ES2541953T3 - Boiling gas condenser - Google Patents

Abstract

Boiling gas condensing system comprising: - a condensing vessel (15) fed by liquefied natural gas, comprising: - a boiling gas inlet (14); - two separate zones without interference between them: - a packed contact area for recondensation (11) located in an upper section of the condensing vessel (15) and housing a packed bed (7) for the recondensation of boiling gas from a discharge of boiling gas from the compressors and entering through the boiling gas inlet (14), when it contacts with subcooled liquefied natural gas taken from the discharge of low pressure pumps (32); - a lower accumulation zone (12), located in a lower section of the condensing vessel (15), which serves as a liquid retention drum for high pressure pumps of liquefied natural gas (29), into which a surplus of liquefied natural gas taken from the discharge of low pressure pumps (32) through a level controller (6); - an upper inlet of the liquefied natural gas (22) disposed in the upper part of the condensing vessel (15) and through which the subcooled liquefied natural gas required to condense the boiling gas is introduced; - a lower inlet of the liquefied natural gas (23) arranged in the lower accumulation zone (12) and through which the surplus of liquefied natural gas is introduced up to a specific proportion sent; - a flow controller of liquefied natural gas (1) to control the flow of subcooled liquefied natural gas introduced into the condenser vessel (15) through the upper inlet of the liquefied natural gas (22) to maintain the desired pressure inside of the condensing vessel (15); - a level controller (6) comprising at least one level control valve (3) for controlling the level of liquefied natural gas in the lower accumulation zone (12); characterized in that the system additionally comprises: - a controlled diverting line of liquefied natural gas (24) to divert from the condensing vessel (15) the rest of the flow of liquefied natural gas taken from the discharge of low pressure pumps (32) ; - a flow controller (5) comprising at least one derivative flow control valve (4) for controlling the flow of the rest of liquefied natural gas that circulates through the diverting line (24) between a specific proportion sent to the maximum proportion sent; and because the boiling gas inlet (14) is arranged in the upper section of the condensing vessel (15).

Description

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DESCRIPTION

Boiling gas condenser

Technical field

The aforementioned invention consists of a Boiling Gas Condenser (BOG) to liquefy again the boiling gas generated by Liquefied Natural Gas (LNG).

Background

A terminal for receiving Liquefied Natural Gas and re-gasification serves to connect the continuous process of pressurization, vaporization of Liquefied Natural Gas and shipment of gas, with the intermittent process of securely docking and unloading of Liquefied Natural Gas Transporters using the Temporary storage capacity of LNG storage tanks.

In general, a Liquefied Natural Gas terminal will receive Liquefied Natural Gas from a transporter and store it in a cryogenic state, under boiling conditions, at approximately -160ºC and slightly above atmospheric pressures in Natural Gas storage tanks. Smoothie. The Liquefied Natural Gas from the storage tanks for Liquefied Natural Gas is pressurized with Low Pressure pumps and combined with Boiling Gas recondensed in the condenser. Then, Liquefied Natural Gas is pressurized by means of High Pressure pumps and sent to vaporizers, where it is vaporized at high pressure and sent to the network through the measuring station.

It is required to maintain cryogenic conditions throughout the terminal, including the discharge lines through which the LNG circulates.

As a result of the leakage of ambient heat to the LNG storage tanks and all terminal lines, steam is generated in the LNG storage tanks. This steam is called Boiling Gas. This BOG must be recovered in order to minimize atmospheric emissions and economic losses.

The most common way to recover the BOG is to take it from the LNG Storage Tanks through the BOG compressors, where it is compressed and sent to the BOG condenser for new liquefaction and combination with the flow sent.

The condenser is an apparatus where the BOG generated in the terminal is put in contact with subcooled LNG, recovered and mixed with the rest of the flow sent from LNG.

There are two main functions that a condenser could perform: one of them is to condense and recover the BOG and the other is to serve as a regulator of the LNG producing a retention between LP (low pressure) pumps and HP (high pressure).

Some condensers have both functions and others do not.

