William P. Schmidt Janet Firley Mitchell Matthew J. Okasinski Jeremy D. Beard

William Schmidt, Air Products

OPTIMUM COMPRESSOR CONTROLS FOR CLOSED LOOP REFRIGERATION

Air Products and Chemicals

Abstract This paper discusses compressor control for closed refrigeration systems used in LNG liquefaction. Similarities and differences between open and closed systems are presented. This paper focuses on one key difference: an open system’s mass inventory can change; it is often controlled by adjusting a compressor flowrate. However, in a closed system, the refrigeration circuit inventory does not change (that is the definition of a closed system), so it cannot be changed using compressor flowrate. This paper focuses on the mixed refrigerant (MR) system frequently used in LNG liquefaction processes. Specific results presented are:

A simple model demonstrates a key feature of a closed refrigeration system: it has two degrees of freedom; selecting two parameters specifies the system.

For LNG liquefaction, the required refrigerant mass flowrate is determined by the LNG production rate, leaving a single degree of freedom. Compressor speed is often chosen as this degree of freedom.

Steady state process simulations show that over a range of production rates, power consumption is minimized by running the compressor at the design speed and adjusting the refrigerant system inventory.

At turndown production, less refrigerant flowrate is needed. The process efficiency can be improved by lowering the MR system inventory. If the MR inventory is not changed, slightly reducing compressor speed improves process efficiency, but not as much as reducing the MR inventory.

The MR compressor speed should not be varied to control suction pressure, because the suction pressure is mostly affected by circuit inventory and mass flowrate. These will compete with the speed control. Changing the MR compressor speed has a large impact on compressor head, which can affect process efficiency.

LNG process control strategies can be improved by automatically setting compressor speed based on refrigeration demand. This can be effectively integrated into the overall plant control strategy.

INTRODUCTION

Liquefied Natural Gas (LNG) that is produced in large baseload plants is an increasingly important clean energy source. The vast majority of the world’s LNG is produced in Precooled Mixed Refrigerant (MR) LNG Processes. These processes cool and liquefy natural gas using two refrigeration systems: precooling and main (Figure 1). The precooling system cools the feed and main refrigerant stream to between -25 to -50°C. The main refrigerant then liquefies and subcools the natural gas, converting it to LNG in the Main Cryogenic Heat Exchanger (MCHE), which

is a Coil Wound Heat Exchanger (CWHE) (1). The LNG leaves between -140 and -160°C and is sent to storage.

Figure 1 – LNG Liquefaction Process

The main refrigerant is a mixture, typically containing N2, C1, C2 and C3, so it is called “Mixed Refrigerant” (MR). The precooling refrigerant is most often pure C3, in which case the liquefaction process is referred to as the Air Products AP-C3MR

TM LNG process. In other cases, the precooling refrigerant is a mixture of hydrocarbon

components from C1 to C4, referred to as the Air Products AP-DMRTM

LNG processes (DMR stands for “Dual Mixed Refrigerant”).

In the MR refrigeration circuit, the high pressure MR is cooled against propane to approximately -35°C, and it partially liquefies. The stream is separated in the HP MR separator into MR Liquid (MRL) and MR Vapor (MRV). The MRL enters the MCHE, where it is subcooled, removed from the MCHE at an intermediate point, flashed over a Joule-Thomson (JT) valve and sent to the MCHE shell. The MRV enters the MCHE, where it liquefies and is subcooled. It exits at the LNG temperature, flashes over a JT valve, and returns to the shell. The MRV and MRL boil on the shell side, providing the refrigeration to liquefy and subcool the incoming natural gas and MR. The vapor MR which leaves the MCHE is compressed in a two or three stage centrifugal compressor.

Concepts that are used throughout this paper are “closed” and “open” systems. In a closed system, mass neither enters nor leaves. An open system has mass entering and/or leaving. To operate an open system at steady state, the system inventory must not change. The inventory is measured as shown below:

Liquid inventory is measured by liquid level.

Vapor inventory is measured by pressure.

To maintain a constant inventory in an open system, the inlet and outlet flows must be equal.

