Download detection of sucrose in boiler feed water

Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
DETECTION OF SUCROSE IN BOILER FEED WATER
By
M.A. GOOCH and A. WIENESE
Sugar Milling Research Institute, c/o University of KwaZulu-Natal, Durban 4041
[email protected]
KEYWORDS: Boiler Condensate Measurement.
Abstract
NUMEROUS authors have outlined the importance of preventing sucrose entrained in
condensate from entering the boilers particularly where higher pressure boilers are in
use. The condensate from a raw sugar mill, sugar refinery or a raw sugar mill with an
attached back end refinery contains different types and quantities of impurities. Because
of this, the appropriate technology to measure impurity levels varies between
applications. This paper is a review of the different techniques to detect the
contamination of sucrose in boiler feed water.
Introduction
A loss of steam production can cause a slowdown or shutdown of a sugar mill. To ensure
reliable operation of the boilers, it is necessary to maintain a good quality boiler feed water. Poor
water quality can cause internal boiler scaling, corrosion and impurities in steam. Impurities in
boiler feed water are mainly dissolved inorganic solids, dissolved organics, iron and copper ions,
dissolved oxygen and acidic ions.
It is universally accepted that it is necessary to prevent sucrose contamination in boiler feed
water. Volatile organic acids can be produced by the chemical breakdown of sucrose. Sucrose
contamination in boiler feed water causes three distinct problems:
!
Scale deposition in the boiler.
!
The lowering of pH due to sugar breaking down into organic acids in the boiler thus
causing corrosion of the boiler tubes.
!
Impurities in the steam that cause scale deposits on turbine blades with consequential
imbalance, vibrations and corrosion.
Sucrose in boiler water
Some signs of sucrose contamination in boiler feed water are a drop in pH, increased TDS,
discoloured water and a sugar smell. There are numerous analytical chemical tests for detecting
trace concentrations of sucrose in boiler feed water. Phenol-sulfuric acid, resorcinol and the alphanaphthol tests are used in the South African industry.
The phenol-sulfuric acid test is simple to perform but it is not specific to sugar, and other
carbohydrates may interfere. The resorcinol test is more specific to sugars but is more complicated
to perform than the phenol-sulfuric acid test.
The alpha-naphthol test is easy and quick to perform and is ideally suited for routine checks.
It is a qualitative test that is used to determine whether a sample contains traces of sugar. Figure 1
shows a typical boiler water system. The protection of boilers from sucrose contamination is carried
out at two levels. At the first level, samples are taken at regular time-intervals at various points in
the system (points 1 to 5). These samples are analysed not only for sucrose, but also for pH,
hardness, alkalinity, suspended solids, TDS, silica and chemical treatment residue.
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Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
The outcomes of these analyses are used to determine the type and level of chemical
treatment and boiler blow down to ensure an appropriate water quality. The time-intervals vary
between one and 24 hours and the response times are therefore slow.
Make-up Water
HP Steam
1
Steam
Drum
Chemicals
Softener Plant
Let Down
Steam
Turbine
5
4
Mud
Drum
2
Gasses
Blow Down
De-superheat
Station
Deaerator
Chemicals
Vapour
BFW
Pump
Exhaust Steam
Process Streams
Condensate
Storage
Tank
Process
Condensate
3
Accept/Reject Valve
Sweet Water
Sweet
Water
Tank
De-superheat Water
Fig. 1—Boiler water system.
At the second level, a condensate accept/reject system differentiates between clean and
contaminated condensate. This does not require an accurate measurement but rather a rapid estimate
of the level of sucrose.
If the level of sucrose contamination exceeds a chosen value, an alarm needs to be activated
and appropriate action taken to prevent that water from entering the boiler. Ideally this system,
which forms the topic of this paper, should be on-line with a short response time.
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Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
Measurement techniques evaluated by SMRI
Heating apparatus causing change in conductivity
In 1986, the SMRI investigated the theory that, when sucrose is heated to 265°C for several
minutes, there is a change in pH and conductivity. The results of tests to determine the relationship
of the sucrose concentration on pH and conductivity are presented in Figure 2. Reid and Dunsmore
(1991) reported on an instrument constructed at the SMRI using this measurement technique to
continuously monitor sucrose traces in condensate.
