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.

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.

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.