Liquid-phase Claus Process Medium-scale Hydrogen Sulphide Removal Technology
a report by David M Seeger,1 Bryan J Petrinec2 and Jenny M Dávila3 1. Chief Technology Officer; 2. Vice President, Engineering; 3. Senior Engineer, CrystaTech, Inc.
CrystaSulf Technology CrystaSulf™ uses a patented non-aqueous liquid-phase Claus
reaction that combines hydrogen sulphide (H2S) with sulphur dioxide (SO2) to form dissolved elemental sulphur in a single step.1
A single
CrystaSulf unit can be used in place of an amine unit, a Claus unit and a tail gas treatment unit combined.
With CrystaSulf, H2S is removed from sour gas in an optimised tray absorber. Within the absorber, H2S is scrubbed from the gas and then reacts with dissolved SO2 to produce dissolved elemental sulphur by a liquid-phase Claus reaction:
2 H2S + SO2 3/x Sx + 2 H2 O (1)
By design, excess SO2 is chemically bound in the solution as a stable intermediate which is always available as an excess reagent for the
reaction. Therefore, the solution is effectively buffered against spikes or
sudden changes in H2S loading to the system. The solution also has a very high affinity for SO2, thus ensuring SO2 is not emitted into the sweet gas. This combination guarantees the ability of the technology to meet outlet gas specifications and provides for simpler unit operation.
The solution containing the dissolved sulphur then passes from the bottom of the absorber to a flash tank, in the case of high-pressure applications, to remove associated gases. The solution then flows to a low-pressure crystalliser, where the temperature is reduced and elemental sulphur crystals are formed. The sulphur crystals are then
CrystaSulf™ uses a patented non-aqueous liquid-phase Claus reaction that combines hydrogen sulphide with sulphur dioxide to form dissolved elemental sulphur in a single step.
removed by the filter system. The crystalliser/filter area is the only area where sulphur solids exist within the process. The lean solution flows to a surge tank, where the solution temperature is raised to the operating temperature of the process, normally 150°F, before being pumped back to the absorber. A simplified diagram of CrystaSulf is shown in Figure 1.
Sulphur Operational Problems with Aqueous Systems Elemental sulphur has traditionally been a problem in aqueous systems because sulphur is extremely insoluble in water (~10-7g–mol/l) and
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thus nucleates, rapidly forming small particles. The problem is complicated because the small sulphur particles are also hydrophobic, meaning that the tiny particles float on the liquid surface because water will not wet them. To address this problem, surfactants must be used to wet these particles. Next, these small sulphur particles must adhere to each other, forming aggregate particles large enough to be filtered and removed. The result is that these surfactant-coated particles and agglomerates can be found everywhere within aqueous sulphur recovery systems, including the contactor and other high–pressure equipment. It is the presence of these tiny sulphur particles and surfactant in the absorber that provides the potential for uncontrollable foaming and plugging at elevated pressures. The result is that surfactant concentration control in aqueous processes can be difficult to manage, resulting in an ‘operational art form’ instead of reliable process operation.
In contrast, the CrystaSulf solution dissolves elemental sulphur, eliminating the presence of sulphur solids in the contactor and other critical equipment and also produces much larger, purer sulphur crystals that are formed only in the low-pressure solids handling equipment without the use of surfactants. Sulphur crystallised from the CrystaSulf solution, compared with the sulphur produced from an aqueous-iron liquid redox system, is shown in Figure 2. The root cause of sulphur plugging and foaming problems experienced in water-based H2S removal processes is eliminated by CrystaSulf.
Effects of High Carbon Dioxide Partial Pressure In aqueous sulphur recovery systems, typically a pH over 8.0 must be
maintained in order to effectively scrub H2S. However, at higher pH, carbon dioxide (CO2) readily absorbs and bicarbonate precipitation problems can occur, requiring the operator to reduce the pH, but at the
lower pH operation, reduced H2S removal capability may occur. These two counteracting effects restrict the use of aqueous sulphur recovery
systems when treating streams with high CO2 partial pressure. In comparison, CO2 has no effect on CrystaSulf, no sodium bicarbonate forms and no economic operating penalty is incurred when using
CrystaSulf to treat gas streams with high CO2 partial pressure. Again, the root cause of the problem is eliminated by CrystaSulf.
High Carbon Dioxide
High CO2 partial pressures adversely affect all aqueous H2S removal processes, whether it be the traditional alkanolamine (amine) type or the aqueous sulphur recovery/liquid redox type. Amine systems
tend to remove CO2, thus high concentrations of CO2 increase the loading, size and operating expense of amine systems. If the acid gas from the amine system is to be fed to a Claus sulphur recovery system to recover sulphur, then extra measures, such as multiple amine
systems, are often necessary to process high CO2 gas streams and gas streams with a high CO2:H2S ratio; these extra measures can
© TOUCH BRIEFINGS 2011
Sulphur
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