In an LNG liquefaction process, the LNG circuit is an open system. Vapor natural gas enters the MCHE and leaves as a liquid. In most LNG liquefaction process, the refrigerant circuits are closed systems; the refrigerants are recycled, so that none is added or subtracted when the plant is running at steady state.

A key question for the LNG liquefaction process is how to control the MR compressor. This paper will first look at the general principles of controlling open and closed systems, and then apply these to controlling the MR compressor. It will primarily focus on the proper control schemes to keep the LNG process operating at a steady state, and it also discusses how these control schemes function when ramping the process between steady states.

Pre-coolingFeed

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Mixed Refrigerant (MR)

MRL

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Pre-cooling

William Schmidt, Air Products

OPEN LIQUID SYSTEMS

Figure 2 is a simple example of an open system, where a liquid flows into a vessel and is withdrawn by a pump. When the inlet and outlet flows are equal, the system is at steady state and the vessel level is constant. The question is how to control the pump so that the two flows are equal.

Figure 2

Figure 3a shows one possible control method. The pump recycle valve is manipulated to maintain a constant liquid level. Figure 3b uses speed to control the outlet flowrate; speeding up the pump increases the outlet flowrate; slowing it down decreases the outlet flowrate.

Figure 3a Adjust recycle valve Figure 3b Adjust pump speed

This example illustrates two key points:

It is not necessary to measure flowrate of FI-1 or FI-2 to match the flowrates. Flowrates are matched by maintaining a constant level.

The vessel can be in material balance (i.e., no gain or loss of mass) at any level.

OPEN VAPOR SYSTEMS

Vapor and liquid systems are similar. However, because a vapor is compressible, the inventory is measured by pressure, and a constant pressure indicates steady state. Figures 4a and 4b show two possible control schemes for an open vapor system. In Figure 4a, adjusting the compressor recycle valve will keep the vessel pressure constant. In Figure 4b, the speed is adjusted to maintain the pressure setpoint.

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Figure 4a Figure 4b

The same two basic principles from the liquid system apply, except that “pressure” is substituted for “level”:

It is not necessary to measure flowrate of FI-1 or FI-2 to match the flowrates. Flowrates are matched when the vessel pressure is maintained constant.

The vessel can be in material balance with no accumulation at any pressure.

COMPRESSOR EFFECTS ON OPEN VAPOR SYSTEMS

Baseload LNG facilities use centrifugal or axial refrigerant compressors. These compressors have a key relationship between head and suction volumetric flow, and Figure 5 shows the general relationship: as suction volume flow increases, the head decreases

Figure 5 – Typical Relationship between Flow and Head

An important point for compressor control is that head and pressure ratio are linked through the equation below:

𝐻 = 𝑍𝑅𝑇𝑖𝑛𝑛

𝑛−1 ((

𝑃𝑜𝑢𝑡

𝑃𝑖𝑛)

(𝑛−1)𝑛⁄

− 1) (Eqn 1)

The implications of Eqn 1 are that for a fixed head:

Higher inlet temperature (Tin) decreases the pressure ratio (Pout/Pin).

Lower suction pressure (Pin) decreases the discharge pressure (Pout) because the pressure ratio is constant

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The characteristics as expressed in Figure 5 and Eqn 1 give additional control possibilities when using a centrifugal compressor in an open vapor system:

Throttle the compressor discharge (Figure 6a) to raise the discharge pressure and lower the volumetric flow through the compressor. In turn, this decreases the mass flowrate. The mechanism is that the higher discharge pressure and the same inlet conditions increases the pressure ratio. This moves the operating point to the left on Figure 5, reducing the volumetric and mass flowrates.

Throttle the compressor suction (Figure 6b). The lower suction pressure requires a higher head for the same discharge pressure, so the compressor volumetric flow falls, reducing the mass flowrate. The mass flowrate is further reduced due to the lower suction pressure and density, relative to the pressure/density in the tank; i.e., the same volume of flow contains less mass.

