncoated α-Al2O3 foams are used as heat shields. To avoid bypassing of gas, the catalyst and the heat shields are tightly wrapped in alumo-silicate paper. The reaction profiles are measured by sliding a 20 cm long fine quartz capillary through a channel, diamond drilled through the centerline of the catalyst. The lower end of the capillary is connected to a ported micro volume tee. The opposite port is used to feed a thermocouple into the capillary. The tip of the thermocouple is aligned flush with the open end of the capillary to measure species composition and temperature simultaneously at each point in the catalyst. The side port of the tee is connected to a stainless steel capillary, which is inserted into the inlet valve of a mass spectrometer vacuum chamber. At the end of the stainless steel capillary, a rotary vane pump generates a vacuum of about 500 mTorr, forcing a permanent flow from the end of the quartz capillary positioned in the catalyst to the sapphire seat of the MS inlet valve. The tee is mounted on a micrometer screw so that the capillary can be moved up and down with sub-millimeter resolution. All profiles are measured by sliding the capillary tip down (i.e., against the flow direction) from a position 3 mm downstream to the end of the catalyst through the catalyst up to about 5 mm into the front heat shield. Using this technique, the open channel is left downstream of the capillary tip and does not influence the sample composition at the tip position.
In a further paper, Journal of Catalysis, 249 (2007) 380-393, R. Horn, K. A. Williams, N. J. Degenstein, A. Bitsch-Larsen, D. Dalle Nogare, S. A. Tupy and L. D. Schmidt describe a modified reactor and capillary sampling system. The sampling capillary comprises a side sampling orifice and a thermocouple aligned with the sampling orifice. The quartz sampling capillary and thermocouple meet in a stainless steel tee, the third port of which is connected to a stainless steel capillary that discharges into the inlet valve of a mass spectrometer. A pump generates a vacuum at the end of the stainless steel capillary, forcing the gases from the sampling orifice into the mass spectrometer. Moving the capillary/thermocouple assembly up and down allows measuring species and temperature profiles along the centerline of the catalyst. The measured temperature is a gas temperature as the thermocouple is in thermal contact with the flowing gas but not with the catalyst surface. The reaction cell is self-supported on a steel mounting frame with stainless steel inlet and outlet tubes. The sampling capillary is guided in steel capillary liners to avoid any bending leading to uncertainties in the sampling orifice position. The compact and rigid self-supported construction of the reactor provides precise geometric alignment and is easy to dismantle and reassemble. The capillary is moved by a stepper motor mounted underneath the reactor. This allows automatization of the experiment such that larger experimental campaigns may be conducted.
To understand heterogeneous catalysis in more detail it is desirable to follow in situ how the reactants are transformed into products and how the catalyst changes along the reaction coordinate. Although it has been demonstrated that spatially resolved measurements of gas species and temperature may be performed in situ inside a reactor, the catalyst has been treated so far as black box.
The problem underlying the invention therefore is to provide a reactor design that enables in situ monitoring of a sample, e.g. a catalyst, in a reactor under working conditions. In a preferred embodiment the reactor design should additionally also allow monitoring changes of the fluid species (gas or liquid) and solid and fluid temperature measurements.
This problem is solved by a reactor as defined in claim 1. Preferred embodiments are defined in depending claims.
The reactor according to the invention is equipped with a sensor for collecting spectroscopic information. The sensor is situated inside the reactor chamber and, therefore, allows collecting information directly from a sample placed inside the reactor chamber without any disturbances introduced by optical sampling ports, e.g. a window. Spectroscopic information allows in situ monitoring of the sample, e.g. a catalyst, and an interpretation of the geometric and electronic structure of the catalyst as well as an in situ investigation of active sites or surface species bound to the catalyst.
Spectroscopic information is understood to be information on intensity and/or frequency of electromagnetic radiation emitted, absorbed, reflected or scattered by a sample. Spectroscopic information may be obtained at a particular energy level, e.g. the intensity of an emission, absorption or scattering of electromagnetic radiation at a particular wavelength. Preferably, however, spectroscopic information is obtained within a particular range of energy, e.g. a particular wavelength range, such that an intensity of emission, adsorption, reflection or scattering is determined dependent on the energy, i.e. frequency, of the corresponding electromagnetic radiation, such that a spectrum may be obtained. The spectroscopic information is preferably collected by optical spectroscopy.
Basically every type of reactor may be equipped with the sensor for collecting spectroscopic information to obtain a reactor according to the invention. A reactor is understood an apparatus comprising a reaction chamber in which a process may be performed during which process at least one alteration of a parameter takes place that can be measured by a spectroscopic method, in particular an optical spectroscopic method. Such reactor may be e.g. a reactor for performing a chemical process, in particular a chemical synthesis, or may be a reactor for performing physical processes, e.g. mixing of several components or grinding a solid component. An exemplary mixing process is compounding of polymers. However, the term “reactor” also comprises apparatuses used e.g. in food production wherein in such apparatus a cooking or baking process is monitored by a spectroscopic method.
According to a preferred embodiment, the reactor is a reactor used to perform chemical reactions.
