Patent ID: 12203875

DESCRIPTION OF EMBODIMENTS

The disclosure relates to an apparatus and method for detecting solids formation, in particular an apparatus and method for detecting solids formation in cryogenic heat exchangers, such as cryogenic heat exchangers used for LNG production. The disclosure also relates to a system and method for remediating blockages in cryogenic heat exchangers.

General Terms

Throughout this specification, unless specifically stated otherwise or the context requires otherwise, reference to a single step, composition of matter, group of steps or group of compositions of matter shall be taken to encompass one and a plurality (i.e. one or more) of those steps, compositions of matter, groups of steps or groups of compositions of matter. Thus, as used herein, the singular forms “a”, “an” and “the” include plural aspects unless the context clearly dictates otherwise. For example, reference to “a” includes a single as well as two or more; reference to “an” includes a single as well as two or more; reference to “the” includes a single as well as two or more and so forth.

Each example of the present disclosure described herein is to be applied mutatis mutandis to each and every other example unless specifically stated otherwise. The present disclosure is not to be limited in scope by the specific examples described herein, which are intended for the purpose of exemplification only. Functionally-equivalent products, compositions and methods are clearly within the scope of the disclosure as described herein.

The term “and/or”, e.g., “X and/or Y” shall be understood to mean either “X and Y” or “X or Y” and shall be taken to provide explicit support for both meanings or for either meaning.

Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

The term “about” as used herein means within 5%, and more preferably within 1%, of a given value or range. For example, “about 3.7%” means from 3.5 to 3.9%, preferably from 3.66 to 3.74%. When the term “about” is associated with a range of values, e.g., “about X % to Y %”, the term “about” is intended to modify both the lower (X) and upper (Y) values of the recited range. For example, “about 20% to 40%” is equivalent to “about 20% to about 40%”.

Specific Terms

A solid is a fundamental state of matter in which atoms or molecules are closely packed together in a regular geometric lattice (crystalline solids) or irregularly (amorphous solids) and contain the least amount of kinetic energy. The term ‘solids formation’ as used herein refers to solids arising from a phase transition, such as a gas to solids phase transition or a liquid to solids phase transition, whereby the phase transition is caused by a change in pressure and/or temperature, in particular a decrease in temperature.

The phrase “directly detect solids formation” as used herein refers to the ability to sense or measure a change in a physical property caused by the presence of solids arising from a phase transition. Direct detection of solids formation may be distinguished from indirect detection of solids formation whereby an apparatus, instrument or sensor is capable of measuring or sensing a physical property, such as temperature or pressure, at which solids are predicted or anticipated to form as a result of phase transition.

The term “cryogenic” when used herein to describe a heat exchanger refers to heat exchanger configured to operate at very low temperatures (from about −140° C. and below) for the purposes of liquefying one or more gases. Such cryogenic heat exchangers are commonly used for liquefied natural gas (LNG) production, and production of liquid gases, such as liquid nitrogen, liquid oxygen, liquid hydrogen, liquid helium and liquid argon.

The term “fluid” as used herein refers to a homogenous gas mixture or a homogenous liquid mixture, optionally with one or more trace impurities. A “trace impurity” is a compound having a concentration in the fluid of less than or equal to 3000 ppm.

The trace impurity may be a “freezable compound”, in other words a compound capable of forming a solid at a higher temperature than a phase transition temperature of the fluid. Examples of freezable compounds in a fluid such as natural gas include, but are not limited to, water, sour gases such as carbon dioxide and hydrogen sulphide, carbon disulphide, carbonyl sulphide, mercaptans (R—SH, where R is an alkyl group having one to 20 carbon atoms), sulphur dioxide, aromatic sulphur-containing compounds, and heavy hydrocarbons including aromatic hydrocarbons such as benzene, toluene, xylene, naphthalenes, and so forth. The term ‘heavy hydrocarbon’ as used herein may be referred to by the symbol ‘C5+’ and refers to hydrocarbons having a carbon chain or skeleton of five (5) or more carbon atoms. Other examples of freezable compounds may include waxes.

It will also be understood by those skilled in the art that two or more compounds may potentially combine to form a clathrate which may be also be a “freezable compound”. For example, methane hydrate is a clathrate comprising methane and water which solidifies at a higher temperature that the solids formation temperature of methane or water.