Another important feature is the operating pressure in the condenser. It could be set at a certain value, but the operator could vary its value.

There are regasification terminals where the condensers only perform one of the main functions described above.

Many condensers operate at a fixed pressure. This implies very high costs associated with high energy consumption due to the large load of the compressor. A condenser with different operating pressures will reduce the operating cost in compressors with high shipping rates by decreasing its operating pressure and minimizing the minimum shipping by increasing its operating pressure.

Some condensers reported difficulties and instability in pressure control.

Many condensers have large dimensions due to high retention times for HP pumps. Reducing this size, maintaining residence time and operating with a referral will be a good solution.

The instrumentation is simple in some condensers, which leads to out of control situations.

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Patent document US2007 / 214831-A1 discloses a boiling gas condensing system capable of working in a wide range of operating conditions by using a condensing vessel with multiple packed beds. However, it is a complex system with improved results in terms of stability in pressure control.

Description of the invention

The invention presented here is a condenser with two separate functions, without interaction between them.

It also includes a controlled shunt that allows reducing the regulation or accumulation characteristic through sophisticated instrumentation and the controls described below. This results in a smaller accumulation zone.

It is a fixed pressure condenser, but this pressure could be changed within a range based on operator preferences. This reduces the cost of operation (energy saving) in compressors with high shipping rates by reducing the operating pressure of the condenser and minimizes the minimum shipping by increasing the operating pressure of the condenser.

The objective of the present invention is to supply an apparatus (and its surrounding controls) that re-liquefies all the BOG generated in the terminal, mixes it with the rest of the LNG sent and that provides a retention time between LP pumps and HP pumps. This apparatus is a container comprising two separate sections without interference between them:

-a top section that houses a packed bed as a contact area, in which the Boiling Gas of the compressors, coming from the discharge of the Boiling Gas, is in contact with Liquefied Natural Gas, taken from the discharge of the pumps LP, for recondensation.

-a lower section that acts as an accumulation zone, which serves as a drum for liquid retention for HP pumps of Liquefied Natural Gas, in which surpluses of Liquefied Natural Gas are introduced up to half of the maximum shipment through at least one level control,

so that the excess of Liquefied Natural Gas of half of the maximum shipment is diverted from the condenser through at least one derivative flow control valve.

The BOG condenser is a bifunctional system (it serves two functions) that solves the problems indicated above. These two functions are:

-The function of condensing the Boiling Gas (BOG) generated in the terminal.

-The accumulation function of Liquefied Natural Gas (LNG), generating a retention time for HP pumps. It is an intermediate storage for the liquid between the LP pumps and the HP pumps.

These two functions are carried out in the condenser system, which is divided into two different parts:

a) The top section of the condenser houses a packed bed in which the BOG is contacted with the LNG for recondensation. The LNG is taken from the discharge of the LP pumps and the BOG comes from the discharge of the BOG compressors. BOG recondensation is possible since this vessel operates at a higher pressure (operating pressure of 4 to 8 barg) than the LNG storage tanks. The LNG pumped from the tanks is subcooled at this higher operating pressure. The LNG could increase its temperature from the pumping temperature to LP to the bubble point in equilibrium at the recondensation pressure. All this thermal load is used by the BOG to cool it to its dew point and then be condensed. This recondensation is the result of a heat transfer by direct contact between the two phases (both downward flow) in the packed bed. With a higher operating pressure in the condenser, a higher bubble point temperature could be reached, and less LNG is required to condense the same amount of BOG.

b) The lower section serves as a drum for liquid retention for LNG to HP pumps. The condenser rises from the base and the height of the edge along with the level of the liquid in the container (plus some degree of subcooling) provide the NPSH required for HP pumps.

These parts are separated in such a way that the packed contact area of recondensation is located above the lower accumulation zone. The resulting container has two different diameters, the lower one being

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The one with the greatest dimension. The two zones are completely separated, and there is no interference between them (the packed bed is not allowed to flow with the LNG (Liquefied Natural Gas)).

