Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/76—Devices for measuring mass flow of a fluid or a fluent solid material
  • G01F1/86—Indirect mass flowmeters, e.g. measuring volume flow and density, temperature or pressure
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/59—Transmissivity

Definitions

• This invention relates to an apparatus for detecting the transmission of material through a conduit .
• Preferred forms of the invention concern a catalyst flow meter, in particular one for use in a polymerisation plant.
• Polymers such as polyethylene are typically manufactured in a pressurized reactor using a so-called slurry system where the polymer is continuously -formed as the reactants are circulated around the loop reactor in the liquid state.
• the polymer product forms as a solid that is suspended within the liquid.
• the ethylene gas, diluent and powdered catalyst are fed into the loop reactor where they are rapidly circulated by means of a pump.
• the reactor core is typically maintained at a temperature of the order of
• the reactor contains about 40wt% polyethylene.
• polymer particles start to form and the larger ones precipitate as "fluff" and enter a settling zone from which concentrated slurry is periodically discharged.
• bimodal polymers These are polymers containing polyethylenes with different molecular weights.
• the Borstar ® process can be used to create bimodal polymers directly. This process combines loop reactors with a gas phase reactor. This enables polymers with a wide range of densities and molecular weight distributions (MWDs) to be produced. In this system, the polymer slurry from the loop reactors is transferred to a gas phase reactor. • Several other materials are also introduced to the reactor, such as ethylene, hydrogen and comonomers . It will be appreciated that the control of the polymerisation process is highly complex and sophisticated computer-based systems are often used to do this .
• the reactor conditions determine the size of polymer molecules (i.e. the molecular weight) that in turn determine the density of the polymer.
• the distribution of molecule sizes i.e. the molecular weight distribution - MWD likewise depends critically upon reactor conditions.
• the catalyst in particular is very influential on the final MWD of the polymer.
• a grade of polyethylene may be specified as having a certain mean molecular weight and a given molecular weight distribution.
• a modern production plant is controlled by a computerised automatic control system which uses various input measurements on the basis of which it controls the flow of reactants into the reactor. It also controls the reactor conditions, etc.
• the reaction is typically catalysed using chromium, Ziegler Natta or metallocene catalyst.
• the catalyst is stored in a tank and is fed into the reaction loop in plugs . These plugs are formed by use of a catalyst feeder.
• a standard catalyst feeder is shown in FIG 1.
• the catalyst is stored in a tank positioned above the feeder 100.
• the catalyst is fed into the feeder 100 through funnel 102 into a passage 103a of rotating valve 103.
• the catalyst powder is suspended in a hydrocarbon diluent, such as isobutane (i.e. the same diluent as in the reactor) under pressure as "mud" .
• the valve 103 is rotated 90° in the direction of the arrow.
• This channel contains a flow of isobutane or other diluent leading to the reactor.
• the plug of catalyst contained in the passage 103a is pushed through the channel 105 by the isobutane and into the reactor.
• the speed of the plug depends on the flow rate of the diluent, which can be controlled as desired. For example, if it takes the elements within the reactor 9 seconds to do a complete circuit it is preferable for the catalyst plug to be delivered into the loop over the same period. In this way all the ethylene within the loop receives a fresh dose of catalyst.
• the flow of diluent can therefore be adjusted so that the catalyst is released into the loop over 9 seconds .
• a second passage 103b of the valve 103 is now in position under the funnel 102 and fills with catalyst. Passage 103b is at 90° to passage 103a, but the passages are kinked so they do not to intersect. Therefore, upon every 90° rotation of the valve 103 a plug of catalyst is delivered to the reactor.
• the present invention provides an apparatus for detecting the transmission of material through a conduit, comprising an optical radiation source located outside the conduit, a light path through the conduit, and an optical radiation detector.
• the radiation source is arranged to direct optical radiation through the light path such that it may be detected by the detector.
• the conduit may be a catalyst feed pipe leading to a loop or gas phase reactor.
• material such as a catalyst plug
• the source, light path and detector can be arranged such that light is only detected by the detector when material is passing through the conduit (e.g. because the material reflects the light towards the detector) , or the amount of light detected is increased when material passes through the conduit .
• the user can determine from changes in detector output that catalyst is being supplied to the reactor.
• the output will fluctuate as plugs of catalyst pass by.
• the user can deduce that the catalyst has run out and that the tank needs to be replaced/refilled.