The term “bulk fluid” as used on its own or in conjunction with “composition” or “properties” refers to the fluid excluding the one or more trace impurities or the composition or properties of the fluid in the absence of the one or more trace impurities.

The term “operating temperature margin” as used herein with respect to a cryogenic heat exchanger refers to a permitted variance in temperature at which the cryogenic heat exchanger may be operated. The permitted variance may vary according to the size and type of cryogenic heat exchanger and the bulk fluid composition of the fluid cooled in the cryogenic heat exchanger, and may be predetermined with a thermodynamic simulation program for solid liquid equilibrium (SLE) calculations (as will be described below).

The term ‘remedial fluid’ as used herein may refer to a gas or a liquid comprising a compound or a mixture of two or more compounds capable of removing the solids deposited in the cryogenic heat exchanger. The one or more compounds in the remedial fluid may be capable of increasing the solubility of the solids in the fluid passing through the cryogenic heat exchanger by varying the composition of the fluid. Alternatively, the one or more compounds in the remedial fluid may be capable of moving the conditions of solid-liquid equilibrium in the cryogenic heat exchanger such that the solids undergo a solid-liquid or solid-gas phase transition. In some examples, the one or more compounds may be hydrocarbon compounds.

Apparatus to Directly Detect Solids Formation

Embodiments described herein generally relate to an apparatus to directly detect solids formation in a fluid. In particular, various embodiments relate to an apparatus configured to detect solids formation in a fluid comprising freezable compounds.

While the disclosure is made in the context of LNG production, it will be appreciated that the disclosure has general application in the cryogenic production of other gases and liquids where it is undesirable for impurities in said fluids to form solids. Other examples where the apparatus as described herein may have a general principle of application include, but are not limited to, cryogenic production of liquid gases such as nitrogen, argon, hydrogen or helium; determination of hydrate formation temperatures in gas (and oil) production systems; determination of wax formation temperatures in gas (and oil) production and so forth.

Referring toFIGS.3a-3d, where like numerals refer to like features throughout, there are shown various embodiments of an apparatus10for directly detecting solids formation. The apparatus10includes a cylindrical pressure vessel12comprising a lower portion14sealingly coupled to an upper portion16with a plurality of fasteners18and a sealing member20, such as an O-ring. In use, the lower and upper portions14,16define an electromagnetic resonant cavity22. The electromagnetic resonant cavity22operates at frequencies up to and including microwave frequencies, with resonant properties sensitive to the presence of a solid phase as will be described later.

With regard to the electromagnetic resonant cavity22, the lower portion14defines a cylindrical side wall24and a sloping bottom wall26terminating in a co-axially aligned flat-bottomed well28. The upper portion16defines an annular top wall30and a conical protrusion32extending into the cavity22terminating in a flat surface34co-axially aligned with the flat-bottomed well28, with a gap36therebetween of 2 mm or less. It will be appreciated that the size of the gap36is selected to be a balance between sensitivity (which is enhanced by a smaller gap) and robustness in operation. Too small a gap may become fouled or blocked by small solid metal particles that may have passed through upstream filters. Additionally, the stability of the baseline frequency (i.e. frequency under vacuum conditions) of the resonant cavity22may become adversely affected by vibrations or drift in the gap dimension over time. In one embodiment, the gap36may be from 0.1 mm to 2 mm.

Freezing is a stochastic phenomenon which may be manifest by thermal and compositional driving forces as well as the presence of favourable nucleation sites. In various embodiments, the resonant cavity22is configured to favour solids formation in the well28. For example, the respective angles of inclination of the sloping bottom wall26and the conical protrusion32are arranged to promote the flow of fluids towards the well28. Similarly, respective joins between the cylindrical side wall24and the sloping bottom wall26of the lower portion14and the annular top wall30and the conical protrusion32of the upper portion16may be curved or bevelled to avoid stagnant regions and promote flushing of the resonant cavity22. It will also be appreciated that respective surfaces of the sloping bottom wall26and the conical protrusion32may be highly polished to deter solids formation thereon.

