(87a) Improving Design of Hot Oil Unit in LNG Plants Using Dynamic Simulation

Improving Design of Hot oil unit in LNG plants using Dynamic Simulation

AIChE 2014 Spring Meeting

Abstract

In recent years, we see an increased use of dynamic simulation as an effective engineering tool during the engineering phase of a project in addition to its use during plant startup, shutdown, revamp and troubleshooting. This is due to increased awareness in the engineering community especially process engineers to use dynamic simulation for understanding the transient nature of processes. Also the capability of dynamic simulation to predict the design limitations within different operating envelopes helps to validate the design.  Usually different units are designed based on the maximum operating flow or a worst case temperature and so on. But, they are not studied for various different combinations of operating cases. And dynamic simulation is an appropriate tool to validate such designs. Furthermore, identification of any design change during early engineering phase provides significant savings in a plant’s long term.

One such important dynamic study has been carried out for a hot oil system in an LNG plant during engineering phase.  The hot oil is a very integrated closed loop system with heat producers like heaters and waste recovery units (WHRU) and different users all over the plant.  Availability of reliable hot oil is very essential for continuous and steady operation of a LNG plant.  It is the primary source of heat for column reboilers in the feed inlet facilities, amine treatment unit and heavies’ removal unit.  The hot oil utilizes the waste heat turbine exhaust gas in its waste heat recovery unit (WHRU). Hot oil extracts the heat from the exhaust gas and distributes it to the process reboilers and other users. After giving away the heat, the hot oil returns back to the WHRU’s for extracting heat and the cycle continues. In this study, such an integrated heat recovery unit has been analyzed extensively for design verification, process control validation and hot oil unit robustness during various process upset scenarios. In addition, the dynamic response of the system was analyzed during a turbine trip leading to a plant turndown or a shutdown. The study also reflected the plant’s integrity and transient behavior during trips and upsets. The study also identified the need for an operator action required during any of these upset scenarios.

A detailed dynamic model of such a complex, integrated network was developed and utilized to study the plant disturbances.  Among a number of scenarios studied in the hot oil system, this article highlights two major scenario that resulted in the design changes. The changes avoided a plant shutdown due to an unforeseen dynamic condition and assisted in improving an existing design enabling uninterrupted plant operation averting possible equipment damage and downtime for maintenance.

The first simulation predicted a high hot oil temperature to the amine reboilers leading to possible amine degradation. As the hot oil circulates through a number of reboilers in different units, an upset in any one of them impacts the others. This was observed during an upset in the inlet facility resulting in its reboiler trip. Due to this reboiler trip, the hot oil return temperature increased. Such a high hot oil inlet temperature to the amine reboiler overheated the amine.  A prolonged operation with such high hot oil temperature can result in amine degradation. Such amine degradation has been avoided by increasing the size of the hot oil recycle cooler valve enabling more cooling.  This resulted in a significant reduction in the hot oil return temperature lower than amine degradation temperature. Re-sizing the valve during the engineering phase avoided amine degradation. This saved the amine replacement cost, plant down time for maintenance without affecting plant production. 

The second simulation demonstrated a low hot oil flow in one of the smaller WHRU coils as a consequence of a major user upset, which can trip the WHRU.  Such a situation occurred when one of the major users of hot oil shutdown reducing the overall hot oil circulation

A low flow in the WHRU coils will overheat the hot oil resulting in coking inside the coils and can lead to tube rupture or damage. Among the three WHRU’s arranged in parallel, the flow through the smaller WHRU decreases keeping enough flow through the others. The smaller WHRU accommodates two different set of coils, one for heating the hot oil and the other for heating a process fluid. A low flow would cause the smaller WHRU to trip. The trip would lead to WHRU shutdown in order to protect and avoid damage to hot oil coils.  In addition, this would also cut down the process heat and affect the associated process unit. However, tripping the WHRU for this scenario was uncalled for and unexpected. The low flow was prevented by increasing the hot oil valve size for the smaller WHRU. Increasing the hot oil valve size ensured sufficient flow to the WHRU coils avoiding unnecessary trip and a process upset

These improvements in the design could not have been achieved by conventional calculation or steady state simulation.  This is due to the fact that the above scenarios are integrated transient effects of an upset which were predicted well using dynamic simulation analysis.