• the invention provides a simple method of detecting catalyst movement through a pipe.
• a laser source is used. This provides a coherent, narrow beam.
• the detector may be either digital or analogue. If an analogue detector is used its output may subsequently be converted to digital form.
• the detector enables the relative intensity of the received light to be measured, which in turn gives an indication of the density of the material .
• the detector can be positioned on the opposite side of the pipe to the light source.
• a catalyst plug will reduce light transmittance to the detector.
• the source and detector are situated on the same side of the conduit (and most preferably adjacent to each other) , such that light reaches the detector by reflection.
• a reflector such as silvered film or reflector tape, is positioned to direct the light beam towards the detector.
• Detecting reflected light is preferable because this does not rely on the light penetrating the - full thickness of the material. A good indication of the density can be gained without requiring a high powered radiation source. In addition the apparatus size can be kept to a minimum.
• the light path preferably created by two sight glasses, situated in an opposed relationship on either side of the pipe. While standard sight glasses can be used, these tend to be smaller in thickness than the pipe containing them, leaving small pockets on the interior of the pipe which can cause turbulence at the glass and the pockets may collect solid material. This would be disadvantageous in a catalyst feed pipe as a layer of catalyst may form on the glass and cause incorrect results.
• the sight glasses are manufactured so as to be flush with the interior of the pipe. This reduces the build up of catalyst on the sight glass and ensures that any catalyst remaining on the glass will be directly in the path of the next plug, increasing the likelihood that it will be removed from the glass.
• the light source can be positioned along the normal to the conduit axis, in order to avoid reflections from the sight glass interfering with the measurements obtained by the apparatus it is preferable that the light source is offset.
• the light source is offset by up to 25° and most preferably by around 7°.
• the detector can be positioned on the opposite side of the normal to the light source, it is preferable that the detector and source are positioned proximate to each other on the same side of the normal. In this way, reflections from the smooth surfaces (such as those of the light path) are directed away from the detector whereas the rough surface of the catalyst directs reflections back towards the light source and the detector.
• the reflector comprises a surface (such as reflector tape) having a plurality of prisms which direct reflections towards the detector, so that the detector records a greater intensity of light when no catalyst is in the conduit .
• the output from the above apparatus may be used to provide a light intensity curve which gives an indication of the density of the plug as well as the time taken for the plug to pass the light source.
• the detector output (intensity) may be fed to a computer or other device where it can be plotted against time.
• the apparatus is arranged to determine the mass of catalyst forming the plugs.
• the apparatus preferably also comprises means for measuring the speed at which the catalyst flow.
• the invention further comprises means for determining the mass of a plug of catalyst from the flow speed of fluid in the conduit and the time taken for the plug to pass the detector.
• the inventor has found that a useful indicator of the mass flow of material comprising a plug can be found multiplying the detected intensity of radiation reflected from the material by the flow speed. If this quantity is monitored as the plug passes by and then integrated with respect to time then the result - is proportional to the mass of material.
• the present invention provides a method for determining the mass of catalyst plugs transmitted through a conduit leading to a polymerisation reactor, comprising the steps of: repeatedly detecting the intensity of optical radiation reflected from the material; detecting the speed of the material though the conduit; multiplying the detected intensity and speed values to obtain a result curve,- and integrating the result curve.
• an apparatus according to the invention can provide a direct indication of the mass of a catalyst plug.
• the above method and apparatus can therefore be used to monitor the amount of catalyst which has passed through a pipe and consequently how much is left in the tank.
• abnormalities in the amount of catalyst being supplied to the feeder can be detected and the reasons for such abnormalities deduced without interrupting the polymerisation process.
• Limits can be set, wherein any result curve which falls within these limits is classified as "normal” . Result curves falling outside the limits are classified as either "long" or “short” plugs. These can be caused by a number of conditions .
• a long plug may be the result of the feeder valve not rotating into the correct position, resulting in a smaller gap for the catalyst to be flushed through into the channel .
• a viscous plug can also cause an unusually long result curve. If this is a recurring problem, the flow of diluent can be increased to flush the catalyst out with greater force. Short plugs may occur if the diluent flow is too high and is compressing the plug, or if the passage is not being fully filled with catalyst . This second option may occur if the tank is running empty or if the catalyst is somehow being obstructed from entering the valve passage.
• the above method further includes the step of displaying the result curves obtained. An operator viewing the result curves can then adjust the variables involved in order to bring the result curves back within the fixed limits.
