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Title:
PLANT FOR THE LIQUEFATION OF METHANE WITH RELATIVE PROCESS CONTROL SYSTEM
Document Type and Number:
WIPO Patent Application WO/2018/173082
Kind Code:
A1
Abstract:
Plant for the liquefaction of natural gas, comprising a unit (S1A,S1B) for compression of the natural gas; a section (S2) for cooling of the natural gas; and a circuit (S3) for liquefaction which comprises a main line (l)-(2)-(3) traversed by a flow (ml) of gas and whereon a heat exchanger (H-EX) is installed; a secondary line (4) -(5) -(6) which branches off from the main line (l)-(2)-(3) and which comprises a valve (J-T Valve 2) through which a flow (m3) of gas is first laminated to a lower pressure (P3) and then led to traverse said heat exchanger (H-EX), cooling there a transiting flow (m2), difference between (ml) and (m3). Means of regulation and control are provided of the working conditions of subcooling of the flow (m2) of fluid which include a first temperature control valve (TC-Control Valve 1) which regulates the flow (m2) of fluid transiting along said main line (l)-(2)-(3), in relation with the final temperature (T2) required for said fluid, and a second pressure control valve (PC-Control Valve 2) which is installed on the secondary line (4) -(5) -(6) downstream of the valve (J-T Valve 2) and which regulates said secondary flow (m3) on the basis of a pressure (P3) detected on said secondary line (4) -(5) -(6) downstream of said valve (J-T Valve 2).

Inventors:
BARTOLINI CARLO MARIA (IT)
SANTOLINI MATTEO (IT)
SPITONI MARCO (IT)
Application Number:
PCT/IT2018/000039
Publication Date:
September 27, 2018
Filing Date:
March 20, 2018
Export Citation:
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Assignee:
S TRA TE G I E S R L (IT)
International Classes:
F25J1/00; F25J1/02
Foreign References:
US20060213223A12006-09-28
US20060218939A12006-10-05
US20120036888A12012-02-16
FR2848650A12004-06-18
US20120079850A12012-04-05
US6553772B12003-04-29
EP3045849A22016-07-20
Attorney, Agent or Firm:
PALESTRINI, Mauro (Via A. Panzini 2, Senigallia, IT)
Download PDF:
Claims:
CLAIMS

1. Plant for the liquefaction of natural gas, comprising a unit (S1A, SIB) for compression of the natural gas; a section (S2) for cooling of the natural gas; and a circuit (S3) for liquefaction which comprises a main line (l)-(2)-(3) traversed by a flow (ml) of gas and whereon a heat exchanger (H-EX) is installed; a secondary line (4) -(5) -(6) which branches off from the main line (l)-(2)-(3) and which comprises a valve (J-T Valve 2) through which a flow (m3) of gas is first laminated to a lower pressure (P3) and then led to traverse said exchanger (H-EX) , cooling there a transiting flow (m2), difference between (ml) and (m3) , plant characterised in that said compression unit is divided into two parts (S1A,S1B, in a first of which a flow (L2) of gas at low pressure recirculating from the process of liquefaction is brought to a pre-established pressure level for the entry of the natural gas in the plant, in the second part said recirculation flow being mixed with a flow of natural gas and then compressed together with it up to the maximum pressure foreseen for the working conditions of said plant; and in that said plant comprises means of regulation and control of the working conditions of subcooling of the flow (m2) of fluid which include a first temperature control valve (TC-Control Valve 1) which regulates the flow (m2) of fluid transiting along said main line (l)-(2)-(3), in relation with the final temperature (T2) required for said fluid, and a second pressure control valve (PC- Control Valve 2) which is installed on the secondary line (4) -(5) -(6) downstream of the valve (J-T Valve 2) and which regulates said secondary flow (m3) on the basis of a pressure (P3) detected on said secondary line (4)- (5)-(6) downstream of said valve (J-T Valve 2).

2. Plant according to claim 1, characterised in that said cooling section comprises a section (S2) of precooling wherein the methane is cooled before being subjected to the cryogenic process.

3. Plant according to claim 2, characterised in that said section (S2) of cooling comprises a chiller via which the methane cooled in said precooling section is further cooled to a lower temperature level.

4. Plant, according to any one of the preceding claims, characterised in that said liquefaction circuit is divided into a plurality of sections, in at least one of which said means being provided for regulation and control of the subcooling conditions.

Description:
PLANT FOR THE LIQUEFATION OF METHANE WITH RELATIVE PROCESS CONTROL SYSTEM

Technical Field

The present invention relates to a plant for the liquefaction of methane, in particular small in size, with relative process control system.

