The present invention relates to a method for flow control of a closed circuit in which an intermediate medium is circulated and in particular when using the closed circuit for a fluid to be evaporized or condensed within a heat exchanger.

Natural gas is produced from subterranean reservoirs throughout the world. Such a gas in the form of methane, for instance, is a valuable commodity, and various methods and devises exist for the extraction, treatment and transportation of the natural gas from the actual reservoir to consumers. The transport is often performed by means of a pipeline in which gas in the gaseous state from the reservoir is conveyed onshore. However, many reservoirs are located in remote areas or areas with restricted accessibility, involving that utilization of a pipeline is either technically very complicated or unprofitable. One very common technique is then to liquefy the natural gas, NG, at or near the production site, and transport liquefied natural gas, LNG, to the market in specially designed storage tanks, often situated aboard a sea-going vessel.

Liquefying natural gas involves compressing and cooling of gas to cryogenic temperatures, e.g. -160°C. Thus, LNG carriers may transport a significant amount of LNG to destinations at which the cargo is offloaded to dedicated tanks onshore, before either being transported by road or rail on LNG carrying vehicles or revaporized and transported by e.g. pipelines.

It is often more favourable to revaporize LNG aboard the seagoing carrier before the gas is off-loaded into onshore pipelines, for instance. US-Patent No. 6,089,022 discloses such a system and method for regasifying LNG aboard a carrier vessel before evaporized gas is transferred to shore. LNG is flowed through one or more evaporizers positioned aboard the vessel. Seawater surrounding the carrier vessel is flowed through a evaporizer to heat and vaporize LNG to natural gas before offloading to onshore facilities.

According to US-Patent No. 6,089,022 the "TRI-EX" Intermediate Fluid-type LNG vaporizer is capable of using seawater as the principal heat exchange medium. Such a type of evaporizer is also disclosed by US-Patent No. 6,367,429 in principle comprising a housing with a pre-heat and final heating section. The pre-heat section has a plurality of pipes running therethrough which fluidly connect two manifolds arranged at either end of the pre-heat section. The final heating section has also a plurality of pipes running therethrough which fluidly connect two other manifolds at either end of the final heating section. Seawater surrounding the vessel is pumped into a manifold and flows through the pipes in the final heating section and into the manifold before flowing through the pipes in the pre-heat section and into the manifold, from which the seawater is discharged into the sea. In operation, LNG flows from a booster pump and into a looped circuit positioned within the pre-heat section of the evaporizer, which in turn contains a "permanent" bath of an evaporative coolant, e.g. propane, in the lower portion. Seawater flowing through the pipes "heats" the propane in the bath, causing propane to evaporate and rise within the precooling section. As propane gas contacts the looped circuit, heat is given to extremely cold LNG flowing through the circuit and re- condensed as to fall back into the bath, thereby providing a continuous, circulating "heating" cycle of propane within the pre-heat section.

To remedy challenges associated with the solutions above, US-Patent No. 6,945,049 proposes a method and system for regasification of LNG aboard a floating carrier vessel before gas is offloaded comprising boosting and flowing LNG into an LNG/coolant heat exchanger in which LNG is evaporated, and flowing evaporated natural gas (NG) into a NG/steam heat exchanger, in which NG is heated before being transferred onshore as superheated vapour. LNG in the LNG/coolant heat exchanger is evaporated by thermal exchange against a coolant entering the heat exchanger as a gas and leaving the same in a liquefied state. Moreover, coolant is flowed in a closed circuit and through at least one coolant/seawater heat exchanger in which liquefied coolant is evaporated before entering the LNG/coolant heat exchanger, and the pressure in evaporated coolant is controlled.

In the propane loop presented by US-Patent No. 6,945,049, the temperature difference between seawater entering and leaving the coolant/seawater heat exchanger has to be relatively high as to avoid voluminous dimensions. Typically, the evaporation temperature of coolant is 20-25 °C below inflowing seawater and, thus, the temperature out from the coolant/seawater heat exchanger is 25-30 °C below seawater or even lower (preheating). NG is additionally heated within a NG/steam heat exchanger of shell & tube type. The latter could be replaced by a direct NG/seawater heat exchanger in which NG is typically heated from -20 °C until some below seawater within a shell & tube type heat exchanger made from titanium. NG and seawater are directed on the tube side and shell side, respectively (trim heating). High pressure on the NG side make the titanium shell & tube heat exchanger very expensive and, to reduce costs, this is construe- ted like an all welded heat exchanger having straight tubes due to considerably reduced diameter and elimination of the very expensive tube plate compared with a heat exchanger having U-tubes.

