DYNAMICS OF BASELOAD LIQUEFIED NATURAL GAS PLANTS ADVANCED MODELLING AND CONTROL STRATEGIES

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1 DYNAMICS OF BASELOAD LIQUEFIED NATURAL GAS PLANTS ADVANCED MODELLING AND CONTROL STRATEGIES Dr. Matthew J. Okasinski, P.E. Principal Engineer Air Products and Chemicals, Inc. Allentown, Pennsylvania, USA Dr. Myrian A. Schenk Senior Principal Systems Engineer Air Products, PLC. Hersham, England, UK ABSTRACT The production of liquefied natural gas (LNG) is becoming an important source of revenue for many countries around the world. However, building and operating a baseload LNG plant requires significant investment. The liquefaction system alone may represent up to 4% of the capital cost of an LNG process train. Traditionally, LNG plants can be operated with minimal automatic controls. However, this can contribute to operating challenges during the transient periods of the plant operations. In order to provide technical advice to the various operating LNG plants employing our LNG process technology and equipment, Air Products has invested in developing a framework for analyzing different operational strategies. While doing so, we have also been able to expand the use of our dynamic model framework to the engineering phase of the project to help to optimize plant design, for example, providing guidelines for sizing relief valves, through studies of various operation upset scenarios. The dynamic model was built with actual plant parameters so that the interaction of the physical process and the control system for all operating scenarios can be investigated efficiently. This model includes the capability of simulating multiphase flows during plant shutdowns, including flow reversals, to capture the short and long time-scale dynamics, thus providing a platform to improve transient operations. Some of the scenarios that were simulated with our framework will be presented in this paper: Varying productions Loss of equipment services Compressor blocked discharge scenarios Loss of feed flow PO-3.

2 INTRODUCTION Computing power has dramatically increased over the past two decades aiding the engineer s ability to find solutions to dynamic problems. Large sets of differential and algebraic equations (DAEs) are easily solved with today s computers and the size of the equation sets have grown considerably in the last decade []. In the LNG industry, many companies have been using dynamic simulation in various stages of an LNG plant s lifecycle [2, 3]. The life-cycle of an LNG plant starts during the design stage, where detail engineering is used for constructing the plant and, in our case, the design of the main cryogenic heat exchanger (MCHE). The interactions of design and control can be assessed here, giving powerful insights to the future operation of the plant. Once the process equipment design and the PI&Ds are defined, relief analyses can be performed to determine the sizing basis of the flare systems. In recent years, the applicability of multi-variable control techniques (MPC) is becoming common practice [4]. Controlling the plant tightly while maximizing production and anticipating known disturbances has clear economic and safety benefits. Dynamic simulation can also start at early design stages for preliminary control studies, highlighting any operability issues early in the construction of the plant and offering the opportunity to assess and correct if necessary. It can also be a useful training tool to establish proper start-up and shut-down procedures. Finally, during actual plant operations, control procedures and operator s best practices can be simulated to achieve optimal operation. Thus, dynamic simulation, coupled with dynamic optimization, can offer substantial benefits during the entire life-cycle of the plant. Moreover, dynamic simulation can provide insights into the liquefaction process that are not obvious when designing from a steady-state perspective. The accumulation and depletion of mass and energy in different sections of the flowsheet will affect how the overall closed loop refrigeration system responds to a disturbance. For example, while it is possible to obtain conservative relief flow estimates using steady-state assumptions, the time-dependent interaction between the compressors, gas turbine and other process equipment can only be studied using dynamic simulations. The flowsheets presented in this work are built using APCI s proprietary detailed dynamic models and run in Aspen Custom Modeler (ACM) 2..5 with APCI s proprietary thermodynamic database. Specific plant information, such as piping, equipment and vessel volumes, compressor curves, and heat exchanger UAs, etc., can be incorporated into the simulation model. DISCUSSION There are two major liquefaction processes offered by Air Products, the C3MR process and the new large capacity AP-X process. The C3MR process has been used for many LNG plants over the years. Figure illustrates a schematic of a typical C3MR process. Natural gas feed is precooled using propane before entering the MCR Main Cryogenic Heat Exchanger. In the MCHE, the feed is further cooled and liquefied using a refrigerant comprised of a mixture of nitrogen and light hydrocarbons (i.e. methane, ethane and propane). The mixed refrigerant (MR) is in a closed loop where it is compressed, cooled against vaporizing liquid propane and in the tubes of the MCHE and finally expanded into the shell of the MCHE as a cooling medium. PO-3.2

