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Factory Engineering

I DEVELOPMENT OF CONDUCTIVITY AND CAPACITANCE SENSORS FOR PAN BOILING

S.R. Reichard, R. Broadfoot and P.G. Wright

Sugar Research Institute, MacKay, Queensland, Australia , / ' ., i

ABSTRACT -.

In the Australian sugar industry the massecuite condition' in vacuum pan boilings is mainly measured by conductivity transducers. This paper describes the hnsiderable developments which have taken place in the F g n of these transducers since the early use of simple A.C. contacting, type, electrodes. Today radio-frequency types which allow the, measurement of the conductivity and/or the capacitarice of the process material are being widely ,used. Sensors have been developed to suit all pan boiling applications in the raw sugar factory and also for all grades of sugar refinery boilings. A design of heated probe which prevents the formation, of sugar crust on the surface contacting the massecuite has been developed. This is particJlarly suitable for use in continuous vacuum pans boiling high purity massecuites.

INTRODUCTION

Conductivity has been widely used in Australia for the control boilings since its introduction from Java in the 1930s as the 'Cuitqmeter'. The )principles of the control method have been described elsewhere, e.g. Wright7. Ban control in Australia is now almost fully automated;' with the feed control being largely based on conductivity sensing methods. However, there has been a gradual development of conductivity sensors from the early simple A.C. circuitry towards radio-frequency types which have special advantages. This paper briefly outlines these developments.

CONTACTING ELECTRODE TYPES , 1

The first conductivity systems were based on measurements of the current flow in a ~ircuit containing contacting1 type massecuite electrodes poyered by 32V 50 Hz alternating current. The pan electrodes were, of a type standardi- zed by' Smiths and known. in Queensland as the ,'Bureau type'. The arrangement for these electrodes is shown in , Figure l(a). This system consists of a pair of parallel metal cylinders, 16 mm diameter and 45 mm long, spaced about 75 mm apart and mounted at least 125 mm from the wall

, FACTORY ENGINEERING

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(a) Double probe 'Bureau' type.

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TEFLON TEFLON 316 . I ,SPACER , SS,316 '

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, , , , ,, ,\OD) Single Probe Type

FIGURE 1. Examples ,of cqntacting r type elqctrodes. ,

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of thec pan! Tlie conductivity 'cell constant: with sthi 0.0911mS cml, and the opefating masseouite 'resistance?is around 250 ohms' for raw shgar high grade boilings and atound'350 ohms for 'low grade boilirigs. These electrode$ were widely used, St over brass, copper or mild stee18fot the ' \ I , - ! I I , !, l,3! q,, ;, t '

S.R. REICHARD, R. BROADFOOT AND P.G. WRIGHT

A family of simpler single probe electrode units was introduced, into the Australian industry in the mid 1960s (Wright6). In these the active section of the probe is a 25 mm diameter metal tip which is separated from the earthed metal shank of the probe by a teflon spacer. The original distance of separation was 50 mm to give approximately the same cell constant as the earlier double probe type. The single probe type has proven to be more versatile in finding a suitable location in the pan and easier to replace and to clean than the 'Bureau type'. Later it was found to be better (for strength reasons) to reduce the spacer distance to about 25 mm and to use the electrode with commercial instrumentation A.C. resistance transducers which use a more sensitive, higher frequency (1,000 Hz) circuit. A sketch of one arrangement of this electrode is shown in Figure l(b).

'CONTACTLESS' (RF) ELECTRODE TYPES

One common defect with the contacting electrode types is the progressive change in the surface of the electrodes. Formation of scale or crust on the electrodes and erosion of metal by electrolytic action make frequent cleaning of the electrodes necessary to avoid drift in the operation settings. It was common practice in some factories that the electrodes be withdrawn and cleaned every eight hours, while other factories had a policy of 'not cleaning' which allowed the scaling to take place to a level where its effect was relatively constant. To overcome these problems a new method for sensing conductivity at radio-frequencies (RF) was introduced by Reichard and Vidler". The method did not require direct contact of the metal to the massecuite in the pan and was consequently termed 'contactless' conductivity measurement.

