U.S. patent number 3,742,721 [Application Number 05/062,590] was granted by the patent office on 1973-07-03 for method of regulation of the temperature of the liquefied gas or gaseous mixture in an apparatus for the liquefaction of gaseous fluids.
This patent grant is currently assigned to Compagnie Francaise D'Etudes Et De Construction Technip. Invention is credited to Jean-Marie Bourguet, Jean Charles Perret.
United States Patent |
3,742,721 |
Bourguet , et al. |
July 3, 1973 |
**Please see images for:
( Certificate of Correction ) ** |
METHOD OF REGULATION OF THE TEMPERATURE OF THE LIQUEFIED GAS OR
GASEOUS MIXTURE IN AN APPARATUS FOR THE LIQUEFACTION OF GASEOUS
FLUIDS
Abstract
In the liquefaction of a gaseous fluid wherein a refrigerant
fluid having at least two components is used, at least two
conditions parameters of the refrigerant fluid are determined. The
determined values are used to control regulating members of the
flow rates of the refrigerant fluid at specified operations of the
liquefaction process.
Inventors: |
Bourguet; Jean-Marie (Le
Vesinet, FR), Perret; Jean Charles (Paris,
FR) |
Assignee: |
Compagnie Francaise D'Etudes Et De
Construction Technip (Rueil-Malmaison, FR)
|
Family
ID: |
9048749 |
Appl.
No.: |
05/062,590 |
Filed: |
August 10, 1970 |
Foreign Application Priority Data
Current U.S.
Class: |
62/611; 62/224;
62/657; 62/228.1 |
Current CPC
Class: |
F25J
1/0249 (20130101); F25J 1/0022 (20130101); F25J
1/0265 (20130101); G05D 23/00 (20130101); F25J
1/0244 (20130101); F25J 1/0212 (20130101); F25J
1/0055 (20130101); G05D 23/1919 (20130101) |
Current International
Class: |
F25J
1/00 (20060101); G05D 23/00 (20060101); F25J
1/02 (20060101); G05D 23/19 (20060101); F25j
003/00 (); F25j 001/00 () |
Field of
Search: |
;62/19,37,21,228,224,210 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yudkoff; Norman
Assistant Examiner: Purcell; Arthur F.
Claims
We claim:
1. Process for the control of the temperature of a gas or a mixture
of liquefied gases, such as natural gas, derived from a
gas-liquefying apparatus, in which the apparatus uses a refrigerant
fluid of at least various components that are fractionally
condensed at lower and lower temperatures, for cooling down and
liquefying, by heat exchange, the gas or mixture of the gases to
obtain minimum deviation from a predetermined temperature of the
liquefied gases at their lowest temperature level comprising the
steps of:
measuring (35, 36, 37, 38) at least two of the state parameters
comprising temperature, volume, and pressure, of the refrigerant
fluid at least at the lowest temperature level of the refrigerant
fluid;
controlling the flow (32) of the liquid refrigerant fluid at the
lowest temperature level under control of, and based on one of said
measured parameters;
regulating (33) the flow rate of gaseous refrigerant fluid that is
collected after the last cooling stage under control of and based
on one of said measured parameters;
and controlling (34) the flow of those two components of
refrigerant fluid make-up gas which have the lowest liquefaction
temperatures, under control of and based on selected ones of said
measured parameters.
2. Process according to claim 1, for use with liquefying apparatus
which includes a condensing column (1) for the refrigerant fluid,
an exchange column (2) for the cooling and liquefying of the gas or
mixture of gases, and a compressor (3) that collects the gaseous
refrigerant fluid issuing from the exchange column (2)
wherein the step of measuring the refrigerant fluid state
parameters comprises
measuring in the condensing column, the pressure (35) of the
gaseous fraction before condensation and the temperature (36) of
the corresponding liquid fraction after condensation;
measuring the temperature (37) and pressure (38) of the fluid, in
gaseous state, that results from said liquid fraction in the
exchange column;
and using the pressure measurement (38) from the exchange column
for controlling the speed of the compressor (3) and, consequently,
the flow rate of the refrigerant fluid collected in the gaseous
state after exchange.
3. Process according to claim 2, wherein that the step of
controlling the flow of refrigerant fluid comprises
controlling (32) the flow of liquid fraction of the refrigerant
fluid that is used to condense said gaseous fraction in the
condensing column based on measurement of temperature (36) of the
corresponding liquid fraction after condensation of the lowest
temperature;
controlling (33) the inlet flow to the exchange column of the
liquid fraction of the refrigerant fluid at its lowest temperature
based on the pressure measurement (35) of the fluid gaseous
fraction before condensation;
and controlling (34) the make-up flow of the components of the
refrigerant fluid that have the lowest temperatures of liquefaction
based on measurement of the fluid temperature (37) of the resulting
gaseous fraction of said liquid fraction.
