THIS MONTH’S PUZZLER
We’re having trouble controlling the reboiler in the regeneration column for diethylamine (DEA) — see Figure — at a refinery we recently bought. The DEA concentration is about 35% by weight. The tower pressure varies 2–3 psig from design. Originally, this DEA process fed our refinery sulfur recovery unit (SRU); as of the last turnaround, it only polishes the gas before the SRU. This system hasn’t been stable since startup.
This tower was built many years before modern simulations existed; plant records lack details about any models used for the column.
Should we move the feed tray, change the feed conditions, or reposition the controlling thermocouple further up the column? Would raising the condenser recycle ratio help? Can we alter the amine or the concentration to get better results? What else should we consider?
Make It Simpler
Your DEA regeneration seems complicated. Let’s see if we can’t make it simpler and address your questions first.
If the amount of acid gas absorbed in the DEA is lower, then the vapor/liquid ratio for the still trays may be below the minimum operating flow range (typically 10–15% for bubble cap trays). If amine flow was trimmed to compensate for reduced loading, then the lower rate could lead to solids settling out on the trays and flows could be channeling.
Generally, units built before good computer modeling (about 1985–1990) are overdesigned. Have an engineer with over 15 years of experience in troubleshooting amine units develop a model that incorporates detailed equipment evaluation to set your capacity limits. Then, you can know if you are operating at a lower or higher than design capacity. Alternatively, you can adjust operating conditions, e.g., amine flow or reboiler duty, to determine maximum capacity of the exchangers, trays, pumps and control valves.
Keeping the pressure steady with small, slow changes is the most critical control parameter for the column. When the pressure drops, so does the bubble point temperature (BPT) throughout the column and reboiler, thus causing increased vapor flow up through the trays. Vapor traffic in excess of tray capacity is the major cause of tray damage. I have also seen large pressure swings move the reboiler and break its foundations. Conversely, a pressure increase raises the BPT: until the reboiler catches up, the vapor traffic is reduced or stops in the tower.
Tower pressure is shown at the top of the column as 7 psig; the pressure varies by 2–4 psig. Typical design pressure for the still reflux drum is 7–10 psig, which depends on the pressure required for the downstream unit plus about 3–5 psig for the pressure control valve (PCV) to operate properly. Because this is a steam stripper and not a distillation column, the pressure sets the temperatures throughout the column. Higher pressure means higher temperatures, which makes condensing easier and reboiling harder: more energy is required.
The simplest solution may be that the pressure controller (PC) and PCV are causing the pressure swings. Normally, the PC is on the reflux drum and the PCV is on the acid gas outlet. The PC is shown on the tower overhead line, which does not account for changes in pressure drop through the condensers. Therefore, if the PC sees a higher pressure and opens the PCV, then the initial increased flow will cause more pressure drop and an even higher pressure, thus making the PC open even more. When it catches up, it will reverse the action and drop below set point, and repeat. Also, if the PCV is a full-line-sized butterfly valve, then its valve characteristics may be causing the oscillations. Consider adding a digital valve controller (DVC) to the PCV or changing the valve to a V-ball. Also, if there is another control valve (CV) downstream, then the two CVs may be fighting each other.
Another control problem may be two-phase flow. Do the lines from the still to the air heat exchanger (HE) and the trim HE to the reflux drum have liquid pockets? Remember, we are working with a condensing fluid and two-phase flow and flow should be continuously downhill. Pockets of liquids can cause slug flow, which would create pressure swings.
Do not move the feed tray. The still feed is shown going to tray 3 (simulation stage 2), which is fine. This tower uses trays 1 and 2 as wash trays. The reflux water washes out the last traces of amine so as to reduce amine loss in the overhead. Many amine stills do not even have wash trays.
Many gas plant still columns have 20 trays. Your 26 trays will provide a bit more efficiency.
Typical amine still tray efficiency is 50%. A proper model may indicate a reduced tray efficiency (less than 25% reliable), which could indicate some tray damage or fouling. During the last turnaround, was the still cleaned and its internals inspected? No other investigative technique is better than putting your own eyes on the internals.
Changing the feed conditions is very difficult. If the amine feed HE is designed and working properly, then the outlet will be about 200–230°F with about 2–5% vapor. Some of the acid gases are released and form steam. Trying to get any higher temperature than this is futile because, as more vapors are formed, the exchanger heat transfer coefficient drops dramatically. Nearly all of these exchangers are designed based on the rich amine liquid properties and, despite what the specification sheet may state, they won’t perform if there is any substantial amount of vaporization. Also, excessive heat input will have to be removed in the condenser, without much benefit. What good is the trim HE on the still feed? I suggest taking the feed trim HE out of service, at least until you get the rest of the unit under control.
Your drawing doesn’t indicate if there is a feed control valve (FCV) and if it is located upstream or downstream of the feed temperature element (TE). If there is not a CV or the TE is located downstream of the CV, then the fluid is essentially at tower pressure and the temperature will be nearly constant. The TE should be upstream of the CV to monitor amine exchanger performance, with a temperature approach of about 50°F.