RU2482405C2 - Method for start of refrigerating circuit containing mixture of hydrocarbons - Google Patents

Abstract

FIELD: heating.

SUBSTANCE: invention relates to the method of starting a unit of natural gas liquefaction, comprising a refrigerating circuit, containing a cooling liquid, which contains a mixture of hydrocarbons, at the same time the method includes serially the following stages: (a) introduction of cleaning gas into the refrigerating circuit and discharge from it; (b) introduction of the first working gas into the refrigerating circuit and (c) introduction of the second working gas into the refrigerating circuit. The average molar mass of the first working gas is more than the average molar mass of the second working gas. The refrigerating circuit comprises a heat exchanger, having a warm input and a cold input, besides, temperature of cooling liquid at the cold input of the heat exchanger reduces at least by 30°C between the start and completion of the stage (b). The invention relates also to the appropriate method of natural gas liquefaction.

EFFECT: using the invention provides for acceleration of start process for facilitation of cooling liquid condensation in a circuit.

13 cl, 3 dwg, 1 tbl

Description

Technical field

The present invention relates to a method for starting a refrigeration circuit containing a mixture of hydrocarbons. Furthermore, the present invention relates to a method for liquefying natural gas using said starting method.

State of the art

To liquefy natural gas, cooling in a refrigeration unit to a temperature of -155 ° C during liquefaction is required, and then to -162 ° C in a tank where the gas will be stored after expansion at atmospheric pressure. As a rule, a refrigeration unit includes a refrigeration circuit (or cycle) or a sequence of refrigeration circuits (or cycles) operating according to the condensation - expansion - evaporation - compression scheme. Coolants used in refrigeration circuits can be pure substances (cascade cycle, Conoco Phillips Optimized Cascade ™ method), mixtures (Axens Liquefin ™ methods, Linde Mixed Fluid Cascade ™ (liquid mixture cascade)), or combinations thereof (cycle APCI C3-MR). All these cycles are well known to those skilled in the art.

In particular, in one or more refrigeration circuits, non-azeotropic mixtures of substances with different boiling points and, in particular, hydrocarbon mixtures (sometimes in combination with other components) are often used.

At the first start of the refrigeration unit, and also after each stop of the unit (for example, for maintenance), a special procedure for starting the refrigeration unit must be carried out. If we take for example a refrigeration unit with two cycles, that is, with a pre-cooling cycle and a cooling cycle itself, where a cooling liquid is used, consisting mainly of an ethane / methane mixture, then the usual procedure is:

- purging all circuits with a cleaning gas, which is called a “defrosting gas" and which, as a rule, is clean and dry natural gas coming from the processing and drying units located in the production cycle in front of the cryogenic units, or nitrogen, in particular, for in order to remove even traces of water in the contours;

- filling the pre-cooling circuit with a suitable coolant (for example, pure propane or ethane with propane), followed by the start of the pre-cooling cycle;

- filling the refrigeration circuit with natural gas (as a rule, “defrosting” gas) until the desired amount of methane in the circuit is reached;

- filling the refrigeration circuit with ethane until the desired amount of ethane in the circuit is reached;

- if necessary, completing the filling of the circuit by adding other minor components to the coolant.

This universally accepted startup procedure causes some difficulties. First of all, condensation of the coolant in the refrigeration cycle (circuit) usually occurs with difficulty; an increase in pressure in the circuit, which would facilitate condensation, is not additionally provided, unless the compressor is specially modified to adapt it, in addition to the normal mode, to the start mode. Thus, the work of the refrigeration cycle takes quite a lot of time. Another difficulty is that the temperature difference between the inlet and outlet of the expander can reach, for example, from 50 to 60 ° C. Such a temperature difference is too high for the mechanical strength of the parts and, in particular, for the mechanical strength of the heat exchanger, which, therefore, is at significant risk of mechanical failure.

Thus, there is a clear need to improve the starting method of the refrigeration unit, which would, in particular, consist in facilitating condensation in the circuit in order to accelerate the process and in minimizing the amplitude of the temperature drop in different parts of the expander of the refrigeration circuit.

