Device for regulating the operation of a chemical treatment plant, to improve efficiency by attenuation of the variances of the regulating parameters

A device for regulating the operation of a chemical treatment plant, by attenuation of the variances of the regulating parameters, has a calculation circuit connected to a measuring device for measuring flows of compounds at the input of the plant and measuring devices for measuring the contents of residual input compounds still present at the output of the plant. The calculation circuit performs coherence processings of the measured values to attenuate the variances of these values. The measurement provides a set-point signal which is used for the regulation of the plant.

DESCRIPTION 
This invention relates to a device for regulating the operation of a 
chemical treatment plant to improve efficiency by attenuation of the 
variances of the regulating parameters. It applies to any chemical 
treatment plant which receives reactive chemical compounds as input to 
provide as output at least one chemical element present in the input 
compounds, the operation of the plant having to exhibit the best 
efficiency for given conditions of operation. 
This invention applies particularly to a plant for obtaining sulfur by the 
reaction of hydrogen sulfide H.sub.2 S with the oxygen of the air. 
It is known that regulating the operation of a chemical treatment plant is 
generally obtained by a servocontrol using the values of parameters 
provided by characteristic sensors of the element or elements or compounds 
obtained at the output of the plant. These values make it possible to 
regulate the operation of the plant thanks to regulating means to which a 
set-point signal is applied, as well as signals corresponding to certain 
parameters (for example, the contents of the residual compounds at the 
output of the plant). These regulating means provide a control signal 
making it possible in particular to control means for regulating flows of 
compounds introduced into the plant. 
This type of regulation, well known in the prior art, uses at least one 
regulating loop and exhibits drawbacks which result from the fact that the 
values of the parameters provided by the sensors are values approaching 
these characteristic parameters and not very precise values. Consequently, 
a regulating device operating directly from the values of characteristic 
parameters provided by sensors does not make it possible to obtain an 
optimum regulating set-point so that the chemical treatment plant operates 
with an optimum efficiency. 
This is the case, for example, for a treatment plant that makes it possible 
to obtain sulfur from the reaction of hydrogen sulfide H.sub.2 S and the 
oxygen of the air, according to the well known CLAUS reaction. 
This type of plant comprises chemical treatment means which have in 
particular, at its input, a furnace connected to an adjustable flow means. 
These means make it possible to inject into the furnace input reactive 
chemical compounds consisting essentially of hydrogen sulfide H.sub.2 S 
and air whose oxygen reacts with a part of the hydrogen sulfide according 
to the reaction: 
EQU H.sub.2 S+3/20.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O 
At the output of the furnace, the reaction products are introduced in one 
or more converters containing catalysts that make it possible, thanks to 
the so-called "Claus" reaction, to cause the sulfur dioxide SO.sub.2 
produced to react on the hydrogen sulfide. This reaction is as follows: 
EQU 2H.sub.2 S+SO.sub.2 .fwdarw.3S+2H.sub.2 O 
The sulfur is therefore available at the output of the converters and the 
total reaction is written: 
EQU 3H.sub.2 S+3/2 O.sub.2 .fwdarw.3S+3H.sub.2 O 
The water produced is eliminated in the form of steam and the residual 
gases of the input reactive compounds can be further partially eliminated 
thanks to an additional catalytic treatment unit such as the one used in 
the "sulfreen" catalytic process or any other process. 
In this example of chemical treatment plant, the regulating of the 
operation of the plant known as "sulfur plant" is performed by acting on 
the combustion air flow introduced at the input of the furnace. 
In a way known in the prior art, the regulation of this type of plant is 
performed in the following manner: thanks to sensors consisting 
essentially of a concentration analyzer, the contents are measured of the 
input residual reactive constituents which are still present at the output 
of the plant. These constituents are, in the example considered, 
essentially hydrogen sulfide H.sub.2 S and sulfur dioxide SO.sub.2. For 
the plant to operate at its best efficiency, it is necessary that these 
contents be as close as possible to 0. The values of these contents are 
applied to regulating means which also receive a set-point signal. These 
regulating means provide a control signal of a solenoid valve for 
adjusting the airflow injected at the input of the plant. 
The set-point signal applied to the regulating means theoretically makes it 
possible, thanks to a supply of additional air at the input of the plant, 
to bring the contents of hydrogen sulfide H.sub.2 S and of sulfur dioxide 
SO.sub.2, present at the output of the plant back to a zero value. This 
set-point signal, of course, depends on the contents of these residual 
gases measured by the analysis means (chromatographic analyzer, for 
example), placed at the output of the plant. However, the values of these 
contents are measured approximately and in this type of known plant, the 
set-point signal applied to the regulating means does not make it possible 
to cause the airflow injected at the input of the plant to vary so that at 
its output the contents of residual gases tend toward zero values. 