This instrument comprised of a sampling system, which passed the sample through a filter,
followed by a combined conductivity/pH cell and a high-pressure pump that passed the sample
through a stainless steel coil heated to 265°C.
The sample was then cooled and passed into a second conductivity/pH cell. It was found that
there was a linear relationship between conductivity change and sucrose concentration.
30
25
5
Cond inc = 0.2866xSuc + 3.7408
R2 = 0.9917
4
20
3
15
2
10
5
0
0
20
40
Dec in pH = 0.0202xSuc + 0.1988 1
R2 = 0.9901
0
60
80
100
120
Decrease in pH
Conductivity increase
μs
35
Sucrose (ppm)
Conductivity
pH
Fig. 2—Relationship of sucrose concentration on pH and conductivity.
Further development of this instrument was terminated because scale built up in the heating
chamber which necessitated frequent cleaning to ensure good repeatability. Other problems
encountered were leaking seals in the high-pressure pump, pressure switch failures, and the
terminals to the heater elements failing. A further disadvantage was the response time of three to
five minutes.
Pulsed amperometric detection
Pulsed amperometric detection (PAD) has become a widely accepted analytical tool for the
measurement of carbohydrate concentrations. The concept of the pulsed amperometric detector
analyser is simple.
A voltage is applied to a gold electrode (50 mV) where oxidation of carbohydrates and some
other compounds produces ions that enable a current to flow across the applied voltage, generating
a current.
This current is amplified and measured and is proportional to the concentration of
carbohydrates. Because oxidation products coat the gold electrode, a pulsed sequence of potentials
is used to remove this coating during operation. This type of detection is reasonably specific for
classes of organic compounds.
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Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
1.6
Response (μA)
1.4
1.2
y = 0.0451x + 0.0489
R2 = 1
1
0.8
0.6
0.4
0.2
0
0
5
10
15
20
25
30
35
Sucrose (ppm)
Fig. 3—Relationship of sucrose concentration on PAD.
Du Boil (2000) described the testing of this measurement technique using a system designed
around a commercially available detector and an anion exchange chromatographic column. Sodium
hydroxide reagent sparged continuously with nitrogen to remove dissolved oxygen and carbon
dioxide was pumped into the chromatographic column.
A discrete short pulse of condensate sample was inserted before the chromatographic
column, and the sample was then measured after the chromatographic column by the PAD for trace
sucrose levels. The system proved to be stable and reliable over a period of several days’
continuous operation.
A calibration plot of the relationship of sucrose to PAD response is shown in Figure 3.
Although this measurement technique worked, a skilled operator would be required to maintain and
calibrate the instrument. Another possible disadvantage is the response time of three minutes and
the volumes of sodium hydroxide and nitrogen required.
Near infrared spectroscopy
Three NIRS instruments were evaluated at the SMRI to determine if they were sensitive
enough to determine trace amounts of sugar in condensates. Standards were prepared ranging from
2 to 20 ppm sucrose in distilled water.
All the instruments were setup to use a 2 mm flow-through cell. The lower cost NIRS
instrument with a fixed monochromator with diode array detection was not sensitive enough and
there was no correlation between the spectra and ppm sucrose.
The other two instruments showed that, although this application is pushing the
instrumentation and procedure to the limit, it is possible to use NIR for measuring trace sucrose
levels. Some results from these tests are presented in Figure 4.
If the factors that influence data scatter at low sucrose concentrations could be understood,
then NIRS could probably be used to measure these trace concentrations.
Near infrared spectroscopy is not noted for its sensitivity but, with enough calibration
samples, could possibly cope with interferences. The other advantages are the robustness of the
instrument, no chemical addition required, and the short response time of 30 seconds.
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Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
25
y = 0.998x + 0.0127
R2 = 0.8492
LAB (ppm)
20
15
10
5
0
-5
0
5
10
15
20
25
NIRS (ppm)
Fig. 4—Trace sugars on the NIR instrument using a 2 mm flow through cell.