Background Art

The technology of liquefaction of methane gas allows a reduction in the specific volume of the natural gas by about 600 times in physical conditions defined as standard, allowing reduced costs for storage and the transport of the abovementioned fluid.

The transport of LNG (liquefied natural gas) takes place mainly via sea, by means of methane tankers or via road, by trucks, while the use of tank-containers for rail transport is becoming increasingly widespread.

The final use is always in any case gaseous and therefore subsequently the LNG will then be returned to the gaseous state in the appropriate regasification stations, at different pressures. High, if it is to be fed into the gas network; lower at the time of its direct use for combustion, both on fixed plants and for feeding means of transport.

The technologies used to date for obtaining LNG vary above all with the size of the liquefaction plant, on the basis of which the technical and economic requirements also change. For plants for large-scale production, plant engineering solutions are adopted that are not suitable for small scale, where the initial cost is of very high importance and therefore where simple solutions are preferred yet which have to maintain adequate standards in relation to adequate values of efficiency, safety and functional reliability.

The field of use of the small-scale plants for liquefaction of gas (micro-scale plants) is mainly: generation of LNG destined for the network of vehicles supply or for storage at large users both civil (municipalities or housing developments) or industrial, with gas coming from a fossil origin or from biogas after appropriate treatment which makes it pure methane; and • re-liquefaction of LNG evaporated in storage depots (boil-off gas) .

The reference thermodynamic cycles are the Linde cycle and the Claude cycle. From them all the main thermodynamic cycles used are obtained, also in the larger plants, in the form of varyingly radical variations of these two types of cryogenic cycles.

More particularly, the Linde cycle is found to be preferable compared to the Claude one, in that simpler from the plant engineering viewpoint and more economical from the viewpoint of the initial cost.

Figure 1 illustrates the basic diagram of the Linde cycle widely used for the liquefaction of air and subsequently for the liquefaction of many gases - a cycle which is of specific interest for the present invention. Said cycle provides for the gas, broadly, to auto- refrigerate. In fact, after having been compressed by means of a compressor, the gas traverses a series of heat exchangers, in sequence: an intercooler (cooling exchanger of the compressed gas) ; a chiller (precooling by means of external unit); and a regenerative heat exchanger, inside of which in fact the gas is cooled by the return of a part of the same gas after its cooling performed in particular by means of a valve, known as Joule-Thomson or lamination valve.

Placed immediately after the regenerative heat exchanger, the Joule-Thomson valve causes a strong drop in pressure which leads to the expansion of the gas almost instantaneously. This process, known as lamination, generates a very high enthalpic drop and the gas cools in a manner directly connected thereto.

The fluid is sent into a tank, where the possible gaseous fraction present is made to return to the entry of the circuit just described (or processed directly with an appropriate auxiliary plant) . The two-phase gas-liquid fluid is conveyed then into a liquid separator where the liquid fraction is separated, while the gas fraction is made to turn back, to the entry of the circuit just described.

As mentioned, the passage of the gas in the regenerative heat exchanger cools the flow before the Joule-Thomson valve, reducing the input enthalpy value and therefore encouraging a cyclical reduction in the temperature in output .

In general, it is possible to state that, the more the gas is cooled before lamination, the greater the fraction of liquid produced will be.

The known solution described above is generally used as a basis for small-scale applications which provide for productions no greater than approximately 50-60 m3/day. There are however many variations proposed which allow an increase in the overall efficiency in terms of flow of LNG produced per k h consumed. These solutions can relate both to the process of compression and the cryogenic process in general.

Currently, most of the processes operating according to the LINDE cycle use vapor-liquid separators to divide the recirculation flow from the main one.

A fundamental disadvantage of this process is however the different chemical composition of the two flows. In fact in the vapor part low-boiling fluids will be present which in natural gas are represented by nitrogen and by hydrogen, with the effect of increasing gradually the concentration of these components in the circuit.

If the concentration increases beyond limit levels depending on the working parameters of the circuit, it is necessary to perform a removal of the nitrogen, which can be carried out by means of:

flash of the LNG for concentrations of approximately 1-2%, by means of suitable techniques of partial evaporation,

• stripping of the nitrogen for higher concentrations, by means of classic techniques of transfer of the gas dissolved in the liquid from the liquid phase to the gas one.

A high quantity of nitrogen, as well as reducing the quality of the LNG, can cause the onset of roll-over in the tanks, which may lead to an anomalous and dangerous increase in pressure.

Disclosure of the Invention

The object of the present invention is that of avoiding these disadvantages, in an effective, safe, simple and economical manner by means of splitting of the recirculation flows in conditions of overheated vapor and of undercooled liquid according to the various stages whereby the process of liquefaction of the methane gas is performed.

