Problems with pneumatic conveying of activated carbon

Posted in: , on 13. Mar. 2008 - 11:42

Hi everybody,

We have an installation for pneumatic conveying of activated carbon (25 micron size). The conveying distance is about 300 m and we are using a positive pressure system (root blowers). The active carbon is dosed into the conveying line by using a gravimetric screw feeder. Between the screw feeder and the conveying line we have placed a rotary valve in order to make some kind of air locking. We stated to run the system and we found a lot of problems with the rotary feeder backflow. The amount of carbon that we dose is about 50 kg/h, but the backflow doesn´t allow it to fall properly and a big amount of it is returning upstream, putting pressure in the discharge of the screw feeder. We tried to vent the rotary valve and have been able to prevent the presssure from going to the screw feeder, but, anyway, the air that we are sucking from the rotary valve is still dragging a lot of material and the dosing amount is not reliable.

It seems that we can not make it work in suction mode since the distance is too long.

Rotary valves always will have this backflow effect, so

What would you suggest to try in order to try to solve this problem. Any other posibility for isolating the screw feeder from the conveying line without using a rotary valve (we were thinking too in some kind of double valve, but we need the dosing to be steady?

Thanks for your help


Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 13. Mar. 2008 - 01:46

Part of resoling issues is always to look for heklp from the conveying system conveying manufacturer. If you have not done so already, I would touch base with the OEM who helped you install the system. Also, since when has this issue become a problem? If it has worked before then I would focus on the wear and tear of the rotary air lock.

There are plenty of system OEM's out there who could help you with installing a system that works in your application. One would be Claudius Peters

But I would deem conveying eductors to be suitable as well, for instance Fox Valve's conveying solution.

Again, it is important to know if this is a new issue or old, and if this was a OEM based solution or yours.

Hope this may help you a little.

Regards, Ralf Weiser (001)-484-718-3518 [url][/url]

Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 13. Mar. 2008 - 06:23

Dear Pablonr,

A rotary valve hampering the feeding of a pneumatic conveying system is not new.

Especially with very fine and light powders, the blow back through the valve pockets and the gap between the rotor and the rotary casing can cause an oscillating pressure.

A better controlled feeding could solve this problem, but will not be easy.

Consult dosing equipment experts.

Running the installation at the lowest possible pressure drop will also help.

The blow back through the rotary lock will be less, when running at lower conveying pressures.

Whether the installation is designed for the lowest possible pressure can be stated by the designer or vendor.

Using the operational data obtained so far, it must be possible to make indicative calculations.

Those calculations should show possible improvements.

As Ralf says: If the problem was not there before then look for changes in time, like wear on the rotary lock




Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 13. Mar. 2008 - 08:17

The problem has been there from the starting up of the system. We, according to the opinion of our pneumatic conveying designer, have already tried different solutions:

- A suction fan in the rotary valve venting line in order to suck the overpressure coming from the backflow. Result: Not only the overpressure was sucked but a high amount of dust too

- Different kinds of inyectors or venturi-type (not properly venturi tubes) devices on the bottom of the rotary valve in order to avoid the pressure to get back. Result: works ok when the pipe is open, but when we cnnect it again to the conveying line (pressure losses) the effect on the rotary is the same again.

- Lower pressure in the blowers and more tigthen rotary valves. Result: Still backflow and dust going through the rotary valve.

We were thinking on installing eductors or venturi tubes, but pressure losses are too high for our blowers (0.3 bar).

We have our own compressed air station, do you think that it would be possible to install a real venturi tube under the rotary valve (maybe about 1 bar pressure losses due to this venturi) and make the pneumatic conveying by using this compressed air (our compressor provides 7 bar and air flow enough?. We can reduce pressure to 3 or 2.5 bar and try. What do you think?

Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 13. Mar. 2008 - 09:04

Interesting idea. I your particular case I would not use a trial and error method though. This tends to costs you more than if you are going to a reputable system OEM and spend the capital money upfront. Any such OEMs (there are a lot of them featured on the web page) should be able to provide you with a good estimate in terms of expected conveying and air flow / pressure requirements. 7 Bar is probably a bit much; usually we see 2-3.5 BarG applications that sometimes needs to be inertized with e.g. N2. You could possibly try contacting e.g Fox Valve at with your system requirements and ask if this is something they would be interesting in pursuing this application with you.

Regards, Ralf Weiser (001)-484-718-3518 [url][/url]

Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 13. Mar. 2008 - 09:11

Dear Pablonr,

The main problem is :

How to dose the carbon from a low pressure region into a higher pressure region, whereby the blow back is sealed off and the carbon can pass freely.

