WO1998012537A1

WO1998012537A1 - Heated fibre optic probe

Info

Publication number: WO1998012537A1
Application number: PCT/US1997/016519
Authority: WO
Inventors: Robert B. Combs; Raymond A. Mangold
Original assignee: The Dow Chemical Company
Priority date: 1996-09-19
Filing date: 1997-09-18
Publication date: 1998-03-26

Abstract

An improved fiber optic probe (10) for chemical analysis, the probe having a body (11) and at least one window (18) attached to the body (11) so that light conducted (16) from an optical fiber (15) can be shown through the at least one window (18) into a medium to be analized. The improvement is a heater (22-22a) in contact with the body (11) so that the at least one window (18) can be heated to an elevated temperature.

Description

HEATED FIBER OPTIC PROBE

The instant invention is in the field of chemical analysis and more particularly it relates to improved fiber optic probes for chemical analysis.

Photometry is an important and well known means of chemical analysis. United States Patent 5,151 ,474, herein fully incorporated by reference, issued to Lange, Denton, Weller, Chauvel, Farquharson, Ruhl and Winter on September 29, 1992 disclosed a photometric cell, see Fig. 3 thereof, suitable for use in a process control method for manufacturing polyolefin. A mixture of solvent, monomer and co-monomer was flowed through the cell and into the polymerization reactor, see Fig. 1 thereof. Infrared light was shown through the windows of the cell, see Fig. 2 thereof, to make a photometric determination of the concentration of monomer and co-monomer flowing through the cell.

United States Patent 5,046,854, herein fully incorporated by reference, issued to Weller, Denton, Lange, Norman, Leugers, Farquharson and Chauvel on September 10, 1991 disclosed a photometric probe, see Fig. 1 thereof, which used an optical fiber to conduct light through a window for the photometric determination of chlorine in phosgene, see Example 1 thereof.

United States Patent 4,707,134, herein fully incorporated by reference, issued to McLachlan and Rothman on November 17, 1987 disclosed a fiber optic probe having a body and a window attached to the body, see Fig. 1 thereof. United States Patent 4,573,761 , herein fully incorporated by reference, issued to McLachlan, Jewett and Evans on March 4, 1986 disclosed a fiber optic probe having a body and a window attached to the body, see Fig. 2 thereof. United States Patent 4,829,186, herein fully incorporated by reference, issued to McLachlan, Chrisman and Leugers on May 9, 1989, disclosed a fiber optic probe having a body and a window attached to the body, see Fig. 1 thereof, wherein the window is in the form of a total internal reflection prism. United States Patent 4,909,588, herein fully incorporated by reference, issued to Harner and Gutzmann on March 20, 1990, disclosed a fiber optic probe having a body and a window attached to the body, see Fig. 1 thereof. United States Patent 5,155,184, herein fully incorporated by reference, issued to Laurent, Martens and Vidal on October 13, 1992, disclosed a process and apparatus for controlling the manufacture of polymers. United States Patent 3,914,342, herein fully incorporated by reference, issued to Mitchell on October 21 , 1975, disclosed a polyethylene blend and process employing a plurality of reactors operated in parallel or in series, see for example Column 4, line 50 to Column 5, line 10 thereof.

Ziegler catalysts are well known as being useful in the polymerization of olefins. More recently metallocene catalysts have become well known as being useful in the polymerization of olefins.

When the photometric cells of the above described United States Patent 5,151 ,474 are replaced with fiber optic probes in a series reactor system like the above described United States Patent 3,914,342, wherein the input and output streams of the reactors are analyzed, and wherein the catalysts used in the reactors include metallocene catalysts, then a problem can arise. The problem is the coating of the probe window(s), over several days to several months, with what appears to be high molecular weight polymer. This coating changes the photometric baseline or blank spectrum of the probe resulting in errors in analysis.

SUMMARY OF THE INVENTION

The instant invention is a solution to the above described problem. The instant invention is an improved fiber optic probe for chemical analysis, the probe having a body and at least one window attached to the body so that light conducted from an optical fiber can be shown through the at least one window into a medium to be analyzed. The improvement comprises a heater in heat communication with the body so that the at least one window can be heated to an elevated temperature.

The instant invention is also an improved method for chemical analysis using a fiber optic probe. The method includes the steps of directing light from an optical fiber through a window into a medium to be analyzed. The improvement comprises the step of heating the window to an elevated temperature. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a drawing, part in full and part in cross section, of one embodiment of an improved fiber optic probe of the instant invention showing a steam jacket surrounding the probe body.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to Fig. 1 , therein is shown one embodiment of an improved fiber optic probe 10 according to the instant invention. The probe 10 has a body 11 and a six hundred pound per square inch rated stainless steel flange 12. The flange 12 has holes 13 bored therethrough for mounting the probe 10 onto a pipe or vessel, the pipe or vessel containing a medium to be analyzed. The body 1 1 has a gap 14 into which the medium to be analyzed can enter. A first optical fiber 15 which conducts light 16 from a tungsten light source not shown enters the back end 17 of the probe 10 and extends along the body 1 1 of the probe 10 and terminates near a first window 18 sealed to the probe body 11. The light 16 is passed through the window 18, through the gap 14 and then passes through a second window 19. The light 16 is then reflected by two mirrors or a prism, not shown, within the front end 20 of the probe 10 and conducted back up the body 11 of the probe 10 by a second optical fiber 21. The second optical fiber 21 directs the light to a light detector, not shown, such as a KVB Analect Brand Fouier Transform Infrared (FTIR) analyzer operated in the near infrared (NIR) region. The elements 11 through 21 describe a conventional fiber optic probe system. The invention is an improvement upon such a system.