In accordance with the invention a plant for the liquefaction of natural gas is proposed as defined in any one of the appended claims.

Brief Description of the Drawings

The features and the advantages of the invention will be made clearer with reference to an embodiment illustrated in the accompanying drawings, in which: Figure 1 is a diagram of a simple cycle of liquefaction of methane gas in accordance with the prior art;

Figure 2 is a diagram of the entire liquefaction plant, in accordance with the present invention;

Figure 3 is a detail of a section of the plant of Figure 2;

Figure 4 is a partial representation of the plant of Figure 3 wherein a particular system of control of the process of liquefaction is highlighted.

Detailed Description of Preferred Embodiment of the Invention

Referring to the figures of the accompanying drawings, Figure 2 illustrates a methane natural gas liquefaction (LNG) plant which, downstream of the appropriate purification processes as a function of its nature and not shown in the diagram, is made up mainly of three sections denoted by reference numerals (SI), (S2) and (S3) respectively indicating a compression unit, a precooling section and a circuit of liquefaction of the methane gas.

More particularly the compression unit (SI) is divided into two parts (S1A, SIB) , in a first (S1A) whereof a precompression phase is carried out wherein the low- pressure recirculation gas (L2) coming from the process of liquefaction is brought to the pressure of feeding of the methane in input to the plant. In the second part (SIB) a second phase is carried out, of compression, wherein two flows of fluid, respectively recirculated and in input to the plant, are first mixed and then compressed by the pressure of feeding up to the maximum pressure provided for the working conditions of the plant .

In the section (S2) of precooling the methane is cooled before being subjected to the cryogenic process. The process of precooling, which is of fundamental importance for the efficiency of the plant, takes place in three distinct and successive steps. In a first step the cooling is performed by the recirculation gases of high (H) and medium (M2) pressure, coming from the liquefaction process. The second cooling (precooling) is instead carried out by means of a chiller capable of bringing the methane to an output temperature considerably lower than its input temperature. In a third step the cooling is completed by the recirculation gases of medium (Ml) and low (L) pressure. The third step of cooling is important to allow a reduction in the load of the chiller which could not reach certain temperatures unless at the expense of a loss of efficiency. Similarly, the first step is important for the purpose of energy efficiency of the process of liquefaction.

In the section (S3) the natural gas, already precooled, enters in fact the circuit of liquefaction, exiting therefrom in the liquid state like the LNG.

In this section (S3) the process at the basis of the cryogenic cycle [which can be linked to the Linde thermodynamic cycle of Figure 1] is repeated several times: the precooled methane enters as high-pressure and low-temperature gas and is laminated several times. At each step of lamination and through the effect of the changes in pressures whereto it is subjected, the methane is cooled by the return lines (the gas auto-refrigerates) and subsequently laminated again. The repetition of the process, which can also be triplicated, has the function of making the system more efficient. This is obtained in particular by means of division of the flows by means of a flow splitters (SPL2,SPL3), lamination through a series of pressure changes performed by means of Joule- Thomson lamination valves (J-T valve 1; J-T valve 2; J- T valve 3; J-T valve 4; J-T valve 5) and heat exchanges performed through heat exchangers (H-EX 1; H-EX 2; H-EX 3) -

Figure 3 schematizes the part of plant for the process of liquefaction which is more particularly involved by the present invention.

Referring to this drawing, at point (1) the methane flows, coming from the precooling section (S2) .

The line formed by points (l)-(2)-(3) is identified as main line. Starting from point (1), a flow splitter (SPL) is first encountered. This component allows a secondary line, identified by points (4) -(5) -(6), to be separated from the main line. The secondary line performs cooling of the main line (which will then give generation of LNG) . From the splitter (SPL), the gas (4) is laminated by means of the lamination valve (J-T Valve 2) . Due to this lamination the gas in (5) will have a much lower temperature with respect to the gas at point (2) (unchanged, instead, with respect to its starting conditions). In the heat exchanger (H-EX), therefore, the gas traversing the main line (l)-(2)-(3), cooled by the secondary line (4) -(5) -(6), goes to point (3), while the gas at point (6), after the heat exchange, returns to the compression unit (SI) in the form of low or medium-pressure recirculation. From point (3), the fluid will then be subsequently laminated, and this process, repeated several times, will thus allow LNG to be obtained .

As shown in Figure 2, the plant numbers three splitters (SPL1-SPL2-SPL3) and the same number of exchangers (H- EX1; H-EX2; H-EX3) of heat and recirculation flows.

The efficacy of the process means LNG is available already at the output of the first exchanger (H-EX1 ) . This process not only allows liquefaction of the methane, but allows it to be cooled to the point that the LNG produced is undercooled. This means that the liquid obtained is colder than is necessary for the purpose of liquefaction. In other words, at the time when inside a cryogenic skid there is the formation of liquid. This will be 100% liquid, without fractions of vapor which would make other members necessary, complicating the plant .