4. Process according to claim 1, for use with liquefaction
apparatus which includes a condensing column (1) for the
refrigerant fluid, an exchange column (2) for the cooling and
liquefying of the gas, or mixture of gases, and a compressor (3)
that collects the gaseous refrigerant fluid issuing in gaseous
state from the exchange column (2),
and in which some of the successive liquid fractions are conveyed
in respective circuits within the exchange column (2);
wherein the step of measuring at least one of the state parameters
comprises:
separating the lowest temperature fraction of the refrigerant fluid
in a chamber (40) that contains the liquid and gaseous phases of
said fraction;
and measuring the state parameters of the fluid in said
chamber.
5. Process according to claim 4, wherein the measuring steps
comprise
measuring the temperature (43) and pressure (42) in said chamber
(40);
measuring the pressure (38) of the refrigerant fluid in the gaseous
state in the exchanger column (2);
measuring the temperature (44) of the gas or of the liquefied
gaseous mixture;
controlling (45) the flow of the liquid refrigerant fraction used
for the formation of the liquid fraction at the lowest
temperature;
controlling (34) the make-up flow of the components of the
refrigerant fluid that have the lowest temperatures of
liquefaction;
controlling the compressor speed (3) and consequently the flow rate
of refrigerant fluid collected in gaseous state, after
exchange;
and controlling (33) the flow of the liquid fraction at the lowest
temperature of the refrigerant fluid at the inlet of the exchange
column (2).
6. Process according to claim 5, comprising the step of
sub-cooling, in the condensing column, the liquid fraction
contained in said chamber (40) before introducing it into the
exchange column (2).
7. Process according to claim 4, comprising the step of
controlling (45) the inlet flow to said chamber (40) of the lowest
temperature liquid fraction in such a way as to cause a pressure
drop in said chamber in relation to the pressure in the condensing
column;
and measuring the temperature (47), pressure (42), and liquid level
(48), and consequently liquid volume in said chamber (40), the
pressure (38) of the exchange column (2) and the temperature (44)
of the gas or liquefied gaseous mixture.
8. Process according to claim 7, wherein
the steps of controlling the flow of refrigerant fluid comprise
controlling (32) the flow of the liquid fraction of the refrigerant
fluid that is used to ensure the formation of the lowest
temperature liquid fraction in the condensing column;
controlling (45) the inlet flow into the chamber (40) of this last
fraction;
controlling (34) the make-up flow of the components of the
refrigerant fluid that have the lowest temperatures of
liquefaction;
controlling the speed of the compressor (3) and consequently the
flow of refrigerant fluid collected in the gaseous state after the
exchange;
and controlling (32) the inlet flow into the exchange column (2) of
the liquid fraction at the lowest temperature of the refrigerant
fluid.
9. Process according to claim 7, comprising the step of
controlling (45) the flow of the gaseous phase of the refrigerant
fluid to the chamber (40);
and wherein the measuring and control steps include
measuring (35) the pressure of the gaseous fraction of the
refrigerant fluid which is transformed in the condensing column (1)
to the liquid fraction with the lowest temperature
controlling (32) the flow of the liquid fraction of the lowest
temperature, as a function of said measured pressure;
controlling (45) the flow of the refrigerant fluid in the gaseous
state that leaves the chamber;
controlling (34) the make-up flow of the components of the
refrigerant fluid that have the lowest temperature of
liquefaction;
controlling (38) the speed of the compressor (3) and, consequently,
the flow of the refrigerant fluid collected in the gaseous state
after the exchange;
and controlling (33) the inlet flow to the exchange column of the
liquid fraction at the lowest temperature of the refrigerant
fluid.
10. Process according to claim 2
wherein the step of regulating the flow of the refrigerant fluid
includes
introducing said fluid into the exchange column, in a controlled
manner, at least at a level immediately above that of the lowest
temperature level.
11. Process according to claim 10, further wherein the measurement
step includes
measuring the flow of the refrigerant that is sent to the exchange
column (2) and
controlling addition of flow of the two components of the
refrigerant fluid that have liquefaction temperatures just above
that of its most volatile component as make-up flow to the
refrigerant fluid, said addition being added at a level just above
that of the refrigerant fluid with the lowest temperature.
12. Process according to claim 2, comprising the step of
adding a make-up of heavy end components to the refrigerant fluid
to increase the condensation temperature of the refrigerant fluid
with the highest temperature to a value approaching the temperature
of the fluid outside the condensing column.
13. Process according to claim 4, comprising the step of
controlling the pressure in the chamber (40), by
venting the chamber.
14. Process according to claim 1, for use with a liquefying
apparatus which includes a single, combined condensing and exchange
column (FIG. 6: 50) and a compressor (51) that collects the gaseous
refrigerant fluid issuing from said single column (50)
wherein the step of measuring the state parameters comprises
measuring the pressure (66) of the gaseous fraction in the single
column;
measuring the temperature (64) of the liquefied gas, or mixture of
gases;
and wherein the control steps comprise
controlling (65) the flow of refrigerant fluid based on a measured
state parameter;
and controlling the speed of the compressor (51) and consequently
the flow rate of refrigerant fluid collected, after exchange, in
said single column based on another one of the state
parameters.