Figure 6a Figure 6b Compressor Control with Suction or Discharge Throttling

CLOSED SYSTEMS

The concepts presented so far have been for “open” systems, in which a fluid enters and leaves the system. However, most refrigeration circuits are “closed” systems; the refrigerant fluid moves within the closed circuit, but no mass is added or removed. Refrigerant systems typically used in LNG facilities (e.g., MR or propane) are closed systems.

Returning to the simple liquid pump example, such a system is shown in Figure 7 (the vapor closed system will be described later).

Figure 7 – Closed Liquid System with Invalid Control Scheme

Figure 7 shows that the inlet to the vessel is now connected to its outlet. FI-1 and FI-2 are now always equal because they are on the same line. Because this closed system operates with a constant inventory, one can see that attempting to change the level by adjusting the recycle valve will be ineffective. Adjusting the pump recycle will change the circulating flowrate, but it will not change the level. This contrasts with an open system (Figure 3a), where it is possible to control the level (inventory) with pump recycle flowrate.

If this fundamental difference between an open and closed system is not recognized, and the control system in Figure 7 is used, the recycle valve will end up in one of two positions, depending on the choice of the level setpoint:

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Operating level below setpoint: The controller will attempt to raise the level by opening the pump recycle valve to reduce FI-2 flow. This action has no effect on the operating level, so the pump recycle valve will be driven fully open, resulting in minimum circulating flowrate.

Operating level above setpoint: The control system will attempt to lower the level by closing the pump recycle valve to increase the FI-2 flow. Because this action has no effect on the operating level, the pump recycle valve will be driven fully closed, resulting in maximum circulating flowrate.

Table 1 below contrasts open and closed liquid systems as shown in Figures 3a and 7:

Table 1 – Comparing Open and Closed Systems

Open System Closed System

Inventory Can Vary Fixed

Flowrates at constant inventory Matches inlet flowrate Can Vary

Now consider a closed system that circulates vapor. In a closed system, the inventory is fixed. The total mass inventory in the refrigeration circuit is the sum of the masses in the high and low pressure volumes:

Mtotal = MLP + MHP

where Mtotal = total mass in the system

MLP = mass in the low pressure portion of the system MHP = mass in the high pressure portion of the system

For an ideal gas, the total mass inventory can be written as

Mtotal = (MW x PV/RT)LP + (MW x PV/RT)HP

where MW = Molecular weight of the vapor (assumed constant)

P = Pressure of the section of the system (high or low pressure) V = Volume of section of the system (high or low pressure) T = Temperature (assumed constant) R = Gas constant

Which can be simplified and re-arranged (“a” and “b” are constants made from the other variables):

PLP = a - b * PHP (Eqn 2)

This shows that for a defined closed system (i.e., fixed volumes, total mass and temperatures), there is a linear relationship between the high and low pressures; if one goes up, the other must go down. (For non-ideal gases, the relationship is conceptually the same, but mathematically more complex). This is a key learning: in a closed system, the high and low pressures are not independent of each other.

Now consider the centrifugal compressor. It has a characteristic relationship between pressure ratio (head) vs. volumetric flowrate, as shown in Figure 5 above. In addition, a different compressor speed gives a different curve. In mathematical terms,

1

Q = f (PHP / P LP, rpm) (Eqn 3)

where Q = Compressor volumetric flowrate rpm = Compressor speed

1 This equation assumes that the compressor Anti-Surge Valve (ASV) is closed. The model can be extended to include this

feature; however, it complicates the model. The simple model is used to illustrate the key features of closed vapor systems.

Finally, because the Molecular Weight (MW) of the fluid is known, the mass flowrate (W) can be computed with equation 4:

W = MW PLP Q / RT (Eqn 4)

where W = Compressor mass flowrate This gives three equations (Eqn’s 2, 3 and 4) and 5 variables: PLP, PHP, Q, W and rpm. Therefore, the refrigeration circuit has two degrees of freedom; if two of these five variables are selected, the other 3 are determined.

This leads to the system in (Figure 8). To make this a working system, an aftercooler is added to remove the heat of compression and a discharge valve is added to provide resistance to flow and develop a pressure rise across the compressor.