Since this is a plant of small size, the role of subcooling for this type of process is very important, in that it allows the cryogenic process to be simplified and at the same time production efficiency to be increased .

If the set-up is as shown in Figure 4.

on the main line (l)-(2)-(3) a system of direct control of the temperature comprising a relative sensor (T2) located on the section (3) of the main line (1)- (2) -(3) and a control valve (TC-Valve 1) piloted by it and placed downstream of the flow splitter (SPL) on the section (2) of the main line; and

on the secondary line a pressure control system which provides, downstream of the valve (J-T Valve) , a control valve (PC Control Valve) piloted by a pressure sensor (P3), it is possible to control the cryogenic process with high efficiency and accuracy.

This can be fully understood with the aid of Figure 4 if we consider a main line (l)-(2)-(3) with initial conditions of flow rate, temperature and pressure pre- established and equal respectively to (ml), (Tl) and (PI) ·

By deviating with a split (SPL) a secondary line (4 ) - (5) -(6), from the main one (l)-(2)-(3), there will be two lines with flow rates respectively equal to (m2 ) and (m3) . In order to ensure the subcooling of the liquid fraction it is necessary for the temperature (T2) of the main line (l)-(2)-(3) at the outlet of the exchanger (H- EX) to remain as unchanged as possible.

Basing on what is said, if this temperature value (T2) were to increase, for any cause whatsoever, there would be unfavorable thermodynamic conditions: a subsequent lamination would have as a result a mixture of vapor and liquid instead of a saturated liquid. Moreover at the time when the temperature (T2) were to rise in an unexpected manner, there would be an imperfect heat exchange in (H-EX) . In other words, the secondary line (4) -(5) -(6) would not find itself cooling in an appropriate manner the primary line (l)-(2)-(3).

The problem is solved with the insertion of a temperature control valve (TC-Control Valve 1) on the main line (1)- (2) -(3). This valve, (TC-Control Valve 2), recognizing an increase in the temperature (T2), will throttle the flow with flow rate (m2) of the main line (l)-(2)-(3) in a progressive manner. By reducing (m2), the second flow with flow rate (m3) which traverses the secondary line (4) -(5) -(6) will consequently find itself cooling a smaller flow, in this way allowing the temperature (T2) to return again to its design conditions.

In parallel, for the same reason, on the derived secondary line (4) -(5) -(6), there will be a simultaneous increase in the flow rate (m3) given that (ml) remains constant and the sum of the flow rates (m2) and (m3) is equal to (ml) . The increase in the flow rate (m3) will go to influence the pressure (P3) . In fact, by increasing the flow - and the area of passage imposed by the valve (J-T Valve) remaining always constant - the fluid will undergo an increase in the velocity. With the increase in the velocity of the fluid the load losses, or pressure losses, increase, through the same lamination valve (J- T Valve) . Given that the pressure upstream of the lamination valve (J-T Valve) remains unchanged and equal to (PI), this entails that the pressure (P3) will decrease. In other words, the lamination valve (J-T Valve) will subject the fluid subsequently transiting to a greater pressure change with respect to the previous one and, consequently, also the temperature (T3) will decrease, sending once again out of control the heat exchange in (H-EX) .

This occurrence, however, does not come about because the system reacts through the pressure control valve (PC Control Valve 2) in the secondary line (4)-(5)-(6). This valve (PC Control Valve 2) in fact, going to control the pressure (P3) , will open at the time when it will detect a decrease in the magnitude controlled. The pressure (P3) will therefore once again rise, and the same thing will occur for (T3) and therefore the heat exchange in (H-EX) will return once again to the rated working conditions .

Definitively, the system described allows controlling in an efficient and punctual manner that the final temperature (T2) remains unchanged in time. In order to do this two control valves are used (TC-Control Valve 1, PC-Control Valve 2), one temperature and one pressure. The first (TC-Control Valve 1), installed on the main line (l)-(2)-(3), allows the regulation of the main flow rate(m2) on the basis of the temperature (T2), while the second one, installed on the secondary line (PC-Control Valve 2), allows the regulation of the secondary flow rate (m3) on the basis of the pressure (P3), balancing in fact the effect of the first regulation.

This control system enables regulation of the flow allowing, due to its nature, instantaneous resetting of the working conditions needed for subcooling.

The invention devised in this way is subject to evident industrial application. It can likewise be the subject of numerous changes and variants, all coming within the scope of the following claims.

All the details can be replaced, moreover, by technically equivalent elements.