15. Process according to claim 14, characterized wherein the step
of measuring at least some of the state parameters comprise the
further step of
separating a low temperature fraction of the refrigerant fluid in a
chamber (57) which contains the liquid and gaseous phases of the
fraction;
and measuring at least two of the state parameters of: temperature;
pressure; and volume of the liquid and gaseous fraction in said
chamber.
16. Process according to claim 14, wherein the control steps
comprise
controlling the flow of refrigerant fluid (65) based on temperature
measurement (64) of the liquefied gas, or mixture of gases;
and the step of controlling the speed of the compressor (51)
comprises controlling compressor speed based on pressure
measurement (66) of the gaseous fraction in the single column.
17. Process according to claim 15, comprising the step of
controlling (63) the make-up flow of components of the refrigerant
fluid that have the lowest temperature of liquefaction;
and controlling (61) the flow of liquid refrigerant fraction used
for the formation of the liquid fraction at the lowest
temperature.
18. Process according to claim 17, wherein control of the make-up
flow (63) is based on measurements providing data representative of
volume (62) in said chamber;
and control of liquid refrigerant fraction flow (61) is based on
temperature measurement (60) representative of temperature of the
gaseous fraction in said chamber (57).
19. Apparatus for the liquefaction of gases, or gaseous mixture,
such as natural gas, by means of a refrigerant fluid with various
components which are fractionally condensed at lower and lower
temperatures, to cool and liquefy the gas or gaseous mixture by
heat exchange,
comprising
means (FIG. 1: 35, 36, 38; FIG. 6: 55, 58, 60, 64, 66) measuring at
least two of the state parameters comprising temperature; pressure;
volume, of the refrigerant fluid at least at the lowest temperature
level of the refrigerant fluid;
flow control means (32, 33) in the path of fluid flow of said
refrigerant fluid at the lowest temperature to control the flow of
the refrigerant, governed under control of some of said measuring
means responsive to one of said parameters;
and flow control means (3, 34) to control the flow rate of
refrigerant that is collected after exchange, and to control the
flow rate of make-up of those components of the refrigerant fluid
which have the lowest liquefaction temperatures, disposed in the
path of said flow of said refrigerant fluid, and governed under
control of selected ones of said measuring means and responsive to
selected ones of said parameters.
20. Apparatus according to claim 19, comprising a condensing column
(1) condensing therein fractionally the refrigerant fluid to lower
and lower temperatures; an exchange column (2) cooling therein and
liquefying, by heat exchange, the gas, or mixture of gases;
and means (8, 16, 23, 28) conveying successive liquid fractions of
the refrigerant fluid through separate conduits to the exchange
column;
and wherein said flow control means comprise a compressor (3)
having applied thereto the refrigerant fluid in the gaseous state
derived from said exchange column (2).
21. Apparatus according to claim 20, further comprising a chamber
(FIG. 3: 40) connected in the flow circuit containing the gaseous
and liquid phases of the lowest temperature fraction of the
refrigerant fluid;
and at least some of said measuring means being connected to said
chamber to measure at least some of the state parameters of said
refrigerant fraction in the chamber (40).
22. Apparatus according to claim 21, wherein the flow control means
comprises
a valve means (45) in the circuit of the lowest temperature
fraction of the refrigerant fluid and controlling flow to said
chamber;
and a second valve means (46) communicating with the gaseous phase
of said refrigerant fluid in said chamber and connected to vent,
the second valve means (46) controlling the pressure in said
chamber and consequently the pressure within the condensing
column.
23. Apparatus according to claim 21, further comprising
a heat exchange circuit (FIG. 3: 41) located within the condensing
column (1) in the circuit of the lowest temperature fraction,
upstream of said chamber (40) and connected to the liquid phase in
said chamber, said further condensing column being located in said
chamber exposed to be cooled, to effect sub-cooling of the liquid
therein before said liquid is directed (39, 28) to the heat
exchange column (2).
24. Apparatus according to claim 19, comprising
a combined condensing and heat exchange column (FIG. 6: 50);
heat exchange means and expansion valve means in said column;
means conveying the successive liquid fractions of the refrigerant
fluid through a first separate conduit in said column;
means conveying said gases, or gaseous mixture to be liquefied in a
second separate fluid conduit path in said column;
said expansion valve means being located in said column for
effecting cooling simultaneously of the liquid fractions in said
first conduit and said gas, or mixture of gases in said second
conduit path, in said column;
some of said measuring means measuring the pressure in said column
and the temperature of one of the fluids
the measured parameters controlling the flow of refrigerant fluid
to the expansion valve means in the column.
25. Apparatus according to claim 24, wherein that said flow control
means further comprises
a compressor (51) having applied thereto refrigerant fluid derived
from said column (50); the speed of
said compressor being controlled by the pressure sensing means (66)
sensing pressure within said column.
26. Apparatus according to claim 24, wherein the measuring means
measure the temperature of the gas, or gaseous mixture.
27. Apparatus according to claim 24, wherein the measuring means
measure the temperature of the liquid fraction.
