Note: Descriptions are shown in the official language in which they were submitted.
WO 95/0683 PGT/EP94/02956
~1~~~~4
A METHOD AND APPARATUS FOR DETERMINING THE CONCENTRATION
OF A COMPONENT PRESENT IN A FLUID STREAM IN DISPERSED FORM
The invention relates in general to a method and apparatus for
determining the concentration of a first fluid which is finely
divided in a second fluid. Generally, a system wherein a first fluid
is finely divided in a second fluid is defined a dispersed system,
i.e. the first fluid (dispersed phase) is wandering about the second
fluid (dispersing medium) in a finely divided form.
As known to those skilled in the art, dispersed systems can be
sub-divided as follows: dependent on the diameter of the dispersed
phase particles, a dispersed system can be a solution (homogeneous
mixture in which no settling occurs and in which solute particles
are at the molecular or ionic state of subdivision); a colloidal
system (an intermediate kind of mixture in which the solute-like
particles are suspended in the solvent-like phase and in which the
particles of the dispersed phase are small enough that settling is
negligible and large enough to make the mixture appear cloudy); or a
suspension (a clearly heterogeneous mixture in which solute-like
particles immediately settle out after mixing with a solvent-like
phase).
In particular, the present invention relates to a method and
apparatus for determining the concentration of a contaminant in a
fluicl stream in dispersed form.
More in particular, the invention relates to detecting and
measuring the concentration of dispersed hydrocarbon (oil) and/or
particulate materials in a water stream.
The continuous on-line measurement of oil-in-water
concentration is becoming increasingly important in order to meet
future effluent quality standard.
A variety of instruments is known for measuring the oil present
in water streams and is installed to monitor produced water
discharges.
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These known instruments have inherent problems when applied to
mixed pollutant waste streams and cannot discriminate between the
oil contamination of interest and the presence of other materials.
Concern over the impact of industrial activities on the
environment, often heightened by public concern and legislative
requirements, has led to a re-evaluation of the wastes and
discharges which routinely occur.
Historically, produced water has been disposed of to the
environment after treatment, either by release to surface waters or
by re-injection into suitable aquifiers or the production formations
themselves. Strict quality levels are imposed by statutory
authorities and these vary from maximum dispersed oil concentrations
of 5 mg/1 for discharge to fresh water systems, to dispersed oil
concentrations of 40 mg/1 for water streams discharged to the open
sea. Current trends will result in reduction of these levels within
the foreseeable future. New limits of 30 mg/1 have been proposed.
This will call for the on-line measurement of the discharge streams,
and closer/improved control of the water treatment facilities, if
the new standards are to be met.
Statutory measurement methods differ significantly throughout
the world. However, the principle methods are based on infra-red
measuring techniques in the 3.2 to 3.5 fun wavelength range. This
requires the sampling of the water discharge, laboratory extraction
of the oil present in a water sample by a suitable solvent, and the
2~ subsequent measurement of the oil concentration. In known solvent
based instruments for determining oil concentrations water is to be
separated from the oil, e.g. by means of halogenated solvents, and
subsequently an infra-red (3.5 )un) analysis is carried out.
These methods have been shown to be time-consuming and
inaccurate in practice. Changes in international convention will
prohibit the use of the solvents used. Numerous commercial on-line
monitors are available, based on a variety of detection principles
e.g. (visible) light scattering, I.R.-absorption and the like. None
have proved satisfactory for use in oil industry applications.
It ~s an object of the invention to provide a method and
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apparatus for determining the concentration of a dispersed
component e.g. a contaminant in a fluid stream, based upon
short wavelength measurement enabling the accurate detection
and measurement of (low) concentration of hydrocarbon and/or
other contaminants present in a fluid stream, concurrently
and independently.
The invention therefore provides a method for
determining the concentration of a component present in a
fluid stream in dispersed form comprising the steps of a)
emitting a light beam in a predetermined range of
wavelengths to a fluid sample to be analyzed, said sample
flowing through a measurement cell; b) selecting a number of
different wavelengths in said predetermined range; c)
measuring a number of light intensities while sample fluid
is flowing through the measurement cell and deriving
therefrom a number of measurement sets which each consist of
a number of measured light intensities at said different
wavelengths; d) measuring a number of light intensities,
while reference fluid is flowing through the measurement
cell and deriving therefrom a number of reference fluid
measurement sets; e) deriving from the data, thus obtained,
information on the concentration of dispersed components in
the fluid stream, characterized in that said reference fluid
measurement sets each consist of a number of measured
reference light intensities at said different wavelengths,
and that step e) comprises calculating normalized light
intensity differences for each of the sample fluid
measurement sets by subtracting and subsequently dividing by
the reference light intensity at the corresponding
wavelength, and fitting the said normalized light intensity
differences to form a linear combination of a determined
number of relative spectral responses which take into
account the manner in which the said normalized light
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intensity at each wavelength changes proportional to other
wavelengths for certain effects responsible for the measured
light intensity differences, in order to give sets of
proportionality constants which are indicative for the
certain effects in question; and multiplying at least one of
said proportionality constants by a calibration factor to
obtain an absolute value of the concentration of dispersed
components in the fluid stream.