Further, the lower portion14may be fabricated from a thermally conductive material, such as stainless steel or aluminium. It may be advantageous for the thermally conductive material in the well28or proximal thereto to have the same or similar material properties as relevant portions of cryogenic equipment, such as a cryogenic heat exchanger. In the embodiment shown inFIG.3a, for example, the lower portion14is provided with a spigot28aextending therethrough in co-axial alignment with the conical protrusion32, wherein a leading face of the cylindrical spigot28adefines the well28. The spigot28amay be fabricated from a different thermally conductive material than the lower portion14, such as copper. In particular, the spigot28amay be fabricated from a material with a higher thermal conductivity than the lower portion14, in particular a portion of the lower portion14surrounding the well28. The difference in thermal conductivities between the lower portion14and said spigot28aamplifies the temperature gradient therebetween, creating a localised ‘cold spot’ in the well28where solids preferentially form. In other words, it is possible to establish a thermal gradient between the well28and the surrounding area to favour solids formation in the well28.

The lower portion14may also be provided with means to control the temperature thereof so that solids formation in the well28is favoured. For example, the lower portion14may be in heat exchange communication with a liquid nitrogen heat exchanger (not shown). Such liquid nitrogen cooling allows the resonant cavity22to be cooled to temperature conditions comparable to cryogenic gas processing conditions (e.g. 90-130 K). In some embodiments the liquid nitrogen heat exchanger may be in combination with resistive heating to control the temperature of the electromagnetic resonant cavity22.

Additionally, or alternatively, the apparatus10may be provided with a thermoelectric cooler, such as a Peltier device, to cool the lower portion14, in particular proximal to the well28, to a desired temperature range.

Said spigot may also be arranged in heat exchange communication with a second heat exchanger and/or thermoelectric cooler (not shown). The second heat exchanger and/or thermoelectric cooler may be arranged, in use, to cool the well28to a lower temperature than the portion of the lower portion14surrounding the well28.

In some embodiments, resistive heating may also be provided to the cylindrical side wall24of the cavity22to establish a thermal gradient therein and ensure that the well28is ‘colder’ than any other region of the cavity22. The arrangement described in these particular embodiments is useful in the absence of the spigot28aas described above.

The lower portion14may be provided with a plurality of calibrated thermometers or thermistors, such as a platinum probe, which may be located with respect to the resonant cavity to monitor temperature therein. For example, a first thermistor38may be disposed in the lower portion14proximal to the well28and may be used to infer temperature (Tfreeze) at which solids form. A second thermistor40may be disposed proximal the cylindrical side wall24to measure the magnitude of a temperature gradient present in the resonant cavity22. A further thermistor42may also be provided in a base of the lower portion14to monitor a control temperature of the apparatus10.

In one embodiment, as shown inFIG.3a, the conical protrusion32of the upper portion16is provided with an inlet44in fluid communication with a co-axial passage46extending therethrough to allow samples of fluid to enter the electromagnetic resonant cavity22for investigation of their phase behaviour and measurement of Tfreeze. The passage46may be dimensioned to allow fluid flow directly to the gap36without impacting the electromagnetic field distribution within the cavity22.

In alternative embodiments, as shown inFIGS.3cand3d, the inlet44is disposed in the lower portion14. The passage46is disposed in fluid communication with the inlet44and extends through the lower portion14. In the embodiment shown inFIG.3c, the passage46extends co-axially with respect to the well28.

Advantageously, the passage46terminates at the gap36between the well28and the flat surface34of the conical protrusion32, thereby allowing ingress of a stream of fluid to purge solids from the gap36subsequent to solids formation. In this way, the cavity22may efficiently recover to baseline conditions following a solids formation detection event.

The annular top wall30of the upper portion16is also provided with a plurality of apertures defining outlets48for egress of the fluid and ports50to receive one or more microwave probes52to excite and monitor an electromagnetic response of the cavity22. The microwave probes52may include any suitable electromagnetic resonance sensor including, but not limited to, a frequency discriminator circuit (separate to or integral to an oscillator circuit), radiofrequency (RF) source and power detector, or a network analyser. Electromagnetic signals may be transmitted through or reflected from the resonant cavity22via sites located in regions of suitable field strength. Accordingly, it is possible to measure resonance frequency via transmission or reflection modes of operation. In a preferred embodiment, the resonance frequency is measured by transmission because the signal quality is better. However, it should be noted that the transmission mode requires two seals, whereas reflection mode requires one seal. Consequently, the reflection mode is more robust from the perspective of a reduced probability that a leak of high pressure fluid within the cavity22may occur.

The apparatus10may be in operative communication with a processor and a controller (not shown) which are used for data acquisition, resonant frequency measurement, thermal control, flow control, and to generate real time data analysis of key parameters including, but not limited to, resonance frequency, temperature and pressure.

