Oct-2016

Reducing hydrocarbons in SWS acid gas

Measures to minimise the level of hydrocarbons in sour water and thereby protect the efficiency of the sulphur plant.

BEN SPOONER and Farsin Derakhshan, Sulphur Experts
DAVID ENGEL, Nexo Solutions

Article Summary

Sour water stripping (SWS) is one of the first stages in the wastewater treatment process in many industrial operations, and especially in refineries. Water streams from throughout a refinery are typically sent to the stripper, which is designed to remove both H2S and ammonia from the water. There are several designs of sour water strippers, all playing upon the same theme of using heat to break down NH4SH in wastewater. This liberates gaseous ammonia and hydrogen sulphide in a produced acid gas. In some cases, the ammonia and H2S are separated and sent to individual destinations, but in the majority of SWS set-ups the effluent acid gas from a sour water stripper overhead is processed in the sulphur recovery unit (SRU).

Hydrocarbons may also be present in the water feeding the stripper. If and when this occurs, most hydrocarbons will be vaporised and flow with the acid gas. This creates problems in the SRU, both operationally and mechanically. In Sulphur Experts’ experience, SWS acid gas containing more than 2% hydrocarbon is a firm indication of problems upstream of the stripper.

This article discusses how to mitigate the presence of hydrocarbons in sour water feeds, thus reducing fouling, corrosion and many associated operating problems both in the SWS and the SRU.

Sour water stripping process
The purpose of a sour water stripper is to remove components that are toxic or cause undesired odour: primarily H2S and NH3 (ammonia) as well as dissolved gases, solids and hydrocarbons. The process involves flowing the water down a tall stripper tower (equipped with either trayed or packed internals) while being contacted counter- currently with steam. When sour water first enters the stripper, H2S and NH3 is dissolved in the water as a salt (NH4HS) and cannot be removed as the salt has no vapour pressure. By heating the sour water, dissolved H2S and NH3 will phase transfer into their vapour forms, which can then be stripped. Any other volatile species is also liberated:

H2S + NH3 ↔ NH4SH ↔ NH4+ + HS-      (1)

There are many wastewater sources in a refinery, all of which have different contaminant compositions, flow rates and pressures. In addition, some sources may be continuous while others are intermittent. As a result, without proper upstream equipment, design, and operation, the chemical composition and flow of water to the SWS may vary significantly. This can result in many operational difficulties both for the stripper and the downstream sulphur plant, the destination for the gases stripped from the water.

The most common sources are: atmospheric crude columns, vacuum crude towers, steam crackers, fluid catalytic cracking (FCC) units, hydrodesulphurisation (HDS) units, catalytic hydrocracking (ARDS) units, coker units, amine reflux, and tail gas treating (TGT) quench towers.

H2S and NH3 concentrations are the highest in water from the HDS, ARDS, and FCC units. Any water stream containing 10ppm or more of H2S should be routed to the SWS.

Proper removal of contamination from the sour water is extremely important for the next steps in waste water treatment; often a biological treater cannot survive under high hydrogen sulphide levels.

Hydrocarbons in sour water
Hydrocarbons in water streams can be present essentially in three forms:
1. Free hydrocarbons These will not interact with the bulk water and will tend to separate within a few minutes. Free hydrocarbons are normally observed by the formation of a top hydrocarbon layer above the water phase (or below, depending on the density difference). The levels of free hydrocarbons can vary from hundreds of ppm to % levels.
2. Soluble hydrocarbons All hydrocarbons will have a certain solubility in water phases. The extent of hydrocarbon solubility in water will depend on the pH of the water, water pressure and temperature, and the type of hydrocarbon. It is impossible to observe dissolved hydrocarbons in a water phase as it is indistinguishable from pure water. In general, the solubility of hydrocarbons in water can range from a few ppm to a few hundred ppm.
3. Emulsified hydrocarbons Under normal conditions, hydrocarbons will be either free or dissolved in a water phase. However, when conditions are right (including the presence of surfactants and energy), the hydrocarbon contaminants can form very small droplets in the water phase. These droplets are stabilised by molecular surfactants (similar to soaps or detergents) and also by small size suspended solids. Emulsion droplet sizes can range from a few microns to about 500 microns. Microemulsions, which are the most stable emulsions available (and can take weeks to separate), are typically found when droplet sizes are below 10 microns.

Sour water sources
In a refinery setting or any plant in general, sour water can be generated in many locations. Water for process applications is used in many ways, such as quench water, steam and wash water, and is also generated by the various distillation fractions where water is co-distilled with certain hydrocarbons. Figure 1 shows a number of sour waters originating at various units.
Primary sour water sources include:
• Atmospheric and vacuum crude towers: water is produced by condensing overhead steam streams. Vacuum towers may also contribute sour water from ejectors and barometric condensers
• Thermal and catalytic cracking units: steam condensate from injection, stripping and aeration. The heavier or more viscous feeds, which contain sulphur, produce H2S when hydrogenated. Ammonia is also produced from hydrogenation of organic nitrogen compounds
• Hydrotreater and hydrocracker wash water from high and low pressure separators
• Cokers – delayed and fluid types plants: water is produced from decoking and quench water
• Flare seals and knock-out drums
• Hot condensates from throughout the refinery, which may have had contact with hydrocarbons. (Often the concentration of contaminates in these streams is low.)
• Any refinery water draw boot: each contains a different sour water composition and flow, depending on crude type and the severity of the process. Manual level controls can also affect the hydrocarbon content of the water, especially if they are accidentally left open for too long.
There are various analytical methods available used to test water samples for hydrocarbon-like substances. The most common are:
• Proton nuclear magnetic resonance (1HNMR) will detect hydrogen atoms attached to carbon atoms in hydrocarbon structures. The technique uses the signals from elections in the hydrogen atom and how these interact with the neighbouring carbon atoms and hydrogen atoms
• Infrared spectroscopy (IF or FTIR) detects alkyl residues, olefin residues and aromatic residues by interaction of infrared energy light with the molecular component. FTIR are IR systems with a special mathematical result treatment using a curve fitting technique called Fourier transform
• Gas permeation chromatography (GPC) is a technique very similar to liquid chromatography. It uses a polymer to separate the various components in a mixture. The separation is obtained by ‘size exclusion’
• Gas chromatography coupled with mass spectrometry (GC-MS) is capable of separating most components in a mixture to provide the exact molecular weight of each component.