WO2004057278A2

WO2004057278A2 - Catalyst flow meter

Info

Publication number: WO2004057278A2
Application number: PCT/EP2003/015031
Authority: WO
Inventors: Karl Nielsen
Original assignee: Borealis Technology Oy
Priority date: 2002-12-23
Filing date: 2003-12-19
Publication date: 2004-07-08
Also published as: AU2003300241A8; RU2324901C2; RU2005120176A; BR0317727A; WO2004057278A3; GB0230052D0; CN100465590C; AU2003300241A1; CN1748126A; GB2398117A

Abstract

An apparatus (10) for detecting the transmission of catalyst plugs through a conduit (105) leading to a, polymerisation reactor, comprising an optical radiation source (9) located outside the conduit which passes a light beam (9a) through the conduit along a light path and an optical radiation detector arranged to detect the light beam. When a catalyst plug passes through the conduit it blocks the light path and thus the light intensity recorded by the detector drops. In this way catalyst plugs can be detected. The apparatus can also be used to measure the mass of an object passing through a conduit by repeatedly detecting the speed of the material and the light intensity, multiplying the speed and intensity values to get a result curve and integrating this curve in order to obtain a value indicative of the mass of the material.

Description

Catalyst Flow Meter

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.

When polyethylene is to be produced in this way, high purity ethylene is supplied to a loop reactor together with a catalyst and certain other materials . A low boiling point hydrocarbon such as isobutane or liquid propane is used to dissolve the ethylene monomer and to suspend catalyst and polymer particles within the reactor. This diluent plays no part in the actual polymerization reaction and so it is eventually recovered, purified and recycled. In addition, hydrogen is added to control the molecular weight of the polyethylene produced and 1-hexene co-monomer is added in order to control product density.

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

100°C and a pressure of the order of 4MPa to 7.5MPa depending on the process being performed. During normal operation, the reactor contains about 40wt% polyethylene. As the process continues, polymer particles start to form and the larger ones precipitate as "fluff" and enter a settling zone from which concentrated slurry is periodically discharged.

This can then be dried and pelletised or undergo further processing, for example to produce 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 . There are a great number of factors that have an effect on properties of the finished product. The reactor conditions, such as temperature, pressure, concentration of reactants etc, 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.

Within each basic type of polymer such as polyethylene, polypropylene etc, products are classified by manufacturers into de ined grades . These each have a set of specified properties that must be met within given tolerances. Thus, a grade of polyethylene may be specified as having a certain mean molecular weight and a given molecular weight distribution.

It follows that in order to produce a given grade of polymer the critical product properties must remain substantially consistent.

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. During the Borstar process 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. In this tank 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" . After a predetermined time, the valve 103 is rotated 90° in the direction of the arrow. This aligns the first passage 103a with channel 105. 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 .

Meanwhile, as the plug is ejected into channel 105, 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.

It is important to monitor the mass of catalyst which is supplied to the reactor. Too much raises the temperature within the reactor and can affect the polymerisation process. The catalyst is also a very expensive material and therefore it is economically beneficial to use the minimum amount necessary. However, if too little catalyst is supplied then the reaction is altered, creating potentially "off-spec" polymers. It is this situation which causes most disruption to the process.

Therefore, an accurate way of determining the mass of the catalyst plugs supplied to the reactor is of great importance .

Traditionally in automatic control systems, the flow of catalyst into the reactor from one rotation of the feeder was always taken to be the same and was based on the volume of the passages 103a, 103b and the density of the catalyst. Therefore, when more catalyst was needed the rate of rotation of valve 103 was increased.

