Abstract:
An optical apparatus for measurement of industrial chemical processes. The analyzer uses Raman scattering and performs measurement of chemical concentrations in continuous or batch processes. The analyzer operates at a standoff distance from the analyte (or analytes) and can measure concentrations through an optical port, facilitating continuous, non-destructive, and non-invasive analysis without extracting the analyte or analytes from the process. The analyzer can measure one or several solid, liquid, or gaseous analytes, or a mixture thereof.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Defined as the analysis of material properties in industrial manufacturing processes, process analysis has been performed for several decades in a wide variety of industries. These industries include chemical, petrochemical, petroleum, pharmaceutical, food &amp; beverage, pulp &amp; paper, and agricultural. A former common implementation of process analysis consisted of manually extracting samples from a process and carrying the samples to a laboratory for analysis. Over time, process analysis evolved from off-line analysis to a continuous on-line analysis where samples are extracted by automated sampling systems and carried in slip streams to process analyzers. 
         [0002]    The primary advantage of on-line process analysis is the reduction of the time interval between sample extraction and data generation. The faster response time provides greater control of manufacturing processes leading to increased product yield, improved product quality (consistency), reduced in-process inventory, reduced operating and maintenance workforce, reduced energy consumption, reduced consumption of raw material inputs, and reduced production of waste streams. 
         [0003]    Several instruments are currently used for industrial process monitoring. Gas chromatographs (GCs), for example, measure differences in molecular mobility to identify multi-component samples. GCs have high specificity and high sensitivity. They require shielded enclosures for protection from the environment, a supply of column gas, frequent maintenance, and water trapping especially in corrosive applications. These instruments are widely discussed in published literature. 
         [0004]    Infrared (IR) instruments rely on material absorption to analyze samples. IR instruments include Fourier Transform Infrared (FTIR) analyzers, IR dispersive analyzers, and non-dispersive IR (NDIR) analyzers. Non-dispersive instruments include filter and non-filter based instruments. IR instruments have displaced other types of instruments due to higher speed, sensitivity, and specificity. IR instruments typically induce a net change in dipole moment in the molecules of a sample as a result of rotational or vibrational motion. The method works well for many species, but fails for homonuclear species such as nitrogen, oxygen, chlorine, hydrogen, and fluorine that cannot have a net change in dipole moment. 
         [0005]    Electrochemical sensors provide other means for quantifying species concentrations. These types of sensors are typically limited to the measurement of a single species and often supplement IR methods. 
         [0006]    An alternative approach for industrial process monitoring includes the use of Raman methods. Raman spectroscopy is based on the inelastic scattering of light off molecules. As a process analysis technique, Raman spectroscopy has advantages over other techniques as it requires no sample extraction or sample preparation, can perform continuous in-situ quantitative measurements, can analyze pipe content through a sight window, can detect molecules that other techniques cannot, and is unaffected by water molecules. 
         [0007]    As a result, Raman spectrometers have found a niche in the market where no other viable solutions exist. Despite these advantages, broad adoption of Raman spectrometers has been hindered because they are very expensive to buy, install and maintain, require frequent calibrations and skilled operators and, in general, lack the robustness necessary to operate in harsh plant environments. 
         [0008]    In order for a Raman instrument to be widely accepted for industrial process monitoring, it must have low cost and have high performance. The present invention uses fewer and more readily available components than other Raman instruments, and is easily manufactured and adapted to different applications. It eliminates the use of optical fiber hence achieves high optical throughput. The invention also uses increased amplification with robust multi-stage photon-to-electron amplifiers, and optimized optical filter designs. Further, the invention can withstand tough industrial conditions and uses low cost and wavelength stabilized laser sources. 