3 LNG Feed C3 Precooling MRV Mixed Refrigerant Liquid Figure C3MR Process Schematic In contrast to the C3MR process, Air Products AP-X process allows for substantially higher production rates relative to a traditional C3MR process for the same size MCHE and compressors. Figure 2 illustrates a schematic of the process. Natural gas feed is precooled using propane before being further cooled and liquefied in the MCHE. Like a C3MR process, the MCHE uses a Mixed Refrigerant stream to condense and begin subcooling the feed. In contrast to a C3MR process, the final subcooling is done in a separate loop using pure nitrogen as the coolant. This debottlenecks the MCHE allowing for substantially higher productions rates compared to a traditional C3MR process. LNG Feed C3 Pre-Cooling Nitrogen Expander Sub-cooling Mixed Refrigerant Liquid Figure 2 AP-X Process Schematic PO-3.3

4 Air Products has successfully run dynamic simulations of both our C3MR and the AP-X processes. The modeling platform developed by Air Products can be used to test overall plant responses to changes in controller setpoints and changes in process or ambient conditions (i.e., air or cooling water temperature). Two examples of the dynamic simulation runs are shown below. The first example illustrates how dynamic simulation is used for relief analyses. The second example illustrates the dynamic behavior of the AP- X process when the N2 refrigeration system is taken out of service. EXAMPLES Propane Compressor Blocked Discharge Event for Flare Sizing During the engineering phase of the life cycle, dynamic simulation can evaluate relief scenarios used for flare system sizing. While it is possible to obtain conservative relief flow estimates using steady-state calculations, the time-dependent interaction between the compressors, gas turbine and other process equipment can give insights into the event and these can only be studied using dynamic simulations. Dynamic simulation also lends itself to more detailed calculations because it accounts for the actual volume and inventory of the refrigeration circuit. Typically the propane compressor blocked discharge scenario is calculated because it determines the height of the flare and its distance from the plant for thermal radiation concerns. Figure 3 shows a process flow diagram of a typical propane refrigeration loop of a C3MR process which was modeled in its entirety. The assumptions used in this simulation were: propane compressors were driven by a single shaft gas turbine and operating at maximum speed process flows and heat sinks remain constant PSV set pressure at the HHP compressor discharge is 8% above operating PO-3.4

5 Figure 3 - Propane Refrigeration Loop The upset can be caused by a vapor blockage between the propane desuperheater and condenser due to a loss of cooling water. For the purposes of modeling, the same effect can be simulated by quickly closing the block valve between the desuperheater and the condenser, as labeled in Figure 3. As the block valve closed, the HHP discharge pressure began to rise, moving the HHP compressor toward surge. The anti-surge controller opened its corresponding anti-surge valve to recycle gas back to the HHP suction drum. This caused the HP compressor discharge pressure to rise and also moved that stage towards surge. This effect quickly cascaded through the compressor string until all stages were on recycle. Figure 4a shows the pressure increase of the suction drums as a function of time. As the suction drum pressures increased, the propane vapor flows from the process evaporators were suppressed very quickly. A representative graph of propane boiloff flow is shown in Figure 4b. This meant that the compressors were completely relying on their recycle flows to keep out of surge and that for the first few minutes no additional mass was being added to the compressor string from the evaporators. Pressure continued to rise in suction drums due to hot gas being recycled. After a few minutes, some of the evaporators with larger duties resumed sending propane vapor to the suction drums. For these evaporators, the rate of pressurization through continued process heat input was greater than the rate of suction drum pressurization caused by recycling hot gas through the anti-surge valves. This effect cannot be accounted for in steady-state calculations. However, dynamic simulation takes into account the time-dependent nature of the disturbance. PO-3.5

6 (a) Selected Suction Drum Pressures (c) Scaled Turbine Power Scaled Pressure MP Suction HP Suction HHP Suction Scaled Power GT bogdown Low Speed Trip Scaled Speed Driver Power Maximum Power Speed Trip Speed (b) Scaled Flowrate LP Evaporator Boiloff Flows GT bogdown Low Speed Trip E54 Boiloff E67 Boiloff E628 Boiloff Figure 4 -- Example Figures At approximately minutes, Figure 4c shows that the compressor power demand reached the maximum available gas turbine power and the gas turbine bogged down. In other words, the gas turbine reduced speed in order to maintain the power balance between itself and the compressors. This reduced the turbine s maximum available power further and the compressor string continued to slow down until the turbine tripped on low speed several minutes later. While this alone did not reduce the pressure rise at the HHP discharge (Figure 4d), the rate of pressurization decreased as the compressors slowed down and thus made less head. Several minutes after the bogdown began, the gas turbine tripped on low speed and shut down the compressor string. At this time, Figure 4e shows that the suction drum pressures have not reached the set pressures of their respective PSVs. In addition, Figure 4d shows that the HHP discharge pressure is also well below the set pressure of the HHP discharge PSV. So there was no relief to flare in this case. (d) HHP Discharge PSV (e) Selected Suction Drum Pressures.9 7 Scaled Pressure GT Bogdown Low Speed Trip Inlet Pressure Set Pressure Scaled Pressure MP Suction HP Suction HHP Suction MP PSV HP PSV HHP PSV Figure 4 -- Example Figures PO-3.6

