New separation approach saves catalyst and energy

New separation approach saves catalyst and energy

The catalysts used in acrylonitrile reactors are expensive. Typical losses for a reactor with a conventional cyclone system are 200 lbs per day to 400 lb per day. There are also indirect costs associated with catalyst losses, such as those caused by plugging and downstream erosion.

Substantial savings and fast payback can often be achieved by installing improved cyclone systems to recover more of the valuable catalyst.

DuPont's Installation
DuPont installed just such a cyclone system in an acrylonitrile reactor at its Beaumont, TX, plant.

The reactor is a fluid-bed unit in which catalyst promotes the chemical reaction that yields acrylonitrile and other organic compounds used primarily in the production of plastics.

The hydrocarbon feed stream passes through the fluidized bed, where it comes into intimate contact with with the catalyst. Cyclones within the reactor reduce the loss of catalyst from the system.

Prior to installation of the new system, DuPont's reactor was equipped with a conventional three-stage cylone system consisting of first- and second-stage cyclones in series, followed by third-stage cyclones.
There are several problems associated with the conventional method. They include:

  • Often, very high efficiencies are achieved at 10 microns, bu the cyclones individually and as a system are unable to collect particles smaller than 5 microns efficiently, the category into which most emissions fall.
  • Using standard performance tools to achieve higher collection efficiencies (e.g. higher pressure drop) often results in higher-than-desired inlet velocity, which can increase abrasion and severely reduce the life of the equipment.
  • Because each cyclone stage has a fixed minimum pressure dropm a three-stage system may automatically result in a total pressure drop higher than allowable for good design within a given reactor.

Using a two-stage cyclone
To eliminate the problems associated with a three-stage cyclone, a two-stage cyclone system was selected.

The two-stage cyclone system uses a first-stage, high-capacity cyclone, with relatively low inlet velocity and pressure drop as a way to remove the bulk of the catalyst loading (98+%) and ensure that the particle size of catalyst to the second stage is small.

By reducing the loading and particle size to the second-stage cyclones, the rate of erosion is greatly reduced, allowing for higher inlet velocities and pressure drop without increasing wear.

Instead of using single cyclones for the second and third stages, the improved system uses parallel twin cyclones for the second stage and eliminates the third stage entirely.

This takes advantage of a well-known cyclone design rule stating that with all else equal, the use of two smaller cyclones in parallel will be more efficient than the use of one larger one of the same cyclone family.

In addition, using a number of smaller cyclones frees enough space to permit use of ultra-high-efficiency cyclones for the second stage. The result is a significant increase in the collection efficiency of particles less than 5 micron.

For example, 88% collection efficiency can be achieved versus 34% at the 2-micron level.

Total catalyst losses are greatly reduced. The distribution of particle sizes in the bed becomes finer.

The new arrangement also has a lower pressure drop, resulting from two stages instead of three and from reduced inlet velocities.

A lower inlet velocity decreases the rate of erosion exponentially and extends the life of the cyclones.

It is well understood that one of the most significant causes of permanent catalyst loss in reactors operating at design rates or higher is minute particles going up the stack as emissions.

In the new system, the lower inlet velocities and the relatively large diameter of the second-stage cyclones (considering the amount of gas handled) results in better recovery of finer catalyst and a lower permanent net loss.

There are several advantages of using the two-stage cyclone system with multiple parallel, high-efficiency, second-stage cyclones over a conventional three-stage cyclone system. They can be summarized as follows:

  • Significantly reduced catalyst emissions;
  • Lower energy consumption as a result of reduced pressure drop;
  • Increased catalyst selectively (greater acrylo-capacity potential);
  • Lower erosion rates resulting in longer equipment life;
  • Reduced frequency of cyclone plugs resulting in lower emissions;
  • Finer catalyst size distribution in the fluidized bed;
  • Less downtime and lower maintenance costs.

Source: McCallion, John. "New separation approach saves catalyst and energy." Chemical Processing, July 1996.

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