Other measurement techniques
Electrical conductivity
A conductivity measurement is the conventional method for accepting or rejecting
condensate water in a sugar mill. Conductivity instruments are simple to operate and maintain. It
should be noted that sucrose is non-conducting and cannot be detected using conductivity
measurements.
In a raw sugar mill, any sucrose contamination is associated with conducting impurities due
to the inorganic components in sugar cane. The relationship between sugar content and measured
conductivity depends on the inorganic impurities composition and on the inorganic impurities to
sugar ratio. Conductivity measurements are also influenced by temperature. Hill (1966)
demonstrated that conductivity was strongly influenced by temperature. He suggested that a 2.1%
rise in conductivity per degree at 30°C be used as a standard. The ICUMSA method described by
Devillers (1974) recommends a temperature correction of 2.3% per °C. Correction of conductivity
to the required temperature is important when setting up background and triggering levels.
Conductivity is not strictly proportional to sugar concentration. Hill (1966) found a general
relationship of conductivity α (concentration) 0.9 and conductivity per unit concentration
α 1/concentration. The inorganic impurities fraction increases when sucrose is removed by
crystallisation, increasing the conductivity to sugar ratios. Conductivity should correlate with sugar
concentration and purity. The fast response times and the simplicity of conductivity detectors make
this instrument the most widely used in raw sugar mills for detecting trace sugars in condensate.
Total Organic Carbon Analysers
Total organic carbon (TOC) analysers measure the amount of carbon bound in organic
compounds. A typical analysis for TOC measures both the total carbon present as well as the
inorganic carbon. The TOC is then calculated by subtracting the inorganic carbon from the total
carbon yields. Virtually all TOC analysers measure the CO2 formed when organic carbon is
oxidised or when inorganic carbon is acidified. Oxidation is performed either through Pt-catalysed
combustion or with an ultraviolet/persulphate reactor.
Tayfield et al. (1993) reported on the installation of a TOC analyser at Huletts Refinery,
South Africa for measuring sucrose contamination in boiler feed condensate. Unlike in a raw sugar
factory where the contaminating streams have significant inorganic impurities contents, in a sugar
refinery the inorganic impurities level is too low for the conductivity measurements to be
meaningful.
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Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
Because TOC analysers measure total organic carbon and not sucrose specifically, the
presence of other organic compounds in the background influence the measured value and
consequently the accuracy of sucrose estimation. These variable background effects together with
the long response time (seven to ten minutes) are the major disadvantages of the instrument.
Although low maintenance on-line TOC instruments are available, they still require a skilled
operator to ensure reliable operation.
Flame photometer
Flame photometry has been the proven standard method for the analysis of sodium and
potassium. The flame photometer technique when applied in the sugar industry is based on
analysing the condensate for potassium and estimating the amount of sucrose by means of a known
ratio of sucrose to potassium. Dale and Lamusse (1977) discussed the installation of a flame
photometer at Gledhow Mill, South Africa. It was installed to monitor sucrose entrainment from
evaporator vessels and pans. A sample obtained from the vapour line was condensed and the
potassium level in the condensate was recorded continuously.
This is an indirect procedure and assumes a constant sucrose to potassium ratio. When this
instrument was tested on injection water, problems were encountered due to blocking of the
capillary by suspended matter in the water. Due to the high purity of refinery streams the potassium
flame photometer cannot be used to measure sucrose entrainment in refinery condensate. Although
this instrument is not sucrose specific, the rapid response time is an advantage.
Colorimetric auto-analysers
Colorimetric auto-analysers have been used successfully to monitor condensates for boiler
feed water. These instruments pump a sample of condensate water to a manifold where mixing with
reagents occurs. The mixture is then heated to 95°C for about five minutes and the colour, which is
proportional to the sucrose content, is measured in a colorimeter. Auto-analysers have worked well
in a clean laboratory where supervision is good. Schaffler (1978) described the use of an autoanalyser for monitoring sugar entrainment. Continuous use of the instrument in the factory
environment was not successful due to a sediment build up in the colorimeter. Fowler (1977)
described the use of a colorimetric auto-analyser for monitoring refinery wastewater.