According to a preferred embodiment, the reactor is designed as a flow reactor through which are transported fluid reaction components in gaseous or liquid form. The reactor may e.g. take the form of a packed-bed reactor or may be the type of a column reactor. However, the reactor may also be of the type of a batch reactor, e.g. a stirred tank reactor. Basically, no particular restrictions apply to the type of reactor.
The reactor comprises a reactor chamber in which chemical reactions are intended to take place or physical processes such as transport of heat and mass. The reaction chamber may be filled with a solid phase, e.g. with a catalyst which may be fixed to a solid carrier. The reactor may be of the type of a fixed-bed reactor or of a fluidized-bed reactor. According to a preferred embodiment, the solid phase comprising the carrier and/or the catalyst is provided in the form of a monolith. The monolith may comprise parallel channels, e.g. having a hexagonal cross section, or may have the form of a reticulated foam. A suitable refractory material that may be used as a carrier is e.g. α-Al2O3. Preferably, the catalyst is coated on a suitable carrier. However, a solid catalyst may also be coated directly to the surface of a wall of the reaction chamber.
According to the invention a sensor for collecting spectroscopic information is disposed inside the reaction chamber. A sensor is understood to be a device for collecting spectroscopic information, in particular for collecting electromagnetic radiation emitted, absorbed, reflected or scattered by a sample. The collected spectroscopic information may be processed directly or may be guided to a place outside the reactor chamber for further processing. The sensor may be situated at every suitable place inside the reaction chamber and may be placed e.g. close to the center of the reaction chamber or may be placed close to an outer wall of the reaction chamber. Preferably, the sensor is placed at a location inside the reaction chamber at which the reaction conditions will be representative for a chemical reaction or physical process performed inside the reaction chamber. When using a reactor equipped with a solid catalyst placed on a carrier for performing a heterogeneously catalyzed reaction, the sensor preferably will be placed in close proximity to the surface of the solid catalyst, preferably at a symmetry line of the catalyst bed, e.g. the center of the catalyst bed. This allows obtaining spectroscopic information on processes occurring on the catalyst surface or processes influencing the catalyst structure.
The sensor preferably is connected to a device for processing and displaying the spectroscopic information collected inside the reaction chamber. Those devices preferably are situated outside the reactor chamber. Such device may e.g. be a spectrometer, spectrograph or computer for processing the data collected by the sensor or may be a display or a chart recorder for displaying the spectroscopic information. A suitable connection, e.g. a fiber, may then be used to transfer the spectroscopic information from the sensor to e.g. a computer for further processing.
The sensor may be designed for only collecting electromagnetic radiation emitted from a sample inside the reaction chamber. However, according to an embodiment, the sensor for collecting spectroscopic information may be combined with a radiation source for exciting a sample inside the reaction chamber. For this purpose, a light source may be placed close to the sensor and the sample may be irradiated by light from the light source. The sample is excited by appropriate radiation, e.g. through a laser light source. The excited sample may then emit, absorb, reflect or scatter electromagnetic radiation that is collected by the sensor as spectroscopic information.
According to an embodiment, the sample is illuminated by laser light of a defined wavelength which is scattered by the sample. The elastically and inelastically scattered light will then be collected by the sensor. In this embodiment, the inelastically or the elastically scattered light may then be used for in situ analysis of processes occurring inside the reactor chamber, e.g. by means of Raman spectroscopy. The sensor may act as a light source as well as a collector of spectroscopic information. However, it is also possible to provide a separate radiation source and to use the sensor only for collecting spectroscopic information.
According to a preferred embodiment, the sensor for collecting spectroscopic information comprises at least one radiation guide for collecting radiation emitted, absorbed, reflected or scattered by a sample and at least one radiation guide for guiding radiation onto the sample for excitation. This allows placement of equipment used for analyzing the radiation emitted, absorbed, reflected or scattered by a sample or for generating light to excite the sample outside of the reactor. The radiation guide for collecting radiation emitted, absorbed, reflected or scattered by a sample and the radiation guide for guiding radiation onto the sample for excitation may combined or may be provided as separate devices.
A radiation guide is understood to be a medium that is transparent to electromagnetic radiation collected from a sample or used for excitation of a sample and may transport the radiation to or from the sample from or to a distant place, e.g. outside of the reactor.
According to an embodiment, at least one of the group formed by the at least one radiation guide for collecting radiation emitted, absorbed, reflected or scattered by a sample and of the at least one radiation guide for guiding radiation onto the sample for excitation has the form of a fiber. The fiber is transparent to electromagnetic radiation collected from a sample and to electromagnetic radiation used for excitation of the sample.
The sensor may according to an embodiment take the form of a fiber transparent to light used for excitation and to light emitted, absorbed, reflected or scattered by the sample. The sensor may then take both functions, i.e. may be used for excitation of a sample as well as for collecting spectroscopic information. For example, light emitted from a laser source may be guided by the transparent fiber to the sample and scattered by the sample, e.g. a solid catalyst placed in the reactor chamber. The same fiber may also collect light emitted from the sample. Excitation of the sample and collection of spectroscopic information may take place simultaneously or consecutively.
However, according to an embodiment, also separate devices may be used for excitation of a sample and for collecting spectroscopic information, e.g. by using separate transparent fibers for each purpose.
