Hobby project: Creating a batch process and seeking help in the design process!

Hello engineers/technicians of PLCS.net! I require your assistance, if you may be so kind to provide it. I hope this is an appropriate query to make to you fine folks! Okay, so my first post is going to be a long winded one..

I am taking on projects from my graduating University as a way to refresh myself on control technologies and to build up my portfolio in hopes of making myself a more lucrative prospect for possible employers.
One of the projects I have conceptualized is a physical, simulated batch process. Here’s a run-down of my project idea:

The batch process is a simple mixing station. The tanks/vessels/containers I plan to use range from 5-15 gal. I want to make my system compact enough to cart around (for demonstration purposes).

I have 3 ingredients: continuously circulated supply of WATER, finite supply of RED DYE, and finite supply of BLUE DYE.

I can create 3 products – RED product (water and red dye mixture), BLUE product (water and blue dye mixture), and PURPLE product (water, red and blue dye of equal parts mixture).

When I select the product and the amount of product (through my HMI) I want to create, the ingredients are sent to a mixing vessel. The ingredients are mixed and I get my finished product. The finished product is then sent to a product recycling tank so I can remove the dye. To remove the dye from the water/dye solution, I’ll be using a chlorine/bleach solution. Why? Because of the dye’s I am using:
http://brightdyes.com/FAQ.html#10 (A tracer dye that is strong visually and relatively cheap)

From the FAQ, it states a chlorine/bleach product should successfully remove the dye concentration and eliminate any color from the water/dye solution. I then take the chlorine/water solution and use a water filter (not sure what type of filter to use) to remove the chlorine from the water. The filtered water is then pumped back into my water tank.
This process continues until there is no more dye ingredient(s) or chlorine left.

To control this process I am using a Siemens S7 PLC/IPC. I am quite comfortable with the programming side of things and feel I can accomplish this. Design/feasibility of overall process is an issue.
http://www.automation.siemens.com/m.../en/industrial-pc/Panel_PC/Pages/Default.aspx (Link to the IPC I plan to mount to my system)
I have a link to the sample P&ID I scratched up on my notebook to give you an idea of the process I had in mind:

http://i.imgur.com/4yVYOei.jpg

Sorry if the sketch is a poor attempt at a P&ID. Just brainstorming.


My problems/concerns:

1. I plan to use 6 solenoid valves to control the step-by-step process of my system. Is this the best way to do so? Everything from adding of the ingredients to mixing and recycling will be controlled by the solenoid valves. I will have ultrasonic level sensors (suggestions on a better sensor?) to measure the ingredient tank levels and I will have to create an algorithm to calculate how much of each ingredient is to be added to the mixing vessel based on operator input. So again, is this the best method to go about doing that?
Small note: The thought is using solenoid valves will be the best method of controlling the input to the mixing vessel because my ingredients would be gravity fed. I was thinking there would be no need for small DC motors to be used. The ultrasonic sensors are being used because they are the ones available to me currently in the lab. Please educate me if I am going about this completely wrong.

2. I believe chlorine is the best way of removing the dye from my product. I hope there will be no toxic byproduct, such as gasses, from mixing the chlorine with the water/dye. The dye manufacturer says their product is non-toxic, so no ill effect should come of the mixture, right? RIGHT?! The system I want to create will in a lab setting with no other harmful chemicals around, but the chlorine will be in an enclosed container. Will the chlorine pose any health hazards?

3. The water filter. I'm a bit hazy on what route to take when removing the chlorine. I've done Google searches and saw recommendations of using an acid, such as citric products to neutralize the base or of reducing the pH. But, I don’t want to introduce contaminants into my water supply. I’m not sure what the best type of water filter to use is. I suppose a simple home filtration system should suffice? The costs, though, I've seen are a bit exorbitant.

I've seen recommendations of using activated carbon or ion exchange resins to see if they can remove the dye. I have samples of the dye on its way so I can perform some tests and see if this is a better solution! I can then create my own simple filter.4. Lastly, does this project seem like a worthwhile venture? I wanted to create a system that was very sleek in design. I plan to use clear containers/tanks so people could view the process as it takes place. I am going to use a touch-screen HMI that allows for simple operability and intuitive to any user. I want to create a model that can be improved by future project teams and can be presented/demonstrated in the classroom and for visitors alike. Before this project can be accepted at the department’s expense, I need to make sure I have a solid proposal. Help a brother out?

Please throw whatever advice/criticism my way. If you need more information, let me know! I plan to document my project and use it to build up my portolfthe time I complete this project.

TL;DR I want to create a physical batch process. I have some technical issues I need addressed.. I'm not a smart man.

Welcome to the site
I dont have the software to help you with Seimens.
but you have taken on an interesting project

Small note: The thought is using solenoid valves will be the best method of controlling the input to the mixing vessel because my ingredients would be gravity fed. I was thinking there would be no need for small DC motors to be used.