The invention further provides an apparatus for
l0 determining the concentration of a component present in a
fluid stream in dispersed form, comprising means for
emitting a light beam in a predetermined range of
wavelengths to a fluid sample to be analyzed, said sample
flowing through a measurement cell; means for selecting a
number of different wavelengths in said predetermined range;
means for measuring a number of light intensities while
sample fluid or reference fluid is flowing through the
measurement cell and means for deriving from said number of
light intensities a number of measurement sets which each
consist of a number of measured light intensities at
different wavelengths; and a number of reference liquid
measurement sets, which each consist of a number of measured
reference light intensities at said different wavelengths;
and means for deriving from the data, thus obtained,
information on concentration of dispersed components in the
fluid stream, characterized in that said reference liquid
measurement sets each consist of a number of measured
reference light intensities at said different wavelengths,
and that means are present for calculating normalized light
intensity differences for each of the sample fluid
measurement sets by subtracting and subsequently dividing by
the reference light intensity at the corresponding
wavelength, and fitting the said normalized light intensity
. ,N. ", ~....~.",",~,. ".,~,~"",",,~.~"..,.,."u"
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differences to form a linear combination of a determined
plurality of relative spectral responses which take into
account the manner in which the said normalized light
intensity at each wavelength changes proportional to other
wavelengths for certain effects responsible for the measured
light intensity differences, in order to give sets of
proportionality constants which are indicative for the
relative spectral response in question; and multiplying at
least one of said proportionality constants by a calibration
factor to obtain an absolute value of dispersed components
in the fluid stream.
Advantageously, the said predetermined range of
wavelengths is in the near-infra-red range (1.0-2.5 ~cm).
The invention is based upon the application of a
short wavelength, optical principle, based upon multiple
wavelength measurement, comparison of differently treated
samples of a test fluid, and statistical methods, which can
detect and analyse small spectral differences between the
samples caused by the presence of contaminants at different
concentrations. The principle is based upon a combination
of the scattering, and absorption properties which are
related to refractive index and size distribution of the
contaminants.
Further, the invention combines the principles of
light scattering (sensitive to suspended components) and
light absorption (sensitive to dissolved components).
Advantageously, visible light (0.4-1 um) can be
emitted simultanesouly with the near infra-red radiation and
at least one wavelength is selected in the said visible
light range.
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In particular, the invention is further based upon
a differential measurement system to take two separate
measurements under different sample conditions and allow
comparison of the differences between them.
It is remarked that WO-A-85/04478 discloses an
oil-in-water measurement wherein a reference sample is
applied.
However, this measurement is only based upon
absorption in the 3.4 to 3.5 ~,m wavelength range and the
specific technique of the present invention has neither been
disclosed nor suggested.
Further, FR-A-2685775 discloses a method for
providing the polycyclic aromatics content in a hydrocarbon
mixture using near-infra-red spectro photometric analysis in
the 0.8-2.6 um wavelength range and DE-A-3633916discloses a
method for determining concentrations using absorption of
light in the infra-red to ultraviolet range.
However, the specific technique of the invention
based upon the combination of scattering and absorption has
not been disclosed.
The invention will now be described in more detail
by way of example by reference to the accompanying drawings,
in which:
fig. 1 represents schematically the principle of a prior art
oil-in-water measuring instrument;
fig. 2 represents schematically the principle of the present
invention.
Referring to fig. 1 a block scheme of a prior art
oil-in-water analyzer is shown.
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Block 1 represents the oil-in-water sample to be
analyzed.
Usually oils comprise hydrocarbons and are not
water-soluble.
R'O 95/068y3 PGT/EP94/02956
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The hydrocarbons comprise CH-chains. Each of these chains has a
different energy-absorption band in the 3.4-3.5 Eun range of the
infra-red absorption spectrum.