Routine maintenance consisted of keeping sample lines clear and the optical system clean
which consumed approximately four to five man hours per week. Calibration checks with known
sucrose solutions were required daily. This instrument should preferably be installed in the control
laboratory where skilled personnel can monitor the operation. The disadvantages of this instrument
are the quantity of reagents that are required and the five minute measurement response time.
pH
Kakuni (1991) described work in comparing conductivity or pH for monitoring the presence
of sucrose in condensate. The work showed that pH had a more reliable correlation with sugar than
conductivity. The other advantages were the fast measurement response time and pH also provided
a measurement of the condensate’s corrosive condition. The disadvantages are the difficulty of
maintaining pH instruments on-line in condensate lines and that pH is an indirect measurement of
sugar traces.
Summary and discussion
Table 1 shows a comparison of the various instruments used to measure trace sucrose levels
in condensates. Although most measurement techniques worked to some degree, there are always
reliability, measurement response time, and ongoing maintenance costs to consider. The main
purpose of monitoring the condensate is to ensure good quality boiler feed water for reliable
operation. This does not require an accurate measurement but rather a rapid estimate of the level of
sucrose. The sugar mill laboratory should routinely, on an hourly basis, sample and analyse all
boiler feed water condensate for water to detect if there is any trace sucrose in the condensate.
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Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
Table 1—Comparison of instrumentation to measure sucrose
Instrument
Heating apparatus
Pulsed
amperometric
detection
Near Infrared
spectroscopy
Electrical
conductivity
Total organic carbon
Flame photometer
Colorimetric
pH
Detection principle
Operational
problems
Response time
Carbonising sucrose &
detecting conductivity
changes
High pressure pump
seals leaked
3 to 5 minutes
Oxidation of
carbohydrates
Require a specialised
or trained operator
3 minutes
Spectral changes due
to sucrose
Change of conducting
impurities due to the
inorganic impurities
CO2 formed when
organic carbon is
oxidised
Change of flame colour
due to potassium
Change of liquid colour
due to sugar reaction
with a reagent
Reduction of pH due to
sugar
Require a specialised
or trained operator
30 seconds
Limited to the presence
of impurities
Maintenance
Daily/hourly
cleaning of
heating chamber
Daily calibration
& addition of
reagents
Daily cleaning of
flow through cell
1 second
Daily cleaning of
probes
Require a specialised
or trained operator
5 to 7 minutes
Daily calibration
& addition of
reagents
Require a specialised
or trained operator
1 second
Daily cleaning
Require a specialised
or trained operator
5 minutes
Daily cleaning &
calibration of
optical system
Relies on pH probe
5 seconds
Daily cleaning
In a raw sugar mill, conductivity is the favoured parameter used to control the reject valve
on the condensate line. Because conductivity instruments are installed directly into the condensate
lines, care must be taken to prevent flashing at the measuring point. The preferred location should
also be easily assessable to enable daily cleaning of the conductivity probes. Exhaust, vapour and
pan condensate require different set points for the conductivity instrument for accepting or rejecting
the condensate stream. The set point should also take into account the possibility of flashing at the
measuring point. If flashing occurs then a conductivity reading below 20 μS/cm is obtained and the
condensate is rejected. Exhaust condensate is usually considered acceptable when the conductivity
is between 20 μS/cm and 70 μS/cm. Vapour condensates between the ranges of 20 μS/cm and 140
μS/cm are also acceptable.
In a refinery, because of higher purity streams, the conductivity instruments do not work
satisfactorily. The TOC instrument can detect trace sucrose levels in condensate but the high levels
of ethanol that could be present in condensate will cause a significant error in predicting sucrose
contamination. Changing the set point can accommodate this background interference. A NIRS
measurement would be more specific and the errors caused by ethanol and other impurities in the
condensate should be negligible
The NIRS instrument measurement cell will probably require daily cleaning and random
samples of condensate will require accurate sucrose analysis by other methods in order to maintain
the NIRS calibration database. It must be cautioned that the NIRS measurement of the trace sucrose
levels in condensate will not be accurate, because this application is pushing the NIRS
instrumentation and procedure to the region where signal to noise ratio is poor.