SUMMARY OF THE INVENTION

First of all, the present invention relates to a method for starting a refrigeration unit, comprising a refrigeration circuit containing a coolant containing a mixture of hydrocarbons, where the method includes the following steps in sequence:

(a) introducing a purifying gas into the refrigeration circuit and discharging it;

(b) introducing the first working gas into the refrigeration circuit; and

(c) introducing a second working gas into the refrigeration circuit;

wherein the average molar mass of the first working gas is greater than the average molar mass of the second working gas.

In one embodiment, the refrigeration unit is a natural gas liquefaction unit.

In one embodiment, the first working gas contains at least 50 mol%, preferably at least 80 mol%, most preferably at least 90 mol%, ideally at least 95 mol% of ethane.

In one embodiment, the second working gas preferably contains at least 50 mol% of methane, more preferably at least 70 mol% or even at least 80 mol% of methane.

In one embodiment, the cleaning gas and / or second working gas is a dried natural gas with reduced acidity.

In one embodiment, the aforementioned method comprises, after step (a), and preferably after step (b), one or more steps of introducing additional working gases into the refrigeration circuit, each additional working gas preferably containing nitrogen, propane, isobutane, n-butane , isopentane, n-pentane, ethylene, propylene or mixtures thereof.

In one embodiment, the first and second working gases provide at least 50%, preferably at least 60%, even more preferably at least 70%, even at least 80% of all coolant molecules present in the refrigerant circuit at the end startup procedures.

In one embodiment, the refrigeration circuit includes a heat exchanger having a warm inlet and a cold inlet, wherein the temperature of the coolant at the cold inlet of the heat exchanger is reduced by at least 30 ° C, preferably at least 40 ° C, more preferably at least at least 50 ° C, even at least 60 ° C between the beginning and end of step (b).

In one embodiment, the refrigeration circuit includes a Joule-Thompson throttle valve comprising an inlet and an outlet, wherein the temperature difference of the coolant between said inlet and said outlet is below 40 ° C, preferably below 30 ° C, ideally below 25 ° C during the entire start-up.

In one of the embodiments, the refrigeration unit further includes a pre-cooling circuit, and the pre-cooling circuit is launched in the nominal operating mode until step (b).

In one embodiment, the refrigeration unit also includes an additional refrigeration circuit.

In one of the embodiments of the additional refrigeration circuit is a refrigeration circuit containing a coolant that contains a mixture of hydrocarbons, and the startup method includes after step (c) the following steps:

(d) introducing the first working gas into the additional refrigeration circuit; and

(e) introducing a second working gas into the additional refrigeration circuit.

In an alternative embodiment, said refrigeration circuit comprising a cooling liquid containing a mixture of hydrocarbons is the only refrigeration circuit of a refrigeration assembly.

In addition, an object of the present invention is a method of liquefying natural gas in a refrigeration unit, which includes a refrigeration circuit containing a cooling liquid that contains a mixture of hydrocarbons, the method includes starting the refrigeration unit in the manner described above, followed by cooling and liquefaction of natural gas in the specified refrigeration unit node.

The present invention overcomes the disadvantages of the prior art. More specifically, it provides a method for starting a refrigeration unit in which condensation in a cycle is easier than with a conventional method and without the need for special compressor settings. Thus, the starting method according to the present invention is faster than the conventional method. Moreover, the amplitude of the temperature drop in different parts of the expander of the refrigeration cycle is reduced, which significantly limits the risk of mechanical failure, in particular in the heat exchanger (s).

This becomes possible by reversing the filling order of the refrigeration circuit compared to the usual order, that is, with the introduction of heavy components (in particular ethane) before light components (in particular methane).

In certain specific embodiments, the present invention also has the following beneficial properties.

- The temperature difference between the inlet and outlet of the expander remains below 30 ° C, preferably below 25 ° C, even 20 ° C, during the entire launch.

- The compressor discharge temperatures at startup are lower than in the prior art, so that it becomes possible to limit the quality requirements of the metal for the compressor or to avoid the use of an intercooler.

Brief Description of the Figures

Figure 1 is a diagram of a refrigeration unit in which it is possible to produce liquefaction of natural gas to which unit the present invention can be applied.

Figure 2 is a diagram of heat transfer in the heat exchanger 2 shown in Figure 1, while the refrigeration unit is used according to the method of Example 2. The abscissa axis represents the amount of heat in MMcal / h, and the ordinate axis represents the temperature in ° C. Curve 1 represents the sum of the warm flows (incoming lines C2 and A7), and curve 2 represents the cold flow (incoming lines C5). The diagram corresponds to time t5 in example 2.