• the method can include storing the result curves and displaying the last n plugs to have been feed into the reactor. In this way the user can also observe trends within the plugs .
• the conditions in which the above method is carried out may vary. For example, as mentioned above, it is possible that catalyst may stick in the light path in between plugs. Also, the colour of the catalyst may alter depending on the type or even the batch of catalyst used.
• the intensity and speed measurements are reset prior to each new mass measurement. This synchronises the result curves and increases the ease of comparison.
• this resetting occurs when the feeder valve begins its rotation. This gives a period prior to the arrival of the plug at the intensity detector during which the zero level intensity can be set. In this way any catalyst or dirt fixed to the sight glass does not effect the next plug's readings.
• the light intensity readings can be normalised.
• this is achieved by normalising the initial intensity drop when the material first reaches the intensity measurer.
• the gain factor used to normalise this drop is then applied to the rest of the reading.
• This technique can also be used to determine if there is too much catalyst left on the sight glass for a relevant measurement to be obtained. Therefore, preferably, if the gain factor required to normalise the intensity drop is too great a warning is issued. The user then knows to discount the abnormal result curve obtained.
• the method further includes a method of self calibration.
• This method comprises the steps of calculating the mean mass of the plugs produced and comparing this with a theoretical fixed value. If there is a difference between these two values the factor by which the integral is multiplied by is altered by a percentage of this difference. Preferably the percentage is around 40%.
• n mass results are used to calculate the mean mass value.
• the number used is around 20.
• the masses obtained from abnormal result curves are not used to calculate the mean mass .
• this is achieved by only carrying out the self calibration method if the last n, preferably 20, result curves were classified as "normal” .
• the mean mass for calculating the mass flow is obtained from the last n masses acquired, wherein the highest and lowest mass values are disregarded. In this way, disturbances are filtered from the mean value. Preferably the last six masses are used.
• the present invention provides a system for determining the mass of material transmitted through a conduit comprising: means for detecting the intensity of optical radiation reflected from the material; means for detecting the speed at which the material is transmitted; means for multiplying the intensity values and the speed values together to create a result curve; means for integrating the result curve; and means for multiplying the integral of the result curve by a factor to determine the mass of the material .
• the system also includes means to display the result curves and mass values to a user.
• the system also includes means for carrying out the normalisation calculations and self calibration method as described above.
• the system is arranged to carry out the method described above in order to obtain the mass flow of catalyst in a polymerisation reactor.
• the means for detecting the intensity of optical radiation reflected from the material is the apparatus described above . While this aspect has. been described in relation to measuring the mass of a plug, the method could just as easily be applied to monitoring the mass of diluent . In this case, the catalyst could be injected with a dye so that a drop in light intensity would indicate the passage of diluent rather than catalyst.
• the invention provides a system for implementing the method described above.
• the method or system is used to determine the mass of catalyst flowing to a polymerisation reactor and this data is supplied to a computerised control system for use in controlling said reactor. In this way the polymerisation reactor can be more accurately controlled.
• FIG 1 shows a standard catalyst feeder which is used to supply catalyst plugs a preferred embodiment
• FIG 2 shows a cross section of the light intensity detector of the present invention
• FIG 3 shows an example of a pocketless sight glass as used in a preferred embodiment of the present invention
• FIG 4 shows an example of the screen display produced by a program running in accordance with the method of the present invention.
• FIG 5 shows a graph of catalyst input and the resulting polymer composition when the present invention is employed and the resulting outcome when the catalyst runs out.
• FIG 1 shows a standard catalyst feeder 100, which is used to supply plugs of catalyst to the reactor.
• the feeder 100 comprises a circular valve 103 containing two non connecting passages 103a, 103b positioned perpendicular to one another.
• the valve 103 is positioned at the junction between the catalyst tank and a channel 105 leading to the reactor.
• the valve 103 is rotated so that one passage 103a fills with catalyst.
• the valve is then rotated 90° so that the catalyst is flushed out toward the reactor by a flow of diluent along the channel 105.
• the second passage 103b is brought into line with the catalyst tank and is supplied with catalyst. In this way the reactor can be constantly supplied with regular doses of catalyst.
• the intensity detector 10 (see FIG 2) of the present invention is positioned downstream of the feeder 100 along the channel 105.
• the instrument is based on a laser 9 which passes a beam 9a through sight glass 2, the catalyst carrying pipe 1 and second sight glass 7.
