(192a) NGL Product Storage and Boil Off Compression System

Natural Gas Liquid’s (NGL) are components of natural gas that are separated from the gas stream in the form of liquids. NGL’s typically refer to Ethane, Propane, Butanes and Pentanes. In a low gas price environment, NGL’s are growing source of revenue for gas producers. Cryogenic distillation in a LNG facility is provided to separate methane and heavier hydrocarbons or NGL’s from the gas stream. The heavier fraction is sent to the LPG fractionation unit where it is separated into C3, C4, and Condensate products.

In general LNG, C3 and C4 products are stored in double wall/full containment atmospheric storage tank. Facilities are provided for loading the LNG, C3, C4, and Condensate products to tanker ships. This paper focuses on the design, safety and controllability of C3/C4 product storage and associated Boil-Off-Gas generation in an NGL export facility.

The refrigeration and Boil-Off system converts warm and high pressure C3 and C4 liquid into cold and low pressure product streams for storage and transportation. A Direct or open system utilizes product as refrigerant, whereas an indirect or closed system utilizes a closed loop refrigeration system independent from product. Steady state simulation and design of open and closed refrigeration systems for C3 and C4 product storage are discussed in this paper, highlighting the advantages and disadvantages of the refrigeration system.

Dynamic simulation utilizing rigorous mathematical models has become an influential tool in the process design, design validation, relief system design, control system verification, startup support and troubleshooting. Many recent developments in the simulation software and technology have led to the development of large scale dynamic models. Dynamic simulation has been utilized for an existing LNG facility which has an open refrigeration system for cooling and storing C3 and C4 products. The Propane Boil-off-Gas compressor, which is an integral part of the open refrigeration system, has been modeled in detail.

Based on simulation results, modifications pertaining to safety and controllability of the C3 and C4 product storage facility are discussed.