Using all welded heat exchangers result in equipment impossible to opened for maintenance, e.g. to clean fouling on the seawater side and plug tubes in case of ruptures. Such a solution having all welded tube heat exchangers is unfavourably as regards maintenance, for instance. Using seawater as one of the media involves that the titanium heat exchangers needed become very costly when these have to be constructed to withstand high pressures as well.

To further improve the technology presented by the latter so as to reduce costs and to facilitate maintenance, for instance, NO 20093341 discloses a plant for regasification of LNG, comprising at least one pump boosting LNG pressure; a LNG/coolant heat exchanger producing NG from LNG being flowed from the boosting pumps; a closed coolant loop extending through the LNG/coolant heat exchanger and including at least one heat exchangers, a coolant from the respective heat exchanger being passed through the LNG heat exchanger as a gas and leaving in a condensed state as to produce NG by thermal exchange; and a heating medium being used within the respective heat exchanger as to provide coolant in a gaseous state, wherein a NG/coolant heat exchanger is arranged in connection with the LNG/coolant heat exchanger and is connected to the closed coolant loop, whereby LNG is preheated within the LNG/coolant heat exchanger and NG is trim heated within the NG/coolant heat exchanger using liquid coolant from at least one heat exchanger.

Thus, LNG is boosted by at least one pump and evaporized in a heat exchanger by condensing propane therein. An intermediate medium, e.g. propane, is circulated in a closed circuit passing through the LNG heat exchanger and condensed propane is pumped from a propane tank into at least one heat exchanger arranged in the closed circuit so as to be evaporated prior to entering the LNG heat exchanger. When evaporizing propane, seawater is often used.

Flow of propane through the closed circuit is controlled by maintaining a stable fluid level within the tank for condensed propane. Dependent on the capacity of the circuit, a propane pump controls the flow so as to indirectly maintain the level within the propane heat exchanger by keeping up the tank level. Alternatively, the pump flow is controlled as function of temperature of evaporized NG leaving the LNG heat exchanger. The first alternative presupposes accurate level measurements within the propane tank which involves an unfavourable large tank volume to provide for stable measuring signals. The result in a need of large quantities of propane in the closed circuit. The latter is based on the temperature of seawater. When choosing a set point for NG from the LNG heat exchanger, this fact implies difficulties regarding satisfactory adjustments of the propane pump.

The main objective of the present invention is to solve the problems mentioned above and to propose an improved regulation concept for the closed circuit circulating intermediate medium.

This is achieved by a method for regulating a closed intermediate medium circuit when heat exchanging a primary medium within a heat exchanger fed by means of a pump so as to be evaporated or condensed therein , the closed circuit is passing through the heat exchanger and is comprising a tank and pump for condensed intermediate medium and at least one heat exchanger evaporising or condensing intermediate medium to be passed through the heat exchanger for primary medium, wherein controlling flow of intermediate medium in the closed circuit as function of primary medium through the heat exchanger.

To ensure sufficient flow through the closed circuit, flow of intermediate medium being function of primary medium is based on a fixed value of all capacities for a certain liquid fraction in evaporated intermediate medium leaving or entering the least one heat exchanger such as 30% or, alternatively, based on a certain intermediate medium flow having a increasing liquid fraction as function of capasity.

Intermediate medium flow can measured and adjusted using ampere measurements derived at the propane pump, derived from pressure drop in the circuit or over the pump, using a dedicated flow meter, or the like and by means of throttling downstream the intermediate medium pump, frequency adjustment of the pump, combining pump throttling and frequency adjustment, or the like.

By adjusting the closed circuit in the manner specified above, accurate measurements of liquid level within the tank are superfluous. Such level measurements are often incorrect, in particular when measuring fluids at boiling point. Filling volumes of the system could be reduced. The tank can exclusively be configured to ensure sufficient inflow height for the propane pump at all possible changes of volume due to fluctuation of the level within the heat exchanger(s) and expansion of liquid as function of operational temperature.