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Gravity feed using solenoids could result in errors in ingredient mix. Whether this will be acceptable depends on what accuracy you want. Adding liquids (water and the liquid concentrate type of Bright Dye) will be okay, but the amount is going to be time-dependent with not very accurate control of the Parts Per Million in the water.

If your use the dry powder dye, your control will be much worse, because dry powder does not gravity-feed consistently. It tends to clump and stick in the valve and have an intermittent feed rate. You should take into consideration that when you gravity-feed liquids out of a tank, as the level in the tank gets lower, the feed rate also goes down from Maximum to 0 when the tank is empty (as the pressure in the liquid feed output line goes from maximum to 0). You could make it a little more linear by using a large-diameter low-height feed tank, and raising the feed tank vertically as high as possible, and extending your line to the lowest possible point (similar to many city water towers with a large-diameter tank raised on legs). This will make the feed pressure less dependent on the level in the tank, although it still will not produce the desired perfectly constant feed rate.

I hope there will be no toxic byproduct, such as gasses, from mixing the chlorine with the water/dye.

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It appears that the amount of chlorine required to dilute the water/dye mixture will be so low that you could discharge it directly into the city sewer system with no bad effects. In fact, you may need to use distilled water instead of city water, which normally contains some amount of chlorine. Otherwise, your colors may fade out before you want them to go away!

If you leave a chlorine/water mixture sitting in an open tank, most of the chlorine will evaporate as chlorine gas. Of course in a lab, evaporating the chlorine may be worse (due to the strong fumes) than leaving it in the water! Chlorine fumes are toxic and hazardous, but how toxic depends on the concentration. If you can use closed tanks, it will reduce the fumes. If you can put your chlorine-mixing operation under a standard lab hood with an exhaust fan and exhaust duct, then it most likely be okay to operate with no other special equipment.

An interesting approach would be to buy a couple of the color-sensitive photosensors with analog outputs. Then you could set a color level, and simply feed a dye into the mix tank until the color reaches the set level. The photosensors would cost a little more, but would result in very good control of the mixture.

Lancie1 said:

An interesting approach would be to buy a couple of the color-sensitive photosensors with analog outputs. Then you could set a color level, and simply feed a dye into the mix tank until the color reaches the set level. The photosensors would cost a little more, but would result in very good control of the mixture.

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That's a very good suggestion! I've never dealt with such sensors, but I'll definitely look into it. Also, thanks for your other recommendations, Lancie1. Quite insightful and helpful.

If your use the dry powder dye, your control will be much worse, because dry powder does not gravity-feed consistently. It tends to clump and stick in the valve and have an intermittent feed rate. You should take into consideration that when you gravity-feed liquids out of a tank, as the level in the tank gets lower, the feed rate also goes down from Maximum to 0 when the tank is empty (as the pressure in the liquid feed output line goes from maximum to 0). You could make it a little more linear by using a large-diameter low-height feed tank, and raising the feed tank vertically as high as possible, and extending your line to the lowest possible point (similar to many city water towers with a large-diameter tank raised on legs). This will make the feed pressure less dependent on the level in the tank, although it still will not produce the desired perfectly constant feed rate.

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I do not plan on using dry power dye. It will either be the liquid solution or the tablets. I want to use the tablets and create an automated feeding system of the tablets to my ingredient tank (for red and blue dye) and just add water to the tanks when required. Thought it would be a nice addition to the overall system, but requires more thought and I need to find the mechanical part to do this.

I was thinking I may be able to use chlorine tablets as well to remove the dye component from my finished product. Do you think it's a more viable option then having a container of chlorine liquid solution to mix with my finished product?? Doesn't liquid chlorine dissipate at a rapid rate when left at room temperature?

And what are your thoughts on the filtering process, Lancie1?

Thanks again for your well thought-out solutions to my problems. I definitely need to consider every angle possible.

Thought it would be a nice addition to the overall system, but requires more thought and I need to find the mechanical part to do this.

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You might design and build some type of tablet dispenser, where you have the tablets stacked in a cylinder, with a bottom cover and slot, and a solenoid-operated pusher that pushes one table out of the bottom slot each time it is energized. One restriction with using the tablets is that your color mixtures will have to be somewhat limited, with only certain color levels obtainable for a set amount of water, unlike using a liquid dye.

I was thinking I may be able to use chlorine tablets as well to remove the dye component from my finished product. Do you think it's a more viable option then having a container of chlorine liquid solution to mix with my finished product??

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Liquids will be much easier to mix. With tablets, you may need a stirrer or mixing motor. Liquid laundry bleach is widely available, easily mixed (just pour it in the water). You will get some chlorine fumes off the water/dye/bleach mixture, regardless of which form (solid or liquid) of bleach you use. But it should be no more than I get from my washing machine tank when I pour in some laundry bleach.