The gap36between the flat surface34of the conical protrusion32and the well28defines a capacitive region whereby the electrical field therein is concentrated, thereby making the resonant frequency of said cavity22highly sensitive to the dielectric permittivity (E) of material in the gap36. It will be appreciated by those skilled in the art that the frequency of the resonant cavity22may be tuned by varying the size of the gap36.

The measured resonant frequency (f) is related to E according to the

f0=fvacɛ
where the vacuum frequency (fvac) is determined by the geometry of the cavity22.

Various electromagnetic models, analytic and/or numerical in nature can be used to predict the resonant modes of the cavity22. For example, in this particular embodiment, finite element analysis (FEA) may be used to solve electromagnetic field equations, allowing the frequency response to changes in dielectric permittivity (for example as associated with a fluid-phase transition) to be accurately modelled.

The geometry of the embodiment described with reference to the figures exhibits a first resonant mode at ˜2.6 GHz in vacuum. The electric field distribution of this mode exhibits high electrical intensity between the conical protrusion32and the well28in the gap26. The frequency measured with this mode is almost entirely determined by the dielectric properties of the fluid (or solid) in this region. FEA has shown that the presence of small amounts of solids in the gap26can generate a signal response that can be resolved and detected.

For example, a FEA simulation of benzene freeze-out in LNG is capable of predicting the smallest detectable volume of solids in the well28.FIG.4illustrates the expected frequency shift given varying amounts of benzene present in the gas sample when cooled in the well28of the cavity22until benzene solidifies. Extrapolation of the trend line suggests that the presence of only 1 ppm benzene could cause a detectable 50 kHz shift in resonant frequency. Example 1 (described later) illustrates the ability to detect solids occupying as little as 0.0001 v/v % of the volume of cavity22.

In use, a stream of fluid may be passed into the electromagnetic resonant cavity22through passage46via inlet44. The electromagnetic resonant cavity22is then cooled at constant pressure across a temperature range encompassing a solid-liquid equilibrium region or a solid-gas equilibrium region to directly detect solids formation.

Concurrently, the electromagnetic resonant cavity22is excited by microwave radiation by the microwave probes52and the response (i.e. the resonant frequency) of said cavity22is measured.

Typically, the resonant frequency of said cavity22responds approximately linearly with temperature, with a change in slope indicating solids formation in the cavity22, in particular in the well28. The frequency signature representative of solids formation often results in a rapid decrease in frequency, although exception to this trend may occur depending on the composition and morphology of the solids formed. The measurement of the resonant frequency allows relatively small changes in the resonant behaviour of the cavity to be detected.

Subsequent to a solids formation event, the solids may be purged from the well28and said cavity22by directing a stream of fluid through the passage46via inlet44. Fluid egress from said cavity22is via outlets48.

It will be well understood by those skilled in the art that the temperature of solidification (Tfreeze) of the fluid or the bulk fluid composition may be determined with thermodynamic simulation programs for solid liquid equilibrium (SLE) calculations. Such thermodynamic simulation programs include, but are not limited to, Multiflash or thermodynamic calculation programs implemented within multi-phase flow simulation programs OLGA, LedaFlow, HyFAST and CryoFAST. A particularly suitable example includes ThermoFAST which is specifically developed for predicting solid-liquid transitions in hydrocarbon mixtures. This thermodynamic simulation program has been endorsed by the Gas Processor Association Midstream organisation as a useful predictive tool for the thermodynamic properties of natural gas and LNG systems including solid-liquid equilibrium.

Advantageously, the apparatus10for directly detecting solids formation as described above may be readily utilised to determine the temperature of solidification (Tfreeze) more accurately than may be predicted by such thermodynamic simulation programs discussed above.

In some dynamic systems where the bulk fluid is cooled, however, underlying changes in the bulk fluid's dielectric properties may mask deviations in resonance frequency caused by solids formation of freezable compounds. This is particularly relevant for fluids having trace impurities, where the amount and rate of solids formation may be low. While not wishing to be bound by theory, the inventors also opine that the crystal morphology of some freezable compounds may also play a role in this ‘masking phenomenon’.