• a reflector 20, such as a reflector tape having a prism- coated surface, is positioned behind the second sight glass 7 to reflect the laser beam 9a back through the pipe 1 to a detector (not shown) contained in the laser housing.
• FIG 3 shows a suitable sight glass 2, 7 for use in the intensity detector 10.
• standard sight glasses create small pockets on the interior of the pipe which is disadvantageous in the present invention.
• Conventional sight glasses are sensitive to stress, bending etc when subject to mechanical overload and can fail without warning.
• Metaglass ® is employed.
• the sight glass 2,7 is made by heating a metal frame 33 so that it expands, and filling it with molten glass. As the glass and metal cool, the glass solidifies and the metal frame 33 contracts. This holds the glass securely in place.
• This pre-stressed glass 30 provides increased strength and also a smooth surface for the interior of the pipe, leaving no pockets.
• the glass 30 is wedge shaped, with the interior diameter 32 greater than the outer diameter 34.
• the sight glasses 2,7 have an inside surface flush with the inside of the pipe and so have no pockets, where catalyst could get trapped. Further, any catalyst that does remain stuck to the sight glass will be scraped off by the next plug to be flushed through.
• the velocity of the plug is measured by a standard flow or vortex transmitter (not shown) , which measures the flow of diluent upstream of the feeder 100.
• the transmitter must have a fast reaction time, as due to the turning of the valve 103, there will be intervals where the flow rate drops to zero before increasing and pushing the plug towards the reactor.
• the transmitter ideally measures the velocity every 0.1ms, in line with the light intensity detector 10.
• the light intensity and velocity sensors are both analogue devices which operate between 4 and 20mA.
• a further sensor is provided on the feeder valve 103. This sends a digital trigger signal every time the valve 103 begins a rotation. This is used in order to correlate the other signals as will be described below.
• a trigger signal is sent to the control system which resets the light intensity and velocity signals. This synchronises the ensuing result curve with previous curves, allowing easy comparison of each plug.
• a trigger indicator 41 lights up on the display when a trigger signal is received.
• two catalyst feeders are generally used.
• the display can be set up to show the status of both feeders simultaneously.
• the plug usually takes approximately Is to reach the laser beam 9a from the instant the trigger signal is sent. This time window is used to measure the light intensity and set this as the zero level. Therefore if residual coatings are left on the sight glass 2, 7 they are compensated for and do not interfere with the next plug. However, an alarm is activated if the initial light intensity is too low.
• the median value of the light intensity measured between 1 and 2s after the trigger signal is sent is calculated and multiplied by a gain factor so that each initial light intensity drop is the same. In this way the light intensity is made independent of colour. However, if the gain factor required to normalise the intensity drop is too large a warning light 40 is turned on to indicate that the instrument is not gaining a viable light intensity reading, for example because there is a thick layer of catalyst on the sight glass.
• the gain factor is applied to each sample value so that the plug is independent of colour and the only changes in intensity come from the alterations in the density of the plug. In this way, the light intensity measurements can be used to give an indication of density changes along the plug.
• the normalised values of light intensity are then multiplied with the signal from the flow transmitter.
• the result appears as a curve 42 on the computer display and can be visually compared to the previous three result curves, 43, 44, 45 which appear on the screen in different colours .
• the integral of the result curve is proportional to the mass of the plug and therefore can be used to calculate the mass of the catalyst by multiplying it by a factor dependent on the cylinder 103a, 103b volume.
• the mass of the latest plug 46 is shown on the display together with the current mass flow 47.
• the mass flow value is calculated from the latest 6 plug masses .
• the highest and lowest masses are removed and the mean mass is calculated from the remaining four values. This is then divided by the time between the latest two plugs in order to give the current mass flow rate.
• the program continues to self calibrate itself during the operation of the reactor.
• the program indexes each result curve and calculates the mean value of the mass of the last 20 plugs. This mean value is compared with the theoretical fixed value. If these values differ the above factor is adjusted by 40% of the difference between the two values .
• a cut off mass flow value can be determined which depends on the volume of the feed cylinders 103a, 103b (e.g. 2 Kg/hr for a cylinder of 200cm 3 ) . Once this value is reached, if a higher feeding rate is set, the catalyst will not have time to fully fill the cylinders before the feeder valve 103 is rotated. Therefore, any mass flow recorded above the cut off value is not reliable and would preferably then not be used in the calibration against the theoretical value. Therefore, in this preferred form, 'normal' plugs are considered to be those which fall within limits 48 and have a mass flow rate under the defined cut off value.