Figure 8 – Closed System Vapor System with Simplified Control System

Expanding this explanation, the system is specified by setting the following:

Volumes of high and low-pressure circuits

Total mass inventory charged to the system

Head/flow curve (Compressor characteristics)

First consider the case with a fixed speed, leaving a system with 1 degree of freedom. The single degree of freedom can then be chosen as mass flowrate, and the results plotted with mass flowrate as the independent variable. With parameters chosen to give similar values to an MR system of an AP-C3MR LNG Liquefaction Process, Figure 9 shows the suction and discharge pressures as a function of mass flowrate for 2 different inventories. The mass flowrate is reduced by closing the compressor discharge valve, increasing the flow resistance, while simultaneously increasing the compressor head and pressure ratio. The increased pressure ratio lowers the suction pressure and raises the discharge pressure. The magnitude of the pressure change depends on the relative high and low pressure volumes and the refrigerant inventory. This shows that for a fixed inventory, each chosen flowrate results in a single set of high and low pressures.

PI

FI1

FIC2

Figure 9 – Closed Vapor System Operating Map

Figure 9 also shows the effects of adding 10% mass inventory to the system. For a given mass flowrate, the operating pressures increase. Note that for a given volumetric flowrate, as the suction pressure rises, the mass flow increases. Therefore, adding mass inventory shifts the operating curves up and to the right. A key learning from this example is that adding mass inventory allows higher circulating mass flowrates and removing mass inventory allows reduced circulating mass flowrates.

Dual shaft gas turbines are being used more frequently in LNG liquefaction plants, where in the past, single shaft turbines have been used. Because single shaft turbines have a very small speed range (typically 95-101% of design), speed control cannot give a wide flow variation. However, the dual shaft turbines can drive the compressor over a much wider speed range (nominally 70% to 110% of design. Varying speed may help to improve operation at off-design conditions (e.g., turndown).

However, speed has more effects on the system than just capacity. While capacity varies proportionally with speed, head varies with the square of speed. This means that reducing the speed to reduce flowrate also reduces the compressor head, which translates into significantly lower pressure ratio. This will decrease the discharge pressure and raise the suction pressure. Figure 10 shows the simple all-vapor system, where the speed is reduced 5%, with the following characteristics for a given mass flowrate:

The compressor is closer to surge

The suction pressure rises and the discharge pressure falls

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Figure 10 – Closed System Operating Map, with Speed Variation

This simple model shows that it is possible to independently set the circulating mass flowrate and suction pressure with the discharge valve and compressor speed, which then determine the pressures and volumetric flowrate. However, this may have undesirable consequences for the LNG liquefaction process if the discharge pressure is too low to efficiently produce refrigeration.

This section provides some key learnings about closed vapor systems:

The suction and discharge pressures are linked. If the suction pressure goes up, the discharge pressure must come down, by simple material balance.

For a closed vapor system with a centrifugal compressor at a constant speed, there is one degree of freedom, so setting the circulating flowrate determines the suction pressure. Combined with the first point above, the discharge pressure is also determined.

Increasing the system inventory (i.e., raising the suction pressure) allows for higher circulating mass flowrates, and increases the discharge pressure.

Adding speed control gives an additional degree of freedom. It allows the circulating mass flowrate and suction pressure to be independently adjusted. However, changing speed has a large impact on compressor head, which strongly impacts the discharge pressure. Reducing the compressor speed raises the suction pressure but decreases the discharge pressure. As will be discussed below, this can have significant effects on LNG liquefaction processes.

EXTENDING TO MR SYSTEMS

MR systems are more complicated than the all-liquid and all-vapor systems discussed above. However, concepts from these simple studies can be readily applied. The MR systems contain significant amounts of both vapor and liquid.

The vapor phase inventory is the primary factor in setting the MR system pressures. Vapor inventory is adjusted by adding or subtracting the light components, (N2 and C1) and affects the system pressures.

The liquid inventory is primarily adjusted by adding or subtracting the higher boiling components (C2 and C3). The liquid inventory is in the HPMR separator, MCHE shellside and piping..

The biggest difference between the simple systems discussed above and the MR system is that the MR system has some significant constraints placed on it by the LNG liquefaction needs. The constraints are

The LNG feed rate determines how much refrigeration is needed, which sets the required MR flowrate.

