(215d) Natural Gas Liquids (NGL)- Fractionate or NOT to Fractionate at the Gas Processing Plant

NATURAL GAS LIQUIDS (NGL)- FRACTIONATE OR NOT TO
FRACTIONATE AT THE GAS PROCESSING PLANT

B.V. Gangar, A.A. Pilehvari

Department
of Chemical and Natural Gas Engineering, Texas A&M University-Kingsville,
Kingsville, TX, USA

Abstract:

Natural
Gas Liquid (NGL) is the liquid product obtained from the bottom of the
demethanizer in a gas processing plant. It consists of ethane, propane, butanes
and natural gasoline (heavier hydrocarbons). Here in USA, the gas processors in
the planning stages of a new gas plant, are in dilemma whether they should
include the fractionation of NGL at the plant and sell the individual products
or send the NGL to a central processing facility such as the one at Mont
Belvieu, Texas. Overseas, most of the large gas plants fractionate the NGL at
the plant site. However, in USA the gas processors must consider the economic
feasibility before making such decisions, which is determined by supply and
demand relationship and availability of pipelines to transport the NGL and/or
NGL products. In fact, most of the old large gas plants in USA have the NGL
fractionation facility but many recent gas plants have been designed without
the NGL fractionation units. This paper investigates the economic feasibility
or unfeasibility of having the NGL fractionation train at gas plants. The gas
plants can operate in either ethane recovery or ethane rejection mode.

In
case of ethane recovery, most of ethane is recovered from the bottom of the
dmethanizer with NGL. However, in case of ethane rejection some ethane is
allowed to escape with the residue gas by warming up the demethanizer bottom.
Decision whether to recover or reject ethane is made based on the prices of
sales gas and ethane. If the price of sales gas is high and the price of ethane
is low, then ethane rejection would be more economical.

There
are many parameters that affect the economics of whether to fractionate at the
plant site or not. Amongst the important parameters are price of sales gas, NGL
and NGL products, cost of fractionation, transportation and storage of NGL and
fractionation cost of NGL at the hub. However, in this work only sales prices,
and costs of fractionation and transportation were considered.

HYSYS
simulation of NGL fractionation units was carried out for typical shale gas
composition. Three different flowrates (150, 200 and 250 MMSCFD) were
considered and the cases were analyzed in both ethane recovery and ethane
rejection modes. The estimation of total module cost, operating cost and the
resulting economic feasibility was analyzed for both current product prices and
other assumed reasonable prices of NGL, sales gas and NGL products.

Incremental
economic analysis approach was utilized meaning, NGL market value was
considered as a raw material cost for the case of fractionation. For the gas
composition considered and the current range of prices, the preliminary results
are as follows. For both ethane recovery and ethane rejection modes at current
prices, NGL fractionation at the plant site is not economical.