Figure 3 is a diagram of heat transfer in the heat exchanger 2 shown in Figure 1, while the refrigeration unit is used according to the method of Example 2. The abscissa axis represents the amount of heat in MMcal / h, and the ordinate axis represents the temperature in ° C. Curve 1 represents the sum of the warm flows (incoming lines C2 and A7), and curve 2 represents the cold flow (incoming lines C5). The diagram corresponds to time t5 in example 2.

Description of embodiments of the invention

In the following non-limiting description, the present invention will be described in more detail.

Method for liquefying natural gas

For clarity of presentation, in a simplified form will be described (with reference to Figure 1) node liquefaction of natural gas (refrigeration unit), allowing the implementation of the present invention. This liquefaction unit is a dual-circuit unit and includes a primary heat exchanger 1 for performing pre-cooling and a secondary heat exchanger 2 for carrying out liquefaction and supercooling. These heat exchangers can be coil-shaped heat exchangers or heat exchangers with soldered aluminum plates.

Designations starting with the letter A correspond to the elements of the natural gas circuit that is being processed at the site; the designations with the letter B correspond to the elements of the pre-cooling circuit, and the designations with the letter C correspond to the elements of the refrigeration circuit (responsible for liquefaction and subcooling).

In normal operation, natural gas, preferably pre-acidified and dried (that is, treated in such a way as to remove water and acid gases such as H ₂ S and CO ₂ ), is introduced into the assembly through a natural gas supply pipe A1 . It is pre-cooled by passing through heat exchanger 1. At the outlet of heat exchanger 1, pre-cooled (and partially condensed) natural gas is pumped through a discharge pipe A2 and introduced from below into the separation column A3 in order to remove its heaviest components (such as benzene, for example ), which otherwise may subsequently crystallize. Such heavy components are collected at the bottom of the column and removed through the intake pipe A10.

The gas leaves the column A3 through the top of the discharge pipe A4 and again passes through the heat exchanger 1 to again enter a partially condensed state. At the outlet of the heat exchanger 1, the exhaust pipe A5 sends a partially condensed gas to the chamber A6, which makes it possible to separate the liquid fraction from the gaseous one. The liquid fraction is compressed by the pump and used to irrigate the A3 column. The gaseous fraction A7 is introduced into the secondary heat exchanger 2, where it is liquefied. Liquefied natural gas is collected through the A8 outlet pipe and has a typical temperature of about -155 ° C.

Pre-cooling in the primary heat exchanger 1 is carried out using a conventional pre-cooling cycle equipped with compressors (in the embodiment presented here, the first compressor B6 and second compressor B7), a condenser B8 and expanders (in the embodiment presented here, the first expander B3 and the second expander B9 )

More precisely, this circuit includes an inlet pipe B1, which feeds the primary heat exchanger 1 with coolant. The coolant is supercooled when passing through the heat exchanger 1 (its temperature drops below the boiling point) and is collected at the outlet of the heat exchanger 1 through the outlet pipe B2; then it expands with the help of expander B3, and after expansion, the coolant is supplied to the heat exchanger 1 through the inlet pipe B4. Then, after expansion, the coolant evaporates, which is accompanied by supercooling of the compressed coolant coming from the inlet pipe B1, condensation of the fluid in circuit C and cooling of the natural gas. Evaporated coolant is collected through the exhaust pipe B5 and is compressed as it passes through two compressors B6 and B7. The compressed coolant then undergoes preliminary condensation in the condenser B8 and enters a new cycle through the inlet pipe B1.

According to the scheme presented here, we have provided a second level of evaporation, that is, for the liquid during the condensation process in the heat exchanger 1, a drain is provided. This lead returns the liquid for evaporation to the heat exchanger 1 after expansion using the expander B9; at the outlet of the heat exchanger, the evaporated liquid enters the main circuit at the level of the second compressor B7.

These two levels of evaporation were described here as an example, but the composition of the cycle (circuit) and, in particular, the number of levels of evaporation can vary depending on the decision of a specialist.

Similarly, cooling at the level of the secondary heat exchanger 2 is achieved using the refrigeration circuit C, which is a conventional circuit equipped with a compressor C7, a heat exchanger C8 and an expander C4.

