Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Le A 30 780-USlHalm/S-P 21 7 2 3 5 9
Method for on-line analvsis
Back~round of the Invention
S The present invention relates to a method for on-line analysis, of analytes present
in a liquid medium to be tested, in accordance with the addition method for
calibration. To this end, the medium cont~ining the analyte and suitable reference
liquids are continuously supplied stepwise, in a recurrent and automatable rhythm,
to a detector for the purpose of detecting a signal characteristic of the analyte. The
method according to the invention additionally permits an internal check of the
trueness of the analysis system.
Analytical methods for chemical products and intermediates and for mixtures of
different substances play a major part in all fields of chemical process engineering.
This is true, firstly, for products which are delivered to the consumer or for further
proces~ing7 since, as a rule, laid-down specifications are very largely required and
must be met. Equally, however, this applies to intermediates which are to be
processed further within a plant or in an interconnected system, if meeting certain
specifications has an impact, for example, on the on-stream time of a catalyst or
the size of the yield in a subsequent step. Process engineering in terms of the
preparation of chemical products has increasingly been the subject of improve-
ments which permit the preparation of pure chemical products to a previously
unknown degree; this has resulted, on the one hand, in the abovementioned
requirements to meet increasingly strict specifications, but also, at the same time,
in the need to be able to cope with smaller and smaller amounts of impurities and
subsequently with more exacting requirements with respect to analytical methods.Furthermore another demand which has become increasingly urgent is that for
technical simplification of a plant, for example the ~limin~tion of intermediatestores serving solely for analytical purposes and for the replacement of
complicated analytical procedures, both for the purpose of economizing on
investment as well as labour costs. - -
Given the exacting requirements they are expected to meet, analytical methods inchemical processes are confronted with fundamental difficulties:
The first to be mentioned in this respect relates to the location and the type of
sampling, so as to obtain a representative sample suitable for the analytical
35 method. This applies, for example, to intermediate stores where, as a result of the
interim settling of the product, separation may occur of the desired substance and
Le A 30 780-US 2 1 7 ~ 3 5 9
_ - 2 -
undesirable impurities. A separation of this type may relate, for example, to
incompletely dissolved impurities present in colloidal form or to separation
phenomena which result from polarity differences, for example owing to the
presence of polar analytes such as metal salts in nonpolar hydrocarbons or their5 mixtures or from other differences. Further problems result from possibly extended
distances over which the sample taken is transported, from the work-up of the
samples and perhaps the use of required aids and reagents in the course of the
analysis being carried out, resulting, in principle, in the risk of traces of substances
being introduced which hinder or falsify the detection of the analyte.
10 Just by way of example, by no means to be regarded as exhaustive, for such ananalytical task mention may be made of the continuous accurate determination of
the sodium and potassium contents in cycle stocks of the petrochemical industry.Such cycle stocks are produced as the bottoms of fractionating columns of various
petrochemical products (hydrocarbon mixtures) and are used for the production of15 carbon black for the rubber industry. In such cycle stocks enrichment takes place
of the non-volatile Na and K compounds although, in spite of said enrichment,
they are still present only in the mg/kg range. Nevertheless, even such low levels
in the carbon black are harmful to rubber materials, since cryst~lli7~tion of alkali
metal salts takes place, which leads to embrittlement and rapid fatigue of such
20 rubber materials. Consequently, the cycle stock produced is first passed to aproduction tank and then passed on into an analysis tank and cannot be transferred
to a shipping tank until after a check of the NalK levels has been carried out and
after it has been cleared.
For the purpose of reliable sampling while avoiding the abovementioned risks of
25 separation, the production tank and the analysis tank must be equipped with astirring or recirculation means, in order to ensure that samples drawn are
representative at all times. For the purpose of determining the Na/K levels, thesamples drawn, in accordance with conventional analytical methods, are either
extracted with hydrochloric acid or are mineralized with oxidants such as
30 H2SO4/HNO3. Only then can the samples worked up in this manner be passed on
for example to a flame photometer system or an atomic absorption spectrometer
system serving as detection instruments. Owing to the high ubiquitous
concentrations of sodium and potassium, additional analytical procedures of thistype, such as, for example, extraction or a mineralizing digestion inevitably mean,
35 however, the introduction of random and systematic errors.
