Some process considerations on the issue that may result useful to someone....
The commercial development of methyldiethanolamine (MDEA), and of all the various blended and promoted versions of MDEA, opened up a world of opportunity for the refiner and the gas processor.
MDEA has a number of positive attributes that can bring some real benefits to plant operators.
Compared to the more conventional MEA and DEA solvents, MDEA has a Iower heat of reaction with H2S and CO2, is non-corrosive at much higher concentrations, and is more resistant to degradation.
These properties can offer considerable energy savings, or increased plant capacities relative to MEA or DEA.
In its blended form MDEA can virtually ignore CO2 or, by changing a few parameters, it will gobble up CO2 as well as any amine available.
A solvent formulation can be made to suit almost any application.
Unfortunately, in the early days there seemed to be an air of the miraculous about MDEA.
Many conversions to MDEA were made from MEA and DEA without a full understanding of all the factors involved.
A great many of these conversions were successful, that is, they accomplished exactly what they set out to accomplish.
Several, however, were not so lucky.
As these experiences filter through the industry, plant operators and engineers are becoming more sophisticated about solvent- changes, and fewer failures are seen.
Here follows some of the factors to be considered in comparing amine solvents.
Make sure the comparison is apt.
The MDEA vendor’s proposal wilI be optimized to the Nth degree.
Don’t compare that calculation with yesterday’s run sheet data to estimate savings.
Your plant probably isn’t optimized, and may not even be running well.
Have the vendor run his computer simulation on your present solvent as well, so you can compare optimum to optimum. You may even learn a little about your present operation in the process.
Know what vour plant steam balance looks like: MDEA requires Iess steam for solvent regeneration than DEA, no question about that.
But, what is the steam savings really worth? Many refineries and some gas plants are chronically Iong on low pressure steam, and may even be venting.
In this case you will Iose money by reducing the steam rate to the reboiler. The stripper reboiler is a good Iow pressure steam condenser because the condensate is recovered for reuse.
Look carefullv at the gas stream to be treated. lf it contains HCl from the reformer, or formic acid from the *****er, or any of a host of other acidic contaminants, beware! Some plants have to bleed off some solvent from time to time to control the build up of heat stable salts. Since MDEA has Iower inherent Iosses, and costs more, the HSS wiII buildup even faster, and the bleed off wilI cost 5 or 6 times as much as it did before the change over.
This unexpected cost can easily outweigh the anticipated savings. In short, a switch to MDEA won’t solve a HSS problem.
Look elsewhere to solve oxygen problems. VVhiIe MDEA itself is much less affected by oxygen than MEA or DEA, it still may not help with oxygen contamination problems.
Oxygen reacts with H2S to form thiosulfuric acid, a common component of heat stable salts. In pure carbon dioxide systems, hydrogen plants for example, there is evidence that oxygen promotes corrosion in the amine system.
Understand both sides of selectivitv. The sulfur plant wiIl operate hotter and more efficiently if you reduce the CO2 pickup in the amine plant, and converting to MDEA is an excellent way to do this.
But, you have to consider where that CO2 ends up when it is not absorbed in the amine plant. At best it wiII reduce the heating value of the plant fuel gas a bit, but could have a big impact on a downstream processing plant, a cryogenic ethane recovery system, for example.
The other source of disappointment in MDEA conversions stems from not fully appreciating the difference in physical properties when changing to a higher solvent concentration.
The differences in these properties can influence a number of areas in your amine plant.
Some places to Iook for traps include the following:
• Lean amine cooler - the higher viscosity and the Iower thermal conductivity wIII lower the exchanger U value, and you may not be able to get the lean amine as cool as before.
• Reboiler - a higher boiIing point may mean a lower delta T on the reboiler if the steam pressure is Iow and could limit heat input to the stripper. Conversely, lower steam rates will mean Iower steam chest pressure if the control valve is on the steam side. This could cause problems getting condensate into the condensate system.
• Liquid - liquid treaters - Stokes law says raising the viscosity of the solvent means Ionger resident time required to disengage hydrocarbon droplets in the bottom of the treater, i.e., possible carryunder to the sulfur plant. Raising the temperature to keep the viscosity down requires a higher operating pressure to suppress vaporization, i.e., possible overpressure, There is an optimum treating temperature and it may not be where you want it.
• Absorber froth height - The lower heat release and the innately higher viscosity of MDEA will increase the height of the froth on the absorber trays. lf the column is being pushed this could trigger the symptoms of foaming. Packed, and some trayed absorbers may have a minimum liquid rates higher than the desired turn down rate.
• Circulating pumps - If the conversion to MDEA is to gain treating capacity rather than save energy the increase solvent density may overload pump drivers. Higher viscosity will mean more delta P on the Iean amine filters, also adding to pump horsepower.
• lnstruments - Flow and level devices may have to be recalibrated to account for the higher density of the solvent.
• Operator training - While operators’ perceptions aren’t physical properties, they can cut into the benefits of conversion just as quickly as any of the above factors. Most amine plant operators have steam and circulation rates at which the plant runs well fixed firmly in mind. In this context “welI’ usually means minimum trouble. lf the operator doesn’t fully understand why the changeover to MDEA has been made, and what the costs and benefits are expected to be, he (she) wilI try to keep the plants rates just where they were before. The plant willl work just fine, but you’re now losing money instead of saving it.
Reply With Quote
Bookmarks