According to an embodiment, the sensor for collecting spectroscopic information is formed as a bundle of fibers. Some of the fibers may be used to distribute electromagnetic radiation to a sample for excitation and some of the fibers may be used to collect electromagnetic radiation emitted, absorbed, reflected or scattered by the sample.
According to a preferred embodiment, optical spectroscopy is used for collecting spectroscopic information. Preferably, electromagnetic radiation in a range covered by infrared, near infrared and ultraviolet/visible radiation is used for collecting spectroscopic information. Preferably radiation of a wavelength within a range of about 10−7 to 10−3 m is used for collecting spectroscopic information. According to a preferred embodiment the sensor for collecting spectroscopic information is selected from the group of IR-, NIR-, UV/Vis-, Raman- and fluorescence sensitive sensors.
The reactor according to the invention allows in situ analysis of processes occurring inside the reactor, i.e. under actual reaction conditions without disturbance, e.g. heat losses, induced by an optical sampling port such as a window.
According to an embodiment it is preferred that those parts of the sensor used for collecting spectroscopic information are placed inside the reactor chamber whereas devices used for processing the collected spectroscopic information are placed outside the reactor.
According to a preferred embodiment, the sensor for collecting spectroscopic information comprises at least one fiber located inside a capillary wherein one of the fiber and the capillary acts as radiation guide for collecting radiation emitted, absorbed, reflected or scattered by a sample and the other acts as radiation guide for guiding radiation onto the sample for excitation.
Accordingly, light used for excitation of the sample may be guided to the sample through the fiber from e.g. a laser source and light emitted, absorbed, reflected or scattered by the sample is collected by the capillary and guided towards a device for analysis. The same configuration can also be used in reverse order, i.e. irradiation through the light guiding capillary and collection of emitted, absorbed, reflected or scattered light by the fiber located inside the capillary. Preferably, light for excitation of the sample is guided through the fiber and light collected from the sample is guided through the capillary.
A single combination of a fiber situated inside a capillary may be used as a sensor. According to an embodiment, a bundle of such combinations may be used as sensor for collecting spectroscopic information. The bundle comprises at least two, according to an embodiment from 4 to 20 combinations of a fiber sitting inside a capillary.
According to this embodiment, the sensor for collecting spectroscopic information is composed of a fiber located inside a light guiding capillary and radiation for spectroscopic analysis is emitted from the fiber tip and collected after interaction with the sample by the light guiding capillary or vice versa. Fiber and capillary are transparent for radiation used to excite the sample as well as for radiation emitted, absorbed, reflected or scattered by the sample and collected by the sensor. A transparent fiber/capillary sensor allows collecting information with minimum requirements for space and the sensor may be placed very close to a sample, e.g. a catalyst surface. Further, a fiber/capillary sensor allows a very easy transfer of the collected spectroscopic information to a device placed outside of the reactor were the collected information may be processed. Further, a fiber/capillary sensor may be designed flexible such that the fiber may easily be placed inside the reactor chamber. The fiber and the light guiding capillary may have very small diameters.
According to an embodiment the diameter of the fiber is less than 1 mm, according to a further embodiment is less than 600 μm and according to a further embodiment is less than 500 μm. The fiber may have a diameter of only few 100 μm. According to an embodiment, the diameter of the fiber is at least 10 μm, according to a further embodiment at least 100 μm. The fiber may comprise a single fiber or may be a bundle of fibers. Some of the fibers may be used for transmitting light for excitation of the sample whereas others may be used to collect radiation emitted, absorbed, reflected or scattered by a sample inside the reactor chamber and to guide the light to a device for processing. When using a fiber bundle, the outer diameter of the individual fibers may be selected within a range of down to about 10 μm.
When using a combination of a fiber sitting inside a capillary, the inner diameter of the light guiding capillary is chosen to match closely the outer diameter of the inner fiber or the inner fibers being used. The thickness of the capillary wall preferably is selected within a range of 10 μm to 500 μm, more preferred 50 μm to 200 μm.
The fibers or capillaries making up the optical sensor are made of a material transparent to the radiation emitted by a sample and used to collect spectroscopic information. A suitable material is e.g. fused silica that also may be doped with a suitable dopant to increase transparency of the fiber. Another suitable material is sapphire. Also photonic crystal fibers are suitable.
The individual fibers of a fiber bundle may be arranged in any order but preferably are arranged in circular closed package. According to an embodiment, the fiber bundle takes the form of a circular closed package at its end located inside the reactor chamber whereas at the other end the fiber bundle is separated into individual fibers which are connected to a radiation source or a device for analyzing and processing the collected spectroscopic information. In an analogous manner also a combination of a fiber sitting inside a capillary may be used in the form of a bundle.
The sensor preferably comprises a tip and the spectroscopic information is preferably collected at the tip of the sensor.
When collecting spectroscopic information inside a reactor it may be of interest for some embodiments to exactly know the site where the spectroscopic information is collected. This is of particular interest in a reactor in which reaction conditions differ depending on the position within the reactor, e.g. in a flow reactor. According to an embodiment, a sensor is used that receives spectroscopic information from a limited and well defined area of the sample.