Description
The present invention relates to a method of regulation of the
temperature of the liquefied gas or gaseous mixture in an apparatus
for the liquefaction of gaseous fluids, i.e gas or gaseous
mixtures, natural gas for example, by means of a refrigerant fluid
with a number of constituents, which is condensed by fractions at
lower and lower temperatures in order to cool and liquefy the gas
or gaseous mixture by exchange, and to a liquefaction apparatus
arranged so as to carry this method into effect.
Liquefaction apparatus for gas or gaseous mixtures are known which
operate with a refrigerant fluid having a number of constituents,
which is condensed by fractions at lower and lower temperatures in
order to cool and liquefy the gas or gaseous mixture by
exchange.
U.S. Pat. No. 3,364,685 in the name of the present Applicants
describes apparatus of this kind which comprises a condensation
column for the refrigerant fluid, an exchange column for the
cooling and liquefaction of the gas or gaseous mixture, and a
compressor fed with the refrigerant fluid issuing from the exchange
column in the gaseous state.
The pressure prevailing in the condensation column is relatively
high, while the exchange column works at a substantially lower
pressure.
The use of a refrigerant fluid having a number of constituents
implies that the boiling temperature of the various liquid
fractions varies in dependence on the composition of these liquids,
so that the temperature required for correctly condensing (or
strongly refrigerating the gas which is liquefied (or to be
liquefied) is liable either not to be attained, or to be exceeded.
As the quantity of refrigerant fluid in circulation is relatively
small, the presence of leakage may result in a fairly large
variation in the composition of this fluid.
If the liquefied gas is insufficiently refrigerated, there results
during storage in containers under a pressure which is usually
close to atmospheric pressure, a considerable vaporization or
"flash", which makes it necessary to discharge the vapours.
Conversely, if the liquefied gas is at too low a temperature, the
insufficient pressure which prevails in the storage containers
causes the entry of air ; in addition, this gives rise to
substantial power consumption, since the cost of cooling at low
temperatures increases very rapidly with the reduction of the
temperature. It is therefore necessary to check continuously the
composition of the refrigerant fluid, and to add the necessary
make-up to this latter, when so required.
Analysis of the compositions is difficult, long and not very
accurate, so that this cannot be considered as an acceptable
solution.
The invention has for its object to carry out, with the maximum
simplicity and a minimum response time, accurate measurement which
can be employed for the regulation of the temperature of the
liquefied gas or gaseous mixture within a range of temperatures
which is as narrow as possible, preferably .+-. 1.degree.C.
According to the invention, the method is characterized in that at
least some of the condition parameters of the refrigerant fluid are
measured, at least at the level of the lowest temperature of this
fluid, and these measurements are utilized to control the devices
which regulate the flow-rate of refrigerant fluid at the level of
the lowest temperature, together with the devices which regulate
the flow of refrigerant fluid recovered in the gaseous state after
exchange, and to control the flow-rate of those two constituents of
the refrigerant fluid having the lowest liquefaction temperatures,
added by way of gaseous make-up to the fluid recovered after
exchange.
It has been observed that the coldest refrigerant fluid, which is
in the liquid state may be considered practically as formed of two
constituents as regards the temperature achievable for cooling and
liquefying the gas or gaseous mixture ; the proportion of the other
constituent has a negligible effect on this temperature. The result
is that the measurement of certain condition parameters of the
fluid (i.e the pressure, the temperature and the volume in
accordance with thermo-dynamic definition) at the level of the
lowest temperature, makes it possible to act instantaneously and
directly on the flow rates of fluid at the critical points of the
apparatus.
SUBJECT MATTER OF THE INVENTION
The method may be applied to a liquefaction apparatus comprising a
condensation column for the refrigerant fluid, an exchange column
for the cooling and liquefaction of the gas or gaseous mixture and
a compressor fed with the refrigerant fluid issuing from the
exchange column in the gasous state, the condition parameters of
the refrigerant fluid measured at the level of the lowest
temperature of this fluid are, for the condensation column, the
pressure of the gaseous fraction before condensation and the
temperature of the corresponding liquid fraction after condensation
and, for the exchange column, the pressure and temperature of the
fluid in the gaseous state resulting from the said liquid fraction,
the measurement of the pressure in the exchange column being
employed to control the speed of the compressor and, in
consequence, the flow-rate of the refrigerant fluid taken in the
gaseous state after exchange.
According to another preferred embodiment a chamber containing the
liquid and gaseous phases of the fraction at the lowest temperature
of the refrigerant fluid is placed into the circuit of the said
fraction, and the measurements of at least some of the condition
parameters of the fluid in this chamber are utilized.
In this case, the temperature and pressure in the same chamber are
measured, as are also the pressure of the refrigerant fluid in the
gaseous state in the exchange column and the temperature of the
liquefied gas or gaseous mixture, and these measurements are
employed to control, a valve regulating the flow-rate of the liquid
fraction of refrigerant fluid which serves to ensure the formation
of the liquid fraction at the lowest temperature, a valve
regulating the flow rate of the topping-up constituents having the
lowest liquefaction temperatures, the speed of the compressor and,
in consequence, the flow-rate of refrigerant fluid in the gaseous
state taken after exchange, and a valve regulating the flow-rate of
the liquid fraction at the lowest temperature of the refrigerant
fluid at the inlet of the exchange column.