The inventors have discovered that this ‘masking phenomenon’ may be overcome by measuring changes to the dielectric permittivity (Δε) of the fluid, as a function of the difference between a theoretical dielectric permittivity (εcalc) of the bulk fluid and a measured dielectric permittivity (εmeas) of the fluid, according to formula (1):
Δε=εmeas−εcalc,  (1)

The measured dielectric permittivity (εmeas) of the fluid is unitless and may be calculated as a function of the measured electromagnetic resonance frequency of the cavity (fmeas) in the presence of the fluid relative to the electromagnetic resonance frequency of the cavity measured under vacuum (fvacuum), according to formula (2).
εmeas=(fvacuum/fmeas)2(2)

The theoretical dielectric permittivity (εcalc) of the bulk fluid may be calculated with thermodynamic simulation programs for solid liquid equilibrium (SLE) calculations as described above, in particular ThermoFAST software, for known temperature and pressure conditions.

Measurement of changes in the dielectric permittivity (Δε) of the fluid removes the systematic effect of changing bulk fluid dielectric properties by subtracting a theoretical dielectric permittivity (εcalc) for the bulk fluid, calculated using fluid property software and the measured temperature and pressure, from the directly measured dielectric permittivity (εmeas), calculated from the measured frequency shift of the cavity's resonance from the cavity's baseline frequency under vacuum. Using this null measurement algorithm, the presence of solids is identified as a deviation from zero or a baseline value.

The solidification temperature Tfreezemay be determined as the temperature at which the change in dielectric permittivity (Δε) of the fluid deviates from a baseline value. This method of determining Tfreezeis also more accurate than the thermodynamic simulation programs discussed above. Notably, the solidification temperature Tfreezedetermined by measuring the change in dielectric permittivity (Δε) of the fluid coincides with the solidification temperature Tfreezedetermined by measuring the resonance frequency of the cavity

Determination of Tfreezeby cooling a fluid containing freezable solids in the apparatus10as described herein by measuring resonance frequency of the cavity or changes in dielectric permittivity (Δε) of the fluid will now be described with reference toFIGS.5a,5b,6aand6b.

Referring toFIGS.5aand5b, the liquefaction temperature of pure methane and the solidification temperature Tfreezepredicted by SLE calculations using ThermoFAST for a mixture of 100 ppm CO2in methane at 9.5 MPa was 90.9 K and 103.5 K, respectively.

The mixture was cooled at a rate of 1 K/min from 110 K to 96 K, then held constant.FIG.5ashows that there was a deviation from a linear relationship between temperature and measured resonance frequency of the cavity at 97.0 K, showing a clear signal of solids formation at Tfreeze=97.0 K (indicated by the star). Similarly,FIG.5bshows that there was a deviation from a baseline value with respect to change in dielectric permittivity (Δε) of the fluid at 97.0 K, showing a clear signal of solids formation at Tfreeze=97.0 K (indicated by the star). Both these measurements compared well with a visual identification of solids formation at 95.5 K for the same mixture at 10 MPa. Notably, both methods of determining Tfreezewere the same and 6.5 K lower than Tfreezepredicted by ThermoFAST.

Referring toFIGS.6aand6b, the liquefaction temperature of pure methane and the solidification temperature Tfreezepredicted by SLE calculations using ThermoFAST for a mixture of 100 ppm benzene in methane at 9.5 MPa was 90.9 K and 149.7 K, respectively.

The mixture was cooled at a rate of 1 K/min from 148 K to 134 K, then held constant.FIG.6ashows that there was no deviation from a linear relationship between temperature and measured resonance frequency of the cavity over the temperature range, even in the presence of solids, and therefore it was not possible to identify Tfreeze. On the other hand,FIG.6bshows that there was a deviation from a baseline value with respect to change in dielectric permittivity (Δε) of the fluid at 140.1 K, showing a clear signal of solids formation at Tfreeze=140.1 K (indicated by the star). Measured Tfreezewas 9.6 K lower than Tfreezepredicted by ThermoFAST.

Said apparatus10may find general application in determining the pressure and/or temperature at which solids form in a fluid of uncertain composition. In particular, said apparatus may be useful to determine the temperature at which solids form in contaminated natural gas as it passes through a cryogenic heat exchanger, and thereby determine an operating temperature to avoid solid blockages in the cryogenic heat exchanger.