Refrigeration per unit mass of refrigerant is strongly affected by the pressure ratio. While there are specific requirements of each facility, the optimum MR compressor suction pressure is typically 2-4 barg and the

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P,disch @ 100% speed

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P,suct @ 95% speed

P,suct @ 100% speed

discharge pressure is 45 to 60 barg. If the pressure ratio deviates from the optimum, the process becomes less efficient, and if pushed too far, then the process no longer functions.

This latter observation puts an additional constraint on using compressor speed to control MR compressor capacity. For the MR system, it is better to think of speed primarily changing the compressor head, with a more subdued effect on flowrate.

CASE STUDY – STEADY STATE SIMULATION

The goal of this case study is to determine the best compressor speed for a given production rate. As the production decreases, less refrigeration is needed. It has been proposed that less refrigeration demand means that the MR compressor is “too big” and reducing its speed will reduce energy consumption. To test this hypothesis, the optimized operating conditions have been calculated at several turndown production rates using steady state process simulations.

The study covered the liquefaction process, with the following assumptions:

The simulations minimize the power consumption for a fixed LNG production.

The precooling system is kept constant; these studies only consider the main MR refrigeration system.

A typical compressor curve was used to determine the suction and discharge pressure.

The compressor speed was allowed to change from 70 to 100%.

The circulating MR composition at the compressor is fixed.

The compressors will recycle to prevent surge, if needed.

Inventory in the system is not constrained. The inventory is resultant from the system pressures.

Steady state solutions were found, and the results are shown in Table 2. Production is decreased in cases SS-1 through SS-3, while the speed is calculated for optimum performance. Case SS-4 is at both reduced speed and production. (“SS” signifies “Steady State”).

Table 2 – Optimum Performance Steady State Simulations

The learnings from this study are

1) As the production decreases from SS-1, the specific power decreases for SS-2 and SS-3. This is because the MCHE and other equipment are oversized for the given LNG flowrate. This improves the heat transfer in the MCHE and reduces the pressure drops.

2) As the production decreases, less refrigeration is needed, which reduces the MR flowrate. Options to reduce the flow are:

a. Reduce suction pressure by removing inventory. b. Maintain the suction pressure and recycle the compressor. However, this consumes more power.

Cases SS-1 through SS-3 show that the most efficient method to reduce the MR flowrate is to lower the suction pressure, i.e. reduce inventory, and keep the compressor from recycling.

3) Reducing the compressor speed raises the suction pressure and lowers the discharge pressure (SS-4), and also increases power consumption.

Case SS-1 SS-2 SS-3 SS-4

LNG % Nameplate 100% 90% 84% 85%

MR compressor Speed 100% 100% 100% 95.3%

Suction Pressure (bara) 4.32 3.77 3.45 4.85

Discharge Pressure (bara) 48.88 45.49 40.05 44.53

Ratio 11.30 12.05 11.60 9.17

Power to recycle 0% 0% 0% 0%

MR Spec Power (relative) 100% 96.4% 96.1% 101.4%

a. In cases SS-1 through SS-3, the simulation optimization was free to adjust the speed which minimized power. In all three cases, the design speed gave the minimum power, primarily because it kept a high pressure ratio.

b. In case SS-4, the upper limit on speed was set to 95.3% of design. At this lower speed, the head decreased significantly, reducing the pressure ratio from over 11 to nearly 9.

4) Comparing cases SS-3 and SS-4 show there is no benefit to decreasing speed at turndown; decreasing the speed increased the overall power consumption.

5) This study allows the inventory to vary, as shown by the reduction in suction and discharge pressures. This is effective for long term turndown operation; however, for short term operation, it is impractical to vent refrigerant.

CASE STUDY – DYNAMIC SIMULATIONS

To address the issue of turndown with a constant MR inventory, a constant inventory study was performed using dynamic simulation. This case study also investigates if speed control could provide significant benefits for turndown or off-design operation.