This avoids measurement of temperature in the exchange column, and
knowledge of the pressure and temperature of the fluid in the
chamber is sufficient to determine the composition of this fluid
and therefore its temperature at the inlet of the exchange column.
In addition, a supplementary condition parameter is introduced,
namely the volume of liquid in the chamber which is determined by
measuring the level.
Liquefaction apparatus according to the invention comprises;
devices regulating the flow-rate of refrigerant fluid at the level
of the lowest temperature, the flow-rate of this fluid in the
gaseous state taken after exchange, and the flow-rate of
constituents added by way of gaseous make-up, these devices are
connected to and controlled by measuring devices for measuring the
chosen condition parameters of the refrigerant fluid
In the circuit of the lowest-temperature fraction of the
refrigerant fluid, a chamber is provided containing the liquid and
gaseous phases of this fraction, which makes it possible to measure
at least some of the condition parameters of the refrigerant fluid
in this chamber.
Other particular features and advantages of the invention will be
brought out in the description given below, by way of explanation
and not in any limitative sense, reference being made to the
accompanying diagrammatic drawings, wherein the parameters sensed
are indicated in the legends, and in which:
FIG. 1 is a general view of a liquefaction apparatus with two
columns, arranged for a first method of application of the method
according to the invention;
FIG. 2 is a partial view showing the last stage, or the lowest
temperature stage, following an alternative form of embodiment;
FIG. 3 is a partial view showing the last stage of a liquefaction
apparatus arranged for another application of the method according
to the invention;
FIG. 4 is a partial view similar to that of FIG. 3, of a
liquefaction apparatus arranged for a preferred way of carrying out
the method according to the invention;
FIG. 5 is a view similar to FIG. 4, showing an alternative form of
embodiment;
FIG. 6 is a general view of a liquefaction apparatus with a single
column arranged in a manner similar to that of FIG. 1, for carrying
into effect the method according to the invention.
FIG. 1 shows an apparatus for the liquefaction of a gas or gaseous
mixture, natural gas for example, by means of a refrigerant fluid
having a number of constituents which is condensed by fractions at
lower and lower temperatures in a condensation column 1 in order to
cool and liquefy by exchange the gas or gaseous mixture in an
exchange column 2. The condensation column 1 has three stages and
the exchange column 2 has four stages, but the number of stages may
obviously be greater or less, the preferred number being comprised
between two and four for the condensation column and between three
and five for the exchange column.
A low-pressure compressor 3 delivers into a high-pressure
compressor 4 through the intermediary of a cooler 3a, the vapours
of refrigerant fluid coming from the exchange column 2; the
compressor 4 also takes the refrigerant fluid vapours coming from
the condensation column 1. The compressed refrigerant fluid is
directed towards a condenser 5 in which it is cooled and partly
condensed. The condenser 5 is cooled by an external fluid, for
example air or sea water.
A mixture of liquid and vapour then leaves the condenser and is
separated in a tank 6 located at the level of the first or bottom
stage of the condensation column 1. The liquid phase is directed on
the one hand into an exchanger 7 at the first stage of the
condensation column 1 and on the other hand into an exchanger 8 in
the first stage of the exchange column 2.
After its passage through the exchanger 7, the liquid is directed
towards an injection head 9 which injects it into the condensation
column in order to cool the liquid in the exchanger 7 on the one
hand, and to partially condense the gas coming from the tank 6 and
directed into an exchanger 10 on the other. With regard to the
liquid directed towards the exchanger 8 of the exchange column,
this is injected by an injection head 11 into this column in order
to cool the liquid in the exchanger 8 on the one hand and to cool
the gas to be liquefied passing through an exchanger 12, this gas
having been compressed if required, by a compressor 13 before its
introduction into the exchange column 2.
The same process is repeated at the following stages of the
condensation column and the exchange column, the fraction of
refrigerant fluid coming from the exchanger 10 being introduced
into a tank 14 located at the level of the second stage, in which
tank this fraction is separated into two phases, one liquid and the
other vapour. The condensation column also comprises at this stage
an exchanger 15 for the liquid, followed by an injection head 17,
and an exchanger 18 for the gas to be condensed.
Similarly, the exchange column comprises an exchanger 16 for the
liquid, followed by an injection head 19, and an exchanger 20 for
the gas to be liquefied. At the third and last stage of the
condensation column are provided a tank 21, exchangers 22, 25 and
an injection head 24, while at the third stage of the exchange
column are provided an exchanger 23 and an injection head 26,
together with an exchanger 27 for the gas to be liquefied. The
refrigerant fluid which leaves the exchanger 25 is in the liquid
state and is directed to the last stage of the exchange column into
an exchanger 28 followed by an injection head 29 in order to ensure
the liquefaction and the refrigeration of the gas to be liquefied
at the level of the last exchanger 30.
The liquefied gas passing out of the exchange column 2 is directed,
through a valve 31 which enables its pressure to be brought to a
pressure in the vicinity of atmospheric pressure, towards storage
tanks (not shown).