According to an embodiment, the fiber used as a sensor to collect spectroscopic information is beveled at its tip located inside the reaction chamber. According to a preferred embodiment, the fiber tip takes the form of a cone. By suitably adjusting the cone angle at the tip of the fiber it is possible to select an angle relative to the fiber axis in which light is emitted from and collected by the sensor. According to an embodiment, the cone angle of the fiber tip is selected such that light used for exciting a sample leaves the fiber tip at an angle about perpendicular to the fiber axis. The angle between the fiber axis and the direction of the light emitted by the fiber for excitation of the sample is preferably selected within a range of 80 to 90°.
Since light collected at the fiber tip takes about the same route as light scattered by the fiber, although in opposite direction, also spectroscopic information collected by the fiber stems from a defined and limited area inside the reaction chamber.
According to a particular preferred embodiment, the optical sensor for irradiation of the sample and collection of the spectroscopic information consists of a fiber located inside a light guiding capillary. Both ends of fiber and light guiding capillary located inside the reactor are beveled to emit and collect light at a defined angle to the fiber axis.
The ends of the fiber and the light guiding capillary can be aligned or displaced from each other by a defined distance. According to the latter embodiment the fiber projects from the tip of the capillary.
According to an embodiment, the ends of the fiber and the light guiding capillary outside the reactor are longitudinally distant from each other to allow independent acceptance of light from an external light source and collection of the light emitted or scattered by the sample.
In a reactor with inhomogeneous reaction conditions, e.g. a flow reactor, it is of particular interest to obtain spatially resolved spectroscopic information from inside the reactor chamber. This allows recordation of a reaction profile along e.g. a longitudinal axis of a reactor, i.e. in or against the direction of fluid flow.
According to an embodiment the sensor for collecting spectroscopic information is arranged moveably inside the reactor chamber, in particular is movable parallel to an axis of the reactor, preferably a longitudinal axis of the reactor. For example, for investigating reaction processes occurring in a flow reactor, it is preferred to move the sensor in the direction or against the direction of the fluid flow to obtain a reaction profile. The movement of the sensor preferably is performed with high precision and in small steps, e.g. manually by a micrometer screw or by a motor or by a step motor. When using a motor for moving the sensor inside the reaction chamber collection of spectroscopic data may be automated.
According to a further embodiment, the sensor for collecting spectroscopic information is rotatable around its longitudinal axis. A catalyst bed may be inhomogeneous, e.g. when using a reticulated foam as a carrier for the catalyst. By rotating the sensor around its longitudinal axis such inhomogeneity will be averaged. This is of particular interest when using the sensor for collecting spectroscopic information together with other types of sensors.
According to an embodiment, the sensor for collecting spectroscopic information is combined with at least one temperature-sensitive sensor. This combination allows obtaining spectroscopic information from a particular site inside the reactor chamber and at the same time to measure the temperature at this reaction site. Preferably, the sensor for collecting spectroscopic information and the at least one temperature sensitive sensor are arranged in close proximity to each other such that the corresponding information originates from the same place inside the reaction chamber.
Depending on the type of temperature sensitive sensor used, it is possible to obtain information on the temperature of the catalyst, i.e. on the stationary phase, or on the temperature of a gas or liquid phase, i.e. the mobile phase. When using a thermocouple the temperature sensitive sensor is in direct contact with fluid. This allows determination of the temperature of e.g. the fluid phase. When using a pyrometer fiber as a temperature sensitive sensor, thermal radiation emitted e.g. from a catalyst surface can be collected and transformed into a temperature of the solid using a pyrometer. The pyrometer is preferably operated in quotient mode as the emissivity of the catalyst bed might change along the profile coordinate.
When using a beveled fiber tip of a distinct cone angle light emitted from the fiber is distributed in a direction having a defined angle relative to the fiber axis. Light emitted from the catalyst bed will be collected from the same acceptance angle. This allows spatially resolved temperature measurements, e.g. of a catalyst surface when moving the temperature sensitive sensor along e.g. a longitudinal axis of the reactor.
According to a particular preferred embodiment the sensor for collecting spectroscopic information is combined with a device for collecting a sample inside the reaction chamber, preferably a sample of the fluid phase. This embodiment allows kinetic measurements on the fluid and the solid phase and optical spectroscopy at the same time wherein the measurements according to an embodiment may be performed in a spatially resolved manner.
The device for collecting a sample preferably is arranged in close proximity to the sensor for collecting spectroscopic information such that information obtained by the sensor for collecting spectroscopic information may be correlated to kinetic data deduced from analysis of the collected samples. The device for collecting a sample preferably is connected to an analytical device that allows qualitative and/or quantitative analysis. For analysis the sample may be transferred e.g. to a mass spectrometer, a gas chromatograph or an HPLC-apparatus.
According to a preferred embodiment the device for collecting a sample inside the reactor chamber is a sampling capillary comprising an orifice for sample collection. The sampling capillary preferably has an inner diameter of less than 3 mm, according to a further embodiment of less than 2 mm, according to a still further embodiment of less than 1 mm. The sampling capillary preferably is made of a material that is inert under the reaction conditions present in the reaction chamber. According to a particularly preferred embodiment, the sampling capillary is made of a material transparent to electromagnetic radiation emitted, absorbed, reflected or scattered by a sample and collected by the sensor for collecting spectroscopic information. According to an embodiment, the material is transparent to UV/VIS, NIR and/or IR-radiation. A suitable material for the sampling capillary is e.g. fused silica.
