Question from April's Chemical Processing
A contractor has been asked to increase the plant capacity of an existing helical-coil vaporizer. The furnace already has an economizer. The results are disastrous the plant maximum rate has been reduced by more than 10%. The vaporizer is running so hard the fins of the economizer are starting to burn off. The flame is too long (Figure 1). What can we do to eliminate this dangerous problem and increase the vaporizer rate while accounting for NOx limitations? Although the space between the coils is adequate, there is some concern about the coil skin temperature if we change the flame. What are your thoughts?
Figure 1. The flame length is too long, forcing us to reduce the feed rate to the vaporizer.
Recirculate flue gas
Flue gas recirculation will help control NOx. It also will increase the velocity, probably increasing heat transfer and lowering the overall temperature through mixing, helping to protect the coil fins. In other words, by increasing velocity, heat transfer should go up and by recirculating the flue gas, you can achieve NOx reduction. The burner also will likely need some work to change the flame pattern and prevent the over-length flame without greatly increasing the flame diameter.
I would put a sight glass in the zone(s) I expected maximal heating (from the burner design) and use an optical pyrometer to ensure the tube fins are safe. I expect if there is enough flue-gas recirculation, and some spin put on the inlet gases to the burner, the middle zone will be okay and the heat transfer should actually go up across the coil while keeping lower surface temperature. It might take a high recirculation percentage to accomplish this but by putting variable spin dampers and using a pyrometer to check the fins, one should be able to tune it to optimal performance fairly easily.
It also would likely be possible to tune flame shape. My guess is that they would be able to increase production rate, make their regulatory authority happy with NOx reduction, and keep the maintenance guys from having to work on the coil very often. It would take some real data and dimensions to calculate, but if they look at the adiabatic flame temperature, the exhaust temperature, and calculate how much recycle it would take to keep the temperature within the limits of the coil material, it should be possible unless the coil is really badly selected. In the worst case, they may have to look at the refractory properties of the metal and be sure its suitable both for the flame and the material contained.
Larry Cummings, director, safety and environmental engineering
AMPAC, Cedar City, Utah
Add another vaporizer
The vaporizer is simply undersized. Compare the existing duty to the nameplate. Burner rate was increased to handle the heat duty and now the combustion chamber is too small. In addition, this has caused a heat flux problem, indicating the surface area of the helical coil is inadequate.
The solution is to reduce the duty on the vaporizer. A second vaporizer should be added in parallel with a low NOx burner. The second vaporizer should probably be a similar size as the existing vaporizer. Both can operate at a comfortable and safe 66% to 75%. When maintenance is needed, one unit can be brought down and the other unit can be operated at 90% to 100%, but not beyond 100% as in the current situation.
Terry Evans, PE, plant engineer
Schirm USA, Inc., Ennis, Texas
Replace the burner tip
The probable solution could be:
- Change the burner tip to reduce the fuel gas velocity to reduce the flame length, hence better heat transfer in the radiation zone.
- The coil size can be increased, if required, to reduce velocity inside the coil. Increase in heat transfer rate and hence vaporization can lead to higher tube skin temperature and higher internal velocity (erosion).
- The coil material of construction may need to be changed if metallurgy doesnt permit the higher temperature.
- The economizer may require an increase in heat transfer area.
Kamal Parikh, senior general manager
Reliance Industries Ltd., Surat, India
Consider three options
There are three different possible solutions to improve the situation:
- Increase firebox pressure. The higher the pressure, the higher the firebox temperature, which gives you a larger ∆T and should improve the heat transfer rate through the coils. This can be accomplished by shutting down the damper (assuming there is one if there wasnt one, it should be added). So pressure, instead of a larger flame, can be used to supply more heat. The negative aspect to this move is that it can increase the risk of fire and flames escaping from the fire box. Also, you must make sure that the coils are designed for a higher temperature.
- Install a soot blower. A soot blower can be used to keep the fins of the economizer clean and avoid hot spots. The soot blower will remove soot and increase heat transfer.
- Add a shield section. At the bottom of the economizer, install a section of tubes without fins to serve as a shield against the flames on the firebox. These tubes are designed to withstand the flames and protect the more fragile fin tubes above.
Suggestion No. 1 should serve you well enough and would be where I would start.
Jared Klein, operations manager
Symrise Inc., Bushy Park, S.C.
This really isnt my area of expertise but my guess would be that the draft in the vaporizer is such that the heat from the burner(s) is pretty much bypassing the vaporizer coils and damaging the economizer. The solution would seem to be to slow the passage of the combustion products down and/or increase their contact rate with the coils. As a result of this situation, there also may be an excess of air. I would suggest increasing the retention time by adding an exhaust damper and at the same adding angled inlet dampers to increase the swirl inside of the combustion chamber to increase the contact rate of the flame with the coils. Increasing the fuel rate or steam injection may be used to reduce flame temperature and limit NOx while maintaining adequate heat to provide the required vaporization rate.
Patrick Richards, sr. instrument designer
Fluor Canada, Saint John, N.B.
Our fractionating column is notorious for its problems. Oil is injected into heated feed to separate an impurity (Figure 2). This approach eliminates the impurity well, perhaps too well. A sludge collects in the bottom of the reboiler feed tank, clogs the sieve column and plugs the reboiler and pre-heater. There is some concern that separation efficiency is affected by plugging of the trays but the main problem is frequent fouling of the heaters, which have fixed heads and are difficult to bore out. The reboiler pump fails often and we must call in a vacuum truck frequently and unexpectedly to clean out the reboiler feed tank. Start-up is often a problem because the steam plant is far away. The oil frequently burns, which adds to our fouling woes. What can you recommend to reduce sludge build-up in the trays, heaters and reboiler feed tank?
Figure 2. A variety of problems lead to excessive downtime.
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