In last post we learned about one type of sulfur removal which is using iron sponge unit. Brief review, iron sponge is economically applied for gas containing small concentration of hydrogen sulfide, usually below 300 ppm. Iron sponge unit can not be used to remove carbon dioxide.
In this post, I want to continue the topic about iron sponge unit. More specifically about how to preliminary size iron sponge unit.
At least there are four parameters that we want to know in sizing iron sponge units:
- Minimum diameter of iron sponge vessel
- Maximum diameter of iron sponge vessel
- Required bed height
- Quantity of iron sponge
- Bed life of iron sponge between change out
Why do we need to know minimum diameter of iron sponge vessel?
There are two constraints that drive minimum diameter of iron sponge vessel. First, maximum superficial velocity. Second, maximum rate of deposition.
The superficial velocity of gas through iron sponge bed is usually limited to maximum 10 ft/min to promote proper contact with the bed and to protect against excessive pressure drop. And maximum recommended rate of deposition is 15 grains of H2S/min/ft2 of bed cross-sectional area. This allow for dissipation of the heat of reaction. We need to calculate the minimum vessel diameter due to both constraints. Then, we select larger diameters between those two as minimum vessel diameter.
Vessel diameter also has a limit due to low superficial gas velocities (less than 2 ft/min). We need to prevent gas channeling through the bed by limiting maximum diameter of vessel.
We also need to consider contact time. A contact time of 60 seconds is considered a minimum in choosing a bed volume. Assuming a minimum contact time of 60 seconds, any combination of vessel diameter and bed height that satisfies the following is acceptable:
d = vessel diameter, in
H = bed height, ft
Qg = gas flow rate, MMscfd
T = operating temperature, oR
Z = compressibility factor
P = operating pressure, psia
In selecting acceptable combinations, the bed height should be at least 10 ft for H2S removal and 20 ft for mercaptan removal. This height will produce sufficient pressure drop to assure proper flow distribution over the entire cross-section.
Quantity of Iron Sponge
Iron sponge is normally sold in the U.S. bu the bushel. The volume in bushels can be determined as it is a function of vessel diameter and bed height.
The amount of iron oxide that is impregnated on the wood chips is normally specified in units of pounds of iron oxide (Fe2O3) per bushel. Common grades are 9, 15, or 20 lb Fe2O3/bushel.
Bed Life for Iron Sponge Between Change Out
Bed life of iron sponge is a function of iron oxide grade, vessel diameter, bed height, efficiency, gas flow rate, and concentration of H2S.
For reference, the equations used to calculate those parameters can be checked in Surface Production Operations Volume 2 by Ken Arnold and Maurice Stewart.
In last post, I said that I love making calculator so much. So, again, in this post, I want to share the iron sponge sizing calculator.
I hope you find this post useful.