The present invention is now to be explained in more detail by means of preferred embodiment illustrated in the accompanying drawings, in which:

Fig. 1 shows schematically a traditional heat exchanging loop for a primary medium by means of a closed circuit in which an intermediate medium is circulated;

Fig. 2 shows schematically the new concept according to the present invention wherein flow of the intermediate medium is controlled based on flow of the primary medium;

Fig. 3 shows schematically a general heating loop in which the present invention can be used; and

Figs. 4 and 5 show schematically some systems in which the present invention is useful.

As mentioned above and schematically illustrated in Fig. 1, the medium to be heat exchanged such as LNG is boosted by at least one pump Al and evaporized in a heat exchanger B by condensing an intermediate medium therein so as to produce NG. The heat exchanger is preferentially a compact printed circuit heat exchanger PCHE. To evaporate LNG, a closed circuit is used, in which the intermediate medium circulated is in the form of propane, for instance. After condensing the propane is passed into a tank H and, then, pumped by means of a pump E into at least one heat exchanger Gl, G2 arranged in the closed circuit so as to be evaporated prior to entering the LNG heat exchanger B. When evaporizing propane in the heat exchanger(s), heating medium is usually seawater although any appropriate medium could be used.

The new and improved regulation concept disclosed herein is based on controlling flow of condensed propane as function of the flow of LNG being passed into the pump(s) Al, see Fig. 2. The most simple is to presuppose a propane flow being direct proportional to the LNG flow as a certain quantity of LNG evaporated at a particular pressure and discharge temperature is requiring a decided quantity of condensing propane.

By direct determination of propane flow relatively to LNG flow without other parameters of the system using appropriate means such as a controller C, it should be expected an occurrence of unequal propane distribution. If LNG is to be evaporated at alternating pressure and discharge temperature, the energy demand per unit of LNG varies to some extent. The flow measurements of LNG and propane can additionally be somewhat inaccurate.

To be able of regulating in accordance with the concept above, the presumption is that propane gas discharged from the heat exchanger(s) Gl, G2 has a varying fraction of liquid as function of flow and theoretical level of liquid therein. The result of increasing propane flow and theoretical liquid level in the heat exchanger(s) is increasing liquid fraction therefrom and liquid level therein, respectively. Such a propane heat exchanger is typically configured to have 25 % theoretical liquid level and 30 % mass fraction at maximum flow. Thus, if no liquid fraction is present in propane gas, theoretical flow through the closed circuit is approximately 30 % higher than the flow needed. An estimate of propane flow as function of LNG flow has to account for the liquid fraction at maximal flow through the heat exchanger.

As long as propane flow is set sufficiently high to LNG flow, the level of propane within the heat exchanger(s) Gl, Gl is automatically adjusted. Thus, it is achieved a liquid fraction in propane gas discharged providing for a quantity of condensed propane at a correct level relatively to the quantity of LNG. A certain propane flow as function of the LNG flow is consequently either based on a fixed value of all capacities of a specified liquid fraction in propane gas discharged from the heat exchanger at maximum flow, e.g. 30 % as mentioned above, or is based on a certain propane flow having a increasing liquid fraction as function of capacity.

Flow of LNG can be measured in any appropriate manner and a few examples are:

• Derived from an ampere measurement at the LNGpump(s).

• Measured using a dedicated flow meter.

Flow of propane can be measured and controlled in different ways and some examples are specified below. i) Flow measurement:

• Derived from an ampere measurement at the propane pump.

• Measured using a dedicated flow meter. • Derived from pressure drop in the circuit, over the pump, ii) Flow adjusting:

• By throttling a valve downstream the propane pump E.

• By frequency adjustment of the pump E.

• By combining pump throttling and frequency adjustment.

Thus, the present invention can be used in any possible corresponding heating systems schematically illustrated in Fig. 3 in which: a) A medium i.e. a heated medium is to be heated by means of an intermediate medium undergoing a phase transition in a closed circuit.

b) An intermediate medium is to be condensed within at least one heat exchanger using a heating medium.

c) An intermediate medium is to be evaporated within at least one heat exchanger using a heating medium.

d) A medium in the form of liquid is to be heated, gas to be heated, or gas to be partly or completely evaporated.

e) A medium in the form of liquid is to be cooled, gas to be cooled, or gas to be partly or completely condensed.