         [0009]    Raman spectrometers are part of a general class of instruments called optical analyzers. Optical analyzers are generally based on one of six phenomena: absorption, fluorescence, phosphorescence, scattering, emission, and chemoluminescence. These phenomena can occur in the ultraviolet, visible, and infrared portions of the spectrum. A typical instrument contains five basic elements: a radiation source, a sample container, a spectral element to look at a specific region of the spectrum, a detector that converts photons to electrons, and a signal processor. Raman is classified as a second order scattering process in that Raman scattered photons are created from the inelastic interaction of incident light photons with the molecules of the sample. These second order photons are weak, typically 10 6  to 10 7  times less intense than first order elastically-scattered photons. 
         [0010]    U.S. Pat. Nos. 4,648,714, 4,784,486, 5,521,703, and 5,754,289 use Raman scattering to perform gas analysis. Gases flow through a section of tube while a laser beam is directed into it. These inventions require a slip stream or redirection of the sample away from a pipeline or reactor. Most use a filter wheel in conjunction with a single detector. U.S. Pat. No. 5,521,703 differs slightly from the other three in that its multiple detectors are arranged along the length of gas sampling cell within a laser resonator configuration. U.S. Pat. No. 5,754,289 teaches the use of a filter wheel in conjunction with an integrating sphere for the sample. The related U.S. Pat. Nos. 5,386,295, 5,357,343, and 5,526,121 teach the use of a filter wheel spectrometer coupled to reference and sample elements using fiber optic probes. U.S. Pat. Nos. 5,963,319 and 6,244,753 teach the use of a dispersive spectrometer and fiber optic couplers for industrial process monitoring. Fiber optic couplers are known to limit optical throughput. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a photometric analyzer incorporated into a small, low cost, and robust package for in-situ industrial process monitoring applications. The analyzer can measure homogeneous or inhomogeneous chemical mixtures made up of one or several solid, liquid, or gaseous analytes. The analyzer uses Raman scattering and maximizes optical throughput, increases the signal-to-noise ratio of the system, and incorporates on-board quantification of process concentrations. The present invention requires neither extraction nor redirection of material from the originating process pipeline or vessel. It can analyze chemical concentrations remotely from a process, i.e. the analyzer can be separated or physically detached from the process pipeline or vessel. The present invention can analyze chemical composition in processes operating under a wide range of conditions. Examples include process pressures from sub-atmospheric to thousands of psi, process temperatures from sub-zero to hundreds of degrees Celsius, and process flows from stagnant to hundreds of liters per minute. 
         [0012]    The analyzer comprises a laser radiation source, which may be any type of laser but preferably a solid state laser diode. The temporal characteristics of the laser radiation are controlled by an integrated laser controller module that, in turn, is controlled by a microprocessor. The laser output is spatially shaped and directed across a free-space light path toward the process sample. Free-space propagation is defined as the propagation of an optical beam predominantly through gases or vacuum with discrete optical components and windows to control focus, spectral characteristics, and other properties. The laser radiation is then incident upon the sample located outside the analyzer. The Raman radiation scattered by the sample is collected by the free-space shaping optics of the analyzer, which adjust the spatial characteristics of the scattered radiation for transport to the detector module. One or more spatial and/or optical filters are used to reduce or eliminate the amount of radiation at the excitation wavelength that is introduced into the detector module without substantially reducing the amount of desired Raman signal. One or more additional filters are used to extract targeted spectral bands from the Raman signal. The signal of each spectral band passes through low-noise, high-gain amplifiers that increase analog signal levels without introducing significant perturbations. The analog signal levels are quantitatively measured by means of analog-to-digital signal converters. The resulting digital signals are processed by a dedicated embedded microprocessor or some other data control system. Using calibration information from known sources such as chemical samples or suitable reference standards, the signals generate a quantitative measurement of the analytes either directly or via a mathematical deconvolution. 