For example, said apparatus10may be conveniently configured with respect to the cryogenic heat exchanger to provide real time measurement of a sample of feed fluid and to directly detect solids formation and determine the temperature of solidification (Tfreeze) before the feed fluid enters the cryogenic heat exchanger. The sample of feed fluid may be directed to said apparatus10via a bleed line as shown inFIG.7(as described below). Measurements may be taken intermittently or quasi-continuously to determine (Tfreeze) as an absolute condition or compared in a relative sense to a plurality of samples investigated over a pre-determined period of time. For example, an increase in (Tfreeze) with time may allude to changing feed fluid composition or inefficiencies in upstream units, for example heavy hydrocarbon removal units. Additionally, the magnitude of the signal response to solidification may also provide operators with a qualitative indication relating to the severity of solids formation downstream.

System and Method to Prevent or Remediate Solids Deposition in a Cryogenic Heat Exchanger

In respect of LNG production, there are practical difficulties in measuring the specific composition of heavy hydrocarbons in natural gas entering the main cryogenic heat exchanger (‘MCHE’), and therefore it is difficult to ascertain whether the fluid contains appreciable amounts of freezable compounds. Consequently, LNG plants are generally operated with a significant margin of safety to avoid potential solids deposition on the MCHE. This margin of safety is of an “open loop” nature.

The apparatus for directly detecting solids formation in a fluid disclosed herein enables measurement and control of an operating parameter not previously available: ΔTfreeze. The solids formation temperature, Tfreeze, may be obtained by taking a sample of fluid cooled by, or intended to be cooled by the cryogenic heat exchanger, and then progressively reducing the temperature of said sample until solids formation is detected. The parameter, ΔTfreeze, is the difference between the operating temperature of the cryogenic heat exchanger Tliquidand Tfreeze(i.e. ΔTfreeze=Tliquid−Tfreeze).

Integration of said apparatus as described herein in an LNG plant allows ΔTfreezeto be monitored so that corrective action may be initiated when ΔTfreezevaries from a predetermined operating margin. The predetermined operating margin may be from 1K to 20K, or from 1K to 10K, even from 2K to 5K. Said margin may vary depending on the historical stability of the cryogenic process. In this way, it becomes possible to control ΔTfreezein a closed loop fashion.

Reliable knowledge of variation of ΔTfreezefrom the predetermined operating margin and likelihood of solids deposition in the MCHE enables various units in the LNG plant that remove impurities (such as a heavy hydrocarbon removal unit) prior to the MCHE to be operated less conservatively. This means less gas is burnt to provide the energy necessary to remove the impurities, and hence overall plant efficiency is improved and a greater proportion of the gas entering the plant can be liquefied.

Referring toFIGS.7and8, in accordance with various aspects of the present disclosure, there is shown a system100of remediating solids deposition (“the remediation system100”) in a cryogenic heat exchanger integrated with a natural gas liquefaction system200. The natural gas liquefaction system200as described herein is described for illustrative purposes only and it will be appreciated that various embodiments of the remediation system100may be similarly integrated into alternative natural gas liquefaction systems200to facilitate remediation of solids deposition. Additionally, it will be appreciated that the remediation system100may be similarly integrated into a cryogenic gas liquefaction system for a gas other than natural gas, such as hydrogen, where impurities such as water, carbon dioxide, carbon monoxide or nitrogen may be present at sufficiently high concentrations to provide a risk of solids formation within the cryogenic heat exchanger.

A feed gas is introduced to the natural gas liquefaction system200via line1to a compressor202where the feed gas is compressed and subsequently cooled with a propane refrigerant. The cooled compressed feed gas is then fed via line3to a scrub column204for separation and removal of condensed heavy hydrocarbons (C5+). A C5+-depleted overhead vapour product of the scrub column204is passed via line5to compressor206where said overhead vapour product is compressed and subsequently cooled with a propane refrigerant. The compressed overhead vapour product is passed via line7to separator208to remove any condensed liquid hydrocarbons which are subsequently returned to the scrub column204via line9.

An overhead vapour product of separator208(“gas stream”) is then passed through a C5+analyser110via line11. The C5+analyser110may be a gas chromatograph or any other suitable analyser that is capable of determining a cumulative content of heavy hydrocarbon compounds in the gas stream. Additional analysers (not shown) may also be provided to monitor the concentrations of other freezable compounds (e.g. water) which may freeze at cryogenic temperatures but which cannot be monitored by the gas chromatograph. It will be appreciated that the C5+analyser110may not be capable of determining a content of any one specific C5+compound in the gas stream from separator208. Nevertheless, the skilled person will appreciate that such C5+analysers110may be capable of providing an estimate of the content of freezable hydrocarbon compounds, such as benzene content, as well as providing information about the composition of the gas stream.

