Shutdown Learning - Corrosion in Exchangers

In this shutdown, we openend many process exchangers, heaters, coolers etc for inspection and found different types of corrosions.

The most happening incident was in a cooler used as intercooler for ASU compressor. The strange thing was that all the baffles on shell side (cooling water) were gone. There was no indication of there existence except some marks on tube bundle as you can see in larger version of the below image.

Related references

• Galvanic Corrosion - Wiki Reference
• Galvanic Materials in Series - In Waster Service
• Key to Steel
• Download - Anodic Protection - Its Operation & Applications
• Download - Cathodic Anodic Protection

The cooler is an interstage cooler in ASU unit for oxygen compressor. The cooling water flow is on shell side while gas was on tube side. Tubes were of SS & shell including baffles were of CS carbon steel. It seems that heavy corrosion has eaten away all the baffles.



If you can enlarge the photo & see it carefully then you will find marks of existence of those baffles.

Another example of similar type of corrosion is seen in other exchanger. This is a condenser for heavy glycol ether vapors. Cooling water is on tube side. So the corrosion is observed on tube sheet & partition plate.



The opposite side was also no better. In fact the damage is more here.



This is mainly due to electorlytic corrosion induced by presence of salts in cooling water. The easiest way to reduce it is to provide a Zinc Anode so that preferential corrosion or sacrificial corrosion saves your exchanger without much effort. One example is shown below.



These examples indicate that the cooling tower which is supplying cooling water to these exchangers doesn't have proper makeup water & has build up the concentration of salt beyond desired levels.

One should keep in mind that the contact of Zinc on the surface to be protected should be proper. In fact, one should prepare it by buffing or some other means to make it very clean.

Also keep in mind that there should not be any paint or coating on Zinc Anode as it is seen on some points in above image. This defeats the purpose of providing zinc anode. It is an usual thing that when you are in a hurry during shutdown you leave the job on contractor. The labourer does his best by providing coating on Zinc Anode also...............HOW Zzzzz THAT????????

So be careful, sometimes good thing are not so good to do.

Yes as an initial step we provided Zinc anode in these two exchangers.

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Ammonia Plant Simulation

My first major job in a Ammonia Urea complex was to prepare a complete plant simulation program starting from natural gas feed to final product using Excel's Visual basic interface.

It took me 60 complete mandays of working for which I thank my BOSS Mr. Pankaj Shah who spared me totally from other day to day jobs for the preparation of this package.

In the first part it uses complete kinetic models of Primary & secondary reformer based on inlet temperature & catalyst volume & predicts outlet composition of reformed gas including heat load for primary reformer. Then comes steam generation in WHB downstream of SR. Next part includes shift reactor models with all waste heat recovery. I have skipped CO2 removal section modelling as it required absorption mass transfer etc. So it considers CO2 in the clean gas at absorber outlet. Methanator model includes only temp calculation as CO & CO2 are generally in ppm level.

Second part includes complete ammonia synthesis loop using generalized correlation of EOS and other solubility data derived from a book by Slack which is very popular in Fertilizer industry.

This package is capable of following

1. Simulation of entire ammonia plant
2. Can be used for predicting changes in operating parameters based on change in certain consitions e.g. Feed, Temp, Pressure etc.
3. Can be used for Optimizing process parameters based on considered objective
4. Can predict performance of equipments
5. Can predict errors in measurements
6. Can predict problem in particular equipment / process part
7. Can be used for evaluation of modification schemes

The package is flexible in making changes in the process as it was developed in Excel. It is having 19 Excel sheets in one file with ~25 routines & takes ~2-3 Hrs
of computing.

The result of this package were matching with the evaluations of Technology Supplier Haldor Tosoe.

So if anyone requires simulation of ammonia plant I can do it.

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Shutdown Learning - Ejectors

An ejector, is a pump-like device that uses the Venturi effect of a converging-diverging nozzle to convert the pressure energy of any motive fluid to velocity energy which creates a

low pressure zone that draws in and entrains a suction fluid and then recompresses the mixed fluids by converting velocity energy back into pressure energy. The motive fluid may be a liquid, steam or any other gas. The entrained suction fluid may be a gas, a liquid, a slurry.

