ChemE.info – Chemical Engineering Consulting

Modeling acid gas scrubbing requires the use of fairly specialized physical property models and, in many cases, one cannot assume equilibrium between phases.

Using an appropriate physical property model is key to successfully modeling any unit operation. General purpose simulators like Aspen Plus, PRO II, etc. have dozens of physical property models to choose from. These can be divided into two classes: equation of state models (e.g. ideal gas, Redlich-Kwong-Soave) and activity coefficient models (e.g. UNIFAC, UNIQUAC, NRTL).

Acid gas scrubbing involves species that act as molecules in the vapor space but behave as dissociated ions in the aqueous phase. As far as I know, no equation of state model will properly represent that. All the simulators capable of modeling acid gas scrubbing do so with some type of activity coefficient model.

For systems where they are appropriate, equation of state properties will work with only the most basic information about the molecules in the mixture. They generally do not need information about the interactions between the specific molecules in the mixture.

For acid gas scrubbing systems and, in fact, any electrolyte system; the activity coefficient models need species-to-species interaction parameters. These need to be regressed against laboratory data (from data libraries such as DeChema). With the Aspen Plus simulator, the activity coefficient model of choice was eNRTL and there were various physical property packages that provided pre-regressed parameters for common systems. For example, one would model a Benfield system using the “HOTDEA” package of parameters.

One also needs to keep in mind that many acid gas scrubbing systems do not operate at equilibrium. In other words, there are mass-transfer limitations between the vapor phase and the liquid phase. A common example of that is nitric acid scrubbing.

We used to model nitric acid absorption columns with a column model called RATEFRAC which could represent non-equilibrium mass-transfer. That was several years ago and I imagine that other general purpose simulators now have a similar non-equilibrium tower models (and there have long been specialized simulators for representing non-equilibrium acid-gas systems).

I have been fortunate enough to visit quite a few places in the course of my work but I think the most “middle of nowhere” place I have ever been was the bauxite mine in Nhulunby/Gove in the Australian Northern Territories. [Nhulunby is the aboriginal name for the area, and Gove was an Australian pilot who crashed and was killed flying out of the airfield in WW II.]

The bauxite mine and it’s supporting town (definitely a “company” town) are accessible only by sea or by air. There are no road or rail links to the outside world. The town and plant are in the middle of an aboriginal reservation and the only people who live in the area are employees of the mine, a few government employees (like postal workers and teachers), and the aborigines.

There is an airport and regular air service although I understand that the runway runs right over the bauxite deposit and will eventually get dug up.

Having never been this far into the wilds before, I frequently made a bit of a fool of myself. When I got off the plane in Gove, my host (whom I’d never met) walked right up to me and said hello. When I asked how he had known it was me he laughed and said that he’d known all the other people getting off the plane.

And when I rented a car (rather surprisingly they have a Hertz franchise) the manager asked when I’d be leaving and I said the following Saturday morning. She told me that they weren’t open on Saturday but that I could just leave the key on the counter. I asked if she wasn’t afraid of someone stealing the car… And she laughed and said where would they go with it (Gove only has about 5 miles of paved road).

I was staying in a small (and only) hotel in Nhulunby which is, I gather, heavily subsidized by the bauxite plant. Perfectly comfortable except that they didn’t seem to have any cold water in the bathroom. At least I seemed to be getting hotwater out of all the taps. It turns out the “cold” water was stored in a cistern on the roof. And in Nhulunby it gets very hot on the roof… So no cold water.

On the two visits I made to Nhulunby I had to stay over a couple of weekends so I had a chance to “see the sights.” There are very nice, pristine sandy beaches but I was warned (severely) not to go swimming and not to even walk along the beaches at night… Why? One reason was the stinging jelly fish that infest the waters for about 6 months of the year and the other was because of the salt water crocodiles in the area that like to snatch meals off the beach at night.

They may have been pulling my leg but the plant personnel told me that a previous visitor (from their parent company in Switzerland no less) had not heeded the warnings and had been snatched off the beach one night and eaten. Certainly, I did see crocodile tracks on the beach when I went walking during daylight hours. (And they were impressively big)

The other weekend amusement was being taken to the Nhulunby Yacht Club (a cinderblock building overlooking the water) where we sat on the porch out front and defended our lunches from swarms of flies (It is where I got to perfect my “Aussie Salute”… a quick swipe of the hand across one’s face to discourage the flies!)