GB2397887A

GB2397887A - Ultrasonic gas composition analysis device

Info

Publication number: GB2397887A
Application number: GB0402009A
Authority: GB
Inventors: Andrew Carrington; Philip Ake Mansson Rydin-Orwin
Original assignee: FLOTEC UK Ltd
Current assignee: FLOTEC UK Ltd
Priority date: 2003-01-30
Filing date: 2004-01-30
Publication date: 2004-08-04
Also published as: GB0402009D0

Abstract

A measurement cell, suitable for analysing near vacuum gas mixtures, comprises a piezoelectric ultrasonic transmitter 4 and receiver 5 closely spaced and facing each other in a flow cell 1. The speed of sound in the gas 12 is calculated by measuring the transit time between the sensors, and the gas is identified from the speed of sound. The physical dimensions of the measurement cell are arranged so that the transit time of the ultrasound pulse through the gas is less than the transit time of the ultrasonic transmission which travels from the transmitter 4 to the receiver 5 though the solid structure which comprises the transducer housings 2,3 and the flow cell 1. Also, the materials for the housings 2,3 and the flow cell 1 are chosen to have high acoustic attenuation and low acoustic sound speed. A temperature sensor 14 or 15 (eg thermocouple, RTD or infra-red pyrometer) and a pressure sensor 19 are also provided. The measurement cell may analyse exhaust gases from a centrifugal evaporator.

Description

Ultrasonic F id Analysis Cell.

Field of the invention

This invention relates to ultrasonic transmit receive-sensing wherein pulses of ultrasonic energy are propagated through fluids. This invention relates to a particular application wherein the sensors are housed in a novel mechanical arrangement that overcomes a serious limitation when operating such sensors in a near vacuum gas mixture.

Background

Ultrasonic methods can be used to analyse a gas by measuring the speed of sound within that gas. This is done by measuring the time of flight (commonly called transit time) for the ultrasound from one sensor to another over a fixed distance. By dividing the distance by the measured time we obtain the speed of sound for the gas. For different gases the speed of sound is different so it is possible to identify gases from the measured speed of sound. It is also possible to calculate the ratios of known gases in a mixture.

In order to do this a measurement cell similar to figure 1 is used. This arrangement allows one piezoelectric transducer to transmit an ultrasonic signal to another transducer through the gas. The piezoelectric transducer (4) in figure I generates ultrasonic energy when excited by an electrical signal. Ideally 10()% of the energy should emanate from the front of a transducer, but in practice energy is also radiated out to the sides and to the rear of the device. In order to mechanically protect and electrically insulate the piezo-ceramic transducer it must be contained within a transducer housing (2). To reduce the spurious energy radiated into the flow cell, absorbing material can be positioned all around the transducer sides and rear lace (7 and X) ensuring that almost all of the energy does in fact pass through the front of the transducer, but the transducer still has to be in direct contact with its mechanical surroundings through its front face in order to couple ultrasound through to the gas.

This means that energy is introduced into the solid material of the mechanical assembly. Generally ultrasound will propagate much more readily through a solid material, due to many factors, but primarily because energy transfer is much more efficient through solid structures that have strong chemical bonds, rather than through a gas that obeys the kinetic gas laws.

Imagine an electrical signal applied to the transmitter (4). In this case most of the energy moves in a direction towards the receiver (5) but also a small amount of energy is reflected back up the transducer housing (3) around the flow cell (I), through the opposite transducer housing (2) and into the receiving transducer (S). This problem is compounded by the fact that typically ultrasound travels 20 times faster in a solid than in a gas and is attenuated hundreds of times less. Therefore when a pulsed transmission occurs the time taken for the ultrasound to travel from the transmitter to receiver via the gas (Tg) must be less than the time taken for the ultrasound to travel around the mechanical housing (Tm); Tg < Tm If this condition is not met, the useful gas borne ultrasonic transmission can become completely swamped in the energy that travailed through the solid material of the

L

now cell and is thus undetectable. In practice due to the detection method a measurement margin is required, typically; 2.Tg ≤ Tm Provided that this condition is met, then the time for a pulse of ultrasound to travel from the transmitter to receiver via the gas can be measured and computed by the measurement cell electronics (13).