Using dynamic simulation to study off-design conditions is a more realistic model of the plant. There are two primary benefits:

The system is maintained at a constant mass inventory. As shown in the simple vapor-only study, the inventory has a large effect on the operating conditions. Ignoring this, as the steady state simulations did, misses a key factor in setting the operating conditions.

Dynamic simulation shows that it is possible to move from one state to another and provides a control strategy to get there.

Three cases were run (“DS” stands for “Dynamic Simulation):

Case DS-1 – Baseline at 100%, to match the steady state design run SS-1

Case DS-2 – Turndown to 85% production, keeping the system inventory constant, while maintaining the MR compressor speed at 100%

Case DS-3 – Turndown to 85% production, keeping the system inventory constant, reducing the MR compressor speed by 4.7%.

The table below shows the results for these three cases, along with the results from SS-3, the optimized steady state simulation at turndown production (effectively, turndown with the optimum reduced system inventory).

Table 3 – Dynamic Simulation with Constant MR Inventory

Case DS-1 DS-2 DS-3 SS-3

LNG % Nameplate 100.0% 85.0% 85.0% 84%

Speed 100.0% 100.0% 95.3% 100.0%

Suction Pressure (bara) 4.28 3.79 4.45 3.45

Discharge Pressure (bara)

44.84 45.72 43.94 40.05

Compression Ratio 10.49 12.05 9.88 11.60

Power to recycle 0.0% 4.5% 2.7% 0%

MR Spec Power (relative) 100% 106.8% 103.8% 96.1%

The following observations can be made:

Case DS-2 shows that if the compressor is not slowed down and the inventory is maintained constant, the compressor must recycle to prevent surge. This contrasts with Cases SS-2 and SS-3 of the steady state simulation study, which reduce the refrigerant inventory which lowers the suction pressure and mass flowrate and prevents the compressor from recycling.

Comparing cases DS-2 and DS-3 show that at turndown, slowing down the compressor does decrease the power consumption, relative to turndown at full speed. The benefit comes from reducing the recycle flowrate. Note that although the production is reduced by 15%, the speed is only reduced by 4.7%.

As expected from the simulations above, slowing the speed from DS-2 to DS-3 dramatically affects the system pressures. The suction pressure rose from 3.79 to 4.45 bara, the discharge pressure fell from 45.72 to 43.94 bara and the compression ratio decreased from 12.05 to 9.88.

Comparing DS-2 and DS-3 to the steady state simulation SS-3 shows that although reducing the speed from DS-2 to DS-3 did decrease power consumption, SS-3 shows that the power could be further reduced by reducing the MR inventory and speeding up the compressor. This would be the best operating mode if the plant were to run at turndown for prolonged periods.

These studies also show that the MR compressor speed should not be controlled with the MR suction pressure. It is true that the speed does impact the suction pressure, as can be seen when comparing SS-3 vs SS-4 and DS-2 vs DS-3. However, other variables have a far greater impact, such as MR inventory and required MR mass flowrate. In particular, the MR inventory will have a large impact on the suction pressure, and this will dominate any impact of speed variation. Also, varying speed has the unintended consequence of significantly changing the discharge pressure, a key parameter in LNG liquefaction. Therefore, if the speed is varied, it should be varied based on MRL flowrate and the change in speed should be small.

PRODUCTION RATE CHANGES

The previous discussion focused on the optimum operating parameters for a steady state. An additional study was performed to demonstrate that the Air Products Enhanced Control Scheme (AP-ECS, described in references (2) and (3)) can be extended to include speed control when moving between production rates. The AP-ECS operating philosophy treats LNG liquefaction as a “refrigeration-led” process: the refrigeration is set, and the LNG flow is varied to absorb it. The extension that follows from the reasoning is that as the MRL flowrate is reduced, the compressor “size” can be reduced by decreasing speed. That is, a smaller MRL flowrate requires a smaller MR compressor. Work done shows that the fractional speed reduction should be much less than the fractional MRL flow reduction, primarily to prevent excessively decreasing the compressor head. However, once the MR compressor anti-surge system becomes active and opens the anti-surge valves (ASV), the speed control is disabled. This prevents the speed control and anti-surge control (ASC) from simultaneously trying to control the MR compressor performance.

