T
he most effective ways to respond to changes in feed gas flowrate and composition or process operating goals depend on where the process is running on the operational map and what constraints are either in effect or are close to becoming active. For example, absorbers are almost always either lean-end pinched, rich-end pinched, or mass transfer rate limited. The most appropriate response to an increase in raw gas flowrate, for example, depends on the regime in which the absorber is operating. The most common challenges to operations are posed by changing feed gas composition and flowrate. These can occur at any time but they are especially prevalent around initial start-up when it is almost invariably learned that the design basis for the treating unit is not the reality that prevails at the time. Often units are designed with the ability to accommodate reasonable departures; however, sometimes such is not the case. Indeed, depending on the process configuration, a substantially lower gas flowrate occasionally cannot be treated at all. This will be the first example of intuition gone wrong.
Failure to treat when gas flow is too low Figure 1 shows a CO2 unit that counterintuitively failed to treat to 50 ppmv CO2 when presented with only 10% of the design CO2 flow. This was a shock to everyone concerned. The cause was identified to be the low temperature of the rich solvent from the absorber. CO2 absorption is exothermic and it can cause quite a large temperature bulge to appear within the absorber. The heat of absorption is partially transferred to the gas which carries heat up the column. There, the gas meets cooler solvent and transfers some of its heat into the liquid phase. The solvent then carries the heat downwards where it meets a cooler incoming gas and transfers some of its heat back into the gas. This back-and-forth exchange often results in high temperatures within the absorber. But independent of a temperature bulge, the heat of absorption must eventually be carried out of the absorber in the treated gas and in the amine solvent.
Ralph H. Weiland, Optimized Gas Treating, Inc., USA, explores how to respond to changing process conditions in amine treating.
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