Sciencemadness Discussion Board - Nitrogen Rejection in the Contact Process

Nitrogen Rejection in the Contact Process

watson.fawkes - 24-8-2008 at 12:30

My request first: Could someone provide Joule-Thomson coefficient data for sulfur dioxide (SO2

in the range 0-20 atm and 250-500K? (I think that's all the regime I'll need.) I've looked around and I can't find it online.

Here's the reason for the question. I've been looking into what it would take to build a miniature-scale contact process reactor for making sulfuric acid. The scale I'm thinking of is several kg of product per run. I'm concerned with getting a reactor without significant sulfur emissions. With microscale synthesis, you just put unreacted SO2 up the flue, which is fine for the small amount of gas released. For the scale I'm looking at, I find that approach unacceptable. I'm not assuming the most efficient catalysts, for example. I'd also like to eliminate nuisances and neighbor problems.

In the contact process, the catalytic conversion of SO2 to SO3 never goes to completion. After washing the SO3 out into sulfuric acid, the remaining gas is primarily N2 and SO2. (I'm assuming that most all the atmospheric O2 was used up as oxidizer.) The issue is what to do with this SO2. You can simply dissolve it in water; what this does is to change a gas disposal problem into one of sour water disposal. You could simply neutralize it, sure, at the cost of acquiring and managing another reagent. I prefer a recirculating system where the SO2 is fed back into the catalyst chamber. The problem is now the nitrogen. If there were no nitrogen, that is, if you were to use pure oxygen instead of atmospheric air, then the gas could be fed straight back into the catalyst chamber. But since there's nitrogen, it must be rejected before feeding back the SO2. Using pure oxygen is tantamount to rejecting nitrogen from atmospheric air rather than from a mixture with SO2. It seems much easier to do the latter.

My original idea was to use a cold finger. The boiling point of SO2 is only -10 C, which is easily achievable with a consumer grade freezer, perhaps with slight modifications. First cool off the gas to about 50 C with ambient air, then through a heat exchanger with the refrigerant loop. The SO2 liquefies and you run it back through a vaporizer. Simple enough. But then I got thinking that this approach requires managing a separate refrigeration loop. This was when inspiration struck. SO2 makes a good refrigerant itself! It's a highly non-ideal gas, just what you want to use with an expansion orifice. And if you get the design right, the SO2 will be liquid at atmospheric temperature when it leaves the orifice.

This technique yields the following reactor design. After cooling to 50 C with ambient air, the output gas from the washing stack goes into a compressor. The high-side compressor line, having heated significant, goes to a second heat exchanger with ambient to cool the high-pressure line. After that it passes through an orifice, cooling it off. The output of this orifice is N2 and SO2, but this time also together with compressor oil. The N2 will still be gaseous; at this point it vents to the exhaust stack. The SO2, however, liquefies upon expansion. The mixture of liquid SO2 and compressor oil goes to a vaporizer that vaporizes the SO2, which goes to the catalyst chamber, and pipes the oil back to the compressor. The vaporizer in this version needs temperature regulation to stay cool enough to avoid vaporizing the compressor oil as well.

If I've got this right, the only SO2 emissions from this reactor arise from the vapor pressure of SO2 at the temperature it leaves the orifice. This seems acceptably small.

My request for J-T coefficient data comes from a desire to do some proper engineering before I build a prototype. I'd like to minimize the amount of time I'm venting SO2 while screwing around with operating parameters (temperature, pressure, compression ratios, etc.) while testing out a prototype.

I've also found a vendor of synthetic compressor oils that states their lubricant is compatible with SO2 as a process gas.

not_important - 24-8-2008 at 20:25

A properly adjusted contact process system is going to convert 98% or more of the SO2 to SO3, industrially 99.5% conversion is reached. I suspect the effort of recovering the remaining SO2 as such from the greatly diluted off gases isn't worth the effort. Run it over damp Na2CO3 and live with the bisulfite formed.

watson.fawkes - 25-8-2008 at 11:10

I don't want to assume "properly adjusted", which was the reason I pursued this line of inquiry in the first place. Since I anticipate publishing plans for other people to build, I'd rather not assume "proper" adjustment. In addition, a recirculating design has two additional merits. (1) It doesn't need vanadium oxide catalyst if, for whatever reason, it's not available. It can use iron or chrome oxides. (2) It continues to work in the face of catalyst degradation.

