May 31, 2008

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.

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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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May 29, 2008

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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May 28, 2008

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

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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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May 23, 2008

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.....???.

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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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May 09, 2008

Value addition - Do Simple Things

In Distilleries where molasses or other sugar based raw material is fermented to Ethanol, the byproduct FUSEL OIL is a great nuisance & is often sold or used at fuel value, whereas it has a great potential of value addition due to its components majority of which is pentanol or Amyl alcohol.

Recently I successfully tested a process for value addition to it though its only first part but still opens up doors for further value addition / recovery of pure components.

In distillery processes where sugar based raw material is fermented, byproduct (FUSEL OIL) is generated at ~3 Kgs/Te of Ethanol. This means that India alone produced ~5.6 Million TPA of Fusel Oil in 2006-07 based on Ethanol production of 1850 MTPA.

The typical composition of Fusel Oil is as below - It contains

Moisture - 12%
C2 - 08%
C3 - 01%
C4 - 09%
C5 - 70%

Clearly majority of the organics is valuable C5 Alcohols.

The moisture content of 10-15% is a big problem in case of utilization of fusel oil either as fuel or if it is used for C5 recovery or if it is used for conversion into other derivatives. Therefore, first step is to remove moisture content in a cost effective manner.

The conventional methods of moisture removal are
  1. Ordinary Distillation

  2. Very poor yield of ~10%, Need lot of energy, Tall tower for separation, yet sensitive to disturbances.

  3. Extractive distillation

  4. Need third solvent, additional cost in terms of operation hardware etc. Not cost effective
Since the boiling points of each component do not differ greatly and they form more than 5 azeotropes, the distillation is really very complex & requires either multiple columns or very large column for separation. Actually, C5 in this mixture is further divided in two components n-C5 & i-C5 both are having a BP difference of only 2.8°C & relative volatility of 1.08.

Now come to the financial part, as I wrote that India produced ~5.6 Million Metric Tonnes of Fusel oil in 2006-07. This quantity at fuel value of Rs. 20 / Kg of fuel oil is Rs. 112 Million for manufacturers, whereas 70% of 5.6 i.e. 3.9 Million TPA of Amyl alcohol at an average price of Rs. 100 / Kg can generate 390 million Rs. which is 3.5 times of fuel value. So it is worth considering value addition project for Fusel Oil.

In view of above, I used a different process i.e. salting out the moisture - this is a common practice but is not followed at large scale. This reduces the moisture from 12 to 5%. In this process wet fusel oil is taken in the agitated tank. Now Sodium carbonate is charged ~20% of the total weight i.e. if you take 1000 Kg of fusel oil charge ~200 Kg of salt. Agitate it thoroughly at high speed for ~45 Minutes & then let it settle - decant the layers for separation. Now you have a fusel oil with 5% moisture.

In the next step I used another conventional process (Contact Me for more details) to bring down the moisture to <1%. The benefit of this process is that it reduces the cost of moisture separation using well known methods.

The learning from this exercise is that you dont need too complex processes for either cost reduction, value addition or for energy conservation. First understand the properties of the material & analyse its data and then go for simple processes which can increase the productivity of your plant.

In this case, the further value addition would come from either the recovery of valuable components e.g. C5 or from the conversion of these alcohols to some other useful derivatives e.g. Amyl nitarte which is also called Banana oil & is used as a fragrance material for Banana like odor.

Here the most common method stated in the literature is to convert this mixture into esters using acids e.g. Amyl acetate or butyrate. But if I go for this again the separation of C2, C3 esters is difficult as they also have very close boiling points. So my strategy here is different. I will first react them in such a way that C2, C3 will separate on its own due to volatile nature thus leaving behind majority of C5.

After this step, I can recover C5 easily & will convert it back to final product.
Cost is of course the criteria in selecting various components.

Dont you think this is a better way then going for large distillation column & wasting lot of energy at a reflux ratio of 20 - 30???

Think on it......Do some initial Financial calculation for estimating the benefits against your time to justify the efforts in planned direction.............then SHOOT IT.

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