7 Dynamic simulation has shown that it is possible to bogdown a single-shaft gas turbine and prevent relief of refrigerant to the flare system. This can lead to economic savings as it removes the propane blocked discharge as the governing case for flare sizing. The flare system can be sized for a less severe scenario resulting in possible savings in pipe diameter, length and flare height. The use of pressure relief valves in parallel to the anti-surge valves in order to bogdown a single shaft gas turbine is the subject of a recent Air Products patent. [5] The modeling platform can be also used to simulate other disturbances and formulate responses to allow the plant to safely continue operation. The second example illustrates this concept in the Air Products AP-X process. Transition from AP-X to C3MR Process for Continuous Operation In case of a nitrogen refrigeration system outage of the AP-X plant, it will be advantageous to continue the plant operation at lower production. In the second example, our dynamic simulation platform can quantitatively show a transitional path from an AP- X process to a C3MR process at lower production over a period of several hours. During the nitrogen system outage, the N2 compressors and companders were put on recycle and the LNG Subcooler was bypassed. The LNG production was ramped down to a predetermined production level over a couple hours as seen in Figure 5a. This reduced the cooling load on the MCHE and reduced the amount of flash gas generated. Simultaneously, the MR flowrate was lowered proportionally (Figure 5b) to avoid low LPMR return temperatures. This caused the warm end (i.e. bottom) of the MCHE to get slightly warmer. The MCHE warm end temperature difference was monitored and the MR flowrate increased as the MCHE pinches. Because the AP-X cycle does not completely subcool the feed in the MCHE, the AP-X mixed refrigerant contains less noncondensibles (e.g. nitrogen) than the mixed refrigerant of the C3MR processes. Nitrogen addition to the MR stream was another key step of the transition process. Figure 5c shows the nitrogen mole fraction in the MR composition as a function of time at three different points in the MR loop, LP MR suction drum, the warm JT valve and the exit of the MCHE. Note that there was a lag between the composition at the LPMR suction drum where nitrogen entered the loop until it made its way around the MCHE. Dynamic simulation takes into account the actual volume of the system and hence the time it takes for the nitrogen to move around the loop. With the altered MR composition, the MR could now subcool the LNG within the MCHE as in a C3MR process. Uninterrupted production, at a lower rate, has been achieved. Dynamic simulation could also be used to determine a suitable path to transition back to an AP-X process once the process or feed upset has been resolved. PO-3.7

8 (a) LNG Production Rate (b).2 MR Circulation Rate Normalized Flowrate Time [hr] Normalized Flow.5 Higher final flowrate aids subcooling. Decreased along with Feed Warm Bundle Shell exit too warm, increase MR flowrate Time [hr] (c) N2 Concentration in MR (d) Colder LNG Rundown Temperature.6.5 Mole Fraction WB Exit LP Suction WJT Normalized Temperature Feed bypassing Subcooler N2 addition completed Middle Bundle subcooling Feed Time [hr].5 Warmer Time [hr] Figure 5 -- Example 2 Figures REFERENCES CITED [] Mandler, J., Modeling for control analysis and design in complex industrial separation and liquefaction processes Journal of Process Control : [2] Cameron, D. et al., Integrated Distributed Dynamic Simulation of the LNG Value Chain Chemical Engineering Research and Design 83(A6): [3] Valappil J. et al., Virtual Simulation of LNG Plant LNG Journal January/February 24. [4] Bakker, K., A Step Change in LNG Operations through Advanced Process Control presented at AIChE Spring 26 Meeting, Paper 38g, Orlando, FL. [5] Lucas, C. et. al., Utilization of Bogdown of Single-Shaft Gas Turbines to Minimize Relief Flows in Baseload LNG Plants US Patent #7,69,733. PO-3.8