Sour water strippers

In 2006 a terrible fire and explosion killed 15 people and injured 500 other contractors and employees at the BP Refinery in Texas City. The main cause of the disaster was a relief valve on a naphtha splitter that vented to the atmosphere via a blowdown stack rather than to the 

The theory for the location of the relief valve location and disposition was, I imagine  

Locate the relief valve on the overhead vapor line. 

If the relief valve opens, only vapors would be emitted, as only vapors are present at the top of the tower. 

Hydrocarbon ambient emissions during a plant upset were legal. 

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If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

As an introduction to the technical aspects of the conference, the results of some studies conducted by the writer on two relevant subjects are presented below. The first commentary is concerned with the design of sour-water strippers and the effects of thermodynamic data on these designs the second commentary is concerned with the calculation of enthalpies of steam-containing mixtures, essential to the design of coal processing and related plants. 

For most sour water stripper design work, a computer is used to perform the calculations. Several of the proposed sour-water modules were incorporated into a tower program and a series of designs on a typical sour-water stripper have been undertaken. 

At the low ionic concentrations encountered in sour water strippers, the effect of dissolved ions is probably small. Thus at a 1% concentration of sodium acetate the volatility of ammonia only increases about 2.5% due to the salt. This is within the prediction accuracy of the ammonia volatility data and no correction is therefore required. However significant ionic effects could exist in the condenser where high concentrations of the ionic components could exist. 

Miles, D. H. and Wilson, G. M. "Vapor-Liquid Equilibrium Data for Design of Sour Water Strippers", Annual Report to the American Petroleum Institute for 1974, October 1975 (Data in this report are also summarized in reference 2). 

Sour water strippers are designed primarily for the removal of sulfides and can be expected to achieve 85-99% removal. If acid is not required to enhance sulfide stripping, ammonia will also be stripped, the percentage varying widely with stripping pH and temperature. Depending on pH, temperature, and contaminant partial pressure, phenols and cyanides can also be stripped with removal as high as 30%. 

Some refineries neutralize the caustic with spent sulfuric acid from other refining processes, and charge it to the sour water stripper. This removes the H2S. The bottoms from the sour water stripper go to the desalter, where the phenolics can be extracted by the crude oil. 

Figure 18 Cyanide generation and disposal in a typical refinery. Cyanide and other gases are formed in FCC or coker units during cracking of organics and go overhead on the fractionating column. Wash water dissolves these gases and becomes sour water. Part of the cyanide is removed by the sour water stripper and the rest goes to the sewer and eventually to the wastewater treatment system. (From Ref. 48.)...

Figure 10.3 shows a simple sour-water stripper. The steam is used to remove NH3 and H2S, dissolved in the waste, or sour water. In the diesel oil stripper discussed above, all the stripping steam went out the top of the stripper. But what happens to the stripping steam in a water stripper It is used in four ways ... 

Normal vertical knockout drums are designed for a K value of about 0.20 to 0.25. If we are installing a KO drum ahead of a reciprocating compressor—and they really hate liquids in their feed—a K value of 0.14 might be selected. If we really do not care very much about entrainment, a K value of 0.4 might be selected. An example of this would be venting waste gas to the flare from a sour-water stripper reflux drum. 

Most of the filtrate is recycled as wash water. However the overall system generates a net water surplus, which must be treated before final discharge. First the surplus water passes through a Sour Water Stripper to remove dissolved gases that are then incinerated. The water is then treated in a biotreatment facility180. 

Until recently the ability to predict the vapor-liquid equilibrium of electrolyte systems was limited and only empirical or approximate methods using experimental data, such as that by Van Krevelen (7) for the ammonia-hydrogen sulfide-water system, were used to design sour water strippers. Recently several advances in the prediction and correlation of thermodynamic properties of electrolyte systems have been published by Pitzer (5), Meissner (4), and Bromley ). Edwards, Newman, and Prausnitz (2) established a similar framework for weak electrolyte systems. 

Sour water strippers where complex chemical and ionic equilibria occur in the liquid phase affecting the vapor-liquid equilibria. 

Sour Water Stripper Sour water is fed to a stripper to remove ammonia and hydrogen sulfide. The wet ammonia/hydrogen sulfide overhead is fed to the sulfur plant. In cases where there is an appreciable amount of ammonia, a two-stage stripper is used to produce separate hydrogen sulfide and ammonia/products. 

For convenience, the discussion of materials for these various processes is divided into five chapters. Crude units and utilities are discussed in this chapter. FCCs, fluid cokers, delayed cokers, sour water strippers, and sulfur plants are covered in Chapter Two. Desulfurizers, reformers, hydrocrackers, and flue gas are discussed in Chapter Three. Hydrogen plants, methanol plants, ammonia plants, and gas treating are discussed in Chapter Four. Underground piping, pipelines, production equipment, and tankage associated with the refinery industry are covered in Chapter Five. Discussed throughout these chapters are many common environments and equipment (e.g., sour or foul water, distillation, etc.) that appear in the various types of refinery process plants. 

Fluid Coking and Cracking, Delayed Coking, Alkylation, Sulfur Plants, and Sour Water Strippers... 

The main corrosion problems in a nonacidified condensing sour water stripper occur in the overhead system. Exchanger tubes in the overhead condenser are often made of commercially pure titanium. The reflux pump is often alloy 20 (CN7M). In very corrosive waters, such as those containing phenols or large quantities of salts, Hastelloy1141 C276 is used. Two API surveys (API Standards 944 and 950) concluded that the location and severity of corrosion vary with the type of unit as follows ... 

API Standard 950. "Survey of Construction Materials and Corrosion in Sour Water Strippers. Washington, DC American Petroleum Institute, latest revision. 

Hildebrand, R.L. Sour Water Strippers—A Review of Construction Materials. Materials Performance 13, 5(1974) p. 16. 

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