Le A 30 780-US 2 1 7 2 3 5q
- 3 -
It was desired, therefore to move from discontinuous analytical methods of single
samples drawn to an on-line method. Here, however, those skilled in the art wereconfronted with the problem that, for example, hydrocarbon mixtures and metal
salts exhibit a high polarity difference which did not appear to provide a simple
option for a direct measurement and, in particular a calibration, as it appearedimpossible to tailor the calibration solution to the composition of the sample
solution. Furthermore, the problem of identifying physical and chemical mal-
functions was entirely unsolved.
Summarv of the Invention
With the aid of the inventive on-line analysis method in accordance with the
addition method for calibration, an easily managed, robust method was found,
which is readily m~int~ined and can be operated economically and which leads notonly to accurate and true analytical values, i.e. those comparable with independent,
reliable analytical procedures, but which also makes it possible to detect physical
and chemical faults by validation tests. To this end, the calibration solution is
tailored to the composition of the sample solution by both of them being made
homogeneously miscible with one another in any proportion. Faults which can be
recognized in this context are, for example, faults relating to the liquid medium
containing the analyte (cont~in~tions), testing of perhaps chemically and
physically unstable calibration solutions (chemostability test) and faults in the
detector instrument (analytical instrument system test).
The invention relates to a method for on-line analysis of analytes in a liquid
medium to be tested in accordance with the addition method for calibration,
characterized in that a detector is supplied stepwise, in a continuously recurrent
and automatable rhythm, with the following liquid substances for detecting a
signal characteristic of the analyte:
a) an analyte-free solvent (I), which does not produce a signal, for the purpose of establishing the signal baseline;
b) the medium (II) which contains the analyte and may or may not have
solvent (I) added in a constant mixing ratio, for the purpose of detecting
the analyte signal; and
Le ~ 30 780-US 21 7 2 3 5 9
-
c) a homogeneous mixture of (II) with a calibration solution (III), which is
homogeneously miscible in any proportion with (II), in a constant mixing
ratio (II):(III) for the purpose of detecting the additive analyte signal;
or the steps b) and c) are carried out in inverse order and the detected signals are
5 evaluated as measurement data, wherein, for the purpose of checking the adequate
chemical and physical stability of (III) and the validity of the analysis, the said
rhythm is interrupted by
d) detecting the signal of a test solution (IV) for the purpose of checking the
trueness of the system
10 and is then continued as before.
Detailed Description of the Invention
An essential feature of the method according to the invention is the use of the
addition method, which permits a comparison of the observed measured values of
the liquid medium to be tested with those of a mixture of medium to be tested and
15 a calibration solution.
The detected signals characteristic of the analyte are transmitted, as measured
values, to a display instrument or are made visible and continuously readable via a
written recording method. It is particularly advantageous, however, employing
continuously improved documentation technology, for the signals obtained to be
20 fed to a computer and to be compared continuously with predefined specifications;
when permitted deviations are exceeded, provision can be made for an appropriatealarm to be raised.
Detectors for detecting the signals characteristic of the analyte are known to those
active in the art; these are, for example and non-exhaustively, optical displays25 (flame photometers, atomic absorption spectrometers for detecting individual
elements, in particular for detecting metals; observation of the absorbance or the
rotation of the plane of polarization, optionally subsequently to characteristic fast
chemical reactions of the analyte) or electrical methods (measurement of the
conductivity or of the electrical capacitance as a function of the amount present of
30 the analyte in the medium to be tested).
Le A 30 780-US 21 7 23 5 ~
- 5 -
Sampling of the medium to be tested, which contains the analyte, takes place in a
continuous rhythm at any point in the manufacturing process, whenever the
drawing of a representative sample is ensured, preferably however at a transfer
line immediately after the preparation of the liquid medium to be tested.
5 For the purpose of performing the inventive analytical procedure, the liquid
medium (II) to be tested, drawn off in a continuous rhythm, is supplied to an
integrated analysis station, set up as closely adjacent as possible, where at the
same time an analyte-free solvent (I) which does not produce a signal, the
calibration solution (III) which is homogeneously miscible with (II) in any
10 proportion and, if required, the test substance (IV) are available. Said indicated
analysis station further comprises the requisite detector and means for recording,
processing and documenting the signals. Via a suitable tran~mi~sion means, a
regularly performed check of these signals (measured values) and an alarm to be
triggered, if required, when permitted tolerances are exceeded, can be made
15 available, via a remote display, to the operating and monitoring personnel of the
chemical production plant. The said liquids (I), (II) and (III) and, if required, (IV)
are supplied to the detector in a continuous rhythm, adjustable metering pumps
being employed for this purpose in the usual manner. Preferably, commercially
available multichannel metering pumps with an adjustable mixing ratio, for
20 example for (II)/(I) mixtures, (II)/(III) mixtures or (II)/(I)/(III) mixtures and a
downstream mixing section are employed (e.g. HPLC pumps with a rapid-action
valve for gradient formation).