The sampling capillary has an orifice for collecting a sample. The sample may be transported through the capillary by a pressure gradient between the reactor chamber and the end of the capillary situated outside the reactor chamber.
According to a preferred embodiment, the orifice for collecting a sample is situated at a sidewall of the sampling capillary, preferably distant from a closed end of the sampling capillary. The diameter of the orifice depends on the size of the sampling capillary and preferably is less than 200 μm and according to an embodiment is within a range of 50 to 150 μm.
According to an embodiment, the distance between the closed end of the sampling capillary and the orifice located in the sidewall of the sampling capillary is selected such that the end of the sampling capillary remains seated within a bearing when the orifice is in a position corresponding to the maximum deflection of the sampling capillary. Such bearing may be formed e.g. by a channel drilled inside the catalyst bed.
In a particularly preferred embodiment, the sensor for collecting spectroscopic information is arranged inside the sampling capillary used for collecting a sample inside the reactor chamber. When providing the sensor for collecting spectroscopic information in the form of a transparent fiber or in the form of a fiber sitting inside a light guiding capillary these optical sensor components may be guided through the interior space of the capillary. A small gap is formed between the outer wall of the optical sensor and the inner wall of the sampling capillary which allows transportation of the collected sample to a corresponding device for analysis. The gap preferably has a width of at least 50 μm and according to an embodiment is selected within a range of 100 to 500 μm.
According to a further preferred embodiment, the sensor for collecting spectroscopic information has the form of a fiber or of a fiber located inside a light guiding capillary and the tip of the sensor is arranged at a position corresponding to the orifice provided in the sampling capillary for sample collection. The sensor may comprise a single fiber or a single combination of a fiber sitting inside a capillary or may have the form of a bundle of fibers or a bundle of capillaries with a fiber located in the interior of the capillary. This arrangement allows an immediate correlation of the spectroscopic information and the composition of a sample collected at the same position. When using an arrangement wherein the sensor for collecting spectroscopic data is situated in the interior of the sampling capillary the sampling capillary is preferably formed from a material transparent to radiation used for collecting spectroscopic information.
For spatially resolved collection of spectroscopic information as well as of other data, e.g. temperature or composition of the fluid phase, it is advantageous when the sensor may be moved with high precision inside the reactor, preferably without causing pressure losses. According to an embodiment, the sampling capillary for collecting a sample is guided in a liner, e.g. a stainless steel liner. The length of the liner is selected to allow a secure guidance of the capillary and to avoid deflection of the capillary when sliding the capillary up and down.
According to an embodiment, the length of the liner is at least 30 mm and is selected according to a further embodiment within a range of 50 to 200 mm. The inner diameter of the liner is selected such that the capillary fits neatly into the liner still allowing up and down movement of the capillary. The liner is fixed to a suitable support to allow relative movement of capillary and liner. High viscous grease may be used to fill the gap between liner and capillary and to alleviate sliding of the capillary.
According to an embodiment, two liners are used each mounted to a suitable support. Between both liners is left an open space and this space is located in a reservoir for high viscous grease. When the sampling capillary is moved along its longitudinal axis, its outer surface picks up a thin grease layer. The annular gap between the capillary and the liner has a width of few micrometers, preferably less than 50 μm, according to an embodiment of less than 40 μm. Even at high operation pressure in the reaction chamber the high viscous grease therefore will not be squeezed through the annular gap. On the other hand, the grease allows slow movement of the capillary with minimum force as the pressure difference exerts only small forces.
The reactor according to an embodiment comprises a solid catalyst bed inside the reaction chamber. In a particular embodiment, the solid catalyst is formed from one or several reticulated solid foams. To guide the movement of the sensor for collecting spectroscopic information, in particular in an embodiment when the optical sensor is inserted into a sampling capillary for collecting samples, a channel is preferably provided in the solid catalyst, preferably solid foam, to accommodate the sensor and/or the capillary. The annular gap formed between the channel wall and the capillary/sensor preferably is narrow to avoid bypassing of fluid phase and preferably has a width of less than 100 μm.
According to a further aspect the invention is directed to a method for analyzing a reactor state comprising the steps:
The reactor described above is in particular suited for in situ investigation of a reaction performed inside the reactor. It allows obtaining information on e.g. a structure or electronic state of a component involved in the reaction at a particular site inside the reactor chamber. As already described above, electromagnetic radiation emitted from a sample may be collected to be displayed in appropriate form, in particular in the form of a spectrum. The electromagnetic radiation may be emitted directly by a sample inside the reactor or may be collected after suitable excitation of the sample, e.g. by irradiation of the sample by light, e.g. laser light. Preferably, optical spectroscopy is used to collect spectroscopic information. A suitable exemplary method for collecting spectroscopic information is Raman spectroscopy. Light of particular wavelength, e.g. laser light, is directed onto a sample and inelastically scattered light is collected as spectroscopic information. However, other spectroscopic methods may be used as well. Suitable methods are e.g. UV/VIS-spectroscopy, NIR-spectroscopy, IR-spectroscopy and fluorescence spectroscopy.
According to an embodiment, the method according to the invention allows spatially resolved optical spectroscopy directly at a reaction site inside the reactor. By the method according to the invention detailed information on processes occurring inside the reactor may be obtained. In particular it is possible to obtain a reaction profile by moving the sensor along a defined pathway in the reaction chamber.
