sensor placement for inline mixing

[stevekuech]

I am investigating the use of an in-line mixing tube to deliver a homogenous mixture of two aqueous solutions (h2o and nacl 20% soultion) to a process vessel. The flow rates of the 2 product streams must be controlled to deliver a .9% concentration at the outlet of the mixer during variable flow of the two product streams.

My thoughts for the control were to install a conductivity analyzer in the line and give a analog signal to the pump feeding the nacl solution. Under this scheme, what would be the proper point to place the sensor to avoid system lag caused by the length of the piping/mixer?

The other thought was to use a flowmeter on the water line to give a signal to control the pump for the nacl solution to maintain a volume/volume ratio of the 2 feeds.

Does anyone have experience as to which method will be the most reliable?

TNX for any help!!

stevekuech

 

[jmw]

You could use a variety of different sensors to control this process depending on the accuracy you need and the budget available.

For example:
Density measurement by vibrating element, gamma ray etc
Refractive index, reflection or transmission (a bit pricey perhaps)
Ultrasonic concentration measurement.
etc.
Vibrating element Density meters offer a wide range of prices and performance. Some of the other types of measurement give less flexibility in either.

Select some manufacturers and get them to quote price and performance (accuracy of %concentration)
Don't neglect to ask about long term performance, cleaning re-calibration etc.
The sensor would then be installed a short way downstream of the static mixer.
You use the sensor output to a 1 or 3 term controller which will control the rate of one or both feed streams.
You only need a flow meter if you need to account for usage.

JMW

http://www.ViscoAnalyser.com

 

[HEC]

Stevekuech,
What is the application for dilute NaCl? ie as jmw asked what accuracy and budget do you need to work to.

In the past I have used various methods from flow ratio control, to final mix measurements. If you are worried about control lag or deadtime the flow ratio control system is the best and most responsive. You will need three flowmeters to achiev this. There is a basic weakness of this type of control/process combination. The ratio controller is quite dependent upon the concentration of raw feed stock. Do a quick check of the variation in the neat NaCl concentration variation against the variability of the final mixture for a constant ratio flow. If the variation is acceptable then you have a nice system. If the variability is outside your requirements, then a concentration measurement device will be required. Conductivity is widely used for this. The concentration measurement would then be used to trim the ratio controller.
Trust this helps.
Mark Hutton
hutton4eng@picknowl.com.au

 

[jmw]

PS visit faq124-1007 which discusses caustic blending in line. There are some obvious similarities except that you probably wont have the temperature problem to worry about.

JMW

http://www.ViscoAnalyser.com

 

[unclesyd]

You might want to take a look at this website. We have found their mixers eliminate a lot of problems.

http://www.komax.com/products.html