The inventive method is described below, by way of example, with reference to
the detection and monitoring of Na~K levels in a hydrocarbon mixture (cycle
25 stock), the inventive method being eminently and preferentially suitable for this
purpose. Thus, first of all, a four-channel metering pump with an adjustable
mixing ratio and a downstream mixing section and preferably having a rapid-
action valve is employed to supply an analyte-free solvent (I), which does not
produce a signal, to a i~lame photometer for the purpose of establishing the signal
30 base line. The characteristic signals, in the present example, are the spectral lines
Na and K. The base signal obtained is constant within a short time; it is then, like
all the other signals after they have become constant, recorded by a computer and
processed further. Then, via another channel of the multichannel pump comprisinga rapid-action valve, a sample of the liquid medium (II) to be tested with analyte
35 present therein is passed to the flame photometer in the same manner. Like all the
Le A 30 780-US 21 7~
,
- 6 -
samples supplied it is combusted and, in the process, generates the characteristic
spectral lines. The signals (measured values) obtained in this process are likewise
passed to the computer and compared with the base signal. The liquid medium (II)to be tested, which contains the analyte, can in this case be passed on either as
5 such, or in dilution with added solvent (I), to the flame photometer serving as the
detector. With regard to the question of whether added solvent (I) should be used,
those skilled in the art will take into account conditions of practical handling and
reliable determination of the analyte. It is thus advisable that a viscous medium
(II) to be tested be diluted to such an extent that it can be conveyed without
10 mishap through all the lines of the integrated analysis instrument, that
consequently constancy of the output signal (measured value) is achieved rapidlyand reliably and no back-mixing takes place with the other steps in the continuous
analysis rhythm. Such a dilution is also advisable in a situation where, such as in
the case of detecting the Na/K level, the intention is to work within the linear15 range of Beer's law in the flame photometer in order to avoid analysis errors. The
extent of such a dilution which may become necessary is known to those skilled
in the art and can be determined by simple pr~limin~ry trials. By employing a
multichannel metering pump it is possible, in each case, to set, as required, a
constant mixing ratio of (II) to (I). In the following step c), finally, a mixture of
20 the medium to be tested (II) with a calibration solution (III) homogeneously
miscible in any proportion with (II) is, as required by the addition method
employed, supplied to the flame photometer in a predetermined mixing ratio, for
the purpose of detecting the additive analyte signal. With this step it is likewise
possible, taking into account the abovementioned reasons, for solvent (I) to be
25 admixed. In step c) the mixing ratios are likewise constant in the continuously
recurrent rhythm. The mixing ratios which are to be set and to be kept constant
are in principle non-critical and are tailored to the specific problem, for example
the viscosity of (II) or that of m~int~ining linearity of the physical law on which
the detection is based. For the purpose of unambiguous detection and
30 comparability of the analyte signal with the additive analyte signal, (II) and (HI)
are mixed with one another in such a way that the analyte signals in (II) and in(III) are in a mutual relationship of an equal order of magnitude. If, therefore, the
predetermined analyte concentration in (III) is virtually equal to that expected in
(II), (II) and (III) are mixed, for example, in a ratio of equal masses or equal35 volumes; a general mixing ratio is, for example, from 3:1 to 1:3. The admixture of
(I) in step b) or in step c) is likewise non-critical and, if an admixture is required
Le A 30 780-US 21 7 235~
- 7 -
at all, is in the range of from 0.5 to 10 parts by volume of (I) to one part by
volume of (II) or to one part by volume of (II)/(III) mixture.
Of course it is possible for the detection of the analyte signal and the detection of
the additive analyte signal to be inverted, i.e. instead of carrying out the steps in
5 the order a), b) and c) to carry them out in the order a), c) and b).
~ny dilution step required in steps b) and c) is in all cases carried out automatic-
ally.