More precisely, this circuit includes an inlet pipe C1, which is connected to the inlet of the primary heat exchanger 1 to provide condensation of the coolant. The liquid is collected at the outlet of the heat exchanger 1 through a connecting pipe C2, which feeds the secondary heat exchanger 2 with cooling fluid (warm inlet). The liquid is supercooled in heat exchanger 2 to the same typical temperature of about -155 ° C as natural gas, and is collected at the outlet to the SZ pipe, which feeds it to the C4 expander. At low pressure, the liquid enters the heat exchanger 2 through the inlet pipe C5 (cold inlet) and then evaporates when the liquid coming from the connecting pipe C2 is supercooled and the natural gas is liquefied. Evaporated liquid is collected through the outlet pipe C6 and is compressed as it passes through the compressor C7. The compressed liquid is then cooled in the heat exchanger C8 and returned to the cycle through the inlet pipe C1.

The expanders described above are preferably Joule-Thompson butterfly valves. The heat exchanger C8 and the condenser B8 described above can be water or air coolers.

The method of liquefying natural gas according to the present invention is characterized in that the cooling / liquefaction unit is started according to the method of starting the refrigeration unit according to the present invention, as described in more detail below.

The liquefaction method can be presented in a variety of options that are obvious to experts. Thus, many modifications or many additions known to those skilled in the art can be brought into the above-described refrigeration unit. In particular, the liquefaction method according to the present invention can be carried out in one cycle, in two cycles (as described above) or in three cycles, provided that at least one coolant used contains a mixture of hydrocarbons (including in combination with other components for example with nitrogen). As a rule, a mixture of hydrocarbons is contained in the coolant of a cold cycle, regardless of the nature of the pre-cooling cycle (circuit).

By “mixture of hydrocarbons” is meant a mixture of at least two compounds of the formula C _m H _n , where m and n are two integers. Such hydrocarbons may be saturated or unsaturated, linear or branched. Preferably m is less than or equal to 6, preferably less than or equal to 5. Preferably, the hydrocarbon mixture contains ethane and methane, which ideally comprise more than 50 mol.%, More than 60 mol.%, More than 70 mol.% Or more than 80 mol.% Of the whole mixture hydrocarbons.

In one of the embodiments of the method with one refrigeration cycle (circuit), the coolant may contain nitrogen, ethylene, propylene, methane, ethane, propane, isobutane, n-butane and, possibly, isopentane and / or n-pentane. Methane and ethane are preferably the main components.

In one embodiment of the method with two refrigeration cycles, the pre-cooling circuit coolant (the above cycle marked with the letter B) contains ethane and propane or, in another embodiment, pure propane, and the main refrigerant circuit cooling fluid (the above cycle marked with the letter C) contains a mixture of nitrogen, ethane, methane and possibly propane and / or isobutane, n-butane, isopentane, n-pentane, ethylene, propylene. Methane and ethane are preferably the main components. For example, the nitrogen content can be from 5 to 10%, the methane content is from 30 to 50%, and the ethane content is 40 to 60%.

In one embodiment of the method with three refrigeration cycles, the method uses three cycles in a cascade, wherein the coolant of each of the three cycles contains a mixture of hydrocarbons (and, in particular, may contain various of the above components). For example, the coolant of the pre-cooling circuit can be represented by a mixture of ethane and propane, while the coolant of the other two cycles (circuits), respectively, can be a mixture of methane, ethane and propane and a mixture of nitrogen, methane and ethane. In this case, the starting method described below according to the present invention can be applied to the last two refrigeration cycles.

In an alternative embodiment of the three-refrigeration process method, the first cycle coolant is propane, the second refrigerant cycle coolant contains a mixture of hydrocarbons (and, in particular, may contain various of the above components), and the third refrigerant cycle coolant contains nitrogen. For example, the second cycle coolant may contain methane, ethane and propane.

The method of starting the refrigeration unit

By “start-up method” is meant a set of operations that transfer the refrigeration unit from a stop state to a state of normal operation, that is, to a state in which the refrigeration unit is characterized by nominal operating and liquefying natural gas.

When starting up for the first time or after restarting after maintenance, water can enter various circuits. During cooling, the water freezes, in particular, inside the heat exchangers, leading to their clogging or to their deterioration due to an increase in the volume of ice compared to liquid water.