In this method the conductivity of the massecuite surrounding a probe is measured by sensing the degree of damping applied by the massecuite to a tuned circuit resonating at radio frequencies (about 10 MHz). The sensing probe which comprises an aerial encased in a thin walled teflon sheath, forms I

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part of the tuned circuit, with the electrical energy being transferred to the massecuite by capacitive coupling. The RF probe consists of a copper tube

1 aerial 20 mm in diameter and 100 mm long encased in a teflon sheath 2 mm thick. Current pulses of very short duration are generated with a repetition rate of a few hundred kilohertz. Each pulse generates a burst of damped oscillations~in the resonant circuit. An amplitude detector and signal condi- tioning circuitry are used to derive a 4-20 mA output signal that is propor- tional to the average amplitude of the ringing waveform. Little damping is experienced when the probe is immersed in a massecuite of high water content. A more rapidly damped waveform is obtained when the probe is immersed'in a heavy massecuite (of low water content) and low conductivity. This type of transducer was first put into service by Sugar Research Institute (SRI) as a replacement for the conventional system in 1974.

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FACTORY ENGINEERING

Other workers have also studied the electrical properties of massecuites at radio frequencies (Moller et a1.2; Radford and Cdx3) and have developed useful RF transducer systems. As well, at least one instrumentation company (Foxboro) market a RF conductivity transducer. Evaluations ,by Miller and Skippen' of different conductivity systems indicated that some diffiaulties may be experienced with 'electrodes' of toroidal shape as these may suffer reduced sensitivity towards the end of a strike when the massecuite viscosity is high and flow through the toroid is sluggish. However, this type has been quite successful on low viscosity applications.

The SRI contactless conductivity transducer has been enhanced since the early prototypes into an extremely useful and robust system. The current probe is designed to fit into a standard 40 NB BSP socket in the pan wall. The probe contains no electronic components, and is connected by a short coaxial cable to the electronics package which consists of the RF circuitry, the detector, amplifier and power supply .(24V D.C. input), all housed in a metal box. The box can be located in such a way as to protect the electronics against overheating from radiation or steam. A separate oalibrating unit is available which can be connected in place of the probe for initial setting up or for re-calibration. An overall view of the (transducer and calibrator is shown in Figure 2. 1 .

The transducer is widely used both in Australia and overseas for raw sugar boilings where materials have high dielectric loss and hence high Jconductivi- ty. It has been found that it can be used either with the original sheathed probes or with unsheathed @are metal tip) probes as long as the tuned circuits in the latter case have a capacitor in series with the probe tip.

A problem appears when this transducer (with either form of probe) is applied to refinery massecuites or to very high purity raw massecuites because these materials have a very low dielectric loss. This loss is then more affected by small variations in impurities than by variations in the water content. Consequently, conductivity measurements of1 this type are unreliable in very high purity of refinery massecuites. A new version of the electronics, the wide purity range (WPR) version, was developed at SRI fo applications. This is described below.

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THE WIDE PURITY RANGE (WPR) TRANSDUC

If a probe convenient geometry is immersed in , massecuite, the resulting capacitance measured at the probe terminal depends largely on the amount of water surrounding the probe. This is because water has by far the highest dielectric constant of the materials in the vicinity? of the probe. Thu capacitance varies with the water content of the massecuite. ,

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FIGURE 2. The SRI RP rnnd~~ctivitv transducer set with sheathed robe

FACTORY ENGINEERING I

The method for measuring the variable capacitance of the probe immersed in the massecuite uses a resonant circuit That includes a fixed inductor and the variable capacitance. The resonant circuit is excited into" oscillation (at approx. 6 MHz). As the capacitance varies, the frequency of oscillation varies in an inverse relationship to the square root of the capacitance. The signal processing circuitry used in the WPR electronics converts the frequency variation to an output signal of 4-20 mA. Thus the output signal is related to solids concentration (i.e. absence of water).