According to an embodiment the spectroscopic information is collected at a first position inside the reactor chamber to obtain a first data set, preferably in the form of a spectrum. Spectroscopic information is then collected at least at one further position inside the reactor chamber to obtain at least a second data set, preferably in the form of a spectrum. The sensor may then be moved to a third position as well as to further positions inside the reactor wherein at each position spectroscopic information is collected. By following a defined movement program the method according to the invention allows a detailed interpretation of processes occurring inside the reactor.
According to a further embodiment, information obtained by collecting spectroscopic information with the sensor may be combined with temperature measurements. As already described above, e.g. a thermocouple may be used to measure the temperature of a fluid transported inside the reactor. According to another embodiment, a properly beveled pyrometer fiber may be used to collect thermal radiation emitted from a sample inside the reactor chamber, e.g. from a catalyst and transformed into a catalyst temperature using a pyrometer.
In a particularly preferred embodiment, optical spectroscopy is combined with collection of a sample inside the reaction chamber which sample may then be analyzed for its composition. This allows combination of e.g. kinetic measurements and optical spectroscopy. Such measurements may also be performed in a spatially resolved manner as has been described above.
According to a most preferred embodiment of the method according to the invention temperature measurement and sample collection is performed in such way that all data may be linked to a defined position inside the reactor chamber. To allow correlation of the information obtained the temperature is preferably determined and/or the sample is collected at a place inside the reactor chamber located in close proximity to the site for collecting spectroscopy information inside the reactor chamber.
The invention will be explained in more detail with reference to the attached drawings. The figures display:
a schematically shows a section of the setup of the interior of a catalyst chamber in which is situated a moveable sensor for collecting spectroscopic information as well as other information, in particular information on kinetic data and on temperature. A catalytic reaction is conducted in a flow reactor containing a catalyst bed 1 sandwiched between two inert packings 2. The catalyst bed 1 as well as the packings 2 may be formed e.g. by reticulated refractory foams, e.g. α-Al2O3 foams. To obtain catalyst portion 1, the foam may be coated e.g. with a thin layer of a catalytically active material according to known methods. The inert packings 2 are used to check in each experiment for activity of the support as well as pre- and post-catalytic chemistry. Through all, catalyst bed 1 and inert packings 2, goes a channel 3 into which a sampling capillary 4 is slidably introduced. The sampling capillary 4 is sealed at the upper end 5. Channel 3 has a diameter slightly larger than the outer diameter of sampling capillary 4 such that sampling capillary 4 fits tightly into channel 3 and may be moved up and down along its longitudinal axis. Only a small gap is formed between sampling capillary 4 and the wall of channel 3 such that bypassing of large amounts of fluid phase through the gap is avoided. Profile of reactants, intermediates and products are measured by moving sampling capillary 4 with micrometer resolution through channel 3. Sampling occurs via a side sampling orifice 6 drilled at such distance from closed capillary tip 5 that no open channel 3 results from moving sampling capillary 4 up and down. In addition to species sampling capillary 4 may accommodate various other probes, like a thermocouple, a pyrometer fiber or a sensor for optical spectroscopy allowing measuring the fluid temperature, the catalyst temperature and/or spectroscopic information from the catalyst and/or the gas phase, respectively. To correlate information about species sampled through side sampling orifice 6 and temperature or spectroscopic information the temperature or optical sensor used should be aligned with the side sampling orifice 6 in position 7 or should be at any other defined position with respect to the side sampling orifice 6. In an arrangement according to the invention a sensor A can be used comprising a single light guiding fiber 14 (
For kinetic measurements a sample will enter through sampling orifice 6 and will be forced through the gap between sampling capillary 4 and either fiber 14 if sensor A is used or light guiding capillary 15 if sensor B is used for collecting spectroscopic information. The sample will then leave sampling capillary 4 at position 11 where sampling capillary 4 has a gap to be transferred to an analytical device, e.g. a mass spectrometer or a gas chromatograph. To force fluid flow in this direction either an overpressure is applied at side sampling orifice 6 or an underpressure is applied at the gap 11 in sampling capillary 4. To direct the fluid flow from the reactor into the analytical device sampling capillary 4 has to be sealed at location 8.
b and 1c show in detail a section of sampling capillary 4 into which is introduced either sensor A composed of a light guiding fiber 14 for collecting spectroscopic information or sensor B composed of a light guiding fiber 14 inserted in a light guiding capillary 15, respectively. If sensor A is used, the tip 14a of the light guiding fiber 14 is beveled to allow illumination and light collection perpendicular to the longitudinal axis of the sensor. If sensor B is used both the tip 14a of the light guiding fiber 14 and the front face 15a of the light guiding capillary 15 into which the light guiding fiber 14 is inserted are beveled and aligned to allow separate illumination and light collection perpendicular to the longitudinal axis of sensor B. The cone angles of 14a and 15a can be different and there can be a fixed distance between 14a and 15a, however, typically 14a and 15a have the same angle and are aligned flush as shown in
In the embodiment shown in
By moving either sensor A or B along the longitudinal axis, it is possible to perform spatially resolved optical spectroscopy for the characterization of the bed of catalyst 1 and inert packings 2.
