GB2104681A - Apparatus for the continuous investigation of chemical reactions by infrared (IR) absorption - Google Patents

Abstract

An apparatus is disclosed comprising an IR spectrophotometer having a throughflow cell through which flows a continuous sample stream branched off from the reaction container. The cell consists of a cell body (14) and cell covers (15, 16), with inflow and outflow pipes (17, 18) and support (19) for fixing cell in the spectrometer. The cell chamber consists of a bore (20) with cut out portions (2) for receiving a crystal plate (11) and sealing supports. The crystal plate (11) is held free from tension inside the throughflow cell and the sample stream flows over the crystal plate in a direction substantially parallel to the crystal surface. The measuring light enters the sloped off end faces of the crystal plate (11), is propagated inside the plate by total reflection against the surfaces, and leaves through a similarly sloped off surface at the opposite end. The intensity of the emergent light depends on the physical conditions at the optical phase boundary of the crystal plate surfaces (11a, 11b). If in the course of a chemical reaction the reaction mixture is caused to flow continuously over the crystal plate (11), the progress of the reaction can be followed by means of IR spectroscopy by the on-line process. <IMAGE>

Description

SPECIFICATION Apparatus for the continuous investigation of chemical reactions by infrared (IR) absorption This invention relates to an apparatus for the continuous investigation of chemical reactions in the liquid phase by infrared absorption using a throughflow cell through which there is a continuous flow of sample branched off from the reaction vessel.

In chemical production processes, the liquid reaction products are frequently analysed by infrared spectroscopy. The samples are generally removed intermittently for this purpose and investigated by means of a spectral photometer.

These discontinuous methods are highly labourintensive and expensive and generally give no information on the reaction kinetics of the chemical process. Since the quantities of sample analysed are relatively small compared with the total quantity of reaction mixture, conversion of the results to a production scale is in many cases critical. In addition, the intermittent method of measuring entails a time lag between removal of the sample and obtaining of the analytical results so that the values obtained no longer represent the status quo.

Attempts to obtain direct measurements of the main process stream (on-line operation) by means of transmitted light throughflow cells have been successful only in a few exceptional cases.

Universal application of transmitted light throughflow cells in chemical production processes is contra indicated for the following reasons: 1. If liquids are highly absorbent, a substantial loss of intensity occurs in the transmitted light process. in order to obtain a sufficiently high value, the cell must be designed so that light passes only through a relatively thin layer. This means that in investigations carried out in on-line operations, the quantity of substance which can be investigated is relatively small compared with the total quantity. When the sample stream is so small, the value measured is liable to be unrepresentative of the conditions in the reaction vessel.

2. Owing to the thinness of the layer in the transmitted light cell, soiling of the window of the cell readily occurs. This results in uncontrollable changes in the basic absorption, which in turn leads to eroneous readings. Furthermore, the cell is liable to be completely blocked by deposits so that the measuring apparatus becomes unusable.

3. If the product to be tested contains filler e.g.

carbon black, which absorb or scatter light, the substance specific IR band and hence the value to be measured may be completely masked.

It is at this point that the present invention comes into play. The general aim of the invention was to enlarge the possibilities of IR spectroscopy for investigating on-going chemical reactions. In particular, it was an object of the invention to enable IR spectroscopy to be used in on-line operations also for the analysis of highly absorbent reaction products or products containing light-absorbent or scattering additives (fillers).

Acccording to the invention this problem is solved by means of a throughflow cell which is a multireflection cell having a crystal plate which is supported free from tension and which is washed by the sample stream flowing in a direction which is substantially parallel to the surfaces of the crystal plate. In practice, this is achieved by providing the multireflection cell with inflow and outflow pipes arranged so that their axes at their attachment to the multireflection cells are orientated parallel to the surfaces of the crystal plate and perpendicuiarly to the direction in which light passes through the crystal plate.

The multireflection cells and the inflow and outflow pipes are preferably designed so that in operation, the speed of flow over the surfaces of the crystal plate is at least 1 m/min, preferably more than 2 m/min.

Supporting the crystal plate in a manner such that it will be kept free from tension is advantageously achieved by holding the crystal at both its ends by means of an elastic sealing material extending over the whole periphery of the crystal and attached to the external surfaces of the cell. The choice of sealing material is governed by the consideration that it is preferred to use materials having an infrared absorption which is not superimposed on the wavelength to be measured.