Ultrasonic absorbers can be placed within the measurement cell to reduce the energy that travels around the mechanical housing, as shown in patent (US 5392635). The best materials for use as ultrasonic absorbers tend to be soft elastomers such as silicone or neoprene rubber but these materials break down in the presence of many solvents and are thus totally incompatible for use in equipment containing such solvents unless they are mechanically protected by other materials. Also if the measurement cell is for use in vacuum equipment then it will be necessary to rigidly mount the sensors so that they withstand the forces involved in vacuum equipment.

Energy will therefore couple from the transducer to the mechanical surroundings.

A measure of the ability for a material to transmit sound is given by acoustic impedance (Z in equation l)). A solid material has high impedance whereas a rarefied gas has low impedance. This is simply the density of the material multiplied by the acoustic sound propagation velocity.

Z = p.c Equation l).

Where p = The density.

c - The acoustic velocity.

When materials have similar impedance the sound wave can easily pass from one material to the next, and if they have a different impedance, a wave cannot easily travel from one material to the next. This can be expressed mathematically looking at the Reflection Coelf cient (R) and Transmission Coefficient (T) in equations 2) and 3).

R = (Z2 - Z I) / (Z2 + Z I) Equation 2) T = (2.Z2) /(Z2 + Zl) Equation 3) The'.' symbol denotes multiplication in the equations above.

For example a reflection coellicient value of 0.5 would mean that 5()% of the energy is reflected.

In low pressure systems the gas will have a very low acoustic impedance compared to the acoustic impedance of the transducer. This large mismatch will cause any transmission reception process to suffer from low received amplitudes and high attenuation. An overall insertion loss of 100 dB is not uncommon. This means that in order to conduct such measurements in this environment, the measurement cell must have several design considerations described below.

Brief Description of drawings

Figure 1 is an end on view of a gas analysis cell Figure 2 is a side view of a gas analysis cell s Figure 3 is a block diagram of an evaporator system with the analysis cell positioned between an evaporator chamber and a vacuum pump.

Figure 4 shows two gas analysis cells, one in a gas now and the other connected to the gas flow but located within a regulated temperature chamber.

Detailed description

A low pressure gas (12) flows through a conduit that forms a measurement flow cell (1). A number of transducers are situated within this cell in order to accurately characterize the gas. A piezoelectric transducer (4) is used as a transmitter and a second one is used as a receiver (5). These transducers (4 and 5) are contained within transducer housings (2 and 3). These housings provide a means for positioning the piez.oelectric transducers close to each other, towards the center of the How cell. The material that the housings and the overall cell (1) are fabricated from is selected to have a high acoustic attenuation. low acoustic sound speed and to be mechanically and chemically stable. The thickness of the front of the transducer housing is chosen so as to provide a quarter wave matching layer to improve energy transfer between the transducer (4) and gas (12). The physical dimensions of the transducer housings and cell are chosen such that the ultrasonic transit time through them is greater than that through the gas.

The temperature of the gas within the measurement cell has a direct effect on its sound speed, therefore it is crucial that this is accurately determined. However in near vacuum conditions the rarefied atmosphere causes the heat transfer to be minute.

Therefore any temperature sensor has to be thermally isolated from the surrounding flow cell. A small temperature sensor (14) such as a thermocouple, a miniature platinum resistance defector, or similar, is embedded within a metal block (23). The block material is selected to have a very high thermal conductivity and it is mechanically secured onto the end of a thermally insulating tube (20). The tube has the thin electrical connection wires passing through the centre of it and is back filled with a thermally insulating material. This effectively isolates the temperature sensor from its external surroundings so it can make an accurate temperature measurement of the gas. Vacuum seals are incorporated into the design of the insulating tube (2()) that is made from a good thermal insulator like Alumina. This construction offers a good degree of insulation, but an alternative arrangement can be incorporated into the design.