When the catalyst tank is full this assumption works reasonably well. However, as the catalyst begins to run out, or if the feeder rotation is at a high rate, the entire passage volume is not filled before the feeder is rotated, leading to a smaller amount of catalyst being supplied to the loop per rotation. The drop in catalyst supplied to the loop results in the control program increasing the rotation of the feeder in order to increase the number of plugs inputted to the loop. However, this exacerbates the problem. Given the reaction time of the loop, it can often be several hours before the created polymer can be tested and found to be off spec. Further, with no direct observation being possible, problems with the feeder and their causes are hard to detect . In order to address these problems and allow a continuous and accurate measurement of the catalyst mass flow, Oxford Instruments have developed "Correflow", an inline, real time system which can be used to measure the mass of catalyst feed into a reactor. This instrument is based on the teachings of US Patent Nos 4,074,184, 4,774,453 and 4,619,145.

The Correflow measures both the velocity and concentration of the catalyst within the diluent. This is achieved by placing probes, set a known distance apart, within the pipe. The probes are used to detect the speed of the catalyst plug while metal plates positioned around the exterior of the pipe determine its density through the capacitance measured. However, this system is fairly complex and expensive. In addition, it requires a large amount of maintenance. Therefore, an alternative solution to the problem of real time catalyst mass flow analysis has been created. According to one aspect, 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.

Since the light path passes through the conduit, material flowing through the conduit will affect the detection of light by the detector. This provides information about the flow of material. In the case of a polymerisation reactor, the conduit may be a catalyst feed pipe leading to a loop or gas phase reactor. In one arrangement, when material, such as a catalyst plug, passes through the pipe the amount of radiation which reaches the detector decreases . Alternatively 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 .

Therefore, regardless of what arrangement is used, the user can determine from changes in detector output that catalyst is being supplied to the reactor. Thus, in normal operation of a catalyst feed, the output will fluctuate as plugs of catalyst pass by. When the radiation intensity fails to fluctuate even if the feeder is in operation, the user can deduce that the catalyst has run out and that the tank needs to be replaced/refilled.

Therefore the invention provides a simple method of detecting catalyst movement through a pipe.

While any form of visible or non-visible optical radiation can be used, it is preferable that 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.

Although in a simple form of the invention there may be a simple flow/no flow output, preferably 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 .

In the case of a polymerisation reactor, this can be important as the catalyst plug density is not uniform. As catalyst fills up the vertical passage of the feeder valve, the material at the bottom is compressed. The entire plug can also be compressed if the flow of diluent is strong. Therefore, an instrument which can indicate the density of a plug can be useful in determining more accurately the mass flow of catalyst . The detector can be positioned on the opposite side of the pipe to the light source. Thus, a catalyst plug will reduce light transmittance to the detector. However, it is preferable that 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. In order to aid this process it is preferable that 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.

It is preferable therefore that 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.

While 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. Preferably the light source is offset by up to 25° and most preferably by around 7°.

While 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.

In this instance, it is preferable that 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. Thus, the detector output (intensity) may be fed to a computer or other device where it can be plotted against time. As indicated above, whilst it is useful to be able to confirm that plugs of catalyst are flowing into the reactor, it is preferable that the apparatus is arranged to determine the mass of catalyst forming the plugs. Thus, the apparatus preferably also comprises means for measuring the speed at which the catalyst flow.

There are several standard instruments which can be used to measure the flow of liquid through a pipe. One such device is a vortex transmitter. By determining the speed of a plug from the flow speed of the liquid carrying it and determining the time taken for the plug, to pass the above apparatus, the length of the plug can be derived. Together this information can be used to obtain the mass of the catalyst plugs .

Thus, most preferably, 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.

Therefore, viewed from another aspect, 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.

Although it may be useful to plot the curve generated by this method, it will be appreciated that this is not essential- The term "result curve" merely means a set of data that could be presented as a curve when plotted against time, if desired.

As mentioned above, the integral is substantially proportional to the mass of the material and so it can be multiplied by a factor to gain the absolute mass value. Thus, by suitable calibration, 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. By keeping track of the plug masses, 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. Assuming the catalyst feeder is operating correctly and the catalyst tank is fairly full, the visual appearance of each result curve should be fairly similar. 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.

