NO20100669A1 - Method for controlling a medium medium circuit by heat exchange of a priming medium - Google Patents

Abstract

A method of controlling a closed medium medium circuit when a primary medium is heat exchanged within a heat exchanger (B) fed by a pump (A1) to be evaporated or condensed therein, the closed circuit passes through the heat exchanger (B) and includes a tank (H) and a condensed medium medium pump (E) and at least one heat exchanger (G1, G2) evaporates or condenses intermediate medium to pass through the primary medium heat exchanger (B), where flow of the intermediate medium in the closed circuit is controlled as a function of the primary medium through the heat exchanger .

Description

Procedure for regulation of a

intermediate medium circuit during heat exchange of a primary medium

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

Natural gas is produced from underground reservoirs all over the world. Such a gas, for example in the form of methane, is a valuable commodity, and various methods and devices exist for extracting, treating and transporting the natural gas from the actual reservoir to consumers. The transport is often carried out using a pipeline in which gas in the gaseous state from the reservoir is brought ashore. However, many reservoirs are located in remote areas or areas with limited accessibility, meaning that the use of pipelines is either technically very complicated or unprofitable. One very common technique is therefore to liquefy the natural gas, NG, at or near the production site, and transport the liquefied natural gas, LNG, to the market in specially designed storage tanks, often placed on board an ocean-going vessel.

Liquifying natural gas involves compressing and cooling gas to cryogenic temperatures, e.g. -160 °C. In this way, carriers can transport a significant amount of LNG to destinations where the cargo is unloaded into dedicated tanks on land, before it is either transported by road or rail on LNG-carrying vehicles or regasified and transported with e.g. pipelines.

It is often more advantageous to regasify LNG on board the ocean-going vessel before the gas is unloaded into, for example, pipelines on land. US patent no. 6,089,022 describes such a system and method for regasifying LNG on board a freighter before vaporized gas is transferred ashore. LNG is flowed through one or more evaporators positioned on board the vessel. Seawater surrounding the freighter is flowed through an evaporator to heat and vaporize the LNG into natural gas for offloading to facilities on land.

According to US Patent No. 6,089,022, the "TRI-EX" medium fluid type LNG vaporizer is capable of using seawater as the main heat exchange medium. Such a type of evaporator is also described in US patent no. 6,367,429 essentially including a cabinet with a pre-heating and final heating piece. The preheating piece has a plurality of tubes running through it which fluidically connect two manifolds arranged at each end of the preheating piece. The final heating piece also has a plurality of tubes running therethrough which fluidically connect two other manifolds at each end of the final heating piece. Seawater surrounding the vessel is pumped into a manifold and flows through the pipes to the final heating piece and into the manifold, from where the seawater is dumped into the sea. In operation, LNG flows from a booster pump into a loop circuit positioned within the preheater in the evaporator, which in turn contains a "permanent" bath of an evaporating refrigerant, e.g. propane, in the lower part. Seawater flowing through the pipes "heats" the propane in the bath and causes the propane to vaporize and rise inside the precooling piece. As propane gas contacts the loop circuit, heat is given off to extremely cold LNG flowing through the circuit and recondenses to then fall back into the bath, providing a continuous, circulating propane "heat" cycle within the preheater.

In order to improve challenges related to the solutions above, US patent no.

6,945,049 a method and system for regasification of LNG on board a floating cargo vessel before the gas is unloaded including pressure increase and flow of LNG into an LNG/refrigerant heat exchanger in which the LNG is vaporized, and flow of vaporized natural gas (NG) into an NG/steam- heat exchanger, in which NG is heated before transfer to land as superheated gas vapour. LNG in the LNG/refrigerant heat exchanger is vaporized by thermal exchange against a refrigerant that enters the heat exchanger as a gas and leaves it in a liquid state. Furthermore, the refrigerant flows in a closed circuit and through at least one refrigerant/seawater heat exchanger in which liquid refrigerant is vaporized before entering the LNG/refrigerant heat exchanger, and the pressure in the vaporized refrigerant is controlled.