Furthermore, the sole measurement of the contents of residual gases at the 
output of the plant are not sufficient parameters to make it possible to 
regulate the latter at its maximum efficiency. Finally, although generally 
the flows of the reactive components introduced at the input of the plant 
are measured, the values of these flows are not taken into account for the 
regulation. 
The invention has as its object to remedy the drawbacks of the known 
devices for regulating the operation of a chemical treatment plant, 
particularly the taking into account of the values of a large number of 
parameters, and by a corrective calculation of the values of these 
parameters to make it possible to regulate the operation of the plant at a 
maximum efficiency. This corrective calculation is, in fact, a coherence 
calculation of the values of the measured parameters. 
The invention has as its object a device for regulating the operation of a 
chemical treatment plant to improve the efficiency by attenuation of the 
variances of the adjustment parameters, this plant comprising chemical 
treatment means connected to adjustable flow means feeding respectively 
the chemical treatment means with input reactive chemical compounds, this 
plant providing at output at at least one chemical element present in the 
input compounds, the regulating device comprising measuring means 
providing signals of measurement of the flows of the input compounds, 
means providing signals of the measurement of the contents of the input 
residual compounds which are still present at the output of the treatment 
means, flow regulating means connected to the means for measuring contents 
of residual compounds and to at least one of the adjustable flow means, 
these regulating means receiving a set-point signal to apply a signal to 
the adjustable flow means modifying the flow of the corresponding input 
compound, so as to cause the contents of the output residual compounds to 
tend toward zero values, characterized in that it further comprises 
calculation means connected to the means for measuring flows of the input 
compounds to means for measuring the contents of the residual compounds 
and to measuring means providing measurement signals of contents of input 
compounds, these calculation means performing coherence processings of the 
measurement values of flows and contents to provide on an output the 
set-point signal applied to the regulating means. 
According to a first embodiment of the invention, the regulating means 
comprise a flow regulator providing on an output a corrected control 
signal of a solenoid valve of the adjustable means. 
According to another embodiment of the invention, the regulating means 
comprise a flow regulator one output of which provides a control signal of 
a solenoid valve of the adjustable means, and an adder connected by an 
input to the output of the regulator, another input of this adder being 
connected to the output of the calculation means to receive the set-point 
signal, an output of this adder providing a corrected control signal of a 
solenoid valve of the adjustable flow means. 
Finally, according to another characteristic, the input reactive compounds 
are sulfurous acid gas and air, the element at the output being sulfur. 
The characteristics and advantages of the invention will come out better 
from the following description given with reference to the single 
accompanying figure. This figure diagrammatically represents a device 
according to the invention, for regulating the operation of a chemical 
treatment plant at a maximum efficiency.

Regulating device 1 makes it possible to regulate the operation of a plant 
2, to improve the efficiency, by attenuation of the variances of the 
regulating parameters. 
Plant 2 comprises chemical treatment means 3 connected to adjustable flow 
means 4 that feed respectively chemical treatment means 3, with input 
reactive compounds. These reactive compounds are applied to ducts 5, 6, 7 
of flow regulating means 4, which will be described in detail below. This 
plant provides as output 8 at least one chemical element present in the 
input compounds. 
In case the device is used in a plant which makes it possible to obtain 
sulfur from the reactions described above, hydrogen sulfide H.sub.2 S is 
applied, for example, to duct 5 connected to treatment means 3 by a flow 
adjustment valve or a solenoid valve 9. 
Duct 6 receives the air containing the oxygen necessary for the reaction. 
This duct is connected to treatment means 3 by a flow adjustment valve or 
solenoid valve 10. A duct 7 also receives air necessary for the reaction. 
This duct is connected to treatment means 3 by a flow adjustment valve or 
solenoid valve 11. It is, in fact, thanks to this valve 11 associated with 
regulating device 1, that the plant operates at its optimum efficiency, as 
will be seen in detail below. In this example, treatment means 3 comprises 
the furnace and converters 12 as well as a so-called "sulfreen" unit 13 
mentioned above and provides sulfur with a high degree of purity on its 
output 8. 
Regulating device 1 comprises means 14 for measuring the flows of input 
reactive compounds. In the case of a plant for obtaining sulfur, these 
measuring means 14 comprise flow meters 15, 16 making it possible to 
measure respectively the flows of hydrogen sulfide and air injected into 
the plant. 