In order for a user to analyse the result curves and detect long and short plugs, it is preferable that 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.

More preferably, 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 . During the operation of the reaction loop 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.

So that these occurrences do not affect the accuracy of the above method it is preferable that several normalisation steps are included.

Firstly, it is preferable that the intensity and speed measurements are reset prior to each new mass measurement. This synchronises the result curves and increases the ease of comparison. Preferably 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.

Secondly, in order to make the intensity measurements independent of the material colour, the light intensity readings can be normalised.

Preferably 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.

In order to reduce the maintenance required to achieve reliable results it is preferable that 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%.

In order to reduce the calculation time it is preferable that only the most recent n mass results are used to calculate the mean mass value. Preferably the number used is around 20.

So that abnormal values do not influence the self calibration it is preferable that the masses obtained from abnormal result curves are not used to calculate the mean mass . Preferably this is achieved by only carrying out the self calibration method if the last n, preferably 20, result curves were classified as "normal" . Once the mass of each plug has been obtained it is possible to calculate the mass flow by dividing the mean mass of the plugs by the time interval between successive plugs .

Preferably 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. Viewed from another aspect 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 .

Preferably the system also includes means to display the result curves and mass values to a user. Preferably the system also includes means for carrying out the normalisation calculations and self calibration method as described above.

Preferably the system is arranged to carry out the method described above in order to obtain the mass flow of catalyst in a polymerisation reactor.

Preferably 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.

Viewed from another aspect the invention provides a system for implementing the method described above.

While there are many ways in which the above described method and system could be implemented, it is preferable that 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.

A preferred embodiment of the present invention shall now be described, by way of example only, with reference to the accompanying drawings, in which;

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; and

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.

As described above, 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. At the same time, 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.

When there is no catalyst in the pipe 1, a high level of laser light 9a is recorded by the detector. However, when catalyst passes through the beam 9a some of the light is absorbed and therefore the detector detects a drop in the intensity of the reflected light. The laser 9 is very sensitive as it has a response time of 0.1ms . The mean intensity value is calculated every 100ms. This value is then used as a single point on the generated light intensity curve.

FIG 3 shows a suitable sight glass 2, 7 for use in the intensity detector 10. As mentioned above, standard sight glasses create small pockets on the interior of the pipe which is disadvantageous in the present invention.

An additional problem with creating sight glasses for use in this process is the high pressure within the pipe 1. This is usually between 70 and 83 bar.

Conventional sight glasses are sensitive to stress, bending etc when subject to mechanical overload and can fail without warning.

In order to overcome these problems, Metaglass^® is employed. Here 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. For additional safety, 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.

The method of operation shall now be described. As the feeder valve 103 is turned to bring the (catalyst filled) first passage 103a into line with channel 105, 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. On the user display, shown in FIG 4 a trigger indicator 41 lights up on the display when a trigger signal is received. For any one reactor, two catalyst feeders are generally used. For convenience, 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.

Different types of catalyst, and even different batches of the same catalyst, are different colours, and so to avoid resetting the system every time a new batch is used the plugs are normalised to a pre-established light intensity. This allows a better comparison with previous plugs .

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 .

In order to ensure that this self calibration only takes account of "normal" plugs, if any curve breaks the fixed limits, shown on the display as red boxes 48, it is discounted and the index is re-set to zero. Additionally, the calculated mass flow value can also be used to aid self calibration. 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 200cm³) . 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.

However, using the present invention 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

(which is not effected by non operational tank B) . 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. Towards the end of the graph, 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. For example, 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. In this example, the light source could be positioned towards the bottom of the tank and light could be directed along a light path into the tank. When the tank was full of catalyst the light would be reflected back off the catalyst towards the detector. However, once 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.