The LNG is fed to the vessel through three routes. The upper one is used for BOG recondensation and keeps the pressure stable. This flow is controlled either through a cascade pressure controller or through a function of the actual mass flow of the BOG entering the condenser. The operator could select the way to control the pressure.

The excess of LNG (up to half of the maximum shipment) is introduced in the lower zone (of accumulation) through the level control. This is a simple level of control.

The excess of LNG (from the middle of the maximum shipment) is diverted or derived from the condenser through the flow control. This flow control could be done, either based on the function of the total volumetric flow of the LNG to the condenser, or through a function of the reference value sent to the measuring station. The operator could select the way to control the bypass.

As a result of this scheme, there is an accumulation zone, but it is half the size of the other type of condenser due to the controlled flow deviation.

Controlled diversion control can be done based on:

- the control of the flow of the total flow of the LNG to the condenser (combined flow of the three LNG power supplies),

- the flow control of one or two of the LNG power supplies,

-the reference value of the flow sent, or

- Actual measured flow sent.

Controlled diversion can control the flow between some specific proportion sent (not exactly half of the maximum shipment) to the maximum proportion sent.

The filling gas connection, the controlled exhaust connection (pressure protection controllers) and the PSVs are located in the lower zone. Filling gas and controlled venting working pressures are outside the normal controlled pressure range. The protective level controller (following the same philosophy followed in the pressure control) also acts on the controlled diversion.

The boiling gas condenser of the invention does not have a flow control that always maintains a small flow of LNG that flows into the accumulation zone to always maintain a degree of subcooling in the HP pump, producing an NPSH (Net Head of Positive Suction, for its acronym in English). The NPSH is given only by the height of the condenser.

The advantages of the boiling gas condenser of the invention are:

-Variable pressure control and more stability.

-Without control valves at the BOG inlet.

-The operator has some flexibility to decide how the control strategy for pressure control and diversion control is carried out.

-The accumulation zone is not designed for maximum flow, it is designed for half the maximum flow.

-There is no interaction between the areas of condensation and accumulation to keep them separate.

For a stable operation of the BOG condenser, the operating pressure is controlled; the Level of Liquefied Natural Gas (LNG) inside the container and the flow of Liquefied Natural Gas (LNG) that deviates from the condenser.

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All terminals have limitations on the HHV (High Caloric Value) and the Wobbe Index specifications of the gas sent to the network. In order to adjust the quality of the gas sent, when necessary, nitrogen is mixed.

An adjustment of the Wobbe Index and the HHV is carried out by introducing nitrogen, in the gas phase, liquid phase or both at the same time in the condenser.

Brief description of the drawings The description of the different components of the system supplied above is complemented by a series of drawings in order to facilitate the understanding of their structure and mode of operation.

Figure 1 is a scheme where the main flow paths are described.

Figure 2 is a diagram of the BOG condenser process control loops for pressure, level, and

operational diversion.

The following references are indicated in these figures:

1.-FC1 flow controller

2.-FV1 flow control shut-off valve

3.-LV level control valve

4.-FV2 flow control valve

5.-FC2 flow controller

6.-Level controller, LC1

7.-Packed bed, P1

8.-Isolation valve 1, V1

9.-Isolation valve 2, V2

10.-Pressure controller, PC1

11.-Packing zone of recondensation

12.-Lower accumulation zone

13.-Pressure safety valve, PSV

14.-BOG mass flow, F1

15.-Condenser container

16.-Filling gas control valve, PV1

17.-Loop gas pressure control loop, PC2

18.-Pressure relief valve, PV2

19.-Pressure control loop of the released pressure, PC3

20.-Level controller that acts in case of low level of LNG, LC2

21.-Level controller that acts in case of high level of LNG, LC3

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22.-Top entry of the LNG 23.-Bottom entry of the LNG 24.-Controlled diversion line of the LNG 25.-Maintenance diversion line 26.-HP pump release line 27.-HP bounce line 28.-LNG output line 29.-LNG output line to the HP pump 30.-Filling gas inlet line 32.-LNG supply LP pumps 33.-Controlled discharge connection line Detailed Description The condenser design shown in Figure 1 is based on a type of fixed pressure condenser and includes

a container (15) fed by three lines of Liquefied Natural Gas, in which the container (15) comprises two

Separate areas without interference between them: -a packaged contact area of recondensation (11), located in an upper section of the container (15), which houses a packed bed (7), in which the Boiling Gas, coming from a Discharge of Boiling Gas compressors (14), is in contact with Liquefied Natural Gas (22), taken from the discharge of the LP condensing pumps (32).

