Natural-Gas-Liquids Recovery - Retrofit Breathes New Life Into Old Scrubber

Abstract

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 188200, “NGL Recovery—A New Concept for an Old Scrubber,” by Ankur Jariwala, SPE, Pinkesh Sanghani, and Dag Kvamsdal, Schlumberger, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed. An onshore gas-processing facility in Southeast Asia currently receives rich wet gas from an offshore production unit. Because of an inefficient scrubber design, the gas-processing facility was experiencing natural-gas-liquid (NGL) carryover of 1,550 B/D from the scrubber into the pipelines. A retrofit scrubber was designed to increase the NGL production by 8,540 B/D. This paper presents details of the retrofit scrubber design and shows the importance of using high-efficiency separation internals. Challenges With the Existing Scrubber The condensate scrubber vessel was originally designed to handle 300 MMscf/D of gas. The condensate scrubber had an inlet vane and pack internals along with a mesh pad for final demisting. The customer deter-mined that the scrubber was designed incorrectly because, since the plant startup, it was experiencing large NGL carryover from the scrubber into the gas lines. Because of this inefficient scrubber vessel, the facility was losing approximately 1,550 B/D of NGL. The existing scrubber design was analyzed with governing empirical formulas and computational-fluid- dynamics (CFD) analysis to identify the following challenges. The existing scrubber vessel had limitations in managing maximum gas loading (k-value) before the bulk of the liquid was dragged by the gas to the demister section of the scrubber. Maximum vessel k-value was predicted to be 0.347 m/s, and values were considered to be much higher in this application. The inlet device was designed inadequately, leading to incorrect flow paths and a poor distribution of fluids. Because of the inertia of the gas, the entry flow into the vessel pushed the gas upward in the vane pack, resulting in poor distribution in the vane-pack region such that only part of the vane pack was used. High-velocity zones in the vessel reduced the performance of the vane pack. The overall separation performance for the current separator was estimated to be 51%, resulting in a gross liquid carryover of 8,500 B/D. CFD Analysis of the Existing Scrubber Fig. 1 shows the predicted gas flow in the vessel, illustrated by path lines. The simulation shows that a relatively large fraction of the gas is directed downward against the liquid pad. This could result in re-entrainment from the already-separated liquid, leading to high liquid carryover. The poor flow distribution in the inlet vane is caused by a very large and sudden expansion of the flow area in the vane. This undesirable distribution creates zones of high gas velocity, causing droplet breakup and low separation performance in the vessel.