In the propane loop described in US patent no. 6,945,049, the temperature difference between incoming and outgoing seawater in the coolant/seawater heat exchanger must be relatively high to avoid bulky dimensions. Typically, the evaporation temperature of the coolant is 20-25 °C lower than inflowing seawater, thus the temperature from the coolant/seawater heat exchanger is 20-25 °C lower than seawater or even lower (preheating). NG is additionally heated within an NG/steam heat exchanger of shell & tube type. The latter can be replaced by a direct NT/seawater heat exchanger in which NG is typically heated from -20 °C to slightly below seawater within a shell & tube type heat exchanger made of titanium. NG and seawater are directed to the pipe and jacket side respectively (trim heating). High pressure on the NG side makes the titanium shell & tube heat exchanger very expensive and, to reduce costs, it is designed as a fully welded heat exchanger that has straight tubes due to a significantly reduced diameter and elimination of the very expensive tube plate compared to a heat exchanger with U -tube.

Using a fully welded heat exchanger results in equipment that is impossible to open for maintenance, e.g. to clean fouling on the seawater side and to seal pipes in case of cracks. Such a solution that has fully welded heat exchangers is disadvantageous with regard to, for example, maintenance. Using seawater as one of the media means that the necessary titanium heat exchanger becomes very expensive when these must also be designed to withstand high pressures.

To further improve the technology put forward by the latter to reduce costs and enable maintenance, for example NO 20093341 describes a plant for regasification of LNG, including at least one pump pressurizing LNG pressure: an LNG/refrigerant heat exchanger that produces NG from LNG which flows through from the pressurizing pumps; a closed refrigerant loop passing through the LNG/refrigerant heat exchanger and including at least one heat exchanger, a refrigerant from the respective heat exchanger is passed through the LNG heat exchanger as a gas and leaves it in a condensed state to produce NG by thermal exchange; and a heating medium used within the respective heat exchanger to provide refrigerant in a gaseous state, where an NG/refrigerant heat exchanger is arranged in conjunction with the LNG/refrigerant heat exchanger and is connected to a closed refrigerant loop, whereby the LNG is preheated within the LNG/refrigerant the heat exchanger and NG are trim heated within the LNG/refrigerant heat exchanger by using liquid coolant from at least one heat exchanger.

Thus, LNG is pressurized by at least one pump and vaporized in a heat exchanger by condensing propane in it. 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 to be vaporized prior to entering the LNG heat exchanger. When propane is evaporated, seawater is often used.

Propane flow through the closed circuit is controlled by maintaining a stable fluid level within the condensed propane tank. Depending 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 maintaining the tank level. Alternatively, the pump flow is controlled as a function of the temperature of vaporized NG leaving the LNG heat exchanger.

The first option requires accurate level measurements within the propane tank, which involves an unfavorably large tank volume to allow stable measurement signals. This results in a need for large amounts of propane in the closed circuit. The latter is based on the temperature of seawater. When a desired value is selected for NG from the LNG heat exchanger, this fact entails difficulties regarding satisfactory adjustments of the propane pump.

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

This is achieved by a method of regulating a closed medium circuit when a primary medium is heat exchanged within a heat exchanger fed by means of a pump to be vaporized or condensed therein, the closed circuit passing through the heat exchanger and including a tank and a pump for condensed medium and in at least one heat exchanger vaporizes or condenses intermediate medium to be passed through the primary medium heat exchanger, where flow of the intermediate medium in the closed circuit is controlled as a function of the primary medium through the heat exchanger.

To ensure sufficient flow through the closed circuit, where flow of intermediate medium is a function of the primary medium is based on a fixed value of all capacities of a given liquid fraction vaporized intermediate medium into or out of the at least one heat exchanger such as 30% or , alternatively, based on a given medium flow that has an increasing liquid fraction as a function of capacity.

Medium flow can be measured and adjusted using amperage measurements derived at the propane pump, derived from pressure drop in the circuit or across the pump, using a dedicated flow meter, or equivalent and using throttling downstream of the medium pump, frequency adjustment of the pump, combining pump throttling and frequency adjustment, or equivalent .

By adjusting the closed circuit as described above, accurate measurements of the liquid level inside the tank are redundant. Such level measurements are often incorrect, especially when liquids at the boiling point are measured. The filling volume in the system can be reduced. The tank can only be configured to ensure sufficient inflow height for the propane pump at all possible volume changes due to level fluctuations within the heat exchanger(s) and expansion of liquid as a function of operating temperature.