Operating data from real plants

watson.fawkes - 26-8-2008 at 20:45

Quote:

Originally posted by not_important
A properly adjusted contact process system is going to convert 98% or more of the SO2 to SO3, industrially 99.5% conversion is reached.

I went and looked this up in Sulfuric Acid Manufacture: Analysis, Control and Optimization. They list operating data from 13 plants for their first-contact catalyst beds (many of these plants have second-contact beds for processing wasted gas). One was for spent acid reprocessing; I threw it out because its operating parameters are different. These plants mostly have three beds (one has 2, two have 4). Average efficiency is 95.2% with a range 92.9%-98.2%.

The per-bed efficiency, though, was rather different, averaging 62.7% with a range 48.4%-80.3%. The computation should be taken with a grain of salt, because beds are generally of different depth and different operating temperatures. Recall that oxidation of SO2 is exothermic, so that the bed temperature rises as gas passes through. These plants all have intercoolers between beds.

not_important - 26-8-2008 at 22:11

Yup, they use multiple beds with cooling and sometimes partial SO3 removal between beds. They also use an excess of O2 to help shift equilibrium. Final bed may use cesium sulfate instead of potassium sulfate as the added alkali sulfate so as to operate at a lower temperature.

The more recent plants get the higher conversion rate, in order to reduce scrubbing needed to meet pollution limits.

Quote:

Facts about Double Absorption Process:
• Plant capacities: 50 to 7,900 mtpd Mh
• Specific HP-Steam production: up to 1.4 t / mt Mh
• Conversion efficiency of SO²: up to 99.9 %
• Specific sulphur consumption: < 330 kg/mt Mh
• Specific cooling energy: ca. 2 GJ
• Specific power consumption: 40–60 kWh/mt Mh
• Specific catalyst quantity: 150–200 l/mtpd Mh
Mh = Monohydrate = 100 % H²SO4

from http://www.outotec.com/28817.epibrw

Quote:

Dual absorption, as discussed above, has generally been accepted as the best available control technology for meeting NSPS emission limits. There are no byproducts or waste scrubbing materials created, only additional sulfuric acid. Conversion efficiencies of 99.7 percent and higher are achievable, whereas most single absorption plants have SO2 conversion efficiencies ranging only from 95 to 98 percent. Furthermore, dual absorption permits higher converter inlet sulfur dioxide concentrations than are used in single absorption plants, because the final conversion stages effectively remove any residual sulfur dioxide from the interpass absorber.

from http://www.epa.gov/ttn/chief/ap42/ch08/final/c08s10.pdf

watson.fawkes - 27-8-2008 at 07:26

The real point I was making is that these high conversion efficiencies are only the result of multiple-pass processing. I believe it's misleading to simply state that "the contact process has high conversion efficiency" without simultaneously speaking about all the process steps involved. I could design a 99.9994% conversion plant, but it would be quadruple-contact and cost twice as much as a double-contact plant. High conversion efficiency is a property of a plant, not of the process in the abstract.

Assuming 95% single-stage conversion, the second contact stage produces only about 5% of the total acid production. A hypothetical third stage would be down at about 0.25% acid production. The only economic reason for stages after the first is emissions control. And even though an engineering firm can design a plant with that high efficiency doesn't mean that the operator is going to run it at that efficiency, although that's more likely in the developed world than elsewhere. As a result of this, I'm more convinced that a recirculating refrigeration system is worth trying at the small scale. One sulfuric acid pump is required for each SO3 absorber. If I replace a second contact stage with a refrigeration stage, I'm eliminating a second catalyst bed and absorber, and I'm swapping a pump for a compressor and vaporizer. In other words, cheaper to build.

This design should also be more resilient against a dysfunctional catalyst. Operate the catalyst too hot, and you vitrify the catalyst and have to replace it. Operate the catalyst too cool, and you get no conversion. This is the ordinary mode at startup, when the catalyst beds are not yet in steady-state operation. I'm anticipating this plant will be for daytime operation (not 24/7), so the start-up phase of the plant is a significant fraction of its total operational lifetime.

Quote:

Final bed may use cesium sulfate instead of potassium sulfate as the added alkali sulfate so as to operate at a lower temperature.

As I understand it, they use a mixture of cesium and potassium sulfates. I'm assuming this is an economic decision, that there are diminishing returns as you move to only cesium sulfate. On the other hand, I haven't seen a three-component phase diagram for vanadium oxide and potassium and cesium sulfates. I'd be curious about where the minimum melting point is.