Thus, the method of starting the refrigeration unit includes, first of all, draining the circuits, that is, in the embodiment described here with two cycles, draining the natural gas circuit A, the pre-cooling cycle B and the refrigeration cycle C. For this, for example, it is possible to purge clean and dry natural gas, for example, taken from the processing and drainage units, which in the production scheme are ahead of the refrigeration unit.

This cleaning or thawing gas can, in particular, be pumped in the coldest circuit by a C7 compressor. This gas, for example, can be taken at the inlet A1 of the refrigeration unit or at the outlet of the chamber A6 (this second option is preferable in order to avoid heavy, in particular, aromatic components entering the circuits). The defrosting gas can be replaced with nitrogen, which is also suitable when it is not part of the cooling mixture. Rinsing / drying is carried out with a low gas flow rate and at low pressure: this procedure corresponds to step (a) of the method described in the brief description of the present invention. For example, the pressure in the circuits can be from 1 to 5 bar (absolute). In order to get rid of water throughout this phase, a constant supply and constant removal of the cleaning gas is carried out.

If there is a pre-cooling circuit B (as described here), after drying, they begin to fill it with a suitable coolant. The operating parameters of the pre-cooling circuit are gradually set to their nominal values, and then the circuit is considered brought into operation.

In the second phase, it becomes possible to formulate the composition of the refrigeration cooling cycle C from its various components (that is, add these components to the circuit) in order to start cooling the secondary heat exchanger 2.

To do this, in accordance with the prior art, the natural gas outlet is shut off in order to stop cleaning and start filling the circuit with this gas, which consists mainly of methane. Upon reaching the supply of methane in the circuit, the introduction of ethane and, if necessary, propane (and, in some cases, other components) is passed to obtain a cooling mixture of the composition provided for the implementation of the method.

Lastly, if necessary, an appropriate amount of nitrogen is added. Methane, which is one of the main components of both cooling liquid and defrosting gas, is considered as natural gas in the prior art according to this filling order. Components partially liquefy in the refrigeration circuit as they fill.

However, a thawing gas, mainly containing methane, is difficult to liquefy. To liquefy this gas, it is necessary either to achieve very low temperatures, impossible at the start of the cycle, or to significantly increase the pressure pumped by compressor C7. But the compressor C7 is designed to compress a mixture of hydrocarbons, the average molar mass of which is from 22 to 30, while the corresponding indicator of the “defrosting” gas is in the range from 17 to 19. In the case of centrifugal compressors, the compression ratios are proportional to the average molar mass of the compressible gas. Thus, the use of such a compressor in combination with a gas of a lower average molar mass than provided inevitably leads to a lower discharge pressure, while higher pressure is needed to condense the “defrosting” gas, which is required to cool the cycle. Hence, as indicated in the introduction, there are fundamental restrictions at the level of compressor C7, the start-up procedure is complicated, and the start-up time is increased. In addition, when liquefaction of the “defrosting” gas in the circuit is nevertheless achieved, another difficulty arises related to the expander C4: the difference between the temperatures of the liquid in the pipe C3 and in the pipe C5 (resulting from the liquid / vapor equilibrium in C5) is much higher than when working in nominal mode. This is because the “defrosting” gas is much lighter than the coolant for which the C4 expander and heat exchanger 2 are designed. As indicated in the introduction, the temperature difference in the C3 and C5 streams can reach 40, 50 and even 60 ° C. which creates temperature stresses that can damage the heat exchangers used to liquefy natural gas.

The inventors have found that the above disadvantages are associated with the fact that during the cooling phases, the average molar mass of the thawing gas is too low.

On the other hand, according to the present invention, the order in which methane and ethane are charged into the refrigeration circuit is reversed.

So, step (b) of the method according to the present invention consists in introducing the first working gas into the refrigeration circuit, and step (c) consists in introducing the second working gas into the refrigeration circuit, while the second working gas has an average molar mass lower (but not higher, as in the prior art) than the first working gas. Preferably, the average molar mass of the first working gas is higher than that of the coolant in the nominal operating mode. Preferably, the average molar mass of the second working gas is lower than that of the coolant in the nominal operating mode. Preferably, steps (b) and (c) provide more than 40 mol.% (Or more than 50 mol.%, Or more than 60 mol.%, Or more than 70 mol.%) Of the entire set of coolant molecules in the nominal operating mode. Other working gases can be introduced into the circuit to a lesser extent, either all together or separately, to complete the creation of a coolant, for example propane, isobutane, n-butane, isopentane, n-pentane, ethylene, propylene, nitrogen. Each additional introduction can be made between steps (a) and (b), between steps (b) and (c), or after step (c).

