(28b) Application of LNG Technology Solutions for Optimisation of Liquefaction Processes

Ducote, D., Chart Industries, Inc.
The application of proven, specialised equipment, efficient natural gas liquefaction process technology and modular construction is enabling the export of US natural gas by challenging the paradigm that a large baseload facility provides the best economy of scale. The concept is widely known as mid-scale LNG but the story doesn’t have to end there. The principal reason for cryogenic liquefaction is storage and transportation and the same criteria that make the model work for LNG can be applied to other gases increasingly in demand; specifically ethylene, ethane, liquid air for energy storage, nitrogen, hydrogen and helium.

Key to cryogenic processing is the brazed aluminum heat exchanger (BAHX). A BAHX is a highly efficient, custom designed, compact heat exchange device that provides a heat transfer area significantly greater than conventional heat exchanger technology and also has an inherent multi-stream capability where 8 or more process streams in a single composite is common. Minimizing power consumption is a key design criterion for liquefaction processes, creating an inherent requirement for highly efficient heat exchange. Process efficiency is gained by minimizing temperature approach between the hot and cold streams. As the temperature approach decreases, the required surface area increases. With that in mind, the heat exchanger of choice must offer high surface area economically.

To ensure the robustness of the BAHX state-of-the-art software is coupled with in-house engineering expertise and experience to perform advanced transient thermal analyses. Exchangers are studied three-dimensionally from available operating data or dynamic simulation process software to determine the internal fluid and metal heat exchanger temperatures and the associated thermal stresses imparted on the exchanger. These analyses can be carried forward to predict potential fatigue damage and life expectancy of the equipment. Dynamic simulation is used to simulate operation of the BAHX and plant upset conditions to ensure process technologies are designed with proper controls to mitigate situations that introduce thermal stresses.

Typically refrigeration cycle configurations are focused on two; nitrogen cycle and mixed refrigerant cycle. Each cycle has its advantages and disadvantages when evaluated for use in a specific application. Usually designers study the trade-offs between power consumption and equipment utilization. This analysis allows them to optimize the liquefier for the total plant cost (capital plus operating costs). Comparison of operating costs should focus on the energy consumption of the refrigeration system, as the balance of plant should be essentially the same for all processes.

Nitrogen cycles are simple to operate and eliminate the need for the use and storage of hydrocarbons as refrigerants. They are ideally suited to remote areas that do not have easy access to hydrocarbons for refrigerants. However, simplification comes at the expense of higher specific energy consumption.

Mixed Refrigerant (MR) processes are based on mixtures of light hydrocarbons and nitrogen and process optimization is achieved by varying the mixture of refrigerant components and operating pressures in a manner that essentially allows the heating curve of the refrigerant to mirror the cooling curve of the gas to be liquefied in a tight temperature approach. The brazed aluminum exchanger accommodates the surface area requirements resulting from the tight approach temperatures and log mean temperature differences (LMTD). Its inherent multi-stream capability also allows the process engineer to add or remove refrigeration along the process thermal gradient, wherever needed, to optimize process efficiency.

Because all the capital equipment and technology required is already proven and deployed globally in LNG service, it represents a no risk solution in their application to other liquefaction processes.