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Aug-2014

Sour water stripping 
Part 2: phenolic water

In addition to ammonia, hydrogen sulphide, and carbon dioxide, phenolic sour water may contain a host of so-called heat stable salts (HSSs), hydrogen cyanide, and phenols.

Nathan A Hatcher, Clayton E Jones and Ralph H Weiland
Optimized Gas Treating
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Article Summary
The presence of these components can adversely affect the ability to strip ammonia and H2S. One way to obviate the effect of such compounds is to neutralise them by the careful use of caustic soda. This article focuses on how HSSs affect sour water stripping and how trying to spring ammonia by neutralising them with caustic affects performance.

Introduction
Sour water is generally classified as either phenolic or non-phenolic. Non-phenolic water, also called HDS water because it is produced by hydrotreating in hydrodesulphurisation or HDS units in refineries, contains almost exclusively ammonia, hydrogen sulphide, and possibly a trace of carbon dioxide. Sources of non-phenolic sour water, sour water chemistry, phase equilibrium in sour water systems, and the removal of contaminants in sour water strippers (SWS) were all addressed in Part 1 of this series1. Phenolic (or more broadly, non-HDS) water typically contains heat stable salts (HSSs) and HCN, although phenols and caustic may also be present, depending on how the water has been used in the refinery. Coal derived gas can be quite high in both ammonia and hydrogen cyanide, and coke oven gas is especially high in these components.

As discussed elsewhere2, non-HDS water is produced by FCCs and Cokers. It contains organic and inorganic impurities derived from HSS precursors, especially derived from HCN. Such waters cannot be fully stripped because the HSSs permanently protonate some of the ammonia, turning it into a salt (NH4+) and thereby rendering it nonvolatile. Ammonia protonated as NH4+ can be freed by neutralising the HSSs with caustic soda. It is important to realise, of course, that caustic soda is a very strong base and the HSSs are only weak acids present in relatively small amounts; thus, the operative descriptor with caustic soda is cautious and careful use. Incidentally, spent caustic from Merox-type units is a common source for caustic, but caustic neutralisation of the HSSs in sour water is not usually the sole means of spent caustic disposal.


Chemistry of Sour Water Contaminants
The only molecular components in sour water are ammonia, hydrogen sulphide, carbon dioxide, and water. Ammonia is a relatively weak base capable of being mono-protonated. In water it protonates, but only to a limited extent:
 
NH3 + H+ ⇌⇔〖NH4+                           (1)

All reactions involve ionic species and all reaction products are ions, with hydrogen ion being common to all the reactions. As long as the species are in solution, they exist solely as individual ions, albeit with interactions that result in solution nonideality.

The equilibrium reactions that occur when H2S and CO2 dissolve in solution are the same as in any other aqueous, primary, or secondary amine system:

H2O⇌⇔ H+ + 〖OH–                            (2)

H2S ⇔⇌ H+ + 〖HS-                             (3)
           
HS- ⇔⇌ H+ + S=                                   (4)

CO2 + H2O ⇌ H+ + 〖HCO3-               (5)
HCO3-  ⇔ H+ + 〖CO3=                       (6)

NH3 + CO2 ⇔〖NH2COO- + H+           (7)

By acting as a proton sink (Reaction 1), ammonia drives all these reactions to the right. Like H2S and CO⇔, heat stable salts such as formate, thiocyanate, thiosulphate, and so on are weak acids, too, but unlike the acid gases, they are almost completely nonvolatile:

HSS · COOH⇌ ⇔ H+ +〖HS · COO-

The hydrogen ion produced by the HSS dissolving in water cannot be removed by thermal stripping simply because the HSS is essentially non-volatile. Therefore, the hydrogen ion permanently converts an equivalent amount of ammonia into ammonium ion which, due to its ionic nature, cannot be thermally stripped either. Phenols behave similarly.

Phenols are organic acids formed when an aromatic phenyl group (C6H5-) bonds to a hydroxyl group. The most common example is carbolic acid, more commonly known simply as phenol. When phenol dissolves in water it dissociates not into an hydroxyl anion and a phenyl cation, but into a hydrogen ion and the phenolate ion, also known as phenoxide:

PhOH⇌ ⇔ H+ +〖PhO-

Dissociation into hydrogen ion makes phenol mildly acidic (pK ≅ 10), and the produced hydrogen ion once again converts part of the ammonia into the non-volatile ammonium ion form.

Hydrogen cyanide is another contaminant often present in appreciable amounts in sour water. It too partially dissociates in water, forming more hydrogen ion:
HCN⇌ ⇔ H+ +〖CN-

Again, the hydrogen ion produced by dissociation converts part of the ammonia into non-volatile ammonium ion. Unlike phenol and HSSs, in this case the HCN can be thermally stripped relatively easily because, being an extremely weak acid, it is volatile.

Neutralisation with caustic soda is a simple acid-base titration type of reaction which, in terms of a HSS for example, is just
〖Na+ +〖HSS⋅ · COO-⇌ ⇔ HSS · COONa

Here it is recognised that the species HSS · COONa really exists only in the dissociated ionic form with charge-equivalent concentrations of Na+ and HSS · COO-. However, this also raises a question with an interesting answer: If one were to use caustic soda to neutralise the HSSs in a sour water containing HCN:
HCN + NaOH⇌ ⇔ NaCN + H2O
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