Two valves 32, 33 are provided respectively between the exchanger
22 and the injection head 24 so as to regulate the flow-rate of
liquid serving to ensure the production of the refrigerant fluid at
the lowest temperature in the condensation column 1, and between
the exchanger 28 and the injection head 29 in order to regulate the
flow-rate of this latter liquid injected into the exchange column 2
after refrigeration in the exchanger 28.
A valve system which is a plural-channel valve having elements 34a
and 34b, permits the regulation of the flow-rate of make-up of
constituents of the refrigerant fluid, added in the gaseous state
to the fluid taken after exchange and introduced into the
compressor 3. These constituents are the two gases which have the
lowest liquefaction temperatures, that is to say in principle
nitrogen (valve 34a) and methane (valve 34b). The topping-up
elements are obtained from a suitable source (not shown) which
supplies either gases in the pure state or mixtures which are rich
in nitrogen or methane. In the case of liquefaction of natural gas,
these mixtures are obtained from extractions made from the natural
gas itself during the course of liquefaction.
It will be noted that the temperature of the liquefied gas leaving
the exchange column 2 depends mainly on the temperature of the
refrigerant fluid in the liquid state which is directed towards the
injection head 29. The most critical point of the installation is
therefore located at this level; if the temperature of the
refrigerant fluid is not sufficiently low, the liquefied gas under
pressure passing out of the exchanger 30 will not be sufficiently
refrigerated and, after expansion by the action of the valve 31, it
will cause relatively large formations of vapor which it will be
necessary to evacuate; if the temperature of the refrigerant fluid
is too low, the vapour pressure of the liquefied gas passing out of
the valve 31 will be too low and lower than atmospheric pressure,
which is liable to cause leakages of air into the storage tanks if
a pressure-balancing device by the intake of supplementary gas is
not provided. In addition, the frigories supplied by the
refrigerant fluid at its lowest temperature are extremely costly,
since it is known that the cost of frigories increases as the
temperature diminishes. A variation of 1.degree.C. at the usual
working temperatures of the apparatus results in an increase in the
cost price of cooling of the order of 1 percent. The temperature of
the refrigerant fluid at the level of the lowest temperature is
therefore an essential factor of the economic efficiency of the
installation, which factor should be maintained within a
relatively-narrow range, preferably less than 1.degree.C.
All other things being equal, this temperature is very sensitive to
the composition of the refrigerant fluid at this level, so that the
variations of percentage of constituents due to leakages or to
accidental opening of blow-off or service valves, may result in
large variations of temperature within the space of a few minutes.
It is therefore essential to rapidly compensate for these leakages
by additions of nitrogen or methane for example, so as to maintain
the composition of the refrigerant fluid practically constant at
the level of the lowest temperature.
It has been observed that, although the refrigerant liquid at its
lowest temperature is theoretically a mixture of a number of
constituents, it may be considered in practice as formed by two
constituents as regards boiling temperatures which can be obtained
in the exchange column, the proportion of the other constituents
having only a negligible effect on that temperature. The result is
that if the pressure and the temperature of this liquid are
regulated before its passage into the exchange column, and if the
pressure in the exchange column is also regulated, the temperature
of the liquid before its injection gives an instantaneous
indication of its composition, which enables the supply of the
necessary additions to be initiated.
In addition, the pressure in the condensation column 1 must be
higher than a certain value such that the vapours passing out of
the tank 21 are wholly condensed in the exchanger 25. This implies
the provision of a regulator for ensuring that this pressure
remains higher than the said given value.
It can be seen from FIG.1 that four means of action are available
for regulating the operation of the liquefaction apparatus, namely:
the valves 32 and 33 which respectively control the flow-rate of
refrigerant fluid serving for the production of the coldest
fraction of this fluid and the flow-rate of this fluid before its
injection into the exchange column, the valve 34 intended for the
injection of topping-up elements, the speed of the compressor 3
which controls the flow-rate of refrigerant fluid in the gaseous
state taken at the outlet of the exchange column.
The above means of action are each associated and controlled by one
of the four measurements of condition parameters of the refrigerant
fluid at the level of the lowest temperature of this fluid. The
condition parameters, which comprise the temperature, the pressure
and the volume, are in the present case the temperature and the
pressure. The four measurements made are: the pressure of gas in
the tank 21 at the last stage of the condensation column, the
temperature of the coldest refrigerant fluid in the vicinity of the
valve 33 before its injection into the exchange column, the
temperature of the refrigerant fluid at the last stage of the
exchange column after its injection, and the pressure of the
refrigerant fluid in the gaseous state in the exchange column.
In the example of FIG.1, the valve 33 is controlled by the pressure
in the tank 21, measured by a pressure gauge 35, the valve 32 is
controlled by the temperature of the refrigerant fluid before
injection, measured by a probe 36, the make-up valve 34 is
controlled by the temperature of the refrigerant fluid after
injection, measured by a probe 37, and the speed of the compressor
3 is controlled by the pressure of the refrigerant fluid in the
gaseous state in the exchange column, measured by a pressure gauge
38. The topping-up valve 34 may be a valve with several channels or
it may be constituted by two valves, one controlling the injection
of nitrogen or a mixture rich in nitrogen, and the other the
injection of methane, or of a mixture rich in methane. In this
latter case, the two valves in fact play the part of a single valve
which varies the composition of the make-up elements in the
vicinity of the desired value.
