(640d) Optimal Pipeline Operations for Unloading Refrigerated Liquefied Petroleum Products

There has been a substantial growth in seaborne trade in Liquefied Petroleum Gas (LPG) or its constituents, viz., propane, butane, etc. Seaborne petrochemical trade is forecast to rise from 57 million tonnes in 2007 to 70 million tonnes by 2012. There are several challenges in the design and transport of LPG including loading and unloading operations. Optimal strategies for ?pre-cooling? and unloading refrigerated petroleum products in a typical storage terminal are discussed in this contribution.

Refrigerated liquefied petroleum products such as propane, butane and LPG are transported by ships and stored in tanks in storage terminals. These products are conveyed from the jetty to the storage terminals by over-ground insulated pipelines to a storage tank that is typically situated 12-20 km inland. The products are stored under atmospheric pressure in insulated tanks. The time interval between two successive ship parcels is typically 3-4 days on average and between two successive arrivals, the pipeline can either be partially or completely evacuated.

During unloading operations and due to heat ingress from the ambient, there is a possibility of liquid products vaporizing. In order to maintain the pressure in the tank, vapor is withdrawn using flash/boil-off compressors. An important operational requirement during unloading operations is to minimize material loss due to excess vapour generation. One common strategy is to ?pre-cool? the pipe by recirculating liquid propane through the pipeline, prior to the unloading operations.

Flow of hydrocarbons under different conditions in long pipelines has been studied in literature, e.g., two phase flow caused by sudden depressurization (blow-down) has been studied extensively in literature [1, 2]. However, these models are not directly applicable to the processes described above. Hence, we develop a dynamic lumped parameter model for the pipeline and the storage tank that accounts for phase change, flow, thermodynamics and heat transfer. These models are validated with operating data from a LPG storage terminal.

Optimal pre-cooling and unloading strategies have to take into account the time interval between two successive ship arrivals, amount of material to be unloaded, initial state of the pipeline, ambient conditions, operating costs, and pump and compressor capacities. The objective is to minimize total costs which include cost of material lost due to flaring of excess vapour, energy costs for pumping and compression during pre-cooling and unloading operations and demurrage costs. We use validated dynamic models and dynamic optimization methods to arrive at optimal strategies for deciding the pre-cooling strategy, flow rates of liquid during pre-cooling and unloading, target temperatures and compressor operation sequence.

References

1. Chen J.R., Richardson S.M. and Saville. G. Modelling of Two-Phase Blowdown from Pipelines - I. A Hyperbolic Model Based on Variational Principles, Chem. Eng. Sci, 50, 695-713, 1995. 2. Fairuzov Y. V. Blowdown of Pipelines Carrying Flashing Liquids, AIChE Journal, 44 , 245-254, 1998.