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Stripping sour water: the effect of heat stable salts

Mass transfer rate-based simulation models sour water stripping and assesses the distribution of ammonia in conventional amine treating systems

Optimized Gas Treating, Inc
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Article Summary
In a recent article dealing with a live steam-injected stripper (PTQ, Q3 2012), we discussed how steam usage affected the stripped water residual ammonia and hydrogen sulphide levels, how ammonia distributed itself within the stripper in an unexpected way, and how Murphree (1925) vapour efficiencies varied with location within the tower at various stripping steam rates.4 The discussion was limited to non-phenolic sour water. In the present article, we examine the effect of heat stable salts (HSS) on sour water stripper (SWS) performance, how the injection of caustic soda can spring ammonia from the sour water, and how caustic injection can worsen H2S stripping if it is injected at the wrong place, or too much caustic is injected. The analysis uses a mass transfer rate-based simulation model for sour water stripping and for assessing the distribution of ammonia in conventional amine treating systems.

Sour water sources
The sour water generated in refineries comes from numerous sources. Most refinery sour water systems contain very little CO2, but H2S levels can become very high. The capacity of ammonia solutions for H2S is a direct result of the weak acid-weak base reactivity between H2S and ammonia. The potentially high H2S content can make sour water extremely foul, and H2S removal from the sour water to quite low levels is mandatory to avoid unacceptable pollution levels. Many sour water sources have been noted in the excellent review article by Asquith and Moore.1

Sour water is generally classified as phenolic or non-phenolic. Non-phenolic water contains almost exclusively NH3, H2S and possibly a trace of CO2. It is generated by refinery hydrotreating (hydrodesulphurisation, or HDS) units. When stripped of contaminants, non-phenolic water can typically be recycled for reuse in the HDS unit as wash water, or it can be used as make-up water to the crude desalting process. Phenolic (or more broadly, non-HDS) water typically contains HSS, phenols and caustic.

Finally, it may be useful to point out that ammonia and hydrogen sulphide have almost unlimited solubility in water when they are present together. This is an interesting consequence of the fact that the reactive component of the solvent, ammonia, is volatile and, if present in the gas phase, it will continue to absorb as long as it becomes protonated as a result of H2S co-absorption. Thus, it is conceivable that a particular sour water stream may be a lot more concentrated than the solubility of either ammonia or H2S by itself might suggest.

SWS uses either steam generated by a reboiler, directly injected steam, or even a hot hydrocarbon stripping vapour to shift chemical reaction equilibria by applying heat. Stripping vapour is the “gaseous solvent” used to remove and carry the ammonia and H2S out of the system. It functions by:
• Heating the sour water feed to the boiling point
• Reversing chemical reactions
• Diluting the partial pressure of the gases stripped by furnishing excess vapour.

Figure 1 shows a typical SWS column with heating by the injection of live steam and with the possibility of injecting caustic soda to one of the trays in the column. Typical energy usage in the stripping process is in the range 1.0-1.5 lb of 50 psig equivalent saturated steam per gallon of sour water.

To minimise heat exchange surface, an external reboiler often uses higher pressure (temperature) steam than is typical in an amine regenerator because amine thermal degradation is not a limiting factor. However, there is a practical limit of 400-450°F, where coking heavy hydrocarbons can lead to fouling and solids deposition in the reboiler and, of course, corrosion is always a concern.  
Stripped sour water specifications for NH3 and H2S can be highly dependent on local requirements. Typical targets for NH3 are 30-80 ppmw in the stripped water and undetectable to less than 0.1 ppmw for H2S. Typical recent installations involve 35-45 actual trays.1,3

It is common in refinery cracking units (FCCs and cokers) for the sour water generated to contain organic and inorganic acid impurities from HSS precursors and, just as for amine units, ammonia partially in the protonated form. It cannot be thermally regenerated because the HSS responsible for the protonation is completely non-volatile and cannot be removed by boiling it into the stripping steam. In such cases, it is quite common to inject a small amount of strong base (NaOH) to shift the pH into a range where ammonium ion (NH4+) shifts back to NH3. Spent caustic from Merox-type units is commonly used for this purpose, but care must be taken to ensure that disposal of the spent caustic is not completely reliant on this destination, or the tail will begin to wag the dog.

When adjusting the pH of the water to spring ammonia chemically, the adjustment is usually made by metered injection of caustic onto a tray far enough down the column that most of the H2S has already been stripped out and ammonia is the main remaining component. The metering rate is normally controlled to a set point on the pH measurement in the stripped water after it has been cooled. Caustic injection on a lower tray generally works better than injection directly into the SWS feed itself because the H2S concentration is already small on lower trays. However, pH is extremely responsive to caustic addition, so the measuring and control elements should be as close together in time as possible if rather large fluctuations in pH are to be avoided. As we shall see, no more caustic than is absolutely necessary should be injected because excess caustic can permanently bind H2S into the solution and eventually this will find its way into biological treatment ponds, either reducing the efficacy of the microbial population or unnecessarily increasing the biological oxygen demand.

Traditionally, SWSs have been modelled as a series of equilibrium stages, with stage efficiencies being quoted anywhere in the range from 15% to 45%, that is ranging over a factor of three. However, since the mid to late 1980s, the mass transfer rate-based approach to simulating amine contactors and extractive, azeotropic and reactive distillation has been in successful commercial use. The extension to sour water stripping is a natural progression and, in December 2011, the ProTreat simulation package saw the addition of a commercial mass transfer rate-based SWS model. The remainder of this article uses this model to explore how HSS affect the performance of SWSs and how caustic addition can be tailored to provide the optimum amount of stripping of H2S and ammonia from HSS-laden sour water.

Case study
Figure 1 shows the simplest possible configuration of a SWS with caustic injection. For this case study, the same stripper as outlined previously was used.4 It contained 40 one-pass valve trays on 2ft spacing with 2-inch weirs. Sour water was fed at 235°F to tray 6 (from the top), live steam saturated at 50 psig entered below the bottom tray and, in all cases, the column was sized for 70% of jet and downcomer flood. Caustic could be injected on any tray in the column. Table 1 gives the conditions of the sour water used for this case study.
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