Integration of the new controls with the current system

One of the challenging task after designing all the required control systems is making it work with the current system. For example, if you design an automated system that can send the right command out to control the rectifier to have the proportional load and flow of caustic and brine to the cells but your rectifier interface is manual, your control system is useless!

So in order to make the control systems useful, it is needed to make sure all the interfaces are compatible and integrate them appropriately.

Documentation on all the current systems is a good starting point to understand what needs to be changed or redesigned to play nice with the upgraded controls.

I was able to find the supplier for the rectifier and have them retrofit their control box so it could be automated. I also needed to come up with many actuated valves in order to actuate the control signals, coming out of the Programmable Logic Controller. The rest of the tuning happens when the control system is in place and the process is being carried out.

Most common control commands are either analog signals (4-20mA) or digital output (10VDC). All the sensors give you a similar outputs except RTDs (Temperature Sensors) which most of the PLCs have interface modules that accommodate RTD also.

What are all the key processes that need to be controlled?

The key processes need to be identified so the proper control system could be designed. The followings are the main ones.

1. One of the most important process for the chlorine production is the brine making. Some might even call it the bottle neck. To be more specific, the last part of brine making, which was mentioned in the flow diagram few postings ago, is the ion exchange purification. The is the last stage of brine treatment which takes care of the hardness (Ca and Mg) down to 0.02 parts per million (PPM). It is particularly important to keep the hardness just under 0.02 ppm to prolong the life of the electrolyzer cells (membrane part of the cells). In order to satisfy this need, the ion exchange columns need to be regenerated as soon as the resin exhaustion occurs. Think of resins, as elements that substitute Calcium and Magnesium ions with Sodium ions. At some point they run out of Sodium ions (exhaustion) and need to be regenerated. If this regeneration doesn't occur at the right time, the purity of the brine will be compromised. This is why you'd need the right control system to ensure the right sequence of regeneration occur at the right time not too soon since regeneration is costly and not too late in order to avoid brine hardness above the threshold of 0.02 ppm.

2. Controlling temperature of brine and caustic soda at various points of the plant is crucial.

3. Control of input flow of caustic soda and brine to the electrolyzer with respect to the load on the rectifier which is another factor in production efficiency is critical.

4. Addition of water to the exit caustic soda to thin it down from 32% to 28% in order to recirculate back into the system.

5. Last and most important part of the whole system is controlled start up and shut down of electrolyzer in case of failure at any point of the production which involves a multiple number of fail-safe sequence of actions.

So what's missing?

Now that I have learned the process, the big question is what's needed to make it all work the way it is supposed to?!

One thing I have learned so far about man power in Iran, you can not rely on them doing what they are assigned to do. Based on the experience, the more labor dependant the chemical process, the more likely to have failure in your production.

The lesson learned by my dad, when running the caustic soda flake production, was not to depend on the operator to do what is right to protect the machinery.

Most of the time, the operator is not even capable of handling more than couple tasks at a time, which makes it physically impossible to follow a sequence of actions when an event happens.

For example, if a failure happens, something like a huge leak in the feeding line to the electrolyzer cell, the final action is to shut down the production, in order to get there, multiple actions ought to happen in the right sequence. Some need to happen within few seconds of each other, 100 of meters apart! hence making it almost impossible to handle by any operator.

Automation seems to be the way to go. In fact most chemical plants, or almost any modern plant has control systems in place that handle similar situations automatically. Simply alarming the operator what is happening is the least they are entitled to do. So the goal is to put a distributed control system in place that takes control of all the key tasks that protect the plant and help running the production possible. This system is not part of the existing package, one of many reasons, no one attempted to start up the chlorine production.

A major benefit of distributed control system (DCS) is that you can centralize the control system and be able to oversee everything in one place. All the parameters and events can be logged so in case of a shut down, they could be diagnosed by analyzing the data collected.