When condensing primary medium, it is understood that intermediate medium has to be an appropriate coolant and that the tank (H) and pump for condensed intermediate medium are situated upstream the heat exchanger (B).

Some specific system in which the present invention is very useful are presented by NO 20093341 and US 6945049 and schematically illustrated in Figs. 4 and 5. The publications disclose plants for LNG regasification. The former is using a combined closed propane circuit in which LNG is heated by condensing propane within a heat exchanger B and by propane in liquid phase in a trim heater C. The latter describes a closed propane circuit for the heat exchanger B and a direct seawater based trim heater S&T.

As illustrated in Fig. 4, LNG is fed from onboard tanks, not shown, and into at least one high pressure pump Al, A2 which boosts LNG pressure, and from which boosted LNG is flowed into a LNG/coolant heat exchanger B. Each pump is a multistage centrifugal pump, for instance, being submerged pot mounted. LNG temperature upon entering the LNG/coolant heat exchanger is typically -160 °C, and it is preheated to -20 °C and higher before exit. Preheating is effected by means of phase transition for liquefied coolant. The LNG/coolant heat exchanger may be a compact printed circuit heat exchanger, PCHE, made from stainless steel or any suitable material.

NG leaves the LNG/coolant heat exchanger B in an evaporated state and enters a NG/coolant heat exchanger C in which NG is trim heated before conveyed onshore as superheated vapour. The trim heating is performed by temperature glide for liquefied coolant. The vapour temperature is typically 5-10 °C below seawater inlet temperature.

The coolant circuit is fed from a coolant supply H, e.g. a tank, and driven by a pump E into a semi -welded plate heat exchanger D. Although illustrated as being mounted outside the coolant supply, the pump, e.g. a centrifugal pump, may also be of the submerged pot mounted type like the pumps Al, A2 mentioned above. Coolant is heated by means of seawater passing through the plate heat exchanger opposite of coolant, typically up to 2-5 °C below ingoing seawater temperature. Then, heated coolant is fed into the NG/coolant heat exchanger C to provide for trim heating of NG.

Cooled coolant leaving the NG/coolant heat exchanger C is pressure relieved by means of a control valve F before it enters at least one semi-welded plate heat exchanger Gl, G2. The control valve may be replaced by any suitable means, e.g. a fixed restriction. An objective of the control valve is to maintain pressure from the pump E through the two heat exchangers D, C above boiling pressure of coolant at seawater temperature. Within each plate heat exchanger Gl, G2 coolant is evaporated using seawater, each being passed on opposite sides through the heat exchangers.

Then, evaporated coolant is passed on to the LNG/coolant heat exchanger B to be condensed while LNG is evaporated on each side within the heat exchanger when preheating LNG. Condensed coolant from the heat exchanger is at last returned into the tank H.

Many optional variations are possible. The preheating and trim heating heat exchangers B, C may be combined to one common heat exchanger. Such a common heat exchanger is having one LNG/NG path and at least one separate path for coolant in preheating and trim heating portions, respectively. Seawater being passed into the heat exchanger D may be preheated using an external heater of appropriate type. Any suitable coolant other than seawater is applicable. The plant presented in Fig. 5 differs by having separate closed circuits for preheating and trimheating LNG/NG. The preheating is effected by a separate closed propane circuit passing through the heat exchanger B, whereas trim heating is performed using a shell and tube heat type exchanger S&T. Typically, the evaporation temperature of coolant is 20-25 °C in the below inflowing seawater and, thus, the temperature out from the coolant/seawater heat exchanger is 25-30 °C below seawater or even lower. Both circuits can use seawater as medium for condensing propane within the heat exchangers) Gl, G2 and trim heating NG in the heat exchanger S& T, respectively. The latter could be a direct NG/seawater heat exchanger in which NG is typically heated from -20 °C until some below seawater. NG and seawater are directed on the tube side and shell side.

The discussion above as regards the present invention are to be construed merely illustrative for principles according to the invention, the true spirit and scope of present invention being defined by the patent claims. Although LNG and NG is especially mentioned when discussion the present invention, this fact is actually not excluding that any appropriate type of liquefied gases such as ethane, propane, N ₂ , C0 ₂ is applicable. It is understood that the present method can be used onboard av sea-going vessel, offshore on a platform, for instance, or onshore.