In order to minimize the heat flow through the electrical wires a non contact temperature sensing method can be used. In this case a pyrometer (15) can be placed on the flow cell (1), and infrared radiation transmitted through an infrared window (21). This infrared radiation would strike the target (16). This target is a thin metal plate having a large surface area to aid heat conduction. It is secured inside the flow cell, near to the area between the two transducers (4 and 5) by means of a thermally insulating rod (17) constructed from a material having a low thermal conductivity, such as Alumina or machinable glass ceramic. At the target, the infrared energy would be reBected and received by the pyrometer (15) and the temperature of the target can be determined. Using this technique an accurate temperature measurement of the gas can be established.

By positioning the gas temperature sensor system used close to the ultrasonic transducer housings we can make sure we are measuring the temperature of the gas passing between the two sensors where we are measuring the sound speed.

There is also a pressure transducer (1'3) included within the flow cell. The amount of signal loss within the system is related to both pressure and the actual gas within the measurement cell. This pressure transducer thus measures one of these factors so it can be used in the identification of the gas within the cell.

A third temperature sensor ( 18) is placed in one of the transducer housings (2). This is a small sensor such as an RID or a thermocouple or pyrometer. This sensor would measure the temperature of the material in the transducer housing. As the ultrasonic signal has to pass through this material and into the gas, the sound speed of the material will have an effect on the transit time of the signal. By measuring the temperature of this material we determine the sound speed for the material at that temperature through a look up table. Using this we can determine the time the ultrasound spends within the material and correct the transit time measured to give the true transit time through the gas only.

As the ultrasonic signal loss in a near vacuum condition is so large, electronic circuit boards (6 Andy) are located very close to the actual piezoelectric transducers (4 and 5). One circuit is a transmitter (9) and the other is a receiver (6). The piezoelectric transducers (4 and 5), the absorbing material (7 and 8) and the corresponding electronic circuit boards are contained within metal cups (24 and 25) that provides a 360 degree electrical screen. All of the electrical transducer outputs from the temperature, pressure and ultrasonic sensors are connected to the flow cell computer (13). The Bow cell computer (13) consists of a timer (28), analogue amplifiers (26), signal generator circuitry (27), analog to digital converter (29), memory (30) and digital signal processing unit (3 l) and any other necessary input / output circuitry to make the cell computer operate.

In operation the flow cell computer (13) uses its signal generator circuitry to create a timed pulse or series of pulses that are sent to the transmitting transducer (4) via the transmitter board ('3). This signal then propagates through the gas within the cell and is received by the receiving transducer (5). It is then pre-amplified by the receiving circuit (6). The amplified received signal is then fed into the cell computer (13) for any further amplification required. This fully amplified signal is then fed into an analog to digital (A/D) convertor (2'3) and digitized with the resulting data being stored in memory (3()). The signal processing unit (31) then uses correlation or similar digital signal processing techniques to determine the overall transit time of the ultrasonic signal.

The cell computer (13) converts the signal obtained from the temperature sensor (1 X) in a temperature value and then uses this to find the speed of sound within the transducer front face by using a look up table or through equation. This sound speed enables the cell computer (13) to then calculate the time the ultrasound spends within the front faces of both the transmitter transducer (4) and the receiver transducer (5).

This time is removed from the overall transit time to obtain the transit time within the gas only. From this the speed of sound of the gas becomes known.

The digitized ultrasonic signal held in memory can also be used to obtain a signal loss value for the system by using the system gain set by the flow computer and the peak to peak size of the digitized signal.

The cell computer (13) converts the signal from the gas temperature system used, be it temperature sensor (14) or pyrometer (15), into a gas temperature value. It also converts the signal from the pressure sensor (19) into a pressure value.

These values of gas sound speed, signal loss, pressure and gas temperature can then all be used to determine the gas or gases present within the measurement cell. This can be done through look up tables of the data for different gases or mixtures of gases.

The inclusion of more than sound speed and temperature data allows gases with similar sound speeds to be distinguished from each other and for mixtures of gases to be distinguished from single gases.

As the speed of sound in a gas is very dependent upon the temperature of the gas it is proposed as an extension of the measurement cell that it can be contained within a temperature regulated chamber (43) in figure 4. In this arrangement gas Bows through a pipe (40) containing a measurement cell (1) and a small amount of the gas is diverted and flows through a long capillary tube (42), into the temperature controlled chamber allowing equilibration of the gas temperature. An identical measurement cell (41) is located within the regulated chamber (43) that analyses and identifies the gas.