Raman spectroscopy is a powerful method for in situ studies in heterogeneous catalysis and has been chosen as a test method but other optical methods like photoluminescence spectroscopy, UV/VIS-, NIR- and IR-spectroscopy are applicable in a similar manner.
The gas flow through the reactor setup is arranged from left to right and through the reactor tube 18 from bottom to top. At the inlet side, three 6 mm gas pipes are connected to digital mass flow controllers 29 (Bronkorst, type EL-FLOW® equipped with Vary-P valves for large pressure differences). In oxidation catalysis the feed gas is typically composed of an oxidant, like O2, N2O, a hydrocarbon, like CH4, C2H6, C3H8, and an inert standard, like Ar, He, N2. Therefore, three gases can be mixed using this gas supply. The feed gas mixture enters reactor tube 18 from the bottom, flows through the bed of inerts 2 and catalyst 1 and leaves at the top. After leaving at the top, the reactor effluent gases pass through a tube in a tube heat exchanger 30 to remove all condensable species which are collected in a reservoir 31. Reservoir 31 can be emptied under operation without pressure loss by means of a needle valve. The dry reactor effluent passes through pressure controllers 32 (Bronkhorst, type EL-PRESS®) which expand the gases to atmospheric pressure. For precise pressure measurement in the reactor, a digital pressure gauge (Leitenberger, type TLDMM-A01) 33 is connected close to outlet of the reactor tube 18.
To heat the catalyst, the quartz reactor tube 18 is surrounded by a split furnace 37 (KANTHAL, type Fibrothal, 450 W, maximum element temperature 1150° C., only one half shown for clarity) connected to a power supply (not shown) and a temperature controller (Eurotherm, model 2416, not shown). The heat transfer between heater and catalyst bed occurs primarily via radiation resulting in a maximum temperature of about 1000° C. without accounting for any heat of reaction. Taking into account that many catalytic oxidation reactions liberate significant amounts of heat, temperatures above 1000° C. can result. For methane oxidation on Pt, the reactor has been successfully operated at temperatures up to 1300° C.
As reacting gas mixtures containing fuel and oxidant under pressure are inherently dangerous, a number of safety measures are taken in designing and operating the reactor. In the first place, feeding of potentially explosive mixtures to the reactor can be excluded by providing upper and lower limits to the mass flow controllers 29 which are outside the flammability limits of the gas mixture. Secondly, the reactor is equipped with a ½″ burst disk 34 manufactured and rated to a burst pressure of 45 bar+11−8% (Bormann & Neupert). In case of an explosion, burst disk 34 breaks and releases the pressure instantaneously. In addition to the burst disk, the reactor itself functions like a pressure relief valve, as number and strength of the springs between base plate 22 and cover plate 23 determine the leak pressure of the system. Depending on the application and the target pressure the spring load can be adjusted to a value slightly higher than the target pressure. This value can be anything between atmospheric pressure and 45 bar. To protect the operator in case of a bursting reactor tube, the ceramic split furnace 37 surrounding the reactor tube is enclosed in a metal housing (not shown). Additionally, the entire reactor is shielded by a safety screen (not shown).
For measurement of high resolution gas species profiles a thin quartz sampling capillary 4 (O.D.=700 μm, I.D.=530 μm) runs through the center of the quartz reactor tube 18 and the packing of inerts 2 and catalyst 1 through channel 3. As already explained with reference to
To measure spatially resolved gas and surface temperature profiles, a thermocouple or a pyrometer fiber can be inserted in sampling capillary 4, respectively. If the fiber or the thermocouple tip is aligned with side sampling orifice 6 at position 7, the measured temperature can be assigned to the respective fluid composition measured at the same location in catalyst bed 1. In case a thermocouple is used, direct thermal contact exists between the sampled gases and the thermocouple tip but no direct thermal contact exists between the thermocouple tip and catalyst bed 1. Therefore, the thermocouple measurement is strongly biased to the actual fluid temperature. In case a properly beveled pyrometer fiber is used, thermal radiation emitted from the catalyst surface can be collected and transformed into a catalyst temperature using a pyrometer 35. The pyrometer should be operated in quotient mode as the emissivity of catalyst bed 1 might change along the profile coordinate.
For optical spectroscopy an optical sensor can be inserted into sampling capillary 4. As shown in detail in
The reactor described above was used to study methane oxidation on autothermally operated Pt coated α-Al2O3 foam catalysts at pressures from 1.4 to 15 bar. The pressure was varied to investigate whether gas phase reactions proceed in parallel to catalytic reactions and whether transport limitations occur. The catalyst section was formed by two α-Al2O3 foam catalysts (45 pores per linear inch, Vesuvius Hi-Tech) coated by 0.6 wt % Pt and sandwiched between two uncoated 80 ppi foams which served as front heat shield and as back heat shield, respectively. In the further discussion, taking into account a flow from bottom to top, the front heat shield is denoted FHS, the two Pt coated catalyst foams are denoted Pt and Pt and the back heat shield is denoted BHS. Channels having a diameter of 1 mm for accommodating sampling capillary 4 were drilled before coating the catalyst foams with Pt. A gas mixture consisting of CH4/O2/Ar (911/228/858 ml·min−1, respectively) was fed through reactor tube 18 from bottom to top. To assign a catalyst temperature to each gas species composition, the solid temperature was measured by means of a pyrometer fiber inserted in sampling capillary 4 and aligned at position 7 with the side sampling orifice 6 to obtain an arrangement according to
Profiles were measured for pressures of 1.4 bar, 5.1 bar, 10 bar and 15 bar. All data are summarized in
Although the experimental results shall not be discussed in detail, the profiles reveal immediately interesting details of the methane oxidation mechanism. It can be seen that there exists a short zone behind the catalyst entrance where CH4 and O2 are rapidly converted. The chemistry in this entrance zone which extends from 0-2400 μm can be described by Equation 1 with x taking values between 0≦x≦2 and y taking values between 0≦y≦1.