Ramping simulations were done in which production decreases from 100% to a target over one hour, is stabilized for 1.5 hours, and is then ramped back up in approximately 1 hour. To do this, the production setpoint is ramped. The AP-ECS then adjusts all other process parameters (valve position, compressor speed) to smoothly and efficiently move to the new production setpoint.

Two cases were run, from 100% to 85% production and back, and from 100% to 70% and back. Figures 12 and 13 show the results of these ramping simulations. Figure 12 shows production and speed as a function of time. It shows that when the compressor speed was reduced to about 95% of design, the ASV’s opened to prevent surge, so that control system stopped reducing the speed. These figures also show that the key operating parameters are within the desired range: production, LNG temperature (i.e., the LNG product quality) and MCHE warm end ΔT (efficiency measurement). The production moves smoothly between endpoints. The LNG temperature barely

moves from the desired setpoint. The MCHE WEΔT ranges between 1.5 and 5°C, which is very close to the

setpoint of 2.8°C. (During manual ramping, the WEΔT can vary between 0 and 20°C, with rapid changes.) These

graphs show that the production met the product requirements of flow and temperature while maintaining good efficiency. These dynamic simulations confirmed that the AP-ECS can be effectively used with speed control when varying production

Figure 12 – Production and Speed During Ramping

Figure 13 – WEΔT and LNG T during ramping

.

PRECOOLING REFRIGERANT SYSTEM

This paper has discussed how to control the circulating flowrate for a closed refrigeration system. It has focused on the MR system in an LNG liquefaction process. It has not discussed the precooling system, either in a dual mixed refrigerant (AP-DMR) or propane precooled mixed refrigerant process (for example, AP-C3MR). The precooling systems in these processes are similar to the MR system, in that they are closed systems. They differ in one key aspect—the refrigerant leaving the compressor is completely condensed, whereas in the MR system, it is only partially condensed. Because of this, the precooling compressor discharge pressure is set by the heat sink temperature and the approach temperature of the condensing heat exchanger. The suction pressure is then set by the compressor head. This difference prevents the conclusions from this paper being directly applied to a precooling compressor.

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LNG T, Δ fr Setpt 70% TD

CONCLUSIONS

While open systems need to adjust compressor flow to maintain a steady state inventory, in a closed system, the inventory is not affected by the circulating flowrate. A compressor operating in a closed system has two degrees of freedom; for example, specifying the compressor flow and speed determine the other dependent parameters. In an LNG liquefaction process, the compressor flowrate is determined by the LNG refrigeration requirement, leaving a single degree of freedom.

Speed can be selected as this degree of freedom. For a fixed speed gas turbine, the system is defined. For variable speed drivers, it must be recognized that changing the MR compressor speed has a large impact on compressor head and a lesser impact on flowrate. The compressor speed should not be varied to maintain a constant suction pressure, because while the speed does have a small effect on the suction pressure, other variables have a far greater impact, such as MR inventory and required MR mass flowrate. These will compete with the speed control.

For the steady state cases studied for this paper, power at turndown is minimized by running at design speed with reduced MR inventory. This simulates changing the refrigerant inventory with production. Constant inventory calculations show that the process efficiency may be improved by slightly reducing speed as production is decreased.

This paper demonstrates that it is possible to control the compressor speed based on the MRL flowrate, and speed control can be added to the existing Air Products Enhanced Control Scheme.

References 1. Coil Wound Heat Exchanger Design for an Evolving Market. Dally, John, Chris Butler, Warren Miller, William Schmidt, Jason Styer. Barcelona : GasTech, 2018. 2. An Innovative Control Scheme of a C3MR LNG Plant. Bronfenbrenner, Jim, Matthew Okasinski, Scott Trautmann. New Orleans : AIChE Spring Meeting, 2008. 3. Air Products Enhanced Control Scheme. Air Products and Chemicals Corporate Website. [Online] 1 January, 2019. http://www.airproducts.com/~/media/downloads/e/enhanced-lng-liquefier-control-schema/data-sheets/enhanced-lng-liquefier-control-schema-data-sheets.pdf.