The present invention will now be described in greater detail by means of the preferred embodiment illustrated in the appended drawings, in which: Fig.1 schematically shows a traditional heat exchange loop for primary medium by means of a closed circuit in which an intermediate medium is circulated; Fig. 2 schematically shows the new concept according to the present invention in which the flow of the intermediate medium is controlled based on the flow of the primary medium; Fig. 3 schematically shows a general heating loop, in which the present invention can be used; and Figs. 4 and 5 schematically show 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 pressurized by at least one pump Al and vaporized in a heat exchanger B by condensing an intermediate medium therein to produce NG. The heat exchanger is preferably a compact printed circuit heat exchanger PCHE (compact printed circuit heat exchanger). To vaporize LNG, a closed circuit is used in which the circulated medium is in the form of e.g. propane. After condensation, the propane is fed 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 to be vaporized before entering the LNG heat exchanger B. When propane is vaporized in the heat exchanger ( e) the heating medium is usually seawater, although any suitable medium can be used.

The new and improved control concept described herein is based on flow control of condensed propane as a function of LNG fed into the pump(s) Al, see

Fig. 2. The simplest is to assume a propane flow directly proportional to the LNG flow, as a given amount of vaporized LNG at a particular pressure and outlet temperature requires a specific amount of condensing propane.

When directly determining the propane flow relative to the LNG flow without other system parameters by using suitable means such as a controller C, an occurrence of different propane distribution should be expected. If LNG is to be vaporized at varying pressures and outlet temperatures, the energy requirement per unit of LNG varies to some extent. Flow measurements of LNG and propane can also be somewhat inaccurate.

In order to be able to regulate according to the concept above, the assumption is that propane gas discharged from the heat exchanger(s) Gl, G2 has a varying liquid fraction as a function of current and theoretical liquid level therein. The result of increasing propane flow and theoretical liquid level in the heat exchanger(s) is an increasing, respectively, liquid fraction from there and liquid level therein. 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 the propane gas, theoretical current through the closed circuit is about 30% higher than the required current. An estimate of propane flow as a function of LNG flow must take into account the liquid fraction at maximum flow through the heat exchanger.

As long as the propane flow is set sufficiently high for LNG flow, the propane level within the heat exchanger(s) Gl, G2 is adjusted automatically. Thus, a liquid fraction is obtained in the expired propane gas which provides a quantity of condensed propane at the correct level relative to the quantity of LNG. A given propane flow as a function of the LNG flow is therefore either based on a fixed value of all capacities of a given liquid fraction in propane gas discharged from the heat exchanger at maximum flow, e.g. 30% as mentioned above, or is based on a given propane flow which has an increasing liquid fraction as a function of capacity.

The LNG flow can be measured in any suitable way and some examples are:

Derived from an ampere measurement at the LNG pump(s).

Measured using a dedicated flow meter.

The propane flow can be measured and controlled in various ways and some examples are specified below.

i) Flow measurement:

Derived from an ampere reading at the propane pump.

Measured using a dedicated flow meter.

Derived from pressure drop in the circuit, across the pump

ii) Flow adjustment

When throttling a valve downstream of the propane pump E.

When adjusting the frequency of the pump E.

By combining pump throttling and frequency adjustment.

Thus, the present invention can be used in any corresponding heating system 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 loop. b) An intermediate medium must be condensed within at least one heat exchanger that uses a heating medium. c) An intermediate medium must evaporate within at least one heat exchanger that uses a heating medium. d) A medium in the form of liquid must be heated, gas must be heated, or gas must be partially or completely evaporated. e) A medium in the form of liquid must be cooled, gas must be cooled, or gas must be partially or completely condensed.

When the primary medium is condensed, it is understood that the intermediate medium must be a suitable coolant and that the tank (H) and pump for condensed intermediate medium are positioned upstream of the heat exchanger (B).

Some systems where the present invention is very useful are described in NO 20093341 and US 6945049 and schematically illustrated in Fig. 4 and 5. The publications describe facilities for LNG regasification. The first uses a combined closed propane circuit in which LNG is heated by condensing propane within a heat exchanger N and by liquid-phase propane in the 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 tanks in the system, not shown, and into at least one high pressure pump Al, A2 which pressure boosts LNG pressure, and from which pressure boost LNG flows into an LNG/refrigerant heat exchanger B. Each pump is a multi-stage centrifugal pump, for example, submerged pot (submerged pot) mounted. The LNG temperature at the entrance to the LNG/refrigerant heat exchanger is typically -160 °C, and it is preheated to -20 °C and higher at the exit. Preheating is effected by a phase transition for liquid refrigerant. The LNG/refrigerant heat exchanger can be a compact printed circuit heat exchanger, PCHE, formed from stainless steel or other suitable material.