         [0013]    The present invention can be customized for specific applications by identifying which of several chemical species are to be measured. Ideally, the Raman radiation of the desired chemical species comprises spectral components that are largely independent from the scattered radiation of the other molecules. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic representation of the invention; 
           [0015]      FIG. 2  is a schematic representation of a preferred embodiment of the invention; 
           [0016]      FIG. 3  is a schematic representation of another preferred embodiment of the invention; and 
           [0017]      FIG. 4  is a schematic representation of a portion of the optical layout of yet another preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    A schematic representation of the invention is shown in  FIG. 1 . The black arrows indicate the direction of the primary optical signals; the gray arrows indicate the direction of the primary electrical signals. A radiation module  110  which includes a laser radiation source is controlled by a control microprocessor  150 . The laser source can be of any type but is preferably a solid state laser diode. In a preferred embodiment of the invention, the laser radiation source emits radiation primarily at a wavelength of 785 nm. 
         [0019]    After leaving the radiation module, the laser radiation is transferred via free-space optics to a free-space optics module  120 . In a preferred embodiment of the invention, the optics module includes a selective reflector which treats the laser radiation differently than other radiation, in particular the radiation that comprises the return signal (defined below). The selective reflector may reflect the laser radiation while allowing other radiation to pass through, or may allow the laser radiation to pass through while reflecting other radiation. The selective reflector relies on one, or a combination of several, mechanisms including: (i) spatial selectivity such as the deposition of a mirror or transparent aperture in one portion of the selective reflector; (ii) wavelength selectivity; or (iii) polarization selectivity. In one preferred embodiment, the selective reflector has a large clear area with a small reflective spot in its center to reflect the collimated laser radiation. In another preferred embodiment, the selective reflector comprises a dichroic filter or is coated with an optical thin film that reflects light at the excitation laser wavelength and transmits light at longer and shorter wavelengths. 
         [0020]    After leaving the optics module  120 , the laser radiation exits the analyzer  100  and is incident upon a sample located outside the analyzer. The sample may be any of a large number of chemical materials and may or may not be contained behind a sample port window that is transparent to both the laser radiation and the return-signal radiation. The sample and sample port window are not part of the analyzer. The analyzer relies on Raman scattering of laser radiation by the sample. The physical processes that govern this scattering generally include both inelastic scattering (wavelength-shifting) processes, such as Raman scattering, and elastic scattering (wavelength-preserving) processes such as Rayleigh and Mie scattering. The desired signal (known as the return signal) includes the inelastically scattered Raman radiation; the undesired signal (known as the noise) includes elastically scattered radiation plus parasitical sources such as stray light. A portion of the return signal and noise is collected by the free-space optics module  120  and makes up the return radiation (known as the backscattered radiation). The spatial and spectral characteristics of the return radiation are adjusted by the free-space optics module. One or more spatial and/or optical filters are used to reduce or eliminate the amount of radiation at the excitation wavelength without substantially reducing the amount of desired return signal. After exiting the optics module, the return radiation is transferred via free-space optics to the photometric detector module  130 . 
         [0021]    The photometric detection module includes one or more filters that extract targeted spectral bands from the return signal corresponding to specific chemicals of interest. The filters can be bandpass or notch filters, and can be fixed-wavelength or tunable filters. After separation from the composite beam, the selected wavelengths of light are directed to photo-sensitive detectors capable of detecting photon streams. The detectors generate analog voltage and/or current responses which are proportional to the number of photons detected at a given time. The composite signal-bearing light-stream is divided into discrete paths such that the photo-detectors receive measurable quantities of light in real time. In a preferred embodiment of the invention, multiple filters and multiple detectors are used to measure multiple wavelength components simultaneously. An advantage of simultaneous detection is the reduction of the total signal collection time to achieve a desired measurement sensitivity. Another advantage of simultaneous detection is the ability to distinguish analyte concentration changes from common mode effects such as sample density changes or variations in sample transparency to incident radiation. In another preferred embodiment of the invention, the filters are positioned on a movable stage such as a rotating wheel, linear slide, or other geometric configuration permitting wavelength separation. In a third preferred embodiment of the invention, wavelengths are separated using an electro-optic filter utilizing either Pockels or Kerr media for the monochromator. An advantage of using a tunable filter, electro-optic filter, or movable stage is the ability to use a lower number of detectors. In a fourth preferred embodiment, wavelengths are separated using a diffractive element (such as a reflective or refractive grating) or a dispersive element. In this embodiment, the different wavelength components can be detected with an array of detectors. 