The advantages achieved by the present invention are as follows: 1. Since the crystal plate is held free from tension inside the cell and the liquid flows over it in a direction parallel to its surface, the mechanical tensions which would otherwise be produced due to the liquid pressure do not occur. Such mechanical tensions would falsify the measurements. In the most unfavourable cases, i.e. if the liquid pressures are high, the tensions produced could be so great that the crystal plate would break. This risk is elminated by the arrangement according to the invention. It has been found that the signal produced as measurement remains independent of pressure up to a pressure of 1 80 bar. This means that the new multireflection measuring cell may also be employed under conditions of high excess pressure.

2. The support of the cell in a manner such that it is kept free from tension prevents the occurrence of mechanical tensions in the crystal plate due to temperature changes. This provides the necessary precondition enabling the multireflection cell to be heated. The heating of multireflection cells was hitherto always liable to produce errors in the measured values due to the effect of temperature change.

3. The relatively high flow velocity in the cell due to the particular construction of the apparatus ensures an efficient washing action over the surfaces of the crystal plate, thus preventing the deposition of product on the critical crystal surface. In addition, the construction of the multireflection cell enables a sufficiently large sample stream to flow so that measurements carried out on the sample stream will also accurately reflect the condition of the product to be analysed in the reaction vessel.

4. The new multi-reflection cell may easily be taken apart for cleaning and then reassembled.

The new measuring technique opens up the possibilities of important new applications for online IR spectroscopy. The invention for the first time enables continuous and quantitative IR analysis to be carried out on reaction products with high optical absorption as well as on products containing fillers which have an absorbent or highly scattering effect.

An embodiment of the invention is described below by way of example with reference to the drawings, in which Fig. 1 is an overall flow diagram (process technical) for extracting and measuring the sample; Fig. 2 is a side view of the multi-reflection cell: and Fig. 3 is a top plan view of the multi-reflection cell.

The use of a multi-reflection cell for measuring the optical absorption is known in principle and has been described in numerous publications.

Particular reference should be made to the book by N. J. Harrick, Internal Reflection Spectroscopy, Interscience Publishers, John Wiley 8 Sons, New York, London, Sydney, 1 967. The main difference between this method and IR spectroscopy using a conventional transmitted light throughflow measuring cell is that the measuring cell contains a plate cut out of a crystal so that the measuring light passes through the plate under Multiple Inter-Reflection. The measuring light enters and leaves at the obliquely cut end faces of the crystal plate and is propagated in the interior of the plate by total reflection against the surfaces of the plate.

The intensity of the light leaving the plate depends upon the physical conditions at the optical phase boundary of the surface of the crystal plate. This enables investigation to be carried out on the optical properties (absorption and refractive index) of a substance applied to the surface of the crystal plate. It is well known that, if total reflection occurs at the interface between crystal and substance, a non-homogeneous optical wave enters the adjacent optically thinner medium and the amplitude of this wave fades exponentially.

The depth of penetration of this wave is in the region of the wave-length of the light used for the measurement. The measuring effect is, thus due to the interaction between the exponentially fading light waves and the liquid under investigation. The multi-reflection cell (hereinafter abbreviated to MIR cell) may be used in a conventional commercial spectrophotometer which carries out a comparative measurement between incident and emerging light intensity in known manner.

As shown in Fig. 1 , the chemical reaction to be investigated takes place in a reaction tank 1. A ring pump 3 circulates the reaction product through a bypass duct 2 attached to the tank 1.

No product losses occur with this arrangement.

The sample stream in the bypass duct 2 is measured by a flow meter 4 and regulated to a constant value if necessary. A restrictor valve 5 is provided for coarse adjustment of the sample stream. The combination of valves 6, 7, 8 passes the sample stream through the MIR cell 9 which is installed in the IR spectrophotometer 10. The crystal plate 11 with incident and emergent light beam is indicated in the MIR cell. The measurement is carried out either by passing through a complete spectrum (12) or by recording the absorption at a fixed wave length as a function of time (13).