The gas stream may then be passed via line11to a cryogenic heat exchanger210with cooling provided by one or more refrigerants (including mixed refrigerants) where the gas stream is cooled to an operating temperature (Tliquid) at or below the temperature at which the hydrocarbons in the overhead vapour product condense. Preferably, the gas stream is cooled to a temperature below the methane boiling point at an elevated pressure, for example to a temperature between about −140° C. to −150° C. The liquefied stream may be further expanded and cooled to about −162° C. and atmospheric pressure for storage purposes.

Additionally, prior to entry to the cryogenic heat exchanger210, a small sample of the gas stream may be passed via byline13to the apparatus10for detecting solids formation in a fluid. Said apparatus10may be used to directly detect solids formation and determine the temperature of solidification (Tfreeze) for said sample, as described previously.

Detection of solids in said sample may be indicative that solids have formed (or will form) in the cryogenic heat exchanger210. Accordingly, it is possible to correlate solids formation in said apparatus10with solids formation in the cryogenic heat exchanger210.

The system100of the present disclosure may also include a thermodynamic simulation program for solid liquid equilibrium (SLE) calculations. Such thermodynamic simulation programs will be well understood by those skilled in the art and include, but are not limited to, Multiflash or thermodynamic calculation programs implemented within multi-phase flow simulation programs OLGA, LedaFlow, HyFAST and CryoFAST. A particularly suitable example includes ThermoFAST which is specifically developed for predicting solid-liquid transitions in hydrocarbon mixtures. This thermodynamic simulation program has been endorsed by the Gas Processor Association Midstream organisation as a useful predictive tool for the thermodynamic properties of natural gas and LNG systems including solid-liquid equilibrium.

The composition of the gas stream, including the C5+content, measured by the gas chromatograph110may be used in the thermodynamic simulation program to determine a remedial composition capable of removing solids deposited in the cryogenic heat exchanger210. In other words, the thermodynamic simulation program will determine the nature of the solvent in which the freezable compounds of interest are soluble or a variation in the composition in the fluid which would move the conditions of SLE to where the deposited solids would undergo a solid-liquid or solid-gas phase transition.

The temperature of solidification (Tfreeze) determined by said apparatus10for directly detecting solids formation may be used in the thermodynamic simulation program to determine a remedial temperature (Tremedial) to remove solids deposited in the cryogenic heat exchanger210.

The remediation system100may further include a temperature controller (not shown) for the cryogenic heat exchanger210to raise or lower the operating temperature of the cryogenic heat exchanger210to the remedial temperature (Tremedial) calculated by the thermodynamic simulation program. The temperature controller may be in operative communication with the cryogenic heat exchanger210to vary a refrigeration duty in one or more locations in the cryogenic heat exchanger210to raise or lower the operating temperature to the remedial temperature (Tremedial). Counterintuitively, in some embodiments, lowering the operating temperature of the cryogenic heat exchanger210may facilitate removal of solids (i.e. ‘retrograde’ melting) within the cryogenic heat exchanger210, rather than increased solids deposition, because the solubility of the deposited solids in the fluid in the cryogenic heat exchanger210may increase with decreasing temperature.

Alternatively, or additionally, the remediation system100may also include a remedial fluid dosing means120to adjust the composition of the fluid in the cryogenic heat exchanger210to the remedial composition. The composition of the fluid may be adjusted to the remedial composition by injecting an amount of a remediation fluid into the gas stream prior to entry to the cryogenic heat exchanger210. The remedial fluid may be injected continuously or at intermittent or regular intervals until the solids have been removed from the cryogenic heat exchanger210.

The remediation fluid may comprise one or more hydrocarbon compounds that when added to the gas stream will increase the solubility of freezable compounds. It will be appreciated that the composition of the remediation fluid will vary according to the composition of the gas stream and the operating conditions of the cryogenic heat exchanger210. Nevertheless, it is anticipated that the remediation fluid may be sourced from hydrocarbon fluids which may be conveniently available on site including, but not limited to, refrigerants such as propane refrigerant, heavy mixed refrigerants, light mixed refrigerants, or product fluids such as ethane, LPGs and light condensate and so forth.