The NG leaves the LNG/refrigerant heat exchanger B in a vaporized state and enters an NG/refrigerant heat exchanger C in which the NG is trimheated before being transported ashore as superheated gas vapour. The trim heating is carried out with temperature drift for liquid coolant. The gas vapor temperature is typically 5-10 °C lower than the seawater inlet temperature.

The cooling circuit is fed from coolant supply H, e.g. a tank, and is driven by a pump E into a semi-welded plate heat exchanger D. Although illustrated as mounted on the outside of the refrigerant supply, the pump, e.g. a centrifugal pump, be of the sunken vessel-mounted type such as the pumps Al, A2 mentioned above. Coolant is heated using seawater passing through the plate heat exchanger opposite the coolant, typically up to 2-5 °C lower than the incoming sea temperature. Heated coolant is then fed into the NG/coolant-heat exchanger C to ensure trim heating of the NG.

Cooled refrigerant leaving the NG/refrigerant heat exchanger C is depressurized by means of a control valve F before entering at least one semi-welded plate heat exchanger Gl, G2. The control valve F can be replaced by any suitable means, e.g. a fixed throat. One purpose of the control valve is to maintain pressure from the pump E through the two heat exchangers D, C above the coolant's boiling pressure at seawater temperature. Within each plate heat exchanger Gl, G2, coolant is evaporated with seawater, each is passed on opposite sides through the heat exchangers.

The vaporized refrigerant is then passed on to the LNG/refrigerant heat exchanger to be condensed while the LNG is vaporized on each side within the heat exchanger as the LNG is preheated. Condensed refrigerant from the heat exchanger is finally returned into tank H.

Many choice four variations are possible. The preheating and trim heating heat exchangers B, C can be combined into a common heat exchanger. Such a common heat exchanger has one LNG/NG path and at least one separate path for coolant in the preheating and trim heating sections respectively. Seawater sent into the heat exchanger D can be preheated by using an external heating device of a suitable type. Any suitable coolant other than seawater is applicable.

The plant shown in Fig. 5 is different by having separate closed circuits for preheating and trim heating of LN G/NG. The pre-heating is effected by a separate closed propane circuit passing through heat exchanger B, while trim heating is carried out by using a shell & tube type heat exchanger S&T. Typically, the temperature of the coolant is 20-25 °C in that under the inflowing seawater and, thus, the temperature from the coolant/seawater heat exchanger is 25-30 °C under seawater or even lower. Both circuits can use seawater as a medium to respectively condense propane within the heat exchanger(s) Gl, G2 and trim heating of NG in the heat exchanger S&T. The latter can be a direct NG/seawater heat exchanger in which NG is typically heated from - 20 °C to slightly below seawater. NG and seawater are directed on the pipe side and the shell side.

The above discussion with respect to the present invention is to be considered illustrative only of the principles according to the invention, the true thought and scope of the invention being defined by the patent claims. Although LNG and NG are specifically mentioned in the discussion of the present invention, that fact does not exclude any other suitable liquefied gases such as ethane, propane, N2, CO2 as suitable. It is understood that the present method can be used on board seagoing vessels, offshore on a platform, for example, or on land.

Claims (6)

1. Method of regulating a closed intermediate medium circuit when a primary medium is heat exchanged within a heat exchanger (B) fed by means of a pump (Al) to be vaporized or condensed therein, the closed circuit passing through the heat exchanger (B) and including a tank (H) and a pump (E) for condensed intermediate medium and at least one heat exchanger (Gl, G2) evaporates or condenses intermediate medium to be passed through the heat exchanger (B) for primary medium, characterized in that flow of the intermediate medium in the closed circuit is controlled as a function of the primary medium through the heat exchanger (B). 2. Method according to claim 1, characterized in that choosing the flow of intermediate medium is a function of the primary medium based on a fixed value of all capacities of a given liquid fraction in vaporized intermediate medium into or out of the at least one heat exchanger (Gl, G2), such as 30% or. 3. Method according to claim 1, characterized in that selecting a flow of intermediate medium is a function based on a given intermediate medium flow which has an increasing liquid fraction as a function of capacity. 4. Method according to any of the preceding claims, characterized by measuring medium flow based on amperage measurements derived at the propane pump (E), derived from pressure drop in the circuit or across the pump, using a dedicated flow meter, and the like. 5. Method according to claim 4, characterized by adjusting intermediate medium flow by means of throttling downstream of the intermediate medium pump (E), frequency adjustment of the pump (E), combining pump throttling and frequency adjustment, and correspondingly. 6. Method according to any of the preceding claims, characterized by using an intermediate medium in the form of propane.