Jul-2013

Preventing ingress of HCN into amine systems

Simulation studies provide guidelines on how to avoid contamination of refinery amine systems by hydrogen cyanide

RALPH WEILAND, NATHAN HATCHER and CLAYTON JONES
Optimized Gas Treating, Inc

Article Summary

Hydrogen cyanide (HCN) has far-reaching effects on amine system performance. After hydrocarbon contamination, its presence is one of the primary reasons refinery amine systems suffer from accelerated corrosion, and operability and reliability problems. When HCN enters the amine system, its hydrolysis produces ammonia and formate, a heat stable salt (HSS). Reaction of HCN with oxygen and H2S generates another HSS, thiocyanate. HSSs are known to chelate iron, and the subsequent accelerated corrosion leads to faster formation of particulate iron sulphide, which, in turn, leads to filter element plugging, fouled equipment, lower capacity and more stable foams. Mass transfer rate-based simulation is used in this article to investigate the prevention of HCN ingress into amine systems via water washing.

Background and context
Over any substantial period of operation, amine systems operated in refineries can be expected to show an increasing build-up of HSSs. This is especially pronounced for units that scrub gases primarily from coking and catalytic cracking units, where the HSSs consist primarily of formate (HCOO-) and thiocyanate (SCN-). Amine systems treating gases from various sources experience different rates of HSS accumulation. The actual rate of HSS anion build-up depends upon the incursion rate of HCN into the amine system. The primary sources of HSSs are summarised in Table 1, from which a direct chemistry link between HCN and the common HSS anions formate and thiocyanate is apparent.

If left unchecked, the build-up of HSSs eventually neutralises permanently part of the amine by protonation and results in loss of treating solution capacity. This is the primary acute symptom. HSSs are also known to complex iron ion and accelerate corrosion in the hot, lean section of the amine unit. When the complexed iron contacts higher concentrations of H2S in the absorber, iron sulphide precipitates. These particles can foul equipment and stabilise foam, leading to a loss of hydraulic capacity, so the operator usually resorts to trading these costs for the cost of replacing filter elements. The costs of equipment failure vary with the failure mode, are highly site specific and the timing of the lost production can matter significantly. In general, lost profit from equipment failure can be expected to dwarf filter costs. It is unfortunate, however, that costs such as these are often ignored from the economic planning process because they are difficult to quantify with certainty. The industry is generally not forthcoming with reporting minor mishaps because there is little business incentive to be open about potentially embarrassing operating episodes or revealing mistakes that can amount to significant competitive advantages when solved.

Hydrogen cyanide is a byproduct of cracking the heavier fractions of crude oil in a refinery (either thermally as in a coker, or catalytically as in a fluid catalytic cracker). The gas oil (boiling point 750°F/399°C+) and heavier fractions tend to have greater concentrations of nitrogen than the diesel and lighter fractions. These processes break up the larger nitrogen-containing molecules at high temperatures and low hydrogen partial pressure conditions that may not be as conducive to complete conversion of byproduct molecules such as HCN to ammonia as in a high-pressure hydrotreater or hydrocracker.

Thus, HCN occurs quite naturally in refineries and has many sources. Some processes are high producers; others do not seem to produce HCN at all. Once produced, HCN finds its way into the amine system with the H2S-containing gases. HCN forms in various processes within a refinery, whereas HSSs form in the amine system. Once in the amine system, various conditions and the presence of other contaminants allow the HCN to be converted into HSS anions. An ounce of prevention is worth a pound of cure: HCN is a precursor to HSS formation and its removal from the raw gas entering the amine unit would virtually eliminate all the problems associated with HSSs. In other words, the most obvious way to prevent HSS anion formation in amine systems is to prevent the ingress of HCN in the first instance. This thinking has led refiners to try various schemes to remove HCN before it gets into the amine unit. One approach that has been tried repeatedly by many refiners is water washing the raw gas and sending the wash water to a sour water stripper (SWS). Despite best efforts, however, this approach seems invariably to fail because the amine system continues to experience the build-up of HSS anions in operation.

Water washing refinery gas to remove HCN
Most applications of water washing appear to be founded upon the thinking that only a relatively small flow rate of wash water ought to be sufficient to remove HCN from a large gas flow. However, to ensure realistic tray or packing hydraulics, the tendency has been to use a large flow of wash water recirculating through the wash column, with a small addition of fresh water and a corresponding small purge of water from the tower bottoms stream. Figure 1 shows this scheme with its small addition, small purge and large recirculation. Using a small purge flow rate, which is added to the regular sour water flow to the SWS, prevents the sour water system from being overloaded.

The specific example for this study uses a typical gas intended to be treated for H2S removal in an amine system. Table 2 shows the raw gas analysis and its conditions of temperature, pressure and flow rate. The water wash column contains 20 valve trays sized for 70% of jet and downcomer flood.

Apart from water and H2S, the components HCN and ammonia are of most interest. They are all modelled by the ProTreat simulator based on their mass transfer rates, not solely on assumptions about equilibrium between phases. This approach to column simulation accounts not just for vapour-liquid equilibrium, but also for the effects of chemical reaction rates and the mass transfer characteristics of the tower internals on the rate of separation of all the components whose presence is important to the gas purification process. In the present context, HCN absorption into the wash water is of particular concern.

There are two distinct cases to consider: in the first, the total water recycle flow rate (Stream 31 in Figure 1) is kept constant at 100 gal/min, to which 5 gal/min of fresh water is added as makeup and later purged. The other case involves no wash water recycle at all — only fresh water is used to feed the wash column. Table 3 shows the effect of adding various fresh water flows to the 100 gal/min recycled flow on HCN and ammonia removal. Table 4 shows the effect of eliminating water recycle altogether and pretreating the gas with various flows of fresh wash water only.

At fresh water rates of more than 50 gal/min added to the basic 100 gal/min of water recycle, the use of recycle at all is of questionable hydraulic performance benefit because it certainly is not needed to keep trays functioning properly from a hydraulic standpoint. Above 20 gal/min water makeup, ammonia removal is hardly affected and H2S removal is rather minimal in any case. HCN removal is rather poor unless very high water makeup rates are used, and even then performance is marginal. The next question then is whether using scrubbing with fresh water alone (no water recycle) can achieve a better result.