The sampled gas is then returned to the main flow via a second capillary tube. The outputs from both measurement cells (I and 41) are sent to a cell computer (13) for processing.

As well as being used to measure the transit time in gas between two sensors perpendicular to any gas flow as in figure 1, the measurement cell can be extended as in figure 2. Here two ultrasonic sensors are positioned at angles to or in the direction of any gas flow. In this embodiment each sensor is used as a transmitter and a receiver. The transit time is measured going with and against the gas flow direction and the results are averaged to give the gas flow velocity as per ultrasonic gas nowmeters (US 5392635). This figure can be used with density to obtain volumetric and mass flow within the measurement cell, using flow correction.

As well as measuring the gas flow velocity these sensors can be used as an alternative arrangement for obtaining the speed of sound in the gas. The transit time with the Bow will be determined by the sound speed of the gas speeded up by the flow velocity and the transit time against the flow will be determined by the sound speed slowed down by the now velocity. So by averaging the two transit times we can remove the flow velocity component and just obtain the transit time without gas flow and hence the speed of sound in the gas.

Centrifuga! evaporators.

Centrifugal evaporators spin and heat samples in a near vacuum environment in a vacuum chamber. The chamber is connected to a vacuum pump. Evaporators of this type are well known (UK 2334688). At present it is difficult to know accurately when the evaporation process is complete and impossible to differentiate between various solvents in evaporation runs containing more than one solvent.

Therefore a measurement cell as detailed previously is proposed, to interrogate the exhaust gases through use of a number of sensors. Using such a cell, it is possible to identify the actual solvent evaporating, when it starts and stops evaporating, measure the gas flow rate, determine when all the solvent has evaporated and optimize the evaporation process by providing feedback signals to the evaporator and vacuum pump, The samples are contained within the evaporator (34) in figure 3, the exhaust gas passes through a connecting pipe (35) and through the measurement cell (36) wherein physical characteristics of the gas are sensed, and the outputs from the sensors are passed to the cell computer (13) for processing. After the vacuum pump, the solvents are recovered in their liquid phase and collected in a storage vessel (38). This enables the cell computer to determine the gas flow rate, and identify the gas or gases passing through the pipe (2). This information can then be passed to the evaporator control computer (39) so that it can then adjust the evaporation process. This can be achieved by adjusting the pressure in the vacuum chamber (34) by controlling the vacuum pump (37), changing the heat input to the evaporator and by varying the speed of rotation of the samples within the chamber. This process optimization will save time and energy, improving system performance and throughput.

When evaporating solvents, it is important to detect the end of the evaporation run in order to prevent the samples from being dried too much. This can be achieved as the measurement cell (l) measures the signal amplitude, which is proportional to the vapour density. Therefore as the vapour density decreases, so the ultrasonic amplitude decreases thus allowing optimization of the solvent evaporation process. The measurement cell (l) also measures the flow rate. Therefore as the flow rate l approaches zero, an end of evaporation run control signal would be generated by the control computer which is fed back to the evaporator to terminate the process.

If a compound is dissolved in a single solvent which is to be evaporated, it is possible to precisely determine the end of run and the rate of evaporation through a new process. Firstly, the user inputs the volume of solvent. The samples are loaded into the evaporator and the process is started. The vacuum pump reduces the pressure in the evaporation chamber and the centrifuge starts to boil the solvent. The flow cell can identify when the solvent starts to evaporate. Initially the flow cell detected air being present through use of its sound speed measurements. Once the solvent starts to evaporate, so the measured sound speed would change indicating a change in gas.

This would signal the control computer to start recording the mass flow rate measured through the flow transducers. Alternatively the change in gases within the chamber could also be detected through a change in received signal amplitude, i.e. signal loss, correcting for any pressure changes. In order to measure the mass flow rate the flow cell first measures the transit time difference which results in a measurement of fluid > velocity. This is multiplied by the cross sectional area to give a volume flow rate, taking into account the now profile and applying a correction based on the fluid's Reynolds number. Finally a measure is required of the gas density. This is provided by the amplitude measurement sensors within the measurement cell, or by other means. Ultrasonic signal attenuation is proportional to gas density. Multiplying the density by the volume flow rate results in a mass flow rate measurement.