Non-catalytic methane total oxidation occurs for pressures p≧10 bar as can be seen from
Upon complete O2 conversion, secondary reactions like steam reforming (CH4+H2O→CO+3H2) and watergas shift (CO+H2O→CO2+H2) are taking place until the mixture leaves the catalyst (2400-20500 μm).
It is very interesting to note that the O2 profiles fall on top of each other and are highly linear regardless whether the reactor pressure was 1.4, 5.1, 10 or 15 bar (
In this example the oxidative coupling of methane to C2 hydrocarbons (C2H6, C2H4 and C2H2) on Li doped MgO is investigated. This reaction is debated in the literature in terms of a so-called heterogeneous-homogeneous mechanism which means that the Li/MgO catalyst produces CH3 radicals which desorb into the gas phase and couple there to CH3—CH3 which is further dehydrogenated to CH2═CH2. The methane oxidative coupling, even though highly attractive from an industrial point of view, is still at the research stage as one-pass yields higher than about 25% could not yet been achieved. The reason for this virtual bond is still unclear.
The experiment was conducted in an analogous manner as described in Example 1 with the only difference that two 0.8 wt % Li/MgO coated α-Al2O3 foams served as catalyst bed, much lower flow rates were used (CH4/O2/Ar=25.6/6.4/8.0 ml·min−1) and that the reactor was heated to about 780° C. Very low total flow rates had to be used as methane oxidation on Li/MgO is much slower than on Pt and external heating was necessary as the reaction does not produce sufficient heat for autothermal operation. The spatial profiles are depicted in
The pyrometer temperature profile shows that the reactor is very isothermal in the catalytic section (788±2° C.) which is an important requirement for kinetic measurements. The increase of the temperature curve towards the ends of both heat shields is a measurement artifact as thermal radiation from the much hotter heating coils of the split furnace 37 is reflected by upper and lower mounting clamps 19 and 20, respectively into the heat shields. Longer heat shields would eliminate this problem.
The species profiles reveal a number of interesting details. It can be seen that methane is lost with constant rate by total oxidation to CO2 and H2O. This total oxidation occurs obviously in the gas phase as it is not restricted to the catalyst section. C2 coupling products (at 780° C. only C2H6+C2H4) are not formed before the reactants hit the catalyst section indicating that no pre-catalytic C2 formation occurs. Even though only small amounts of C2 are formed the profile shows clearly that C2 is formed with a constant rate and that the formation continues behind the catalyst section in the back heat shield. This could be due to loose catalyst material that has been transported by the gas flow to the back heat shield. However, as the gas flow is extremely slow and directed against gravity this explanation is not very likely. The post-catalytic C2 production is more likely an indication for gas phase formation of C2 products triggered by the catalyst in line with the postulated “heterogeneous-homogeneous mechanism”. CO seems to be a secondary product as its formation starts quite late in the catalyst section. Virtually no hydrogen is formed at all.
To demonstrate that spatially resolved spectroscopy is possible, Raman spectroscopy was chosen as spectroscopic method. For demonstration of spatial resolution, an experiment as shown in
Sensor B (
In all experiments the scattered light was analyzed by means of a triple filter Raman spectrometer (TriVista S&I GmbH) operated in subtractive mode.
For both optical sensors A and B, the actual measured spectra are a linear superposition of the Raman spectrum of the sensor material (fused silica) and the Raman spectrum of the sample at the place of analysis. The contribution of the signal from the sensor material cannot be fully eliminated experimentally in the spatially resolved Raman measurements demonstrated here, as no optical filters can be used at the tip of either sensor A or B due to the small dimensions of the sensors and the high temperatures in the quartz reactor tube 18. Nevertheless, as shown in
The position accuracy and the spatial resolution of the method was determined for Sensor A and B and evaluated as shown exemplarily for Sensor A in
Finally it will be demonstrated that it is possible to measure also Raman spectra of catalytically relevant materials forming catalyst bed 1 which are typically much weaker Raman scatterers than sulfur which was only chosen for demonstration purposes of spatially resolved Raman measurements. 30% by weight Molybdenumtrioxide, MoO3, a catalyst active e.g. for oxidative dehydrogenation of ethane to ethylene was supported on 1 mm γ-Al2O3 spheres and filled in the quartz reactor tube 18 shown in
In summary it could be demonstrated that spatially resolved optical spectroscopy is possible with the invention described herein.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/08973 | 12/15/2009 | WO | 00 | 8/27/2012 |
