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

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 stripping section 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 the price of sales gas, the cost of transportation and storage of NGL and/or NGL products and fractionation cost of NGL at the hub.

In this study, 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.

The incremental economic analysis approach was utilized meaning, NGL market value was considered as a raw material cost for the case of fractionation. Further, it was assumed that the products are sold on site which eliminates the transportation and storage costs. For the gas composition considered and the current range of prices, the preliminary results are as follows. For the ethane recovery mode it would not be economical to fractionate the NGL at the plant site. However, in case of ethane rejection mode it is economical to fractionate the NGL at the gas processing plant.