Q&A - AMINE CIRCULATION

Amine Circulation

Lee:

Mr. Lieberman, which is a better amine – MEA, DEA, or MDEA – for use in a refinery?

Norm:

MEA, without good reclaimer operation, in a plant with an FCU, is going to promote H₂ assisted stress corrosion cracking. MDEA is used in concentrations of 45%, and hence tends to foam. In practice, I have found using 30 – 35 wt.% DEA is optimum – but occasional reclaiming to reduce heat stable salt accumulation is still required. If you have a sulfur plant tail gas scrubber, then a selective solvent, that rejects CO₂ absorption, such as MDEA may be necessary.

Q&A - SULFURIC ACID ALKY

Sulfuric Acid Alky

Carol:

Our alky unit refrigeration compressor is limited by low suction pressure. The suction pressure gets down to one psig or less, but contactor temperature is 60°F or more. So, we slow down the compressor (turbine driver), to bring up the suction pressure, but then the contactors get hotter still. Any suggestions would be appreciated.

Norm:

May I suggest you reduce your depropanizer bottoms temperature by 5°F. This will allow propane to build in your refrigerant recyle and raise compressor suction pressure, but without raising contactor temperature. You need to make moves on the depropanizer bottoms temperature slowly, as it will take several hours for the refrigerant recycle propane content to respond. Note, that if your compressor discharge pressure is at max, this will not work.

Q&A - CAT CRACKER

CAT CRACKER

Mike:

On our FCU wet gas compressor, we have an orifice plate that operates from 0” to 100” of water DP. The flow is typically high. The meter reads full scale – at an orifice plate DP of 95” H₂O, or 3½ psi pressure drop. As the compressor suction pressure is only 3 psig, it seems to me that we are wasting a lot of compressor capacity on that orifice plate. What do you suggest? Also, high compressor suction pressure backs air out of the regenerator.

Norm:

Eliminate the orifice plate. Use the vapor outlet nozzles of the upstream K.O. drum, as a restriction orifice. Measure the DP between the drum and the compressor suction. You can back-calculate the “orifice” coefficient based on the unit material balance. I’ll show you how to do this, if you send me the operating data. Looks like you can increase compressor capacity by about 10%, with zero investment cost.

Q&A - DELAYED COKER

Delayed Coker

Dale:

My coker fractionation tower top pressure surges higher by 5 psi for a few hours. Tower top temperature jumps by 20°F - 40°F. The level in light gas oil pan drops. This occurs every few days. What’s happening?

Norm:

You’re salting-up the top few trays. The top reflux can’t cool off the upper trays. They get hot, and sublime off the salts. A simple way to prevent the problem, is to periodically heat up the top of the tower, maybe for 30 minutes by 30°F, every few days. I’m not too sure about time and temperature factors. Periodic water washing is also possible, but a lot more complicated.

Q&A - VACUUM TOWER

Vacuum Tower

Will:

In a vacuum tower, what parts of tower internals need to be designed for a large up-lift force, to prevent damage?

Norm:

Only the over-flash chimney tray, if the chimneys are designed for a 0.5 – 1.0 mm Hg DP.

Q&A - CRUDE DISTILLATION

CRUDE DISTILLATION

Terry:

When I increased crude tower bottom stripping steam from 1,200 to 2,500 lbs/hr, I increased diesel yield by 1,000 BSD at the expense of LVGO. Vacuum, in the vacuum tower, improved from 25 to 18 mm Hg. Yet, vacuum resid yield failed to improve. Why not?

Norm:

Reduction in vaporization of diesel in flash zone of vacuum tower, off-set reduction in pressure:

·       % vaporized prop. to  (V/1 • 1/P)