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LNG Process – Liquefied Natural Gas

LNG is purified natural gas converted into a liquid. In this liquid form, NGL takes up 1/600th the volume of gas. This makes transportation via tankers cheaper and attractive.

LNG plants consist of three main processes:

1. Liquefaction and storage
2. Transportation via tankers
3. Storage and regasification.

LNG has become popular because it burns clean. There is high demand for both large-scale LNG plants, medium scale LNG plants and small-scale LNG plants.

LNG Plant Refrigeration Cycles:

Natural gas liquefaction requires a significant amount of refrigeration. There are different options on refrigeration cycles.

Regarding refrigeration cycles, there are various options. The refrigerant can be a single-component fluid (e.g., Nitrogen) or a mixture of light hydrocarbons, generally termed as mixed refrigerant (MR) or can be propane + ethylene and methane in three separate independent closed-loops.

In one of the options - Brayton cycle (BR cycle), the refrigeration is supplied by expanding a single-component fluid (typically Nitrogen) without fluid phase change. For medium to large production plants where high efficiency is important, a compression refrigeration cycle (CRC) with refrigerant phase change is used.

In the BR cycle (Brayton cycle), the cooling duty is provided by expanding nitrogen through a Joule–Thomson valve (JT valve), or through an expander, without causing a change in the fluid state. The Nitrogen pressure is raised from one level to another and cooled at constant pressure to a third level producing the refrigeration needed by the process.

In the Compression refrigeration cycle process (CRC process), heat is removed by evaporation at low pressure, the refrigerant fluid in an exchanger and rejecting heat by condensing the refrigerant vapor at high temperature.

The rejection is done by transferring the heat to another refrigeration system (cascade refrigeration). The refrigeration loop is completely closed-loop - comprises of compression, cooling, liquefaction and then evaporation.

In the cascade process, two or more refrigerants (propane, ethylene and methane) are used in three separate refrigeration cycles. The low-temperature cycle provides the cooling in the evaporator and rejects heat to the other cycle by means of the evaporator/condenser heat exchanger.

Another option is to use Mixed Refrigerant. A single refrigerant fluid must be compressed and expanded to pressures low enough to reach a temperature colder than the process stream. The lower the gas liquefaction temperature, the larger its duty, and generally the more complex the refrigeration system becomes. MR used for liquefying natural gas generally contains a mixture of methane, ethane, propane and nitrogen.

Prior to any advanced process control (APC) project, base-level PID tuning and optimization is a critical prerequisite step for an LNG plant. Unless base-level PID control loops are well tuned, advanced process control (APC) cannot work well, since advanced process control (APC) will be manipulating the setpoints of the base-level PID control loops. Therefore, the first necessary step in the overall process control improvement procedure for an LNG plant is PID tuning and optimization of primary or base-level PID controllers. The benefits of PID tuning and optimization in LNG plant is the reduction of the controller deviations from setpoints. This allows the smoother running of the LNG plant with increased stability in all control loops.

PiControl Solutions LLC has extensive experience in PID tuning and optimization for LNG process PID controllers. PiControl engineers understand and know how to tackle with typical PID control loop problems and have customized PID tuning and optimization software tools to help optimize all LNG plant controllers.

PiControl’s unique and novel closed-loop system identification technology makes it possible to tune and optimize base-level PID control loops quickly, efficiently and precisely. With this closed-loop technology PiControl can perform system identification and PID tuning optimization of the following critical base-level PID controllers easily.

Moreover, all process and data analysis and PID tuning and optimization work can be easily performed remotely by PiControl Solutions LLC process control engineers.

Many LNG plants are new with young and inexperienced personnel in the control room. Young and new engineers, technicians and operators need practical process control training.

PiControl’s SIMCET real-time dynamic PID tuning training simulator allows plant personnel to be trained on the process control simulator. SIMCET can be used for practice, training, testing and certification. Using SIMCET, plant personnel can tune PID control loops in a real-time, real-life mimicking process control environment. Test records can be maintained by management in the employee files and training can be made mandatory and repeated every 2-3 years.

PiControl’s PITOPS software is both an open-loop and closed-loop data analyzer. PITOPS can identify openloop transfer function models with both open loop and closed-loop data in Excel files or using OPC communications technology. PITOPS can tune PID control loops and PITOPS can be also used for designing and tuning Advanced Process Control.

PiControl’s APROMON software is Advanced Process Monitoring technology for monitoring PID control loop and APC scheme quality. APROMON can monitor all PIDs and APC loops in an LNG plant.

All PiControl software work entirely in the time domain and can be installed in just ab out half a day with subsequent control-related benefits.

PiControl has novel and new technology for APC and MPC, different and superior to other MPC and APC vendors.

Before you decide on a MPC vendor, make sure you discuss MPC technology and options with PiControl.

PiControl MPC/APC technology totally bypasses Cyber-Security issues which are important today. This is because PiControl does not use OPC communications like all other DCS and MPC vendors. PiControl uses the more reliable Modbus TCP/IP communications protocol and not OPC and so there is absolutely no issues related to Cyber-Security. PiControl is the only APC/MPC vendor that does not use OPC and uses methodology that totally eliminates any Cybersecurities and vulnerabilities.

PiControl can design and implement the entire APC directly on the existing DCS platform or using Modbus TCP/IP.