-a lower accumulation zone (12), located in the lower section of the container (15), which serves as a liquid retention drum for HP Liquefied Natural Gas pumps, into which the surplus of Natural Gas is introduced Liquefied (23) up to half of the maximum shipment through a level control that includes a level control valve (3),

so that the excess of Liquefied Natural Gas of half of the maximum shipment, the condenser is diverted to

through a flow control bypass (24) comprising a derived flow control valve (4). This condenser has the option of selecting the reference value of the pressure by means of the operator in the range of 4 to 8 barg. This reference pressure will be selected based on the gas flow rate. The desired pressure is controlled with the flow of the LNG to the top of the condenser.

The operating pressure range (in this case described from 4 to 8 barg) could vary based on the requirements of the

operator and customer The condenser pressure could be set from the lowest value of 4 barg with high shipping rates up to the maximum value, 8 barg, with minimum shipping rates.

If the pressure in the condenser is low, more LNG flow is required to condense the same amount of BOG as the required flow at a higher pressure. When a high shipping rate is required, the pressure in the condenser could be reduced since there is no limitation in the flow of the LNG (we are far from the minimum shipping) to the upper contact section. This is desirable since the BOG compressor requires less workload with low discharge pressure, saving energy.

A high pressure (8 barg) is set in the condenser during the minimum shipping requirement. Also during minimum shipping requirements, the generation of BOG is higher, since the level of liquid in the LNG storage tanks falls very slowly.

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When the shipping rates are higher than the minimum shipping rates, the pressure in the condenser could be set between 4 and 8 barg.

The condenser has an inlet for the filling gas from the shipping vaporizers, upstream of the measuring station. This filling gas allows to keep the pressure of the condenser at a minimum value. This flow is controlled by means of a loop for pressure control acting on a control valve. In a sustained form of operation, with high shipping rates, the generation of the BOG in the terminal could be close to zero. Then, the BOG compressors will stop. The pressure of the condenser will drop and fill gas must be required.

The condenser also has a direct controlled vent connection back to the LNG Storage Tanks. This is to avoid high pressure in the condenser and to prevent the PSV from opening. This release is controlled with a loop to control the pressure acting on a control valve.

At least one pressure safety valve (13) is the last protection of the condenser against overpressure. Two pressure relief valves can also be accommodated, one in operation and the other replacement, to facilitate in-line maintenance of one of these valves. These valves are mechanically intertwined, one closed and the other open. Each valve is designed for complete flow release.

The description of the input and output lines of the BOG condenser is shown in Figure 1 in which the main flow paths are described, to follow the description:

Upper LNG input for recondensation (22)

The subcooled LNG required to condense the entire BOG is introduced at the top of the condenser, reaching equilibrium. This flow is controlled to maintain the desired pressure inside (4 to 8 barg).

BOG top input to the condenser (14)

In the upper part of the condenser, the BOG is also introduced from the compressors of the BOG to be condensed.

Lower LNG input to the condenser (23)

When the shipping rate is higher than the flow required in the upper section, the remaining LNG is supplied to the lower section of the condenser more or less until half of the maximum shipment. The bottom retainer of the condenser should be calculated in seconds (between 15 and 60 seconds) of flow of half the maximum shipment associated with the LNG flow.

LNG controlled diversion (24)

If the shipping rates are also greater than half of the maximum shipping associated with the LNG flow, the rest of the sent flow is diverted from the condenser. This could be done using a controlled bypass (control valve is required).