The analyte-free solvent (I) which does not produce a signal is in all cases
miscible with (II), homogeneously and in any proportion. The same applies with
10 respect to the calibration solution (III) with respect to (I) and (II). In the case of
the determination of Na and K in a hydrocarbon mixture, for example cycle stock,the solvent (I) therefore is a hydrocarbon stream which preferably has a lower
boiling point than (II). This ensures, in a preferable manner, that any reduction
required in the viscosity is achieved, since lower-boiling components generally
15 also have a lower viscosity, and that a lower-boiling solvent and diluent is
generally obtained by distillation as the top product or in the side stream and
consequently, unlike a bottom product is not cont~rnin~ted with Na or K to be
detected.
The calibration solution (III) employed is advantageously the solution of Na or K
20 compounds, for example an organic Na salt and an organic K salt, in a hydro-
carbon stream, the additional use of a cosolvent being optional. The hydrocarbonstream in which the said organic Na/K compounds are dissolved may, for
example, be identical with the solvent (I). If the organic salts are salts of medium-
length aliphatic carboxylic acids, such medium length aliphatic carboxylic acids in
25 free form, i.e. not as a salt, deserve consideration as cosolvents. Carboxylic acids
to be contemplated for this purpose are, for example, C4-CI~ carboxylic acids. l~he
amount of Na or K ions in the liquid to be detected is, in each case, both for Na
and for K, defined by up to approximately 2 mg/l of liquid within the linearity
range of Beer's law. For the purpose of detecting the analyte signal and the
30 additive analyte signal the advantageous procedure to follow will therefore be to
effect such a dilution, both of (II) and of (II)/(III) mixture in such a way that the
abovementioned upper linearity limit is reliably not exceeded. Taking possible
dilutions into account, the calibration solution (III) may then also contain a higher
LeA30780-US 21 72359
- 8 -
concentration, for example up to 10 mg of Na and K each per litre. To avoid
unnecessary dilution steps, however, the calibration solution (III) is prepared in the
range of from 0.3 to 2 mg/l of Na and K ions.
Instead of a flame photometer it is also possible to use an atomic absorption
5 spectrometer as the detector for metal ions, preferably alkali metal ions.
For the purpose of controlling automatic filling of tank wagons or tank lorries with
the hydrocarbon mixture, the computer may, as previously mentioned, be provided
with limit values which, when they are exceeded, result in an alarm being trig-
gered.
10 The method according to the invention further permits the stability of the calibra-
tion solution (III) and the trueness of the analysis to be checked, by virtue of the
said rhythm of the steps a), b) and c) or a), c) and b) being interrupted at pre-
determined intervals by a validation step d). In this validation step d), the detector
is supplied with a liquid, in the form of a test solution (IV) which is independent
15 of (I), (II) and (III) and contains the analyte(s) to be detected, said test solution
permitting a validation of the entire system of the method according to the inven-
tion. The system of the inventive method includes not only (I), (II) and (III), but
also the detector, in the present example the flame photometer or the atomic
absorption spectrometer, which, for example owing to fouling of the burner or
20 owing to other external cont~min~tion, provide signals (measured values) which
differ from those intended. Thus it was found, for example, that in the given
example one of the components of the system of the inventive method, viz. the
calibration solution (III) does not keep indefinitely and stably in spite of the low
concentrations of Na and K salts, which again, in spite of the low concentrations
25 of the salts, is due to the nevertheless effective polarity difference between the
salts on the one hand and the hydrocarbon mixture on the other hand. A t-est
solution (IV) to be employed now need not be compatible, for example not
miscible with the medium to be measured (II), the calibration solution to be
employed (III) and the solvent employed (I); the main requirement with respect to
30 the test solution (IV) is its physical and chemical stability. Thus, in the present
case, an aqueous NaCI/KCl solution was found to be eminently suitable, whose
concentration of Na and K should be in the range of from 0.5 to 2 mg/l of liquid.
Of course, other Na and K salts, for example sulphates or nitrates are also suitable
Le A 30 780-US
21 7235~
~_ g
for this purpose. Test solution (I~) is checked for its actual content in independent
analytical procedures.
The initial setting and trueness check of the system on which the inventive method
is based is carried out as follows:
5 1. First, the base line is established with the aid of the solvent (I) by the latter
being supplied to the detector. In the case of a base line check
(cont~min~tion test), solvent (I) is likewise supplied to the detector. If this
check produces the old value of the base line, the continuous analysis
rhythm can be resumed. If a new value is found, the entire analytical
apparatus (multichannel metering pump, mixing section and detector) is
purged; if the result is a constant value, a readjustment can be carried out,
if required, in accordance with the steps further below; if the result is not a
constant value, an alarm is triggered and further troubleshooting is
requlred.