Compared to any of the MPC vendors or DCS vendors offering MPC – Aspen DMC, Honeywell RMPCT, Emerson PredictPro, Yokogawa SMOC and Yokogawa PACE, Schneider Foxboro IA Connoisseur, PiControl MPC/APC technology does not need an external new server, does not need new hardware and software. With all other MPC and DCS vendors, you will be spending a lot of money, time and effort on Systems Engineering, Windows patches and upgrades. The other DCS and MPC vendors also charge a fat annual fee for software and hardware refresh. In contrast, PiControl can show you how to eliminate the need for new servers, new hardware and new software and design and implement APC for LNG is a very much cost-effective manner with benefits outperforming other MPC and DCS vendors.

PiControl LNG APC will always maximize the overall LNG production rate by pushing against the identified constraints.

The active constraint identification is automatic and the push in the direction of higher production rate and maximum profits is stronger and more aggressive compared to MPC options from MPC vendors.

LNG plants see a lot of unmeasured disturbances caused by changes in ambient temperature, feed gas composition changes –change in heavies, and also changes in wind
direction.

Conventional MPC algorithms are slower than PiControl APC algorithm. PiControl APC is able to maximize production rates and maximize profits more aggressively than other MPC methods.

PiControl can design and implement the following APC schemes for an LNG plant. This list is just a sample, as there are many different combinations and options in an LNG plant, PiControl can customize APC as needed and required for a specific process.

• Main Feed Flow Maximization
• Feed Bypass Valve Minimization
• Optimize JT Pressure Valve (Joule-Thomson) located between HRC (Heavies Recovery Column) and Methane Economizer
• Ethylene Economizer Pass Optimization – Shift refrigeration load between Ethylene and Propane circuits.
• Methane/Ethylene Chiller Valves Optimization – Optimize load between Methane and Ethylene systems
• Minimize Compressor Suction Pressure Controllers
• Automatic Adaptive Tuning for Max Production Rate and at Reduced Production due to Loss of Feed Gas from Pipeline or a Compressor Trip

Feed Flow is the most important MV (manipulated variable) in the LNG plant, as it has relationships with almost all important CVs (controlled variables).

• If any of the important CVs are being approached or violated, APC will try to use other MVs first before cutting Feed Flow. This is one of the important APC functions – to maximize feed and use other MVs to protect CVs.
• The Feed Flow maximizing APC scheme will look at almost 20 CVs in the LNG plant – e.g., train feed flow demand, HRC temperature, Delta Pressure Constraints on various coolers, chillers and heat exchangers, sub-cooling delta temperature between HRC and Methane Economizer, various vent flare controllers (to avoid flaring), various suction and discharge pressures.

• The Feed Bypass valve is an important optimizing option in an LNG plant.
• In most cases, keeping the feed bypass closed or at low position is more efficient as it helps to maximize use of the ethylene refrigeration system.
• APC will try to minimize the feed bypass valve but will open it in order to relax some important constraints and allow feed maximization.
• Not using the feed bypass valve in some cases may force cutting back the main feed flow and this is lost profits.
• APC feed bypass valve will be opened to alleviate possible flooding (high delta pressure in HRC) and low temperature limits in the ethylene chillers and coolers.

• APC will maximize the JT pressure in order to maximize rates.
• Maximization of JT pressure helps to improve the subcooling margin that is a hard safety limit. This is turn helps to run at higher production rate (higher Feed Flow).
• The JT pressure is the most important MV for the subcooling limit and hence it is as important as the Main Feed APC scheme.
• The JT pressure can also help to relieve constraints on various delta pressures and temperatures in various chillers and coolers.
• The Main Feed Flow and the JT Pressure are the two most important MVs in an LNG plant and are the most important APC MVs.

• The LNG plant has two or three closed-loop refrigeration circuits: Propane Refrigeration, Ethylene Refrigeration and Methane Refrigeration
• In very cold temperatures, it is necessary to shift the balance of the refrigeration load and this is done by manipulating two important bypass valves:

1. Ethylene Economizer Bypass Valve
2. Methane Ethylene Chiller

• Both these valves are APC MVs and will be minimized at most times.

• LNG demand is projected to be high for the next several years.
• With high demand for LNG, APC will try to maximize the Feed Flow to maximize profits.
• In this case, at high production rate, all three suctions pressures (propane, ethylene and methane) will be riding above their setpoints and the base level PIDs will not be in control. This corresponds to high/max production rate.
• In certain cases (< 10% of the time), a few constraints may need to raise the compressor suction setpoints and this will be done automatically by the APC.

• LNG plants often define two modes of operation:
  • High/Max Production Rate
  • Half Rate/Reduced Rate
• Most of the time, the LNG plant will be in Max Rate mode.
• 15% or less time, due to loss of feed gas from pipeline or a compressor trip, the plant may be operating in half rate mode.
• Dynamics are different for full rate and half rate.
• PiControl’s closed-loop model identification software is the only software in the world capable of using closed-loop data without step tests to identify accurate models for both cases.
• Automatic model switching will be provided by the APC schemes.

PiControl can help you to increase your average production rate by 4-5% which corresponds to a huge increase in the LNG plant profits.

PiControl can show you how to build the APC/MPC without new servers, without new hardware, without new software, without OPC and without cyber security issues.

PiControl APC/MPC costs are lower than conventional MPC from Aspen, Honeywell, Emerson, Foxboro, Schneider and Yokogawa. PiControl design requires less time and less maintenance.

Contact PiControl today for a demo.

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