Crash Course on Chlor-Alkali Plant Process Flow

The best starting point for any implementation is to understand an overview picture of what the process looks like.
In this simple diagram, I have tried to make it as easy as possible. First the saturated brine solution is made (regular NaCl powder or Rock salt is used). The goal is to make sure we have at least 300 grams of salt per 1 litter of Solution.

Next the solution goes through filter press to get rid of physical impurities.

Through multiple chemical addition and settling periods, majority of chemical impurities such as Calcium and Magnesium are precipitated and separated.

At this point, brine needs to be heated up in order to go through ion-exchange columns for super purification. In this section, the goal is to get the brine hardness (Ca+Mg) down to 0.02 ppm. In another word, the brine needs to have almost no hardness left in it. Ion exchange columns use high- tech resins to substitue Ca and Mg ions with Na ions.

Next the purified resin goes through mico filters and are ready to be used in the electrolyser cells.

Heated brine and 28% w/w NaOH solution enter two sides of the cell (Anode and Cathodes).
Chlorine and Hydrogen gases and also thicker NaOH solution (~32% w/w) and thiner brine (210 g/L) exit the system.

Some of the NaOH solution will be used in the cycle by adding water and thining it to 28% w/w. The brine solution needs to be decholorinated before going to the beginning of the line for saturation. Cl2 and NaOH could be used to make different byproducts such as Ferric Chloride (FeCl3) used in water treatment, Bleach (NaOCl) used as a whitening, disinfecting agent in both household and industrial use. The is an overview and needless to say each section has lots of smaller processes that need control and have their own suffistications.

Learning how a Chlor-Alkali plant works

One of the best things I was tought in college was how to teach myself something.

Most research schools, teach you that!

So I started looking for books that could explain how this plant is supposed to operate. I can tell you it was not easy trying to find information on something as practical as a working production plant. Most books talk about theoretical stuff which thanks to our books in school, I already had knowledge of that.

I needed a book that could explain the industry and give me crash course on how someone would go by running such a plant. I finally found a 1600 page book that explains just that!

It's called " Handbook of Chlor-Alkali Technology".

It is a very useful book! I did a lot of reading, and trying to match the contents with the documentation we have at the plant to understand the system I am dealing with.

There is a lot of practical and useful information packed into 1600 pages but you get to choose what is more important to you.

After reading most of the book and reading the documentation, I learned a great deal. I can finally wrap my head around this enormous task.

I will post a simple diagram showing how everything works.

So far I have gotten a great amount of satisfaction, learning something new. I think that is one way I can feel my time is well spent, after being done with school. Experience and learning new things is the least you can do to keep up with this changing and advancing technological world we live in.

About three months, is what it took me to get done with the first task! But I think this was one task I figured would take the longest. Surprisingly, finding the right resources, shortened that quite a bit.

As one of my friends used to say " Knowing is half the battle" "The other half is remebering it on the exam". In this case I have to change the second part and say " The other half is implementing it!"

Start of a challenging Project!

My name is Aref and I am starting this blog to track my progress on a project for ages !

About 6 months ago I decided to move back from US to Iran and start working on a chemical plant revitalization.

The plant in question is mainly owned and operated by my dad for years, except the main part of the plant which is in charge of producing chlorine gas and caustic soda. It needs major upgrades and reconstruction work.

Here I am with a degree in electrical engineering with 4 years of experience in controls , faced with a challenging and rewarding task of helping my dad complete his "legacy".

Tasks to be handled:

1. Understand how a Chlor-Alkali plant operates

2. Figure out all the chemical processes involved and know them all from inside out

3. Find all the missing pieces of the puzzle

4. Design a much needed and non-existent control system for all the key processes

5. Make the current system compatible and integrate the designed control system into the plant

6. Repair and upgrade current machinery required for the process

7. Replace all old pumps and pipes and valves and other needed equipments and instruments

8. Procure all the ingredients needed from local and foreign suppliers (PLCs, Pipes, Valves, Instruments, Fittings, .....)

9. Put together a reliable team of operators and engineers to run and maintain productions at the plant

10. Do all above in a timely fashion and under a certain budget

Sounds challenging?

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You bet!