Does installaton of static inline mixer in place of conventional mix type globe valve for mixing the wash water and crude before desalter help in improving desalter efficiency?
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20/05/2010
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mike moxley, Sulzer Chemtech MRT, mike.moxley@sulzer.com
There are many static mixers operating on this application. The static mixers are used to improve desalter removal efficiency. It seems to be more widely accepted to use static mixers in Europe than in the US. The use of a valve does not have to exclude the use of a static mixer. There are applications where a mixing valve and static mixer are used together. Static mixers can help when used together with a mixing valve to offer better removal rates with fluctuating flow rates. One of the advantages of a static mixer over a mixing valve is that a static mixer can yield a more uniform droplet size. This uniform droplet size creates the optimum surface area for the removal of the salt. The uniform droplet size reduces the number of very small droplets and very large large droplets that can cause carryover and carryunder problems. The static mixer also continually renews the surface area for the transfer to take place as the flow moves along the length of the mixer while the mixing valve creates a dispersion one time with out any renewal of the droplets. The static mixer creates a dispersion with lower pressure drop or energy input than a mixing valve.
Just like any piece of processing equipment, the static mixer should be properly designed for the unique requirements of your application.
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20/05/2010
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sam lordo, Becht Engineering, salordo@comcast.net
While they have the potential to improve mixing efficiency, however, they can create more problems if they are not sized for the charge rate and crude types... which unfortunately is rarely the case.... if you are to use them then make sure that you have a bypass and back up mix valve (double ported). the issue is that they are not adjustable and if the refiner is running a variety of crudes this can become a problem as mix energy can too high for easily to emulsify crudes resulting in undercarry (oil) and overcarry (water).
There are a couple of cases where high solids crudes eroded the internal baffles to the point of collapse.
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19/05/2010
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keith bowers, B and B Consulting, kebowers47@gmail.com
I hate to say this, but , It depends---on several other factors.
IF the present mix valve is not developing a very fine emulsion, desalting efficacy may improve with addition of a 'static mixer' ahead of the valve or even as a replacement. Desalting is a simple extraction of sodium chloride particles and droplets from the crude oil. The finer the mix (smaller droplet size,) the higher the probability the sodium chloride molecules will enter the aqueous phase where they can be separated when the electrical field is applied. The real challenge is how to achieve the thorough near molecular level of dispersion of the 2-5% by volume of wash water into the hydrocarbon continuous phase WITHOUT creating a stable emulsion. Often chemical additives are very effective at modifying the surface tension and other electro-static forces to enable very fine dispersion without forming stable emulsions that are difficult or impossible to separate in the desalter grid.
The generic term 'static mixer' is not a sufficient description of those devices. The number of flow splits, the specific geometry, resistance to clogging, materials of construction (longevity) and important design choices that must be optimized for each application. For instance, in this application, one would want the mixing elements to be oil wet (hydrophobic) and the wash water to be injected from a nozzle in several streams to better ensure even distribution of the water. There are some such devices that enable one to change (control) the number of mixing elements in the mixing zone while in operation. This would seem desirable so one could use as many as possible without making stable emulsions.
There is operating history on both sides of the question of which is best. I strongly recommend you consult with the licensor of your desalter and several of the designers/vendors of static mixers for this specific service. Like so many issues in the extremely complex processing of hydrocarbons, the devil is in the details, and all too often, those details may not be well understood by ones making decisions.
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19/05/2010
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Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
I am not a real expert on this question. The refining industry seems very split about whether inline mixers or mix valves are the best choice. Many in North America seem to favor mix valves.
There are many other factors which contribute to desalter efficiency, with #1 factor being use of 'double desalting'.
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