Controlled diversion control is based on the flow control based on the total LNG flow to the condenser (combined flow of the three LNG supplies).

Maintenance diversion (25)

This diversion is to remove the condenser from service and keep the terminal running. This bypass is located outside the isolation valves (8 and 9).

LNG output (28)

This line connects the output of the condenser with the suction of the HP pumps (29).

HP Pump Release Line (26)

This line connects the vent lines of the HP pumps to the condenser.

Bounce line of HP pumps (27)

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This line connects the bounce lines of the HP pumps to the condenser. This line could contain the flow of two phases.

Trailed gas connection (30) and controlled exhaust connection (33)

The filling gas line (30) connects the outlet of the vaporizers (upstream of the measuring station) with the condenser operating at low pressure. This allows the LNG to enter the shipment to keep the pressure in the vessel at a minimum value.

The controlled vent connection line (33) connects the condenser with the LNG Storage Tanks in the high pressure control. This prevents the pressure in the vessel from increasing by opening the control valve, unfolding towards the head of the BOG.

Figure 2 shows the BOG condenser process control loops for pressure, level, and operational bypass.

a) Condenser pressure control

The BOG condenser operates in the fixed pressure mode. The operator selects the desired operating pressure of the BOG condenser between 4 and 8 barg. You could have another operating pressure range based on customer requirements. The pressure in the BOG condenser is controlled through the flow controller (1) (Flow Controller). The flow controller (1) manipulates the shut-off valve of the LNG (2) that supplies the LNG to the section of the packed bed (7) of the BOG condenser, where the packed bed (7) consists of either a random packing or of a structured packing. The pressure in the BOG condenser can be as low as 4 barg during periods of high flow sending and as high as 8 barg during periods of minimum shipping rate.

The pressure in the BOG condenser can be controlled either through a pressure feedback control loop or through a pressure feedback control loop. The preferred system can be selected by the operator.

a.1) Pressure recognition / pressure control

The main function of the flow controller (1) to control the flow rate of the subcooled LNG to the top is to keep the BOG condenser at a fixed pressure and to operate in a stable operating region.

The reference value of the flow controller (1) is derived from a function of a specific factor and the measured mass flow of the BOG (14) entering the condenser. This specific factor is the result of a function of the reference value of the pressure desired by the operator, the pressure measured in the vessel and a correction temperature for both the LNG and the BOG. Once the operator enters the pressure reference value, depending on the BOG flow, the LNG flow is calculated and fixed to the LNG flow controller (1) as the reference value. The operator can adjust the pressure reference value to achieve the desired operating pressure of the BOG condenser.

This type of control is faster than using only a cascade pressure feedback control over the LNG flow controller (1).

a.2) Feedback pressure recognition / control

The main function of the flow controller (10) is to control the fixed pressure required in the BOG condenser. The pressure controller operates through a cascade control loop, resetting the reference value of the LNG flow controller (1) to keep the BOG condenser at the desired pressure.

a.3) Pressure protective control

The following pressure protection controls are available during the operation of the condenser:

-The condenser has a filling gas inlet (30) sent from the vaporizers, upstream of the measuring station. This filling gas (30) allows to maintain the operating pressure of the condenser at a minimum pressure reference value (less than 4 barg). This flow is controlled by a pressure control loop (17) that acts on a filling gas control valve (16).

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-The condenser also has a controlled vent connection to the head of the BOG. This is to avoid high pressure (greater than 8 barg) in the condenser and prevents the pressure safety valve (13) from opening. This release is controlled with the pressure control loop (19) acting on a control valve (18). This is achieved with the separate loop for pressure control (19).

5 However, only one pressure controller can be included for both the filling gas (30) and the pressure release.

b) Condenser level control

The level in the lower section of the condenser is controlled using the level controller (6) that manipulates the lower inlet valve (3) of the LNG (two level control valves (3) can also be included). If the

10 level, the level controller (6) closes the valve (3). If the level falls, the response of the level controller is the opposite.