         [0022]    In some cases, such as the analysis of low-pressure gas, the detector signals are increased using low-noise, high-gain amplifiers capable of increasing analog signal levels without introducing significant perturbations. In a preferred embodiment, the analog signal paths are divided into stages, each designed for stability and low susceptibility to electronic and thermal noise. The inputs to each amplifier are shielded to prevent electronic pickup of signal from external sources. The analog signal levels are quantitatively measured by means of analog-to-digital signal converters. 
         [0023]    Two-way communication between the photometric detection module and control microprocessor  150  is carried on digital control bus  140 . In a preferred embodiment of the invention, each signal is monitored with a dedicated analog-to-digital converter so as to minimize integration time and add speed to the data analysis without signal degradation. The microprocessor provides digital data such as measurement data and analyzer status information to one or more external devices. In a preferred embodiment of the invention, the analyzer also employs in-situ physical presentation of data which can be monitored without an external device. 
         [0024]    A schematic representation of another preferred embodiment of the invention is shown in  FIG. 2 . Optical radiation is generated by laser source  212 . The radiation source is temperature controlled by laser cooler  214 , and the radiation is shaped spatially and spectrally by beam shaping optics  213 , which in one preferred embodiment, include lenses to control the spatial extent and collimation of the beam, a wavelength selective reflecting element to control and stabilize the central wavelength of the beam, and optical filters to limit the spectral extent of the beam. The temporal characteristic of the laser radiation is controlled by laser controller  211  which, in turn, is controlled by control microprocessor  250 . In one preferred embodiment, the temporal characteristic of the beam incorporates a boxcar modulation at a frequency below 10 MHz to facilitate discrimination against stray optical background. The laser source  212 , laser cooler  214 , beam shaping optics  213 , and laser controller  211  are included in radiation module  210 . 
         [0025]    Upon leaving the radiation module, the laser radiation enters the free-space optics module  220 . The free-space optics module includes a selective reflector  222  with a large clear area and a small reflective area in its center to reflect the laser radiation. After being reflected by the selective reflector, the laser radiation is incident upon objective lens  221  whose function it is to facilitate delivery of the laser radiation to the sample under test. 
         [0026]    The objective lens  221  collects the backscattered radiation from the sample under test. The backscattered radiation travels to the selective reflector  222  which lets a large portion of the signal through and reflects the light in the center of the reflector. The spatial characteristics of the transmitted signal from the selective reflector are adjusted by the return signal shaping optics  223 . The excitation blocking filter  224  comprises one or more lenses and one or more spatial or optical filters that serve to reduce or eliminate the amount of undesired radiation that enters the detector module  230  at the excitation laser wavelength. The excitation blocking filter  224  does not substantially reduce the amount of desired return signal but reduces the undesired elastic component of the return radiation at the excitation laser wavelength. The remaining portion of the undesired radiation is monitored to serve as a diagnostic signal that contains information regarding the operation of the instrument and the condition of the process being analyzed. 
         [0027]    The signal leaving the excitation blocking filter is incident upon one or more mirrors  225  that serve to redirect the signal towards the photometric detector module  230  and facilitate optical alignment of the analyzer. The detector module includes a filter module  231 . Upon entering the filter module, the return signal is incident at non-normal angles upon a multiplicity of optical bandpass filters  232 , each of which selectively transmits radiation at a fixed and narrow wavelength band and reflects radiation of other wavelengths towards the next filter in the chain, thus creating a devious and divaricate optical path within the filter module. Signal that is transmitted through any of the bandpass filters is focused by a detector lens onto a photodetector element  233 . Examples of photodetector elements include Silicon photodetectors, avalanche photodetectors, or photomultiplier tubes. Although six filters and detector elements are shown in  FIG. 2 , the analyzer is operable with any number of filters and detector elements. In an alternate embodiment of the invention, the filters are positioned on a movable stage such as a rotating wheel or a linear slide. 