Conclusion
A variety of methods have been tried in an attempt to find a quick on-line method that is
applicable to contamination by high purity sugar streams. The commonly used conductivity method
is satisfactory for the relatively impure contamination that occurs in raw houses. Various
alternatives have been tested for application in refineries where the sugar streams are relatively
pure. Most of these methods proved unsuccessful, but the NIRS method showed promise, with a
rapid response time, robustness and no requirement for chemical additions. The main disadvantage
of NIRS is the relatively high capital cost.
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Gooch, M.A. & Wienese, A.
Proc. Aust. Soc. Sugar Cane Technol., Vol. 27: 2005
________________________________________________________________________________________________
REFERENCES
Dale, T.B. and Lamusse, J.P. (1977). Monitoring of entrainment by vapour sampling and the use
of a flame photometer. Proc. S.Afr. Sugar Technol Assoc., 51: 116–118.
Devillers, P. (1974). Ash. I.C.U.M.S.A., 16: 222.
Du Boil, P.G. (2000). A prototype instrument for the electrochemical detection of sucrose
contamination in sugar refinery condensates – pulsed amperometric sugar trace analyser
(PASTA). Proc. S.Afr. Sugar Technol Assoc., 74: 298–302.
Fowler, M.J. (1977). Continuous sugar detection in refinery wastewaters. Sugar Industry
Technologists inc., 36: 219–231.
Hill, J.W. (1966). Electrical conductivity methods for monitoring sugar contamination of process
water. Sugar Research Institute Technical Report, 89: 1–48.
Kakuni, S.S. (1991). An alternative to detecting sugar contamination in the evaporator condensate
and boiler feedwater. Hawaiian Sug Technol., 50: F61–F63.
Reid, M.J. and Dunsmore, A. (1991). The protection of boilers from sugar contamination in
feedwater. Proc. S.Afr. Sugar Technol Assoc., 65: 208–212.
Schaffler, K.J. (1978). Sugar entrainment monitoring. Proc. S.Afr. Sugar Technol Assoc.,
52: 123–124.
Tayfield, D.J. and Anderson, E.W. (1993). The use of total organic carbon analysers to
monitor sugar contamination in boiler feed condensate. Proc. S.Afr. Sugar Technol
Assoc., 67: 144–147.
DETECTION DU SACCHAROSE DANS L’EAU D’ALIMENTATION
DES CHAUDIERES
Par
M.A. GOOCH and A. WIENESE
Sugar Milling Research Institute, c/o University of KwaZulu-Natal, Durban 4041
[email protected]
MOTS CLEFS: Mesures des Eaux Condensées, Condensats, Chaudières.
Résume
LES DANGERS causes par la présence du sucre dans l’eau d’alimentation des chaudières ont été
discutés par un grand nombre d’auteurs. Ce problème est encore plus sérieux avec des chaudières de
haute pression. Les condensats de sucreries, de sucreries avec raffinerie attenante et de raffineries,
contiennent des impuretés différentes et les concentrations ne sont pas les mêmes; il faut donc des
technologies spécifiques pour analyser ces condensats. Ce papier présente une revue des techniques
pour détecter le sucre dans l’eau d’alimentation des chaudières.
DETECCIÓN DE SACAROSA EN EL AGUA DE ALIMENTACION DE CALDERAS
Por
M.A. GOOCH y A. WIENESE
Sugar Milling Research Institute, c/o University of KwaZulu-Natal, Durban 4041
[email protected]
PALABRAS CLAVE: Medición de Condensado de Calderas.
Resumen
NUMEROSOS autores han destacado la importancia de impedir que entre a las calderas la sacarosa
arrastrada en el condensado, especialmente donde se usan calderas de alta presión. El condensado
de un ingenio de azúcar crudo, de una refinería o de un ingenio con refinería contiene diferentes
tipos y cantidades de impurezas. Por esta razón la tecnología apropiada para medir los niveles de
impurezas varía según la aplicación. Este artículo es una revisión de las diferentes técnicas para
detectar la contaminación de sacarosa en el agua de alimentación de calderas.
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