Therefore it is possible to measure the quantity of solvent that has passed through the measurement cell and hence how much of the original solvent has been removed from the evaporation chamber. In this manner, it would be possible to estimate how much time remains before the evaporation run is complete. This would be a valuable aid to evaporator operators. It will also be possible to detect when all the solvent has been evaporated when the measured amount through the measurement cell equals the amount initially placed within the chamber.

When the evaporator is used with a number of solvents, the sound speed and signal loss measurements can be used to determine when each individual solvent has started to evaporate. We will also be able to determine the ratios of the resulting gases.

Claims

Claims I) A transmitting and receiving transducer are located in a

mechanical configuration, facing each other, located in relatively close proximity in a measurement cell wherein the time for a pulse of ultrasound to travel from the transmitter to the receiver through the fluid is less than the time for a pulse to travel from the transmitter, through the walls or other solid materials within the measurement cell and into the receiver.
2) The materials in contact with the transducers as claimed in claim I) must have a low acoustic propagation velocity, compared to most metals and the flow cell wall must be substantially thick enough to support the ultrasonic transducers and prevent movement of the transducers in situations of large pressure changes.
3) The materials in contact with the transducers as claimed in claim 1) must have a high attenuation to ultrasound, compared to most metals.
4) A thermally isolated temperature sensor exists in the measurement cell, as claimed in claim I) that provides a temperature measurement that is necessary in order to correctly identify the fluid from the measured sound speed using look up tables stored within the cell computer.
5) The temperature measurement as claimed in claim 4) comprises a noncontact pyrometer arrangement wherein an infrared transceiver located on the outside of the measurement cell radiates infrared radiation through a vacuum sealed infrared window, through the fluid, onto a target, wherein its energy is re-radiated from the target7 propagating back through the fluid and window, wherein it is received by the infrared pyrometer, thus yielding the actual fluid temperature, free from erroneous readings due to thermal conduction through the measurement cell walls.
6) The metal target, as claimed in claim 5, has a low thermal mass, a large surface area and is supported by a thermally insulating support, in order to thermally isolate the target.
7) In a further embodiment to claim 4), the complete measurement cell, as claimed in claim 1) can reside within a regulated temperature controlled cell7 connected to the flowing fluid via long capillary tubes that are themselves substantially located within the regulated cell7 in order that the whole measurement cell7 including the fluid that it contains7 reach a known temperature.
8) The flow cell as claimed in claim 1) has two additional sensors7 mounted on opposite sides of the measurement cell7 angled obliquely to the fluid flow direction in order that the time for a pulse of ultrasound to travel from the transmitter to the receiver through the fluid is less than the time for a pulse to travel from the transmitter through the walls or other solid materials within the measurement cell and into the receiver.
9) The arrangement as described in claim 8), is used to measure the transit time difference between upstream and downstream ultrasonic transmission bursts and hence calculate the flow velocity within fluids.
10) A method wherein volume flow rate can be calculated using the flow velocity value as claimed in claim 9) and multiplying the measurement cell cross sectional area by the fore mentioned flow velocity.
l I) A method is offered wherein the mass flow rate can be calculated by multiplying the volume flow rate as claimed in claim to) by the fluid pressure as measured as claimed in claim 13).
12) The arrangement as described in claim 1)7 is used to measure the transit time of an ultrasonic pulse and hence calculate the sound speed in the fluid by dividing the distance between the transmitter and receiver by the fore mentioned transit time.
13) The arrangement as claimed in claim 1), is used to calculate the amount of signal loss through the fluid such that the acoustic attenuation is calculated.
14) A pressure sensor located within the measurement cell as claimed in claim 1) to allow pressure readings to be taken within the measurement cell.
15) A conductivity sensor located within the measurement cell as claimed in claim 1) to detect the presence of condensation within the measurement cell.
16) A method exists that allows the generation of an alarm signal when the conductivity sensor, as claimed in claim 15) detects condensation.