Introduction of the remedial fluid into the gas stream will inherently and temporarily change the composition, and hence the energy properties, of the LNG produced in the cryogenic heat exchanger210. Significant changes to the LNG composition, particularly if the LNG composition is ‘off specification’, may require the LNG to be recycled or flared. It is anticipated that the financial losses associated with flaring will be insignificant compared to losses that would be incurred from a complete plant shutdown to remove solids blockages which may require several days.

The remediation system100may optionally include a further apparatus10′ for detecting solids formation in fluid communication with an outlet212of the cryogenic heat exchanger210to monitor the effectiveness of the remediation process. A sample of output fluid from the outlet212from the cryogenic heat exchanger210may be passed through said apparatus10′ and the behaviour with respect to solids formation of said output fluid may be investigated across an operating temperature range of the cryogenic heat exchanger210. The presence (or absence) of solids formation in said apparatus10′ may be correlated with the progress of the remediation process within the cryogenic heat exchanger210, with presence of solids formation in said apparatus10′ indicating a need to continue the remediation process and an absence (or cessation) of solids formation in said apparatus10′ indicating a successful or completed remediation process.

The effectiveness of the remediation process may also be monitored by measuring a pressure drop across the cryogenic heat exchanger210, whereby a decrease in the pressure drop may indicate the removal of the solids deposit therein.

It will be appreciated that prior to adjusting the feed gas composition to the remedial composition or raising or lowering the operating temperature to the remedial temperature (Tremedial), such remedial actions may first be tested in said apparatus10for detecting solids formation to gauge the effectiveness of such remedial actions before they are implemented with respect to the cryogenic heat exchanger210.

Various embodiments may be illustrated by the following examples. The examples are provided for illustrative purposes only and are not to be construed as limiting the scope or content of the disclosure in any way.

Example 1

An electromagnetic resonant cavity22of the apparatus10as described herein was filled with a sample of 4% p-xylene, 96% ethane fluid and cooled from −60° C. to −82° C. at constant pressure with concurrent measurement of the resonant frequency response of said cavity.

FIG.9shows that a 76 MHz shift was observed at Tfreeze−76° C. at onset of solids formation which is consistent with FEA simulations. The smallest frequency shift that can be resolved is approximately 25 kHz which, through extrapolation, would suggest sensitivity to p-xylene solid at ˜3 ppm by mole. FEA modelling of said cavity22may be combined with this data to estimate the smallest solid volume that the apparatus10is capable of detecting unambiguously. For the ethane p-xylene system, the apparatus10may detect a solids volume equivalent to 0.0001 v/v % of the volume of the cavity20. This is significant given that the relative difference in dielectric properties of liquids and solids is small for hydrocarbons.

Example 2

Example 2 illustrates a case study in the remediation of a benzene solid deposition after the addition of three different remediation fluids (mixed refrigerant (MR), iso-butane (iC4) and neo-pentane (n-C5) into the gas stream of the cryogenic heat exchanger. The results are based on the solubility of benzene in the remedial composition as predicted by the thermodynamic simulation tool ThermoFAST. The case study involves the deposition of 1 kg of benzene solid in the cryogenic heat exchanger, the composition of the gas stream and flow rates analogous with those experienced by RasGas Train 4 prior to the blockage event in November 2014. Table 2 provides the composition of the gas stream under normal operating conditions for this LNG train. The average mass flow rate flowing through the cryogenic heat exchanger is 146 kg/s with coldest effluent temperature of 124 K.

TABLE 2Estimate of the molar composition of natural gas componentsin a gas stream fed to main cryogenic heat exchanger duringnormal operation at the RasGas plant.ComponentCompositionCarbon dioxide0.00004Methane0.909329Ethane0.057492Propane0.021953Iso-Butane0.004239n-Butane0.006847Iso-Pentane0.000028n-Pentane0.000026Hexane0.000026Heptane0.000016Benzene0.000004

FIG.10a) illustrates the time of remedial fluid addition required to remove the benzene deposit per volume percent added of each component. It can be seen that some remedial fluids perform better per unit volume in the dissolution of benzene, however the optimum choice will depend on price and availability of the remedial fluids. Despite being the worst performer by volume, mixed refrigerant fluid is readily available and stored in large amounts in LNG plants.

FIG.10b) graphically represents the solubility of benzene in the adjusted composition of the gas stream given the addition of the remediation fluids. Intuitively the solubility of benzene increases with increased volume of remediation fluid added, however the amount added by an operator would be constrained by the plant's ability to cope with additional mass flow through its unit operations.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

In the claims which follow and in the preceding description except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.