WPR transducers measure resistance as well as capacitance. It has been found that, to enhance the measurement of massecuite capacitancq, the probe should contact the massecuite, i.e. a bare metal tip probe should be used.

Electrical parameters associated with WPR transducers

The bare tip probe capacitance (including cable and strays) is about 120 pF. During a typical pan boiling cycle, the massecuite capacitance varies over a range of about 8 pF. Thus the typical full scale range of capacitance variation is about 6% which results in a full scale frequency change of 3%. The oscillator and the circuitry deriving the 4-20 rnA signal from such a small variation are required to have excellent stability. This presents a challenge in circuit design but this has been successfully addressed.

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~lectrdnic configuration of WPR transducers I

An electronic chopper technique is employed in the WPR ci;cuitry. The input to the transducer is alternatively switched between a standardizing network and the probe network. This is used for cancellation of errors which may be introduced through variations in ambient temperatures and to provide good stability in the circuitry.

These transducer units are installdd with bare tip probes in batch refinery and raw sugar mill pans and in a continuous high grade pan.

SHEATHED AND B m METAL TIP PROBES

Early RF probes were sheathed because the sheath was widely reported to make the readings less affected by probe contamination and the probe therefore required no cleaning. Later (1982-88) it was found that this only applied where the contamination or encrustation was not severe. In applications such as for evaporator syrup Brix control and control of high grade continuous pans, encrustation can be so severe that it influences the RF distribution around the probe. In batch pans with poor circulation, the


sheathed probes are thought by some operators to be superior to bare tip probes because the sheathed probes are subject to less contamination of the tip with sugar crust and only have to be cleaned very infrequently. In locations where the massecuite circulates vigorously around the probe, the contaminants are likely to be worn away by the sugar crystals and there may well be no advantage in using sheaths.

The sheath is a virtual capacitor interposed between the massecuite and the RF circuitry. The presence of the sheath over the aerial affects the measurement sensitivity because the sheath capacitance tends to swamp any capacitive variations in the massecuite. This is of little importance where conductivity is being measured and hence sheathed probes are suitable for conductivity measurements. The sheath varies slightly from probe to probe and evaluation tests have shown that differences in the response of the transducer can arise due to variations in a number of parameters including the thickness of sheath, the lay of the aerial within the sheath (if the aerial is not a tight fit within the sheath) and the distance from the end of the aerial to the end of the metal shank. These variations are of little consequence for individual, un- disturbed applications, e.g. for control of batch pans using a single probe for conductivity measurement. Nevertheless, close attention is given to consistent manufacturing methods to reduce the magnitude of these variations.

Bare metal tip probes (with series capacitors fitted to the RF circuitry) are not subject to these same variations. Physically similar bare tip probes show , greater electrical similarity than do sheathed probes. Bare tip probes have been operating successfully in white refinery pans, recovery pans and melter liquor tanks and in raw sugar pans .of all grades of massecuite.

The transducer with bare tip probe ofters the advantage that the total transducer unit (RF exciter electronics plus probe) can be ranged for imposed 'massecuite' conditions (resistance plus capacitance). This is undertaken prior to installation of the probe into the pan body by connecting a calibration box across the probe's shank and tip. This feature is of particular benefit for continuous boiling pans where multiple probes with the same calibration are preferably used. It also allows a transducer set to be precalibrated for a known application and so reduces the'time required in the factory for ranging the instrument.

In summary either sheathed or bare metal tip probes are suitable for use with the RF conductivity transducer. Sheathed probes are usually used for batch pan operation and bare tip probes for batch or continuous pan applications. Bare tip probes are used with WPR transducers where a measure of product capacitance is sought.

FACTORY ENGINEERING

The probes are generally supplied with either a long or short shank (total probe length 320 mm and 450 'mm respectively) to provide the appropriate immersion into the process vessel. The probes are usually attached to the vessel through standard screwed sockets (40 NB) although, for, applications where probes may have to be easily removed from process for cleaning, a quick release head (requiring a 50 NB socket) has been developed.