The regulating device also comprises means 17 for measuring the contents of 
the input compounds which are still present at the output of the plant and 
which are therefore residual. In the case of a plant for obtaining sulfur, 
the means for measuring the content of the residual compounds comprise, 
for example, two chromatographs 18, 19 which make it possible to determine 
the contents of hydrogen sulfide gas in the output residual compounds. 
These contents are measured at output 20 of the final converter of the 
treatment means, as well as at output 8 of the "sulfreen" unit. Measuring 
means 17 provides signals for measuring these contents on their respective 
outputs. 
Finally, the regulating device comprises flow regulating means 21 connected 
to means 17 for measuring the content of residual compounds, as well as at 
least one of the adjustable flow means 4. In the example of a plant for 
obtaining sulfur, regulating means 21 are connected to valve 11 making it 
possible to regulate the flow of air introduced into this plant 
(additional regulating air). The regulating means receives a setpoint 
signal on an input 22; this signal makes it possible, as will be seen in 
detail below, to apply a signal modifying the flow of the corresponding 
input compound (airflow in the application under consideration) to one of 
adjustable flow means 4 (valve or solenoid valve 11 in the application 
under consideration). This modification of the flow of the input compound 
makes it possible to cause the contents of the residual compounds to tend 
toward zero values at the output of the plant. 
According to the invention, the device also comprises calculation means 23 
connected to means 14 for measuring the flows of the input compounds, to 
means 17 for measuring the contents of the residual compounds at the 
output of the plant, as well as to measuring means 24 which provide 
signals for measuring contents of input compounds. These means for 
measuring the contents of the input compounds, in the application under 
consideration, consist of a chromatograph making it possible to determine 
in particular, in the treated acid gas, the contents of hydrogen sulfide 
H.sub.2 S, carbon dioxide CO.sub.2, methane CH.sub.4, ethane C.sub.2 
H.sub.6, benzene C.sub.6 H.sub.6, . . . for example. 
Calculation means 23 performs coherence processings of the values of the 
measurements of flows and contents received from the various measuring 
means. Calculation means 23 provides to output 32 the set-point signal 
applied to regulating means 21. The coherence processings performed by 
calculation means 23 will be described in detail below. 
Calculation means 23 comprises, for example, a treatment processor 25 
connected in a known way to storage means 26, to a terminal 27 with a 
keyboard and screen, and to a printer 28. Storage 26 makes it possible, in 
particular, to enter programs necessary for the coherence processings. 
Terminal 27 with keyboard and screen makes it possible to interact with 
processor 25, while printer 28 makes possible an entry of the results of 
the measurements, for example. 
In a first embodiment of the device of the invention, the set-point signal 
available on output 24 of calculation means 23 and resulting from the 
coherence processings performed on the values of the measurements, is 
applied directly to a flow regulator 29; the output signal of this 
regulator directly controls solenoid valve 11 which, in the application 
under consideration, makes it possible to regulate the airflow at the 
input. This regulator 29 also receives the signals measuring the contents 
of the residual compounds at the output of the plant. 
In another embodiment of the device of the invention, regulating means 21 
comprises, in addition to regulator 29, an adder 30 to which the output 
signal of regulator 29 is applied. The output signal of calculation means 
23 is no longer applied to input 22 of this regulator, but to adder 30, as 
connection 31 shows. An output of adder 30 provides a corrected control 
signal of the solenoid valve of adjustable flow means 4. 
The coherence processing will now be explained in detail from a calculation 
example: 
A pipe which transports an incompressible fluid is considered and on this 
pipe are installed two mass flow meters A and B. 
Flow meter A has a turbine sensor and flow meter B has a vacuum-generating 
orifice sensor, for example. A simultaneous plotting of the two 
apparatuses gives: 
For flow meter A, the value m.sub.A =100 
For flow meter B, the value m.sub.B =105 
Under these conditions, there is a measurement of a single magnitude by 
independent means which give two different values of the measurement noted 
as M in what follows. 
It is a matter of calculating two values mA and mB closer to M than values 
m.sub.A and m.sub.B are. The maker of apparatus A indicates that he has 
performed, on flow M, a series of experiments which have given him a set 
W.sub.A of measurements of M. 
The standard deviation of set W.sub.A is s.sub.A =2 for example, and its 
mean is M. 
Set W.sub.A has a normal distribution law, i.e., the probability density of 
the law is, in a known way: 
##EQU1## 
The maker of apparatus B indicates that he has also performed a series of n 
experiments on flow M and that he has obtained the set W.sub.B of the 
measurements of M. 
