Hydrocarbon removal from sour water systems A hydrocarbon and solids removal system allows stripper columns to operate as designed without exchanger fouling
Mathews Thundyil, David Seeger, and Arvind Chaturvedi Transcend Solutions
S our water systems can be found in most refineries and gas plants around the world. Sour water is generated from many sources throughout the facility and is accumulated in large tanks for treatment within the sour water system. In this system, light hydrocarbons, hydrogen sulphide (H₂S), and ammonia (NH₃) are stripped from the sour water. The stripped gas is often sent to the Claus unit reaction furnace for destruction and conversion to nitrogen and sulphur. Examples of sources of sour water within the facility include:1 • Hydrotreater or hydrodesulphurisation units, where sulphur-containing hydrocarbons are reacted with hydrogen and H₂S is formed, and the hot gases from the reactors are quenched with water to reduce the temperature. • The reflux water from the regenerator in the amine unit can be a source of ammonia. • Steam is used in various processes throughout the refinery, where it may contact NH₃ and H₂S. When condensed and recovered, the condensed water can become sour, containing H₂S and NH₃. • Wash water used throughout the facility may contact NH₃ and H₂S and become sour. • Water is recovered in the crude, fluidised catalytic cracker, and coker units, and is also likely sour water. These and many other sources of sour water are accumulated from throughout the facility and pumped into feed tanks to be treated in the sour water system. An examination of the various sources of sour water quickly indicates that it is probably highly contaminated with salts, solid particulates, and hydrocarbons. One method of handling these contaminants is to accumulate the water in large holding tanks, hoping that the solid
particulates will settle to the bottom while the hydrocarbons separate and float to the surface, where they can be skimmed.2 The solid particles are removed during turnaround when the tanks can be cleaned. The issue is that the approach does not work well, as solid particulates and hydrocarbons are not separated in the large holding tanks. Instead, they are pumped into the sour water system, perpetuating problems within it and others connected to it. This ongoing problem at the source of contamination inspired the design of the Torsep system, which aims to remove contaminants from the sour water system more effectively. Case study A refiner on the US Gulf Coast was forced to clean the heat exchanger in the sour water system approximately every three to six months due to fouling. The operators at the facility had come to accept this problem as the normal, routine course of operation. During a meeting to review operations and maintenance, it was discussed that operations should not have to accept heat exchanger cleaning every three months as routine and that the problem could be fixed. It was suspected that the problem was not only solid particulate fouling but also hydrocarbon fouling on the surfaces of the heat exchanger. To better define and understand the issues, in situ solids-loading samples3 and bottle samples were taken from its sour water system. The results of this testing revealed that both problems were occurring. This refiner originally took the approach of holding the sour water in large tanks with a long residence time (one to two days). However, even
Refining India
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