Some types of sheathed and bare tip probes currently in use are shown in Figure 3. These include:-

- Standard sheathed probes (30 mm outside diameter), - Large bare tip probes (approx. 18 sq cm bare tip area), - Small bare tip probes (approx. 10 sq cm bare tip area),

where the area of the bate tip is defined as, the area of the metal tip in contact with the material. Figure 3 also shows a large bare tip probe with a quick release head.

NON-CRUSTING HEATED PROBES

No problems are generally experienced with crusting of sheathed or bare tip RF probes in batch pan applications as these are cleaned sufficiently by pan steamings during pan turnround. However in high purity raw sugar continuous pans and in some refinery batch strikes some sensor drift is caused by

* sugar crust forming on the probe surface.

To address this crusting problem SRI has ,developed a version of the bare, tip probe which comprises an internally heated metal tip and shank. Sufficient heat is applied to ensure the surface temperature of the probe is above the saturation temperature of the mother liquor in the boiling pan. The heated surfaces successfully retard the formation of a tcrystalline crust. ,

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This development results in a transducer which is substantially non-crusting in continuous boiling of high grade massecuites. The probes maintain consistent response for the whole week without having, to be removed and cleaned. Where continuous boiling operations are conducted for times longer than one week between pan boil-outs with watet, it has not yet been established if other (e.g. manual) cleaning of these probes is required.

Trials have yet to be conducted to determine the effectiveness of these probes in overcoming crusting in re / I

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FIGURE 3. Some types of probes currently used by SRI for the control of sugar pans. These components are from the top: sheathed probe; large bare tipped probe; small bare tipped probe (extra short shank); housing for quick release probe; large bare tipped probe with quick release head.

SELECTION OF THE PHYSICAL SIZE OF THE METAL TIPS OF PROBES FOR CONDUCTIVITY AND WPR TRANSDUCERS

It is of little importance which size (area) of tip is used on the bare tip probes in conjunction with an RF conductivity exciter although the larger tip area does provide greater sensitivity.

The size (area) of the tip of a bare tipped probe is important when used with a WPR exciter. It influences whether the WPR is operating in the capacitive or conductive mode and this affects the extent to which purity affects the output. It also affects the sensitivity when operating in the capacitive mode. The larger the tip, the greater the capacitance and the lower the resistance of the probe as seen by the WPR. In highly conductive products, smaller tips are used becaused the increased resistance associated with the smaller tip

FACTORY ENGINEERING

offsets the increase conductivity of the product and so enhances the measurement of capacitance and not resistance. In products of low conduc-tivity, e.g. high grade refinery materials, a larger tip is used because that increases the sensitivity of the tip without moving operation away from the capacitive regime.

In practice, two sizes of bare tip probes are currently used with the WPR exciter. The "large' tipped probe is used in white refinery pans and the "small7' tipped probe is used in melter liquor Brix control (refinery front end), recovery pans (refinery back end) and in raw yugar mill syrups and massecuites.

It is possible to operate the WPR exciter with a large tip probe in conductive materials but, in these situations, the transducer operates as a combination of capacitive and conductive modes rather than capacitive alone. For control purposes this would be quite satisfactory except that the transducer signal would be affected more by changes in the purity of the material surrounding the probe than if the transducer operated purely in capacitive mode. - Nevertheless, this would be affected to no greater extent than would an RF conductivity transducer and would likely be' less affected. In these circum- stances a WPR exciter can replace a conductivity exciter (but not vice versa).

APPLICATIONS FOR RF CONDUCTIVITY AND WPR TRANSDUCERS

WPR transducers are suitable for use in sugar refinery applications including the low purity boilings. On the other hand RF conductivity is not suitable for use in refineries due to the low ash (conductance) of the high purity materials and the variable ash composition in the low purity recovery boilings resulting from variations in ash in the refinery feedstock sugars.