         [0028]    In a preferred embodiment of the invention, the optical bandpass filters  232  are assembled in removable and replaceable cartridges. These cartridges are designed such that the filters  232  are pressed and held against a reference surface to provide a low-cost, easily-reconfigurable, and easily-manufacturable assembly. In the specific embodiment shown in  FIG. 2 , two cartridges can be used each holding half the filters. 
         [0029]    The output currents from the photodetector elements  233  are amplified and converted to signal voltages by photodetector amplifiers  234 . Since the laser source  212  is modulated, the photons scattered by the sample have a distinct identification and are differentiated from undesired photons from other sources. The signals from the photodetector amplifiers  234  are demodulated into DC voltage or electronic current signal levels by lock-in amplifiers  235 . Each demodulator is synchronously tied to the laser excitation modulation to allow for in-phase measurements. 
         [0030]    The analog signal levels are quantitatively measured by means of analog-to-digital signal converters  236 . In a preferred embodiment of the invention, these signal converters perform high-resolution averaging functions. Unwanted noise (including noise generated by the modulation source) is filtered, which provides more accurate DC measurements. 
         [0031]    The photodetector elements  233  are actively temperature stabilized by heating and/or cooling devices  237  coupled with a PID loop temperature controller in order to minimize thermal noise and drift. 
         [0032]    The analyzer is calibrated after assembly using known quantities of reference chemicals or a calibrated fluorescence source. The intensity of the optical signal is directly proportional to the molecular density. This proportionality provides a way to relate observed Raman photon power to sample composition. Alternatively, a quantitative measurement of the chemicals can be generated using a mathematical deconvolution employing information from a combination of photodetector elements. In another embodiment of the invention, the signals from the photodetector elements are used to provide non quantitative information about a process such as changes in concentration as a function of time or operating conditions. With the information provided by the analyzer, a user can adjust or otherwise control process conditions. 
         [0033]    A schematic representation of another preferred embodiment of the invention is shown in  FIG. 3 . This embodiment is similar to the one shown in  FIG. 2  except for the detector module  330 . The detector module contains a single detector element  333 . The optical filtering element  332  comprises either a single variable filter (such as a tunable filter or an electro-optic filter) or multiple fixed filters on a movable stage (such as a rotating wheel or linear slide). The optical filtering element  332  selectively transmits radiation at a desired wavelength band. The transmitted signal is detected by detector element  333 . Examples of detector elements include Silicon photodetectors, avalanche photodetectors, or photomultiplier tubes. A different wavelength band is allowed through the filtering element  332  by changing the properties of the filtering element or by moving the stage. 
         [0034]    The output current from the detector element  333  is amplified and converted to signal voltages by a detector amplifier  334 . The signal from the detector amplifier  334  is demodulated into DC voltage or electronic current signal level by a lock-in amplifier  335 . The analog signal levels are quantitatively measured using a high-resolution analog-to-digital signal converter  336 . 
         [0035]    In another preferred embodiment of the invention, one or more additional filters and detectors are added to the detector module  330  in order to analyze other desired wavelength bands. 
         [0036]    A schematic representation of a portion of yet another preferred embodiment of the invention is shown in  FIG. 4 . Laser radiation from radiation module  410  is incident on free-space optics module  420 . The objective lens  421  is common to both the laser radiation delivery and return signal collection but, unlike in the specific embodiment presented in  FIG. 2 , the selective reflector  422  is used only to deliver the laser radiation to the sample. In the specific embodiment shown in  FIG. 4 , selective reflector  422  need not be wavelength selective. 
         [0037]    Although the invention has been shown and described with respect to several exemplary embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.