By "average molar mass" is meant the average molar mass of the various components of said liquid, calculated from the molar ratio of each of its components. For example, if a gas contains 1/3 of molecules A with a molar mass of M _A and 2/3 of molecules B with a molar mass of M _B , then the average molar mass of such a gas is (1/3) M _A + (2/3) M _B.

Preferably, the first working gas mainly consists of ethane, and the second working gas mainly consists of methane (this second gas may be natural gas or a “defrosting” gas, for example, coming from the pipe A1 or, preferably, from the outlet of the chamber A6).

Thus, according to the present invention, the gas used for cooling initially has a molar mass higher than that of the coolant in the nominal operating mode, and then the average molar mass of the coolant in the circuit decreases as methane is introduced, until the average value reaches molar mass. In this case, the previously identified deficiencies are eliminated.

Since during step (b) the coolant mainly contains components heavier than methane, for example, mainly ethane, its condensation is achieved at a lower pressure. It therefore easily condenses, and thus can more likely provide sufficient cooling by evaporation at low pressure and thus accelerate cooling. In addition, since the average molar mass is higher than in the nominal operating mode, a compressor with the same polytropic index can easily create increased pressure, which facilitates condensation. At the same time, high values of pressure pumped by the compressor are achieved more easily than in the prior art and, at the same time, the pressure required for condensation is lower. Finally, the temperature drop associated with the expansion of the coolant at valve C4 is reduced and no longer jeopardizes the mechanical integrity of the heat exchangers.

Reversing the order of introduction of the components of the coolant thus provides advantages for the operation of the compressor and heat exchangers.

Limiting the level of compression by introducing ethane provides another advantage for the compressor. It is known that the higher the level of compression, the more significant the increase in temperature during compression. The usual procedure, by using a “defrosting” gas as a means to lower the temperature in the gas liquefaction unit, makes it particularly difficult to pressurize it partially to condense. Such high pressure leads to particularly high outlet temperatures, which can cause stresses in the compressor metal or require an additional cooler to limit the final outlet temperature. In any case, additional costs can be eliminated thanks to the startup method according to the present invention.

The method described above for a refrigeration unit with two cycles, the specialist can easily adapt to the unit with one cycle or with three cycles. So, in the case of a single-cycle refrigeration unit with a mixture of hydrocarbons, the present method consists in the implementation of the above steps (a), (b) and (c) without prior cooling of the pre-cooling circuit. In contrast, in the case of a unit with three cycles, the cycles are cooled sequentially in the order of decreasing their nominal temperatures: each cycle containing a cooling liquid based on a mixture of hydrocarbons (or, more generally, on the basis of a non-azeotropic mixture of components with different boiling points) can be cooled in in accordance with the above sequence of steps (a), (b) and (c).

Examples

The following examples merely illustrate the present invention without limiting it.

Example 1. Coolants containing a mixture of hydrocarbons

The following are examples of known methods for liquefying natural gas using coolants based on mixtures of hydrocarbons. Each of these methods can be carried out by the method according to the present invention. Of course, the following formulations are typical formulations, which may be modified depending on the specific application conditions (nature of the liquefied gas, environmental conditions, etc.).

1) APCI method. This method includes two cycles: a pre-cooling cycle with propane and a second liquefaction and supercooling cycle. The coolant of the second cycle has the following composition:

- nitrogen: 5.83 mol.%;

- methane: 44.28 mol.%;

- ethane: 37.30 mol.%;

- propane: 21.59 mol%.

The second cycle of the method can be started according to the above sequence of steps (a) to (c).

2) The Linde method. This method includes three cycles, i.e., a pre-cooling cycle, a liquefaction cycle, and a sub-cooling cycle. The subcooling cycle coolant has the following composition:

- nitrogen: 8.93 mol.%;

- methane: 54.88 mol.%;

- ethane: 36.19 mol.%.