15 2. To enable a comparison of the signals of the medium to be measured (II),
which contains the analyte, with the signals of the test solution (IV), a
reference function has to be recorded; this is done with different dilutions
of calibration solution (III) with solvent (I).
3. Into said recorded reference function the measured Na and K content of the
test solution (IV) is read, which provides the reference value.
4. For the purpose of checking the calibration solution (III) (chemostability
test) the measurement of the desired dilution of calibration solution (III)
with solvent (I) is carried out on the basis of the reference function. If the
old value is found, the continuous analysis rhythm can be resumed. If a
new value is found, a recalculation of the Na/K calibration solution (III)-on
the basis of a new reference function and a new reference value becomes
necessary.
5. For the purpose of an analysis instrument system test, the test solution (IV)
is measured and is compared with the current reference function. If this
produces the old value, the continuous analysis rhythm can be resumed. If
the result is a new value, the reference function has to be drawn up again.
Le A 30 780-US 2 1 7 2 3 5~
- 10 -
Further possible uses of the method according to the invention are the detection of
further individual metals/elements or mixtures of a number of these in individual
liquid media of an organic or inorganic nature, the detection of undesirable by-products, for example empyreumatic products after thermal stress, and further
5 analytical problem solutions known to those skilled in the art. To this end, the
abovementioned detectors are employed, inter alia, for example. The analytes aredetected either directly or their conversion products from a characteristic, fast
reaction are detected. Solvents/diluents (I), calibration solutions (III) and test
solutions (IV) are tailored to each particular problem, in a manner known to those
10 skilled in the art.
Further examples for the use of the inventive method are, for example:
- the testing of eluates for a series of heavy metals from ion exchange resins
for the food sector. In this case, automatic enrichment and automatic cali-
bration are givens, the detector is, e.g., an optical emission spectrometer.
15 - the checking of identified organic substances in effluents with or without
automatic enrichment by means of a detector corresponding to particular
species (e.g. UV detection et al.).
Le A 30 780-US 2 ~ 7~359
1 1
Example
In the case of a hydrocarbon mixture with a boiling range of 100-250C and
having from 8 to 10 carbon atoms, a conventional determination of the Na and K
level resulted in a considerable scatter of the values, giving, for example in the
case of extraction with hydrochloric acid followed by flame photometric determin-
ation, Na values of from 2 to 7 mg/kg and K values from below the detection
limit up to 2 mg/kg; in the case of wet decomposition of the hydrocarbon mixturewith H2SO4/HNO3 followed by flame photometric determination values of from 2
to 8 mg/kg of Na and less than 1 to 5 mg/kg of K were found. Compared with the
inventive method, these values proved too high and too variable. When the
method according to the invention was employed and a value of 0.5 mg/kg of Na
was set in the hydrocarbon mixture with the aid of a further hydrocarbon mixtureof the boiling range 100-250C having from 8 to 10 C atoms as the solvent/diluent
(I) and 0.5 mg/kg of Na was added in the calibration solution (III), the additive
analyte signal of 1.0 mg/kg of Na was recovered, which corresponds to a recoveryof 100%. In comparison, in the case of a calibrated test solution (IV) a value of
0.5 mg/kg was recovered with likewise 0.5 mg/kg, and a value of 1.0 mg/kg was
likewise recovered with a value of 1.0 mg/kg; both these correspond to a recovery
of 100%.
In the case of a content of 1.0 mg/kg of K, set in the hydrocarbon mixture with
the aid of solvent/diluent (I) and an addition of 0.5 mg/kg of K in the form of the
calibration solution (III), an amount of 1.5 mg/kg of K was recovered (100%
recovery). In comparison thereto, calibrated test solutions (IV) having contents of
0.5 mg/kg and 1.0 mg/kg, respectively, were recovered with the values of like-
wise 0.5 mg/kg and 1.0 mg/kg, respectively (100% recovery).
Further illustrative embodiments are
The determination of a number of elements such as, e.g., chromium, lead, zi-nc, --
copper, nickel, manganese, in filtered wastewaters, drinking water, process water
and subsequent determination, via optical emission spectrometry with inductivelycoupled plasma (ICP-OES), the determination of mercury in natural gas conden-
sate with cold-vapour atomic absorption spectrometry (CVAAS) as the detection
method.