The standard deviation of set W.sub.B is s.sub.B =4, for example, and its 
mean is M. 
This set also has a probability density: 
##EQU2## 
In set W.sub.A, the probability of obtaining a value m'.sub.A as close as 
possible to value m.sub.A is expressed: 
##EQU3## 
where dm is the differential element of variable m. 
In set W.sub.B, the probability of achieving a value m'.sub.B as close as 
possible to the value m.sub.B is expressed: 
##EQU4## 
When two events A and B are independent, the combined probability of 
achieving A and B at the same time is expressed: 
EQU Prob (A B) =prob (A).times.prob (B) 
By performing the changing of variables as follows: 
##EQU5## 
The probability of simultaneous achievement, in sets W.sub.A and W.sub.B, 
of values m'.sub.A and m'.sub.B respectively as close as possible to 
observed values m.sub.A and m.sub.B, is expressed: 
##EQU6## 
Examination of the analytical expression which quantifies the desired 
probability shows, obviously, that the probability increases monotonically 
when the expression 
##EQU7## 
decreases. 
In other words: the probability of obtaining values m.sub.A and m.sub.B in 
sets W.sub.A and W.sub.B, simultaneously, is maximum when the expression 
##EQU8## 
is minimum. 
Thus, when: 
##EQU9## 
is minimum, the most probable desired values of m.sub.A and m.sub.B are: 
EQU m=m.sub.A +S.sub.A X.sub.A =M+m.sub.A -m'.sub.A 
EQU m.sub.B =m.sub.B +S.sub.B X.sub.B =M+m.sub.B -m'.sub.B 
Since apparatuses A and B measure a single magnitude M, it is necessary to 
look for the equality of values m.sub.A and m.sub.B. 
The logic constraint on estimates m y=m.sub.A -m.sub.B is noted. The 
numerical problem is then to calculate simultaneously: 
##EQU10## 
minimum under the constraint y=0. 
Since y=0, it is tantamount to minimizing the auxiliary function 
##EQU11## 
where k is a new unknown of the problem. 
Function z has an extremum when the derivatives, in relation to X.sub.A and 
X.sub.B, cancel one another, i.e.: 
##EQU12## 
all calculations performed, these two equations are expressed by the 
system: 
##EQU13## 
Variables X.sub.A and X.sub.B, replaced in the expression of the constraint 
(m.sub.A +S.sub.A X.sub.A =m.sub.B +S.sub.B X.sub.B), then gives: 
EQU kS.sub.A.sup.2 +S.sub.B.sup.2 =m.sub.A -m.sub.B 
i.e.: 
##EQU14## 
The value of K referred to in system (1) gives: 
##EQU15## 
Finally: 
##EQU16## 
The numerical application of the preceding results is: 
##EQU17## 
The most probable value (and certainly not the closest value) of M is equal 
to 101. 
The coherent values of measurements m.sub.A and m.sub.B are: 
EQU m.sub.A =m.sub.B =101 
The certainty of obtaining values m closer to the true value than the raw 
values m is obtained by multiplying the raw measurement readings and their 
processing. 
The reduction of error is 50% for measurement A and 66% for measurement B 
in the case where the true value is equal to 102, and the residual error 
of B then changes direction. 
The effectiveness of the treatment increases with the number of 
redundancies of the raw measurements and with the number of repeated 
processings. 
In the application under consideration of the regulating device of the 
invention at a plant for obtaining sulfur, experience shows that the 
coherence processing performed on the values of the measured flows and 
contents makes possible an operation of this plant at an optimum 
efficiency. In plants of the prior art, which do not use this coherence 
processing in this type of application, and which in particular do not 
treat by coherence flow values of the reactive compounds at input as well 
as the values of these compounds at output, the efficiency is much less. 
The coherence processings also make it possible to establish intervals of 
values inside of which the measured values must be located. When the 
values of one of the measured parameters, for example, are not located in 
the corresponding predetermined interval, it is possible to trigger an 
alarm which optionally makes it possible to stop the operation of the 
plant since under these conditions there is probably a failure in the 
operation of the plant. 
In the application under consideration, the coherence processings make it 
possible to refine the prediction of the airflow necessary for the 
reaction with the input acid gases. 
The coherence processings make it possible to reduce the variances of the 
measurements taken and thus to increase the stability of the operation of 
the entire plant. Consequently, at the established regulating speed, it is 
no longer necessary to take measurements as often as in plants whose 
operation variant is great. 
The regulating device which has just been described, particularly for a 
plant for obtaining sulfur, of course, can be used for any other type of 
chemical treatment plant.