Referring still to Fig. 1 , an embodiment of the invention is shown in the form of a heating jacket 22. The jacket 22 surrounds the body 11 and is welded to the body 11 and the flange 12 to produce a cavity 22a. The flange 12 has a first passageway 23 bored therethrough with an associated first nozzle 24 for admitting steam 25 into the cavity 22a. The flange 12 has a second passageway 26 bored therethrough with an associated second nozzle 27 for exhausting exhaust steam 28 from the cavity 22a. If no steam 25 is admitted into the cavity 22a, then when the probe 10 is used the temperature of the windows 18 and 19 come to an equilibrium temperature depending primarily on the temperature of the medium to be analyzed and the temperature of the flange 12. When steam 25 is admitted into the cavity 22a, then this steam heats the body 11 , which heat is conducted to the windows 18 and 19, so that the windows 18 and 19 are in turn heated to a temperature elevated above the equilibrium temperature of the preceding sentence. A suitable fiber optic probe to which the flange 12 and jacket 22 can be added is available from the UOP Guided Wave Company as Model Number SST.

It should be understood that the jacket 22 is but one embodiment of the instant invention. In its full definition, the instant invention requires a heater of some type in heat communication with the body 11 so that the window(s) of the probe are heated to an elevated temperature. Any suitable heater can be used, for example: a tube wound around the body 11 so that a heated fluid can be flowed through the tube; and an electrical resistance heater positioned within the body 11 so that electricity can be flowed through the electrical resistance heater.

In its method aspect, the instant invention is an improved method for chemical analysis using a fiber optic probe, the method including the conventional steps of directing light from an optical fiber through a window into a medium to be analyzed and then back though the same window or through another window to a light detector. The improvement of the instant invention is to heat the window(s) to an elevated temperature.

The exact degree of temperature elevation of the window is not critical in the instant invention. In most applications it is preferred to elevate the temperature of the window at least five degrees centigrade but less than ten degrees centigrade. In other applications which are more demanding, it is preferred to elevate the temperature of the window at least ten degrees centigrade but less than twenty degrees centigrade. In even more demanding applications it is preferred to elevate the temperature of the window at least twenty degrees centigrade but less than fifty degrees centigrade. The elevation temperature of the window can be more than fifty degrees centigrade in yet more demanding applications. However, it should be understood that in most applications it is best to elevate the temperature of the window only moderately.

The temperature elevation of the window can be measured by cementing a small thermister or thermocouple to any suitable portion of the interior side of the window. The probe is then operated with the heater off so that the window comes to its equilibrium temperature as measured by the thermister or thermocouple. The heater is then turned on so that the window can be heated to a higher temperature as measured by the thermister or thermocouple. The elevated temperature of the window is the difference between these two measurements.

Referring now to Fig. 1 , the length of the jacket 22 relative to the length of the probe body 11 can be used to fine tune the elevated temperature of the window. A relatively short jacket 22 allows more heat to dissipate into the medium to the analyzed before it gets to the windows 18 or 19 even if steam at a relatively even temperature is used as the heat source.

Near Infra Red (NIR) is the preferred photometric region because optical fibers are available that are reasonably transparent to NIR light and because of the relative complexity of the spectra obtainable in the NIR region as opposed to the visible or UV region. The IR region is also preferred but optical fibers typically are less transparent in the IR region. However, as optical fibers become available which are transparent in the IR region and for bands in the IR region which are already usable, then operation in the IR region may be just as preferred or more preferred than operation in the NIR region.

The fiber optic probe system described in reference to Fig. 1 is preferably standardized using known standard solutions over the temperature range that will be encountered in use taking into account the heating of the probe and the spectra therefrom stored in a system computer in second derivative form at each temperature. Chemometric treatment of the photometric data is preferred, for example a partial least-squares factor analysis model produced by Pirouette Brand version 1.2 software available from Infometrix Inc. of Seattle, Washington, operating on the stored second derivative spectra.

Claims

WHAT IS CLAIMED IS:

1 . An improved fiber optic probe for chemical analysis, the probe having a body and at least one window attached to the body so that light conducted from an optical fiber can be shown through the at least one window into a medium to be analyzed, wherein the improvement comprises: a heater in heat communication with the body so that the at least one window can be heated to an elevated temperature.

2. The probe of Claim 1 , wherein the heater comprises a jacket surrounding the probe body so that a heated fluid can be flowed through the jacket to heat the at least one window.

3 . The probe of Claim 1 , wherein the heater comprises a tube wound around the probe body so that a heated fluid can be flowed through the tube to heat the at least one window.

4. The probe of Claim 1 , wherein the heater comprises an electrical resistance element within the probe body so that electricity can be flowed through the electrical resistance element to heat the at least one window.

5 . An improved method for chemical analysis using a fiber optic probe, the method including the steps of directing light from an optical fiber through a window into a medium to be analyzed, wherein the improvement comprises the step of heating the window to an elevated temperature.

6. The method of Claim 9, wherein the elevated temperature is at least five degrees centigrade.

7 . The method of Claim 9, wherein the elevated temperature is at least ten degrees centigrade.

8 . The method of Claim 9, wherein the elevated temperature is at least twenty degrees centigrade.

9 . The method of Claim 9, wherein the elevated temperature is at least fifty degrees centigrade.