The construction of the MIR cell 9 is shown in Figs. 2 and 3. It consists substantially of the cell body 14 and cell covers 1 5 and 16 with inflow and outflow pipes 17 and 1 8 and a support 19 for fixing and positioning the MIR cell in the IR spectrometer. The chamber of the cell properly speaking consists of a continuous bore 20 with rectangular cut-out portions 21 partly extending right through. These receive the crystal plate 11 and the corresponding sealing support. As may be seen from Figs. 2 and 3, the inflow and outflow pipes 1 7 and 1 8 open into the interior of the cell 20 in a manner such that the axes of the pipes at their junctions with the cell extend parallel to the crystal plate surfaces 11 a and 1 b which determine the measuring process.By this arrangement, the light used for measurement enters and leaves the crystal at a certain angle.

Due to this arrangement of the crystal plate 11 and pipes 1 7 and 18, the sample stream washes over the crystal surfaces 1 a and 1 b virtually without applying any pressure to them, which means that there is no normal flow component acting on the crystal plate surfaces 11 a and 1 b.

Another important requirement is that the crystal plate should be head elastically in the MIR cell and yet in such a manner that it is resistant to pressure. As shown in Fig. 3, the plate must be supported in a manner such that surfaces of entry 23 and exit 24 for the measuring light are situated.

outside the body 14 of the cell while the crystal surfaces 11 a and 11 b which are critical for the measuring process are washed by the sample stream. It is necessary that the crystal plate should be attached in an elastic manner since the coefficient of thermal expansion of the plate is generally not identical to that of the cell body 14.

If the crystal were attached in a rigid manner, any changes in temperature would rapidly result in breakage of the crystal plate 11. It is also necessary that the semi-conductor crystal plate 11 should be attached in a pressure-resistant manner since the cell is 100% filled and the flow inside the cell 20 must be sufficiently powerful to keep the crystal clean. For this purpose, the MIR cell is so designed that in operation, the speed of flow of liquid over the surfaces 1 a and 1 b of the crystal plate is at least 1 m/min and preferably more than 2 m/min. This elastic and pressure resistant mounting of the crystal plate 11 is made possible by supporting the crystal plate at both its ends over its whole periphery by means of an elastically sealing material 25 attached to the external surfaces of the cell.The body of the cell is cut out to provide wedge-shaped recesses at its lateral surfaces to receive the seal and is equipped with closing plates 26 which may be screwed to the body 14 of the cell, (screws 27 indicated in Fig. 2). The closing plates 26 ensure that the seal 25 applies a uniform surface pressure against the crystal plate 11. Siloprene rubber has proved to be a suitable material for the seal 25. In its optical characteristics, this material has the advantage that its IR absorption is generally outside the range of wavelengths used for the measurements.

The MIR cell may be temperature-controlled by means of a built-in heating plate 29 or by heating rods directly installed in the cell body 14. Due to the special arrangement by which the crystal plate 11 was supported, temperature changes from room temperature to 1 000C caused neither fracture of the crystal nor any loss in sealing effect. The crystal plate may consist inter alia of geranium, silicon, or zinc selenite. The surfaces 1 a and 11 b and 23, 24 are plane-ground and polished to a high degree of precision.

The multi-reflection cell described above provides a new measuring technique which may be used for measuring on-line infrared spectroscopic data during the reaction in a production process and which may be used directly for controlling further reaction processes or for optimizing the progress of the reaction.

Claims (6)

1. An apparatus for the continuous investigation of chemical reaction processes in the liquid phase, comprising an infrared spectrophotometer with a throughflow cell adapted to accommodate a continuous flow of a sample stream branched off from the reaction container, wherein the throughflow cell consists of a multi-reflection cell with a crystal plate supported free from tension, which crystal plate is washed by the sample stream flowing in a direction substantially parallel to the surface of the crystal plate.

2. An apparatus according to claim 1, wherein the multi-reflection cell is equipped with inflow and outflow pipes having axes which at their entry into the multi-reflection cell are oriented parallel to the crystal plate surfaces and perpendicular to the direction of light passing through the crystal plate.

3. An apparatus according to claim 1 and 2, characterised in that the multi-reflection cell and the inflow and outflow pipes are so designed that, in operation, the velocity of flow of the sample stream over the crystal plate surfaces is at least 1 m/min, preferably more than 2 m/min.

4. An apparatus according to any of claims 1 to 3, wherein the crystal plate is supported at both its ends over its entire periphery by means of an elastic sealing material attached to the external surfaces of the cell.

5. An apparatus according to claim 4, characterised in that the seal consists of a material having an IR absorption which is not superimposed on the wavelength to be measured.

6. An apparatus according to claim 1, substantially as herein described with reference to any of the accompanying figures.