The subcooling cycle can be started according to the above sequence of steps (a) to (c).

3) The method of the connected cascade. In this method, one cooling cycle. This method belongs to the same group as the Prico ™ method. The coolant in this case has the following composition:

- nitrogen: 4.67 mol.%;

- methane: 29.17 mol.%;

- ethane: 34.00 mol.%;

- propane: 11.09 mol.%;

- isobutane: 4.58 mol.%;

- n-butane: 2.25 mol%;

- isopentane: 6.01 mol.%;

- n-pentane: 8.23 mol.%.

A single cycle can be started according to the above sequence of steps (a) to (c).

Example 2. An example of starting a refrigeration unit according to the present invention

This example is based on the refrigeration unit shown in FIG. 1.

They start by cleaning all the circuits with natural gas, or “defrosting” gas coming from the outlet of chamber A6, the composition of which is as follows:

- nitrogen: 4.4 mol.%;

- methane: 87.8 mol%;

ethane: 5.3 mol%;

propane: 1.8 mol%;

- butane: 0.7 mol%.

Then cycle B (pre-cooling cycle), which is the ethane / propane cycle, is put into operation and cooled to approximately -65 ° C.

Then they proceed to start cycle C (refrigeration circuit, corresponding here to the liquefaction and supercooling circuit), where the coolant must have the following final composition:

- nitrogen: 7.5 mol.%;

- methane: 42.5 mol.%;

- ethane: 45 mol.%;

- propane: 5 mol.%.

To do this, the outlet of the “defrosting” gas is blocked, then the components of the coolant are introduced in the following order: ethane, propane, again natural gas (“defrosting” gas) and, finally, nitrogen.

Table 1 below summarizes the change in cycle C parameters during startup. The designations t1-t6 correspond to six consecutive moments during this run. T1 denotes the temperature at the cold end of heat exchanger 2. P1 is the pressure pumped by compressor C7 in absolute bars. T2 represents the discharge temperature of compressor C7. % N ₂ indicates the percentage of nitrogen in the loop. % C ₁ indicates the percentage of methane in the loop. % C ₂ indicates the percentage of ethane in the loop. % C ₃ indicates the percentage of propane in the loop. % C ₄ indicates the percentage of butane in the loop. ΔT represents the largest temperature difference in the heat exchanger 2. Finally, for comparison, ΔT 'represents the largest temperature difference in the heat exchanger 2, which would have been achieved at the same T1, if the prior art procedure was used, which consists in filling the circuit with a “defrosting” gas before injection ethane.

Table 1 Starting the refrigeration cycle t1 t2 t3 t4 t5 t6 T1 -55 ° C -75 ° C -100 ° C -120 ° C -140 ° C -155 ° C P1 3 bar 9 bar 11.9 bar 12.6 bar 13.4 bar 56 bar T2 139 ° C 157 ° C 160 ° C 163 ° C % N ₂ 4.4% 3.1% 2.5% 2.5% 2.4% 7.5% % C ₁ 87.8% 60.1% 50.5% 48.6% 46.6% 42.5% % C ₂ 5.3% 35.1% 45.5% 47.6% 49.7% 45% % C ₃ 1.8% 1.3% 1.1% 1,0% 1,0% 5% % C ₄ 0.7% 0.5% 0.4% 0.4% 0.4% 0,0% ΔT 18 ° C 23 ° C 21 ° C 23 ° C 12 ° C ΔT ' 58 ° C 60 ° C 61 ° C 60 ° C 12 ° C

The moment t1 corresponds to the beginning of the start of the cycle. The composition corresponds to the “defrosting” gas, which was used to clean the circuit. The pressure on the compressor P1 is very low, on the one hand, so as not to initially inject a lot of methane, and on the other hand, because too high pressure is not necessary to clean the equipment with gas. Ethane is then added to lower the temperature T1. Since the defrosting phase is carried out at low pressure, the mass content of the “defrosting” gas is negligible, and the addition of ethane to the final norm allows you to quickly increase the average molar mass of the coolant.

At time t5, the temperature reaches -140 ° C and the ethane content exceeds the corresponding indicator when working in normal mode. A pressure of 13.4 bar (absolute) is now sufficient to condense the coolant, while the compressor in normal mode is able to pump a pressure of 56.4 bar (absolute) when using gas with a lower average molar mass at the same suction pressure.