In the high purity refinery materials WPR transducers, if provided with a probe with the appropriately sized tip, will measure the capacitance of the material. This signal is little affected by the solids composition-(purity) of the material. The WPR transducer will behave similarly to an RF conductivity transducer if used with a probe of 'large, tip area in conductive process materials.

The SRI WPR transducers offer better temperature stability than do the SRI RF conductivity transducers but are more expensive and slightly more difficult to calibrate for factory installation. Importantly, however, they are less affected by purity changes in the process materials and so require less frequent resetting than do RF conductivity transducers in applications of varying impurity concentration and composition.


- ' CONCLUSIONS I

Pan control in Australia is rapidly moving to full automation, largely based on conductivity sensing methods. The sensors available have gradually increased in sophistication to improve their ease of use and to widen their application. This paper has briefly reviewed the development of conductivity sensors from the early simple A.C. :circuitry towards advanced radio-frequency types which I

are proving useful in all types of sugar boiling, including refinery boiling and 1

continuous boiling operations.

I REFERENCES z 1 / I

1. Miller, K.F. and Skippen, N.G. (1989). An evaluation of several sensors for feed control of vacuum pans. Proc. Aust. Soc. Sugar Cane Technol.

2. Moller, G.R., Knovl, E. and Madsen, R.F. (1977). Measuring methods for control of crystallization in massecuites of high purity. Proc. Int. Soc. Sugar Cane Technol. 16: 2811-2818.

3. Radford, D.J. and Cox, M.G.S. (1986). The use of electrical properties measured at radio frequencies for pan boiling and brix control. Proc. South African Sugar Technol. Assn. 60: 94-102.

4. Reichard, S.R. and Vidler, T.L. (1975). Contactless conductivity measurement in massecuite. Proc. Qld. Soc. Sugar Cane Technol. 42: 249-253.

5. Smith, N. (1937). Circulation in coil vacuum pans. Tech. Comm. Bur. I Sug. Expt. Stat., Australia 4: 65-90.

6. Wright, P.G. (1968). Conductivity electrode design. Proc. Qld. Soc. Sugar Cane Technol. 35: 137-140.

7. Wright, P.G. (1983). Pan and pan stage control. Sugar Technology Reviews 10: 39-96.

FACTORY ENGINEERING

LE DEVELOPPEMENT DE SONDES, BASEES SUR LA CONDUCTIVmE ET LA CAPACITANCE,

POUR LES CUITES 1 I - S.R. Reichard, R. Broadfoot et P.G. Wright

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Sugar Research Institute, Mackay, Queensland, Australia

RESUME

'r

En Australie on utilise, en general, la conductivite pour obtenir des mesures sur les massecuites. Ce papier decrit les developpements realises dans ce domaine, depuis les premiers essais avec de simples electrodes A.C. Aujourd' hui la radio frequence est employee et on peut mesurer la conductivite et la capacitance du material etudie. Des sondes ont ete developees pour toutes les applications en sucrerie et en raffinerie, On a develope une sonde chauffee, ce qui empeche la formation d'incrustation sur la surface de la sonde. Cette sonde convient particulierement aux cuites continues pour des massecuites de fortes pouretes.

LE EVOLUCION DE SENSORElS DE CONDUCTIVIDAD Y CAPACITANCIA PARA

TACHOS DE COCIMIENTOS

roadfoot y P.G. Wright

RESUMEN

En la indrustria azucarera Australiana la condici6n de la masacocida en 10s tachos es normalmente medida por sensores de conductividad. Este escrito discute 10s avances que se han realizado en el disefio de estos sensores desde el primer uso de 10s simples electrodos de tip0 de conctacto A.C. Hoy electrodos de radio frequencia que permiten medir la conduclividad o la capacitancia del material en proceso se encuentran en gran uso. Sensores han sido disefiados que encumbren todos 10s grados de cocimientos en la refineria. Tambien ha sido disefiado un sensor calentado que no permite la incmstaci6n de az6car sobre la superficie en contact0 con la masa cocida. Este disefio es particularmente fitil para uso en tachos continuos con masa cocidas de alta

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