( in cement conveying a screwpump is used for such an application. Fuller Kynion)

The particle size of 25 micron is, may be, too small for a rotary airlock.

Depending on the various design parameters, may be, an option could be a sonic choke on the high pressure plant system (taking care of the required airflow) and a “venture-eductor” as suggested by Ralf.

Contacting Foxvalve or a representative is a good idea

Actually redesign by using the obtained experience.

What is the designed pressure, airflow and pipediameter?

take care



Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 14. Mar. 2008 - 01:07

Dear Ralf and Teus,

some time ago, we thought of the posibility of replacing our rotary valve by some kind of eductor. We had some information from Fox products but, when checking, we discovered that maximum recommended distances were not longer than 100 - 120 m and we discarded the idea since in our case we are talking about 300 m (I was thinking in using our root blowers (0.3 bar) for putting air through the eductor, placing it in the conveying line).

Anyway, and thanks to your advises I have been having again a look to this issue and I´ve found a couple of options that maybe could work in our case:

- 1st one: maybe we can replace the rotary valve by the eductor, but not placing the eductor in the main conveying line but feeding it wit our own compressed air and use it for entraining the active carbon in the positive pressure conveying line (0.3 bar). In this case, the required distance for the eductor wouldn´t be longer than 2 or 3 m. (something similar to attached file CS47).

- 2nd one: maybe esier to install. To place the eductor in the vent line of the rotary valve, putting compressed air on inlet side and returning backflow air and material right back in the conveying line (there is another similar file, but i have not been able to attach it).

In both cases I would need to have more information about the effect of the pressure coming from the eductor over the low pressure conveying line. Anyway, I agree with you on the fact that we should contact some expert company before taking any risk.

What do you think?

Thanks a lot for all the help that you are providing


fox_cs47 (PDF)

Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 14. Mar. 2008 - 05:56

Oh boy this sounds a bit too complex for what you need to do. You may be able to limit the air blow by in the valve, but it will remain a factor. If you have already tried the eductor idea I think the screw feeder pump idea from Teus is a very effective alternative. Fuller is a good address with their Kinyon pump and there is a company called WM Meyer out of IL USA that has a similar working device.

They also have an example of a what I call a double dump valve that allows material to drop into your air stream via two gate type valves.

I am a bit out of my area of expertise, but be careful with your air to coal dust ratio - be sure to let your contacts know what you are conveying and how.

I am not sure why you are only getting 300mBar pressure rise out of your blowers, but I would focus on getting the system requirements and then to obtain the blowers to match (unless the pressure is limited due to the conveyed material). There are plenty of positive displacment blower reps out there that will be able to offer new, remanufatured, or used packages to you. Using plant air is usually a huge waste of energy since you would be letting an air compressor bump up the air pressure to at least 75PSI just to expand it in the pneumatic conveying system at less than 15PSI.

Good luck.

Regards, Ralf Weiser (001)-484-718-3518 [url][/url]

Re: Problems With Pneumatic Conveying Of Activated Carbon

Posted on 14. Mar. 2008 - 07:33

Dear Pablonr,

An eductor is a feeding device, that does not know anything about the length of the conveying pipeline after the injection point. The eductor only sees the pressure of the pneumatic conveying system.

How the pressure is generated, depends on the system design and actual Solid Loading Ratio.

An eductor should work on a long pipeline as well.

Your first option looks very elegant.

Nevertheless, it needs thorough engineering and designing to make it work.

The extra injected air from the eductor has to be added to the air flow of the blower, influencing the backpressure in the pipeline and/or the capacity.

The blower acts as a booster. If the eductor air flow + blower air flow is too much for the system, the blower should be reduced in air delivery.

Here you can benefit from the already made observations.

What are the designed pressure, airflow , pipe diameter and reached capacities?




Clyde Rotofeed Injection

Posted on 18. Mar. 2008 - 11:19

Dear Sirs

I represent a compnay called Clyde Materials Handling, we specilise in Pneumatic Conveying are Injection Technologies.

Our Rotofeed injection system, would be ideal for your application, we are very successfully injecting many materials from coal to PFA and copper concentrate.

Our system has not pressure differential across the feeder (conveying line to feed vessel), resulting in no wear and enabling us to convey at high pressure and distances up to 1.5km.

If you require any further information please contact me.


Richard Sagar EngTech AMSOE AMIPlantE Ammermann Partners Pty Ltd Mobile: +61 (0)488 149 361 Direct Fax: +44 (0)2 4372 2033 E-mail: [email][/email] Web: [url][/url]