Improvements to water gas shift process
With a growing population supplies of natural resources are under ever-increasing pressure, their sustainable use has never been higher on the agenda.
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Against this backdrop, Johnson Matthey (JM) seeks to improve the efficiency, effectiveness and sustainable impact of their products, enabling JM customers to achieve high productivity by making more for less.
Through a new patented solution JM has used expertise in purification science and engineering skills to develop an innovative new shift product which extends LTS life allowing ammonia plants to increase their process reliability and production efficiency.
The choice of shift catalyst and how they operate are vital to the economy and efficiency of an ammonia plant. It is essential that the shift catalysts produce the lowest possible exit concentration of carbon monoxide in a stable and predictable manner. The catalysts need to be able to withstand the challenges presented in real plant operations for an increasing period of years between planned shutdowns.
The water gas shift reaction and attributes for good catalysts are discussed in this paper, plus an introduction to new purification technology developed for use in the HTS reactor to further improve the performance of a LTS catalyst.
Water gas shift reaction
The water gas shift reaction plays a major role in ammonia plant design and operation. Good performance of the shift catalysts, and attainment of a close approach to equilibrium thus minimising the CO slip from the catalyst system which is critical to the efficient and economic operation of the plant and ensures maximum hydrogen production from the hydrocarbon feedstock. The water gas shift or shift reaction (1) is highlighted below.
CO + H2O ⇔ CO2 + H2 (1)
The reaction is exothermic and high conversions are favoured by low temperature and a high steam ratio. Ammonia plants usually operate a two-stage system – a High Temperature Shift (HTS) followed by a Low Temperature Shift (LTS) – with a suitable form of inter-bed cooling, as in Figure 1.
HTS catalyst duty
The HTS is a Fe-Cu-Cr catalyst that is relatively insensitive to poisons. HTS catalysts have lower activity and must be operated at higher temperatures (typically 350-470°C) the gas stream leaving the HTS reactor therefore contains a substantial amount of carbon monoxide (usually 2-3%).
The HTS catalyst sits immediately downstream of high temperature reforming systems and heat recovery (boilers and superheaters). The heat recovery equipment is highly stressed which can cause failure leading to leakages and deposits onto the HTS bed. The HTS is the first catalyst bed downstream so can be fouled by refractory or metal dusting debris that may be emitted from the high temperature recovery equipment. The fouling impact of this on HTS can be mitigated using both high voidage inert hold-down media and a high voidage shaped HTS pellet such as KATALCO 71-5F. (Figure 2).
Shaped KATALCO 71-5F offers 12% lower pressure drop than the conventional tablet with increased strength. The shape offers additional voidage to accommodate any deposits that may foul the HTS bed lowering the rate of any resultant pressure drop increase on the HTS from deposits.
Also on top of the HTS bed high voidage inert DYPOR 607 can be used to protect the top of a catalyst bed by both capturing boiler solids and by preventing the impingement of liquid droplets onto the catalyst itself. JM market this solution as SHIFTSHIELD, Figure 3.
Below the HTS bed the use of similar inert media is well established in shift reactors as Johnson Matthey’s STREAMLINE low pressure drop solution. STREAMLINE was originally developed in response to several high temperature (HT) and low temperature (LT) shift reactors exhibiting high pressure drops. On investigation it was discovered that a high portion of the vessel pressure drop was associated with the area around the collector. This was confirmed by detailed computational flow modelling. (Figure 4).
The specific size grades of DYPOR, used as a support media, vary depending on the vessel configuration and particle size of material being supported. JM have a proprietary STREAMLINE software program which is configured for a full range of vessel collector designs. Figure 5 shows a STREAMLINE model of a vessel with a dished bottom collector, along with plant data showing the 60% reduction in pressure drop that resulted from the deployment of STREAMLINE on this vessel.
An enhancement to STREAMLINE technology is now available as discussed later, utilising PURASPEC 2272 as a high voidage support media, which also provides LTS protection from chloride poisons.
The efficiency of ammonia plants depends critically on the ability of the LTS catalyst to convert the maximum amount of CO to H2. For every molecule of CO passing through the LTS there is not only the loss in production of H2 via equation 1, but also a further loss of three molecules of H2 in the methanation of CO, equation 2, and further loss when this extra methane is purged from the synthesis loop.
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