It should be noted that if the start-up was carried out with a “defrosting” gas, the discharge temperature of the compressor 12 could reach or even exceed 200-250 ° C, since high compression would be required to condense such gas.

The butane present in the circuit is introduced with a “defrosting” gas and diluted as the content of ethane, propane and nitrogen increases to a small concentration.

At each of the stages from t2 to t5, the gas content in the circuit increases due to ethane intake. From t5 to t6, first the remainder of ethane is added, then propane, again natural gas (thawing gas) and nitrogen until the prescribed value of each component is reached.

Figures 2 and 3 are diagrams of heat transfer in a heat exchanger 2, respectively, for the above steps t2 and t5. It can be noted that the temperature difference does not exceed twenty degrees, that is, it is acceptable for any cryogenic heat exchangers, and during the start-up procedure from the prior art, the temperature difference exceeds 50 ° C, while the maximum usually allowed by the developers is 30 ° C.

Claims (13)

1. The method of starting the liquefaction unit of natural gas containing a refrigeration circuit containing a coolant that contains a mixture of hydrocarbons, the method includes sequentially the following steps:
(a) introducing a purifying gas into the refrigeration circuit and discharging it;
(b) introducing the first working gas into the refrigeration circuit; and
(c) introducing a second working gas into the refrigeration circuit;
wherein the average molar mass of the first working gas is greater than the average molar mass of the second working gas,
where the refrigeration circuit includes a heat exchanger having a warm inlet and a cold inlet, and the temperature of the coolant at the cold inlet of the heat exchanger is reduced by at least 30 ° C between the beginning and end of step (b). 2. The method according to claim 1, where the first working gas contains at least 50 mol.%, Preferably at least 80 mol.%, Most preferably at least 90 mol.%, Ideally at least 95 mol.% Ethane. 3. The method according to claim 1 or 2, where the second working gas preferably contains at least 50 mol% of methane, more preferably at least 70 mol% and even at least 80 mol% of methane. 4. The method according to claim 1, where the cleaning gas and / or the second working gas are dried natural gas with reduced acidity. 5. The method according to claim 1, comprising after step (a) and preferably after step (b) one or more steps of introducing additional working gases into the refrigeration circuit, each additional working gas preferably containing nitrogen, propane, isobutane, n-butane , isopentane, n-pentane, ethylene, propylene or mixtures thereof. 6. The method according to claim 1, where the first and second working gases provide at least 50%, preferably at least 60%, even more preferably at least 70%, even at least 80% of the molecules of the coolant present in the refrigerator circuit at the end of the startup procedure. 7. The method according to claim 1, where the refrigeration circuit includes a heat exchanger having a warm inlet and a cold inlet, and the temperature of the coolant at the cold inlet of the heat exchanger is reduced by at least 40 ° C, more preferably at least 50 ° C, even at least 60 ° C between the beginning and end of stage (b). 8. The method according to claim 1, where the refrigeration circuit includes a Joule-Thompson throttle valve having an inlet and an outlet, the temperature difference of the coolant between said inlet and said outlet being less than 40 ° C, preferably less than 30 ° C, ideally less than 25 ° C during the entire launch method. 9. The method according to claim 1, where the natural gas liquefaction unit further includes a pre-cooling circuit and where the pre-cooling circuit is started in the nominal operating mode prior to step (b). 10. The method according to claim 1, where the node liquefaction of natural gas also includes an additional refrigeration circuit. 11. The method according to claim 10, where the additional refrigeration circuit is a refrigeration circuit containing a coolant that contains a mixture of hydrocarbons, and the startup method includes after step (c):
(d) introducing the first working gas into the additional refrigeration circuit; and
(e) introducing a second working gas into the additional refrigeration circuit. 12. The method according to claim 1, where the specified refrigeration circuit containing a coolant that contains a mixture of hydrocarbons, is the only refrigeration circuit of the natural gas liquefaction unit. 13. A method of liquefying natural gas in a node for liquefying natural gas, which contains a refrigeration circuit containing a coolant that contains a mixture of hydrocarbons, where the method includes:
- launching a natural gas liquefaction unit according to any one of claims 1-12, after which
- cooling and liquefaction of natural gas in the specified node liquefaction of natural gas.

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