Level protection controllers are installed for protection against a high or low level:

-In the case of a low level, a level controller (20) reduces the flow sent to HP to maintain the level, closing the inlet control valves of the vaporizers, canceling the signal from the measuring station.

15 -In case of high level a level protection controller (21) will cancel, through a selection process, the signal to the lower inlet valve (3) of the LNG, to the bypass control valve (4) controlled by the LNG (two bypass valves for flow control can also be included) and to the shut-off valve for flow control at the top (2) of the LNG, closing them if necessary.

c) Condenser deflection control

20 This deviation is located between the isolation valves of the BOG condenser (8 and 9). The lower section has been sized for an LNG flow rate equivalent to half the maximum shipment. When the shipment through the LNG terminal exceeds this value, the LNG will be sent through an operational diversion to the suction head of the HP pumps.

A flow controller (5) at the operational diversion that receives its reference value in cascade from the value

The shipping reference of the terminal as a whole will open the bypass valve (4) if the shipping flow rate exceeds half of the maximum shipping rate. It is also possible to use the total flow rate of the LNG to the condenser to control this deflection by means of a function. The operator will decide the control he will use.

Claims (5)

5 10 fifteen twenty 25 30 35 40 1. Boiling gas condenser system comprising: -a condenser vessel (15) fed by liquefied natural gas, comprising:

•

a boiling gas inlet (14);

•

Two separate zones without interference between them:

-a packaged contact area for recondensation (11) located in an upper section of the condenser vessel (15) and housing a packed bed (7) for the recondensate of the boiling gas from a discharge of boiling gas from the compressors and which enters through the boiling gas inlet (14), when it contacts with subcooled liquefied natural gas taken from the discharge of low pressure pumps (32); -a lower accumulation zone (12), located in a lower section of the condensing vessel (15), which serves as a liquid retention drum for high-pressure pumps of liquefied natural gas (29), into which a surplus of liquefied natural gas taken from the discharge of low pressure pumps (32) through a level controller (6);

•

an upper inlet of the liquefied natural gas (22) disposed in the upper part of the condensing vessel (15) and through which the subcooled liquefied natural gas required to condense the boiling gas is introduced;

•

a lower inlet of the liquefied natural gas (23) disposed in the lower accumulation zone (12) and through which the surplus of liquefied natural gas is introduced up to a specific proportion sent;

-a flow controller of liquefied natural gas (1) to control the flow of subcooled liquefied natural gas introduced into the condenser vessel (15) through the upper inlet of the liquefied natural gas (22) to maintain the desired pressure inside of the condensing vessel (15); - a level controller (6) comprising at least one level control valve (3) for controlling the level of liquefied natural gas in the lower accumulation zone (12); characterized in that the system additionally comprises: - a controlled diversion line of liquefied natural gas (24) to divert the remaining flow of liquefied natural gas taken from the discharge of low pressure pumps (32) from the condensing vessel (15); - a flow controller (5) comprising at least one derivative flow control valve (4) to control the flow of the rest of liquefied natural gas that circulates through the diverting line (24) between a specific proportion sent to the maximum proportion sent; and because the boiling gas inlet (14) is arranged in the upper section of the condensing vessel (15).

2.

Boiling gas condensing system according to claim 1, further comprising an inlet of filling gas (30) sent from the vaporizers upstream of a measuring station to maintain the operating pressure of the condensing vessel (15) at a value of minimum pressure reference and a controlled vent connection (33) to avoid high pressure in the condensing vessel (15).

3.

Boiling gas condensing system according to claim 2, further comprising at least one filling gas control valve (16) for filling gas and at least one control valve (18) for pressure relief.

Four.

Boiling gas condensing system according to claim 2 or 3, further comprising a pressure control loop (17) of the filling gas and a pressure control loop (19) for pressure release.

5.

Boiling gas condenser system according to any one of the preceding claims, further comprising a flow control shut-off valve (2) controlled by the liquefied natural gas flow controller (1), for supplying LNG to the packed bed.

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