17) The measurement cell, as claimed in claim 1) is connected with electrical wires to a cell computer that takes the sensor outputs from the measurement cell and converts the analogue sensor signals from the measurement cell into digital record samples of the data that are written to memory and stored in order that they are processed by the computer at a later time.
18) The cell computer as claimed in claim 17) has a method wherein all the timing functions necessary in order to conduct ultrasonic pulse-echo, pressure and temperature measurements can be carried out.
19) A method wherein the speed of sound measurement, as claimed in claim 12), enables the cell computer, as described in claims 17 and 18) to identify the gas, or liquids that are evaporating by looking up the sound speed from a data table held in cell computer memory.
20) A method wherein the acoustic attenuation, or signal loss measurement, as claimed in claim] 3), enables the cell computer, as described in claim 17 and 18) to identify the fluid, or fluids.
2] ) A method wherein the cell computer as described in claims] 7 and] 8) uses the ultrasonic amplitude attenuation, ultrasonic sound speed, pressure and temperature to identify the fluid from lookup tables or equations stored in memory.
22) A method is proposed wherein the measurement cell as described in claims 1) is connected between a vacuum pump and an evaporation chamber, in order that the system as described in claims 1) to] 6) can analyse the exhaust gas being removed from the evaporator.
23) A method for use with the systems as described in claim 22 wherein the start of evaporation of a solvent from its liquid state is sensed by a change in the speed of sound, as measured by the system as the air in the chamber is replaced by the solvent gas.
24) A method for use with the systems as described in claim 22 wherein the start of solvent evaporation from its liquid state is sensed by a change in the amplitude of a received ultrasonic energy pulse, as measured by the system as the air in the evaporation chamber is replaced by the solvent gas.
25) A method of generating a control signal for use with the evaporator configuration as claimed in claim 22 wherein the tilted diameter sensors, as claimed in claim 8), detect the flow slowing down to practically zero and cause the cell computer to trigger a zero flow condition.
26) A method to measure flow rate whilst using a temperature regulated cell requires an additional measurement cell located in the main gas flow, outside the temperature regulated cell that is also connected to the cell computer as outlined in figure 4.
27) A method of generating a control signal for use with the configuration as claimed in claim 22) wherein the tilted diameter sensors, as claimed in claim 8), detect the flow starting from zero, or practically zero, and cause the cell computer to trigger an "evaporation has started" condition that is used by the evaporator to optimise the evaporation run and to signal to the system operator that progress has started.
28) A method of generating a control signal for use with the configuration as claimed in claim 22) wherein the tilted diameter sensors, as claimed in claim 8), wherein a change in the flow rate causes the cell computer to trigger a "flow rate changed" condition that is used by the evaporator to optimise the evaporation run and to signal to the system operator that the flow rate has changed.
29) A method wherein a change in amplitude, as claimed in claim 13), is used to optimise an evaporation run, using the configuration as claimed in claim 22).
30) A method wherein a change in sound speed, as claimed in claim 12), creates a control signal that is used to optimise an evaporation run, using the configuration as claimed in claim 22).
3 1) A method which uses one of the "evaporation has started", techniques of either measuring the flow, or sound speed or amplitude change in order that the cell computer can start a timer in order that the total mass, or volume flow rate can be calculated.
32) A method that uses one of the "evaporation has finished" techniques of either measuring the flow, or sound speed or amplitude change in order that the timer proposed in claim 31) can be stopped and the total mass, or volume flow rate can be calculated.
33) A method of estimating the total evaporation time and the time to evaporation run completion can be estimated by subtracting the totalised mass flow rate from the total mass of solvent initially put into an evaporation system.
34) A sensor system substantially as herein before described with reference to and as shown in the accompanying drawings.
35) A control system substantially as herein before described with reference to and as shown in the accompanying drawings.
36) A method substantially as herein before described with reference to and as shown in the accompanying drawings.
37) A sensor system which includes any one or more of the novel features herein before described.
38) A control system which includes any one or more ofthe novel features herein before described.
39) A method which includes any one or more of the novel features herein before described.