FIG.2 shows an alternative form in which the means of action are
associated with the measurements of temperature and pressure
indicated above in a different order, namely: valve 32 --
temperature probe 37; valve 33 -- temperature probe 36; make-up
valve 34 -- pressure-gauge 35; speed of compressor 3 -- pressure
gauge 38. Irrespective of the combination chosen, the speed of the
compressor is associated with the pressure in the exchange column,
measured by the pressure gauge 38. The result is that the number of
combinations available with the four means of action and the four
measurements defined above is six.
The arrangements which have just been considered do not however
take into account the flow of gas to be liquefied. In fact, even if
the temperature of the coldest refrigerant fluid after its
injection into the exchange column is perfectly stabilized, the
temperature of the liquefied gas passing out of the exchanger 30
can vary slightly as a function of the flow-rate of gas to be
liquefied which passes into this exchanger. In order to take
account of the flow-rate of gas to be liquefied, the flow-rate of
the coldest refrigerant fluid is varied before its injection into
the exchange column, as a function of the flow-rate of liquefied
gas, in order to maintain the final temperature of this liquefied
gas constant.
To this end, as has been shown in FIG.3, there is introduced into
the circuit 39 of refrigerant fluid at the lowest temperature, a
tank 40 containing the liquid and gaseous phases of this fluid. The
refrigerant fluid passing out of the exchanger 25 is sent into the
tank 40, in which the liquid and vapour phases are in equilibrium
at the boiling temperature of the fluid and under the high pressure
of the condensation column. The refrigerant fluid is practically
equivalent at this level to a mixture of two constituents,
generally nitrogen and methane, and the mere knowledge of the
pressure and temperature existing in the tank 40 is sufficient to
define the proportions of nitrogen and methane in this mixture.
The liquid phase of the tank 40 is directed to an exchanger 41
provided in the condensation column, as in the previous examples
shown in FIGS.1 and 2. Since the refrigeration actions carried out
on the coldest refrigerant fluid depend essentially on the
exchangers, they are practically constant for given exchangers, so
that the boiling temperature of the fluid practically defines that
of the refrigerant fluid before its injection into the exchange
column.
By maintaining the pressure and temperature of the coldest
refrigerant fluid and also the pressure in the exchange column at
constant values, there is obtained a constant value of the
temperature of the refrigerant fluid after its injection into the
exchange column. By this means, measurement of the temperature of
the fluid in the exchange column is eliminated, this measurement
being in any case difficult and inaccurate unless expensive and
delicate precautions are taken.
In the example of FIG.3, the make-up valve 34 is controlled by the
pressure in the tank 40, measured by a pressure-gauge 42, the valve
32 is controlled by the temperature of the refrigerant fluid before
its passage into the tank 40, measured by a probe 43, the valve 33
is controlled by the temperature of the liquefied gas passing out
of the exchange column, this temperature being measured by a probe
44, and the speed of the compressor 3 is again controlled by the
pressure in the exchange column, measured by the pressure-gauge
38.
In an alternative form, the make-up or topping-up valve may be
controlled by the temperature probe 43, while the valve 32 can be
controlled by the pressure-gauge 42.
In order to avoid the provision of the supplementary exchanger 41,
a different arrangement can be adopted, as shown in FIG.4. In this
case, a valve 45 regulates the flow-rate of refrigerant fluid in
the liquid state passing out of the exchanger 25 and sent into the
tank 40.
The liquid phase contained in this tank is sent directly to the
exchanger 28 of the exchange column, the valve 45 causing a drop in
pressure such that the tank 40 becomes at a pressure less than that
existing in the condensation column, corresponding to the boiling
pressure at the desired temperature of the coldest refrigerant
fluid. Futhermore, the gaseous phase of the tank 40 is coupled to
the evacuation system of the apparatus by means of a valve 46 which
may be termed the negative make-up valve, since it makes it
possible to obtain a reduction of the percentage of nitrogem by
evacuating the excess.
There are then available six means of action, namely (see FIG.4):
the valves 32, 33 and 34 (not shown), the valve 45 which regulates
the flow of refrigerant fluid passing out of the exchanger 25 and
introduced into the tank 40, the evacuation valve 46 and the speed
of the compressor 3 (not shown).
The parameters corresponding to these means of action are as
follows: the pressure in the tank 21, measured by the
pressure-gauge 35 and associated with the valve 32; the pressure in
the tank 40, measured by the pressure-gauge 42 and associated with
the evacuation valve 46; the temperature in the tank 40, measured
by a probe 47 and associated with the valve 45; the volume of
liquid in the tank 40, measured by a level measurer 48 and
associated with the make-up valve 44 (not shown); the temperature
of the liquefied gas passing out of the exchange column, measured
by the probe 44 and associated with the valve 43; the pressure in
the exchange column, measured by the pressure-gauge 38 and
associated with the speed of the compressor 3 (not shown).
