THIS MONTH'S PUZZLER
Our refinery uses a thermal oxidizer (TOX) to dispose of vacuum off-gas. Currently, a steam jet pulls this off-gas because we've had reliability trouble with liquid ring pumps. We've suffered severe corrosion in the TOX's burners and tiles. Startup problems with the TOX — particularly with the fire detectors and igniter system — have caused delays. What's causing the problems and how can we improve the unit's reliability? We also have a related concern: our production manager wants to send the off-gas to a flare while the TOX is out of service for repair, which will take a week. The typical composition of the off-gas by volume is 30% H2S, 5% CO, 2.75% H2, 2.25% N2, 1% CO2, and 0.5% O2, with the remainder C1–C3 hydrocarbons. The vacuum is 40 torr, absolute, and the temperature is 120°F. About 2,500 pounds per hour would go to flare while the TOX is offline. Do we risk damaging our flare? What other problems might arise? What temporary measures would you suggest to avoid environmental issues?
CHECK SEVERAL FACTORS
Your problems could be related to your gas composition, corrosives' content and solids. Burner tips, when plugged, create misfiring and flame impingent. Burner tips' erosion from solids and corrosives also creates these. If you did not customize your TOX for your gas composition and make the internals as robust as possible to withstand your combustion (and corrosive) components, this is what would be expected. This is common with rental equipment.
Sending the off-gas to the flare while the TOX is out of service would be of concern to your reliability and environmental people. Environmental issues depend on your emissions permit.
Regarding damaging your flare, it would depend on what it was designed for (i.e., gas compositions/combustion products, corrosives in gas stream/combustion products). Flares are designed robust enough to withstand natural elements as well as common combustions products. Depending on the gas corrosiveness, robust metallurgies are selected.
A quick calculation using your 2,500-lb/hr gas and its composition plus C1-C3 (assuming 26.3% C1, 6.23% C2, 0.94% C2=, 23.08% C3 and 3.64% C3=) shows the following mass flows to flare: H2 4.5 lb/hr; O2 13.0 lb/hr; N2 51.3 lb/hr; CO 114.0 lb/hr; CO2 35.8 lb/hr; C1 320.6 lb/hr; C2 152.1 lb/hr; C2= 21.5 lb/hr; C3 829 lb/hr; C3= 124.9 lb/hr; and H2S 833.27 lb/hr. Using our flare efficiency and NOX efficiency program, we estimated the following emissions: NOx 2.42 lb/hr (NO 2.30 lb/hr and NO2 0.12 lb/hr); CO emissions would be 1.47 lb/hr and CO2 emissions 0.49 lb/hr. The amount of SO2 to the flare would be 1,568.5 lb/hr. The emission after destruction will depend on your flare destruction efficiency — compare the calculation against the permit to decide if you would incur permit violations.
As for the corrosion, some adjustment may be required. Our plant has three flares. Their service life span is 29 to 36 years. One services a gas stream that contains corrosive products. This one flare tip was changed 5 times. The last tip was re-designed.
Arbues Maymi, senior process engineer
CITGO Refinery, Corpus Christi, Texas
FIND A LONG-TERM SOLUTION
Let's start with the temporary permit required. You'll need such a permit when you install an amine system upstream of the flare. That should take care of any corrosion issues with the flare. There's no way you should run a stream with this concentration of H2S to flare; it's been done, though.
Another option is a stopgap TOX. As with all interim equipment, it will require a temporary operating permit. You'll probably also need to pass a stack test, so you'll want to plan for this during the repairs of the old TOX.
As for the liquid ring vacuum pumps, I would surmise the pumps failed because of contaminated seal fluid. Scale buildup eventually would cause a liquid ring pump to seize up. If hard enough, the scale could damage the bearings, seals and internal components. Vacuum pumps may be less reliable, and therefore, less suitable in this application.
Could there be a connection between the startup problems and the corrosion in the furnace? Perhaps, it's the flue gases. The SOx, formed by burning the H2S, could reach saturation in the brick or on the shell wall of the furnace. This would cause severe corrosion of the brick and shell — periodically necessitating wholesale replacement of the carbon steel plate. The effect of saturated acid gases, such as SOx, partially could be mitigated by selective insulation and even heat tracing. The idea is to reduce the penetration of the acid saturation point into the brick; you'd rather have brick wear away at the hot face than have acid worm its way to the vessel shell where it can cause permanent damage.
Likewise, the sulfur-tar residue formed in burning the gases could be fouling the fire detectors, giving false signals the burner pilot is off; the residue could choke the burner, adversely affecting the flow dynamics controlling mixing of the fuel and combustion air inside the burner tunnel. If the burner tile were severely corroded, enough to cause tile cracking, it would not act as a heat sink, allowing the fuel gas and air to heat to ignition temperature. In this type of situation, where damage may be unavoidable, reliability cost savings are in avoiding unanticipated catastrophic failures and by improving ease of repairs. Sometimes, redundant, isolated in-line spares might be a means to avoid fouling problems with fire detectors — controls alone are not the answer.