It will be appreciated by those skilled in the art that, when
measuring the infra-red absorption of an oil sample between 3.4 and
3.5 Vim, the absorption is related to the oil concentration in the
sample. As water also absorbs energy in the infra-red band between
3.4 and 3.5 dun, it is virtually impossible to determine low oil
concentrations in a water Sample. For this purpose the oil present
in the water should be separated prior to measurement.
Halogenated solvents supplied in any way suitable for the
purpose via line 2 are suitable to separate oil from water for the ,
subsequent infra-red analysis 3 as
a) these solvents are virtually insoluble in water;
b) they have a specific gravity higher than water:
c) they dissolve easily all volatile or non-volatile organic
compounds; and
d) they do not absorb infra-red energy in the range of 2-4.5 dun.
After processing (block 4) of the infra-red analysis data 3,
infoz-motion (block 5) on the oil concentration is derived.
In fig. 2 the principle of the invention will be described in
more detail. No solvent is applied and advantageously a
predetermined number of wavelengths in the optical range, e.g. near-
infra-red (1.0-2.5 Eun) is used.
Light is emitted from any suitable light source 1' via any
suitable optical system 2' to a sample or measurement cell 3'.
Detention takes place via any suitable optical system 4' by any
suitable detector 5' which is suitably connected e.g. via an
amplifier 6' and A/D converter 7' to a computer 8' for data
processing purposes. A set of measured light intensities is provided
which are processed further in a data processing step Which leads to
the measurement of the concentration of contaminant in the fluid.
The sample or measurement cell 3' is connected in any suitable
manner to a flow selection means 9'; reference numerals 10', 11'
represent suitable filters. A and B represent an inlet for
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contaminated fluid and a drain respectively.
In the optical system 2' wavelength selection is suitably
achieved by narrow band interference filters. These are e.g. mounted
on a rotating disc assembly (filter wheel), forming a light beam
chopper, and allowing sequential wavelength measurement of the
measuring system and fluid sample within the measurement cell 3',
under identical conditions. Such sequential measurements can be
carried out at relatively high speed (e. g. 500 r.p.m.). It is also
possible to apply a number of separate light sources, each having a
different wavelength, rather than a filter wheel. The reference and
sample fluids are controlled and fed through the measurement cell
(3') (e. g. 0.6 mm wide) by e.g. electrically controlled valves, with
fluid flowing continuously through the measurement cell to ensure
temperature stability and representative water conditions.
The complete device (not shown in detail for reasons of
clarity) comprises a detector assembly, light source,
chopper/filter, test cell, water control valves, and infra-red
detector/amplifier. The reference fluid stream is produced from the
contaminated fluid by a suitable filter assembly, to produce a fluid
stream free of dispersed oil andlor other contamination. Any filter
suitable for the purpose can be applied, as will be appreciated by
those skilled in the art. E.g. a filter having a pore size < O.OOl~un
is applied in case of dissolved contaminants, whereas in case of
other dispersed contaminants a filter having a pore size of 0.001-
1Nm is suitably applied.
Advantageously, in case of oil-in-water monitoring, both the
concentration of the dissolved and of other dispersed hydrocarbon
components, if any, can be determined by a sequence of two
differential measurements, which differ only by the type of filter
applied to create the reference water stream from the contaminated
water stream. The concentration of dissolved contaminants can be
determined by comparing the contaminated water stream with a ,
dissolved component-free water stream, which is created by feeding
the contaminated water through a suitable filter, e.g. pore size
0.001-1 Eun. The said filter filters out all other dispersed
WO 95/06873 PGTIEP94/02956
companents but does not filter out the dissolved components. Next,
the concentration of dissolved components can be determined by
comparing the said dispersed-free water stream with a water stream
free from both dissolved and said other dispersed components, which
is created by feeding the contaminated water through a suitable
filter (pore size < 0.001 Eun). In the following the general
differential method will be described.
The operation of the invention is as follows:
In case of oil-in-water-monitoring, the hydrocarbon
concentration, as determined by the hydrocarbon-in-water monitor, is
obtained after a sequence of data processing steps on the measured
light intensities. This sequence uses values taken from two
measuring steps. These basic measuring steps and the subsequent data
processing will be described here.
In the first measuring step, during a number of rotations (e. g.
100) of the filter wheel light intensities im are measured while
sample water is flowing through the test cell. For each rotation of
the filter wheel a measurement set im~~ is measured, which consists
of a number of measured light intensities at different wavelengths ~,
(corresponding to the number of filters (e.g. 8) mounted in the
filter wheel). In this example, in total, 100 measurements sets
im~~~n with n = 1-100) are collected, giving 800 measurement
values. The large amount of measurements taken allows statistical
analysis to improve the accuracy of the instrument. From
e.g. 100 measurement sets, e.g. the 10 measurement sets which have
the largest deviation from the average light intensity, are
rejected. Thus, 90 sample water measurement sets remain for further
processing.