And what are your thoughts on the filtering process, Lancie1?

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I have one of those Pur home water pitcher filters that seems to remove 99% of chlorine from the water. That type of filter would be cheap and easy to use. You could use more than one in a series configuration, if needed.

That's a very good suggestion! I've never dealt with such sensors, but I'll definitely look into it.

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Here is a link to one type of color-sensitive photoelectric sensor. The Allen Bradley Colorsight 45CLR with 3 outputs will detect 3 different color ranges, user settable. If you get the RS-485 model, it can detect up to 5 color ranges (but you need a RS-485 comm channel on your PLC).

http://ab.rockwellautomation.com/Se...Sensors/ColorSight-Color-and-Contrast-Sensors

Here is a link to one type of color-sensitive photoelectric sensor. The Allen Bradley Colorsight 45CLR with 3 outputs will detect 3 different color ranges, user settable. If you get the RS-485 model, it can detect up to 5 color ranges (but you need a RS-485 comm channel on your PLC).

http://ab.rockwellautomation.com/Sen...ntrast-Sensors

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A bit beyond my budget I believe..
It would be great to integrate into my build, but I may not be pull this off. I'm sure it can be a consideration for future modifications.

A bit beyond my budget I believe..

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I understand. One cheaper method to improve your mix accuracy would be to measure the amount of dye and/or water that is used (a Loss-in-Weight feedback signal to the PLC. All you would need to set that up would be 2 or 3 cheap load cells or strain gages and milivolt PLC inputs. The bare strain gages in the 1 to 5 lb range they are not that expensive (Omega Engineering Karma type about $31 each). Mount your tanks on a single-point hanger, attach to the load end of the strain gage, and connect the strain gage outputs to a milivolt input points on your PLC. You will need a 10 or 12 VDC supply for the strain gages.

The way a LIW feeder works is that you have an initial setup routine where you read the beginning weight of each tank into the PLC. Then you set your LIW routine for a set amount of dye or water to be gravity fed. Then you open your water or dye solenoid and watch the weight by subtracting the current reading from the initial reading. When your setpoint weight in dye or water is reached, close the solenoid.

Lancie1 said:

I understand. One cheaper method to improve your mix accuracy would be to measure the amount of dye and/or water that is used (a Loss-in-Weight feedback signal to the PLC. All you would need to set that up would be 2 or 3 cheap load cells or strain gages and milivolt PLC inputs. The bare strain gages in the 1 to 5 lb range they are not that expensive (Omega Engineering Karma type about $31 each). Mount your tanks on a single-point hanger, attach to the load end of the strain gage, and connect the strain gage outputs to a milivolt input points on your PLC. You will need a 10 or 12 VDC supply for the strain gages.

The way a LIW feeder works is that you have an initial setup routine where you read the beginning weight of each tank into the PLC. Then you set your LIW routine for a set amount of dye or water to be gravity fed. Then you open your water or dye solenoid and watch the weight by subtracting the current reading from the initial reading. When your setpoint weight in dye or water is reached, close the solenoid.

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Gotcha. Again, a very original solution (at least for me)! I have dealt with strain gages and get the gist of how this setup would work. I'll work on hammering out the other details of my project as soon as I can perform some testing on the dye. Absolutely elated for all the wonderful advice you've provided, Lancie1. Thanks for pointing me in the right direction and teaching me some new methods of control.

You might design and build some type of tablet dispenser, where you have the tablets stacked in a cylinder, with a bottom cover and slot, and a solenoid-operated pusher that pushes one table out of the bottom slot each time it is energized. One restriction with using the tablets is that your color mixtures will have to be somewhat limited, with only certain color levels obtainable for a set amount of water, unlike using a liquid dye.

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Also looking back at something else you pointed out -- I don't plan to have the most accurate system when it comes to having the correct color/dye concentration just yet. I want to get the overall system in place, then in the future think about making adjustments later where I'm able to add a visual control element in place to have more consistency in my batch product.

I had it in mind to use the tablet dispenser, as you pointed out, to add a tablet to my ingredient tank(s) once a certain low-level threshold is reached. I will then either pump or gravity feed water to my ingredient tank until it reaches a sufficient level. Though the tablets are said to dissolve well, I know it won't be the most consistent mix.

I didn't want to make the system overly complex at this stage and continually add more hardware for me to consider. I want to make it where future project teams can perform additional changes and upgrades to the system and think of ways to perform more refined control or implement more complexities to the overall design.

This endeavor is quite new to me. I believe I'm capable of developing the programming logic/HMI, but the design/construction side is not my forte. Hence, why I want to do this from scratch

...but the design/construction side is not my forte. Hence, why I want to do this from scratch.

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Here is a place to begin, with your dye dispenser.

Late reply -- that is so cool! Thank you so much for the schematic! Very well done.