Optical probe for process Raman spectroscopy and method of use

A method for in-line quantitative analysis of at least one process stream of a urea synthesis process, the method comprising:

• providing a spectroscopic system, comprising:
• a narrow band light source adapted to generate excitation light of a suitable wavelength;
• an optical probe, the probe including:
• a probe body having a center axis defining a proximal end and a distal end and including an aperture in the distal end;
• a window affixed in the aperture, wherein the window is substantially optically transparent, and wherein the window is compression bonded in the aperture;
• a flange adjoining the proximal end of the probe body and extending outwardly therefrom, the flange including a sealing surface and a sealing edge, wherein the flange separates an in-process portion of the probe from an ex-process portion of the probe, the ex-process portion including at least the probe body, sealing surface and sealing edge;
• and
• a collar adjoining the flange opposite the probe body,
• wherein the probe body, flange and collar define an inner volume therethrough, the inner volume configured to accommodate an optic cable such that the optic cable can pass through the probe body, flange and collar and be in optical communication with the window, and wherein at least the in-process portion of the probe consists essentially of an austenitic stainless steel material;
• a spectrometer configured to generate Raman spectra from scattered light conveyed to the spectrometer via the probe, the spectrometer including a detector, wherein the optic cable is in optical communication between the probe and the spectrometer;
• and
• a processor configured to control the spectroscopic system and to process and analyze the Raman spectra;
• generating the excitation light using the light source and transmitting the excitation light via the probe to a process sample of at least one process stream of a urea synthesis process;
• detecting the scattered light with the detector of the spectrometer via the probe and processing the scattered light to generate a Raman spectrum;
• modeling the Raman spectrum using a chemometric model, the model including a conversion of a spectroscopy band corresponding to excitation of the carbonyl di-amide bond and of a spectroscopy band corresponding to excitation of atomic bonds in the carboxyl form, the modeling performed using the processor;
• and
• determining a concentration of urea in the at least one process stream using the model of the Raman spectrum based on the modeled excitation of the carbonyl di-amide bond, wherein a concentration in the at least one process stream of carbon-containing molecules other than urea and having a carboxyl group is determined using the model of the Raman spectrum as an equivalent concentration of carbon dioxide corresponding to the excitation of the atomic bonds in the carboxyl form, the determining performed using the processor,
• wherein urea is synthesized from ammonia and carbon dioxide using the urea synthesis process at a pressure in the range of 100 to 300 bar and temperature in the range of 50 to 250° C.

A probe, comprising:

• a probe body having a center axis defining a proximal end and a distal end and including an aperture in the distal end;
• a window affixed in the aperture, wherein the window is substantially optically transparent, and wherein the window is compression bonded to the probe body in the aperture;
• a flange adjoining the proximal end of the probe body and extending outwardly therefrom, the flange including a sealing surface and a sealing edge, wherein the flange separates an in-process portion of the probe from an ex-process portion of the probe, the in-process portion including at least the probe body, the sealing surface and the sealing edge;
• and
• a collar adjoining the flange opposite the probe body in the ex-process portion of the probe,
• wherein the probe body, flange and collar define an inner volume therethrough, the inner volume configured to accommodate an optic cable such that the optic cable can pass through the probe body, flange and collar as to be in optical communication with the window,
• wherein at least the in-process portion of the probe consists essentially of an austenitic stainless steel material, and
• wherein the probe body, flange and collar form one, monolithic body.