In the second measuring step, again during e.g. 100 rotations
of the filter wheel, light intensities are measured, but now while
reference water is flowing through the test cell. Again, after
rejecting e.g. 10~ of the measurements with largest deviation, this
gives 90 reference water measurement sets. From these values, at
each wavelength 7~, the averaged light intensity is calculated, giving
the reference light intensity ix~~ at 8 wavelengths.
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Then, the normalized light intensity differences
d~,,n are calculated for each of the 90 sample water
measurement sets by subtracting and subsequently dividing by
the reference light intensity at the corresponding
wavelength, using
Zm,~.,n Zx,~
dRn -
Zx,A
Different effects are responsible for the measured
light intensity differences. The ones considered here are
the temperature difference between sample water and
l0 reference water, the oil concentration in the sample water
and the particle concentration in the sample water. Now,
from calibration experiments it is known how for these
effects the normalized lights intensity at each wavelength
changes proportional to other wavelengths. This
proportionality for a certain effect k is termed a relative
spectral response yk,a. Therefore, with the same measurement
principle it is possible to address effects from a very
different physical origin.
Next, in the data processing, for each measurement
set of the wavelengths (e. g. 8) the normalized light
intensity differences are fitted using three proportionality
constants P'k to form a linear combination of the three
relative spectral responses, given by
3
(2)
d R,n - ~ .vk,~. ~ p k,n -~Y~.,n
k=1
The fitting is done such that the squares of the residual
differences r~,between measured light intensitites and the
linear combination of relative spectral responses is
minimized (least squares fitting). In this example, in
total, 90 least square fits are performed giving 90 sets
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of 3 proportionality constants, which are indicative for the
temperature difference, hydrocarbon concentration, and
particle concentration.
To obtain absolute values Pk for the temperature
difference, hydrocarbon concentration, and particle
concentration, the proportionality constants obtained from
the fitting procedure are multiplied by calibration factors
Ck. using
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Pk. n = Ck ~ fkn ( 3 )
These calibration factors are determined (e. g. experimentally,
using a predetermined oil-in-water mixture, e.g. created by an
accurate oil-injection). In this example, in total, 90 values for
the 'temperature difference, oil concentration, and particle
concentration are obtained. Finally, from these values the average
oil and particle concentration together with their respective
standard deviation are calculated.
It will be appreciated that the reference water conditioning
system can be selected in such a manner that it is possible to
measure and calculate the concentration of the dissolved and the
dispersed components. This will entail two reference water systems
and applying the above-mentioned calculation technique of the
desired concentrations in order to detect the presence of these
materials from the measured data.
It will be appreciated by those skilled in the art that any
optical wavelength and any number of wavelengths suitable for the
purpose can be applied in the said predetermined range of
wavelengths.
Advantageously, the number of wavelengths applied is four to
ten and in particular eight, e.g. 1.3, 1.43, 1.5, 1.6, 1.73, 2.16,
2.23 and 2.29 Eun. It has appeared that advantageous effects of using
such wavelengths are the following:
7i, = 1.43 ftm, especially sensitive to temperature
~, = 1.30 Eun, ~, = 2.29 Eun, the combination of these wavelengths
is especially sensitive to dispersed oil
~, = 2.23 Hm, especially sensitive to total dissolved
hydrocarbons
~. = 2.16 lun, especially sensitive to dissolved aliphatic
hydrocarbons
. ~. = 2.29 dun, especially sensitive to dissolved aromatic
hydrocarbons.
Further, it will be appreciated that any number of rotations of
the filter wheel and any suitable number of relative spectral
responses can be applied. The measuring principle of the invention
WO 95/06873 PGT/EP94102956
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Z1 ~
is capable of the detection of a large number of quality parameters
of a discharge stream, such as salt concentration, alcohols and
organic acids and similar contaminants.
Further, it will also be appreciated by those skilled in the
art that the invention is not restricted to oil-in-water monitoring
but can be applied for the measurement of a range of fluid based
applications: e.g. dissolved hydrocarbons and other materials in
water, fine dispersed materials in either aqueous or hydrocarbon
streams, chemical quality analysis and the like.
10 Various modifications of the present invention will become
apparent to those skilled in the art from the foregoing description.
Such modifications are intended to fall within the scope of the
appended claims.