Claims

1. Apparatus for detecting the transmission of catalyst plugs through a conduit leading to a polymerisation reactor, comprising; an optical radiation source located outside the conduit, a light path through the conduit, and an optical radiation detector, wherein the radiation source is arranged to direct optical radiation through the light path such that it may be detected by the detector.

2. Apparatus as claimed in claim 1, wherein the transmission of material through the conduit may be detected by the detector via reflection of the optical radiation.

3. Apparatus as claimed in claim 2 , wherein the apparatus further comprises a reflector positioned in the light path such that it may reflect light towards the detector.

4. Apparatus as claimed in any preceding claim, wherein the optical radiation source is offset from the normal .

5. Apparatus as claimed in claim 4 , wherein the optical radiation source is offset by up to 25°.

6. Apparatus as claimed in claim 5 , wherein the optical radiation source is offset by 7°.

7. Apparatus as claimed in any preceding claim wherein the optical radiation source is a laser source .

8. Apparatus as claimed in any preceding claim, arranged to determine the approximate density of the material from the output of the detector.

9. Apparatus as claimed in claim 8 , further comprising means for measuring the flow speed of material in the conduit and being arranged to determine an indication of mass flow of material based on the measured flow speed and the determined approximate density.

10. An apparatus as claimed in any preceding claim used in association with an apparatus for supplying catalyst plugs to a reactor, wherein the apparatus is arranged to identify abnormal plugs from the output of the detector.

11. An apparatus as claimed in any preceding claim, wherein the catalyst plugs are fed into the conduit from a vessel, further comprising means for determining the level of catalyst remaining in said vessel .

12. An apparatus as claimed in any preceding claim, wherein the catalyst plugs are fed into the conduit from a vessel, further comprising means for indicating when the vessel is exhausted

13. A method for determining the mass of material transmitted through a conduit comprising the steps of: a) repeatedly detecting intensity of optical radiation reflected from material; b) detecting the speed of material moving though the conduit; c) multiplying the intensity and speed values to gain a result curve; and d) integrating the result curve; wherein the integral of the result curve is indicative of the mass of the material.

14. A method as claimed in claim 13, further comprising the step of multiplying the integral of the result curve by a factor to gain the approximate absolute mass of the material.

15. A method as claimed in claim 13 or 14, further comprising the step of normalising the intensity values prior to step c) .

16. A method as claimed in claim 14, wherein if the gain factor required to normalise the intensity values is too large, a warning signal is given.

17. A method as claimed in claims 13 to 16, further comprising the step of resetting the intensity value prior to material detection.

18. A method as claimed in any of claims 13 to 17 further comprising the step of storing the result curves and mass values .

19. A method as claimed in claim 18, further comprising a method of self calibration, comprising the steps of a) calculating the mean mass of the last n measurements; b) comparing the mean mass with a theoretical fixed value; and if these values differ, c) adjusting the factor by a percentage of the difference between the mean mass and the theoretical value.

20. A method as claimed in claim 19, wherein the factor is adjusted by 40% of the difference between the mean mass and the theoretical value.

21. A method as claimed in claim 19 or 20, further comprising the step of disregarding measurements whose result curves do not fall within fixed limits .

22. A method as claimed in claim 19 or 20, wherein self calibration is carried out only if the last n measurements came from result curves which fall within certain fixed limits.

23. A method as claimed in any of claims 13 to 22 comprising the further step of calculating the mass flow value .

24. A method as claimed in claim 23, where in the mean mass is calculated from the last n mass measurements.

25. A method as claimed in claim 24, wherein the highest and lowest mass values are removed prior to calculating the mean mass .

26. A method as claimed in any of claims 13 to 25, wherein the method 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.

27. An system for determining the mass of material transmitted through a pipe 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 gain the mass of the material.

28. A system as claimed in claim 27 further comprising display means to display the result curve and mass values .

29. A system as claimed in claim 27 wherein the display means displays the most recent result curves simultaneously.

30. A system as claimed in claim 27, 28 or 29, wherein the method 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.