As compared with the previous embodiments, this solution has the
advantage of permitting regulation of the pressure existing in the
condensation column, by means of the addition of the evacuation
valve 46.
In the alternative form shown in FIG.5, the pressure in the tank 21
is associated with the valve 45, the pressure and the temperature
in the tank 40 are associated with the valve 46 and the valve 32
respectively; the level in the tank 40 is associated, as
previously, with the make-up valve and the temperature of the
liquefied gas is again associated with the valve 33 and the
pressure in the exchange column with the speed of the compressor.
In this case, an increase of pressure in the tank 40 causes the
opening of the valve 46, while a fall in level in this tank causes
the injection of additions of nitrogen and methane in the form of a
mixture having a composition identical with that of the coldest
refrigerant fluid.
Different additional measurements may be taken in order to still
further improve the effectiveness of the regulation.
For example, the temperature of the refrigerant fluid injected at
the last stage of the condensation column by the injection head 24
depends essentially on the temperature of the liquid in the tank
21. This latter temperature can be regulated by controlling the
flow-rate of refrigerant fluid injected by the injection head 17 at
the level of the intermediate stage of the condensation column.
In addition, the flow-rate of refrigerant liquid sent to the
injection head 26 in the exchange column must be sufficient to cool
to the maximum extent the gas to be liquified, since cooling by
this liquid is less expensive by the coldest refrigerant fluid.
A measurement of the level of liquid in the tank 21 may be provided
to control the flow-rate of refrigerant liquid injected by the
injection head 26 into the exchange column. In order to compensate
for an insufficiency of the flow of this liquid, additional
quantities of constituents of the refrigerant fluid having
liquefaction temperatures immediately above that of the most
volatile constituent,(generally nitrogen), may be added in the form
of a mixture practically identical to that of the liquid (generally
methane and ethane). A probe measuring the temperature in the
circuit of the gas to be liquefied after the exchanger 27, or a
flow measuring device provided on the circuit of refrigerant liquid
going to the exchange column, enables this injection of topping-up
quantities to be controlled.
Arrangements may be provided in the lower stages of the
installation in order to utilize the cooling effect available in
the two columns with the maximum efficiency. By this means, there
is also ensured a superheating of the evacuated vapours of
refrigerant fluid, making it possible to prevent the entry of
liquid particles into the compressors.
In addition, a trap operating by gravity and collecting the liquid
particles in the bottom of the condensation column may be used to
control, by the measurement of its level the injection of make-up
of heavy constituents (pentane for example) of the refrigerant
fluid. These heavy constituents have the effect of bringing the
condensation temperature of the coldest refrigerant fluid (first
stage) closer to that of the fluid outside the column and therefore
of utilizing the condenser 5 at its maximum efficiency.
FIG.6 illustrates the application of the invention to a
liquefaction apparatus having a single column 50, in which are
simultaneously effected the condensation of the refrigerant fluid
by successive fractions and the cooling and liquefaction of the gas
to be liquefied.
A compressor 51 takes the vapours of refrigerant fluid passing out
of the column 50 and directs them to a condenser 52. As in the
example of FIG.1, the partially condensed fluid is separated in a
tank 53, and then the liquid and gaseous phases are sent into
respective exchangers at the lower stage of the column 50. The same
arrangements are carried out at the following stages. Valves
regulate he flow-rate of refrigerant fluid injected into the column
at the different stages.
The pressure in the last tank but one, 54, measured by means of a
pressure-gauge 55, is associated with the valve 56 which regulates
the flow of refrigerant fluid introduced into the last tank 57. The
pressure in the tank 57, measured by a pressure-gauge 58, is
associated with the evacuation valve 59 connecting the gaseous
phase of this tank to the evacuation system. The temperature of the
refrigerant fluid in the tank 57, measured by a probe 60, is
associated with the valve 61 which regulates the flow-rate of
refrigerant fluid before injection into the last stage but one of
the column. The level in the tank 57, measured by means of a level
measurer 62 is associated with the make-up valve 63; the
temperature of the liquefied gas, measured by a probe 64, is
associated with a valve 65 which regulates the flow of the coldest
refrigerant fluid before its injection into the column; and the
pressure in the column, measured by a pressure-gauge 66, is
associated with the speed of the compressor 51.
In all the examples of application described, whether the apparatus
is of the type with two columns or a single column, the regulation
of the temperature of the liquefied gas is ensured by simple
measurements having a practically instantaneous response time.
These measurements are only effected on condition or state
parameters: pressure, temperature, volume, which are readily
measured by conventional, accurate and inexpensive devices. The
controls of the means of action, valves and compressor speed, can
be effected in any appropriate manner, by manual or automatic
methods, and advantageously by servo-controls. The problem of
leakage is solved with optimum effectiveness since the injection of
topping-up quantities is effected almost instantaneously.
It will of course be understood that the present invention is not
limited to the methods of utilization and construction described,
which are given only by way of illustrative examples.
Thus, the number of stages of the liquefaction apparatus may be
variable, and the constituents of the refrigerant fluid may be
differently chosen as a function of the nature of the gases to be
liquefied.
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