• a counter 49 displays the number of sequential "normal" plugs to have passed through the light intensity detector. Underneath this is an indicator light 50 which is green if the present plug is normal.
• the current flow speed 51 and the elapsed time since the last plug 52 are also displayed. With this information, abnormalities in the plugs can more accurately be deciphered. For example, if short plugs are recorded the flow speed can be checked and perhaps reduced to see whether the plugs are being compressed. If this does not improve the situation the time between plugs can be increased, in case the catalyst is particularly viscous and is taking longer to fill the passage 103a, 103b. If neither of these actions correct the problem, the catalyst tank itself can be investigated.
• the various self calibration routines carried out by the program mean that no complex adjustments are needed when a new catalyst batch is introduced and limited user input is required.
• FIG 5 shows the how the information gained through implementing the above embodiment can be used to accurately control a polymerisation reactor. If the level of catalyst input into the reactor drops, then the ethylene concentration within it will rise, causing the reactor to start producing "off spec" polymers . The time delay between a drop in catalyst input and a rise in ethylene concentration is approximately 2 hours. Therefore, by the time the ethylene concentration begins to rise, it is often too late to take action to prevent "off spec" products from being produced.
• the mass flow into the reactor can be monitored directly and therefore changes in this rate can be spotted immediately.
• Two catalyst tanks are used to supply the reactor with catalyst. This enables the production process to continue when one tank runs empty or develops a malfunction. Initially only one tank, tank A, is operating. The mass flow supplied to the reactor by tank A is shown by line 62. Each time a plug passes the intensity detector 10, a new mass value is calculated and the graph then displays this value until a new plug passes and a new mass is measured.
• the mass flow supplied by tank B is shown by line 64. At first, this tank is not operational and therefore line 64 continuously displays the last registered mass outputted by the tank. The total average mass flow is shown by line 66
• the dotted line 67 shows the set point value, i.e. the desired mass flow output. Also shown on the graph is the ethylene concentration within the reactor 69.
• the first three quarters of FIG 6 show the catalyst feed running smoothly. Slight variations in the plug mass do not greatly effect the total mass flow 66, which fluctuates around the desired set point 67.
• tank A begins to run out of catalyst. Very quickly, within four plug deliveries, the mass flow 62 drops sharply, mirrored by the total mass flow 66. At this point, the feeder for tank B can be started, thus producing a new source of catalyst for the reactor.
• the total mass flow 66 very quickly recovers and therefore no significant variation is noted in the ethylene concentration 69.
• the present invention therefore provides a method of accurately monitoring catalyst input into a reactor and consequently allows for a better controlled polymerisation process. However, the invention has further reaching consequences and can be used in a number of different systems.
• the apparatus can be used to indicate when the contents of a tank (e.g. catalyst or other material) has dropped beyond a certain point.
• the light source could be positioned towards the bottom of the tank and light could be directed along a light path into the tank.
• the light would be reflected back off the catalyst towards the detector.
• the catalyst dropped below the level of the laser, the light would no longer be reflected back and thus the light intensity received by the detector would drop. At this point an alarm could be sounded to alert the process operators.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Fluid Mechanics (AREA)
• General Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
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• Biochemistry (AREA)
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• Immunology (AREA)
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• Polymerisation Methods In General (AREA)
• Physical Or Chemical Processes And Apparatus (AREA)
• Measuring Volume Flow (AREA)
• Investigating Or Analysing Materials By Optical Means (AREA)
• Other Resins Obtained By Reactions Not Involving Carbon-To-Carbon Unsaturated Bonds (AREA)

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• 2002
  • 2002-12-23 GB GB0230052A patent/GB2398117A/en not_active Withdrawn
• 2003
  • 2003-12-19 CN CNB2003801096853A patent/CN100465590C/zh not_active Expired - Fee Related
  • 2003-12-19 RU RU2005120176/28A patent/RU2324901C2/ru not_active IP Right Cessation
  • 2003-12-19 AU AU2003300241A patent/AU2003300241A1/en not_active Abandoned
  • 2003-12-19 WO PCT/EP2003/015031 patent/WO2004057278A2/en not_active Application Discontinuation
  • 2003-12-19 BR BR0317727-0A patent/BR0317727A/pt not_active IP Right Cessation

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