The diagram below is a typical ejector. It consists of a motive fluid inlet nozzle and a converging-diverging outlet nozzle. Water, air, steam, or any other fluid at high pressure provides the motive force at the inlet

Related Articles & References

• Steam Ejectors Theory.
• Ejector System Troubleshooting
• Ejector Design.
• Vacuum Basics
• Jet Vacuum Ejectors

So in this shutdown when we opened one of the ejectors, we made following observations. Typically in our complex most of the ejectors are used on main equipments for creating vacuum, thus they are normally steam ejectors pulling vacuum from reactor & finally condensing all the steam in downstream condensers.









So this picture shows some corrosion around the throat & nozzle however it seems to be within normal limits in steam service. The important part is that the tip of nozzle is not corroded and the location of nozzle in the downstream pipe is at required location so it seems to be working fine.

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Waste Heat Recovery - Consider Exergy

Waste Heat - Every process engineer is aware of this word. But do you know how much % of total energy requirement in your plant is going as waste heat?....Don't Know......It can be as high as 50%.....!!!!......

Yes, it can be. So what is waste heat...Simply from language it is the heat wasted in the process. So it is not just flue gases which are carrying this waste heat but to your surprise the biggest carrier of waste heat, usually, is cooling water. YES.

How to minimize it.....???.

Related Articles & References

• Recover Waste Heat from Flue Gases.
• Waste Heat Recovery Expert
• Effective Utilization of hot air from DG sets.
• Small Quiz on Waste Heat
• Exergy - Wiki Definition.

Generally the following are the sources of waste heat.

1. Flue gases from Fired Furnaces


2. Distillation Column condensers


3. Power Generation Turbine Condesers


4. Steam Condensate


5. Process Coolers & Heaters


6. Other Sources

You can list out other items in this list for sharing, if you have it.

We all plan & wish to recover this heat to the maximum possible extent & thus do lot of work on analysing different combinations, their technical evaluation, Feasibility etc. For simplest example, All industries where steam is used have lot of flue gases at ~180-200°C. I have already mentioned one article for recovering heat from flue gases which can reduce the flue gas temperature up to 100°C. But the question remains - is it possible? Which stream should I use for heating & finally resulting it in useful work.

The objective is not just heating of any colder stream. The objective is to finally convert it in useful heat or work. Here comes the role of Exergy.

Exergy can be defined in simple terms as - The potential of any stream to produce useful work. So,

Energy is efficiently used when the quality of the source is matched to the quality demanded by the task. Thus, electricity is a thermodynamically sound way to drive the motor that agitates the clothes in the washing machine. It is not a thermodynamically sound way to heat up the washing machine's water. By thermodynamically matching sources to tasks, we can avoid the enormous waste of using high quality energy for low quality tasks, and minimize the growing social and economic costs of energy production

Now let us take an example of heat recovery from LP steam for easy calculations. Suppose steam is available at 140°C saturation. So its Enthalpy is 653.3 Kcal/Kg and its entropy is 1.656 Kcal/Kg/K. Now suppose you use it for heating of cold water from 40°C in an exchanger with steam exit at 102°C. In this case Enthalpy is 640.2 & entropy is 1.752 in same units.

So for this process based on exergy formula (I am not putting them here, you can find them anywhere or in my upcoming article on exergy basics)

E = T0 x (S2 - S1) - (H2 - H1)
= 298.15 x (1.752 - 1.656) - (640.2 - 653.3)
= 28.6 + 13.1
= 41.7 Kcal/Kg

This is positive so we will get work out of it. This is the maximum amount of work which can be extracted from hot stream however what we are getting is only enthalpy gain in cold water i.e. 653 - 640 = 13 Kcal/Kg, so only 30% energy is used.

Compare it with hot water heating you will find that it will be practically 95% efficient & that is why we should consider exergy approach for any heating or cooling system design.

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