jmw
Industrial
- Jun 27, 2001
- 7,435
For whatever reason, perhaps mostly historical, some terms come to be inappropriately applied.
So of all the operations described as "sparge mixing" the some may more accurately be described as " recirculation mixing."
As I am not overly familiar with mixing technology or processes I need some help with basic concepts in order to sort out what are the key problems in these applications.
I will be more than content with some definitions, basic pointers or guidelines, or references to some good web sites. I need a basic understanding to apreciate the problem areas, not to be able to design a system. A sort of "identikit" approach to appreciating what is good and bad about any system I come across would be helpful.
The process is commonly performed around the world but the equipment used appears to vary significantly.
Two liquids are being mixed by a process that is often described as "sparge mixing".
In this industry and in some plants this involves, as my limited knowledge would lead me to expect, using a sparge pipe at the bottom of the mixing tank to blow air through the liquids until the resultant mix is homogenous.
Undoubtedly this is how the process originally was used (even though my own concern would be the significant density and viscosity differences between the components i.e. is it the most suitable anyway?) but the term now seems to be applied to any arrangement of pipe work and as often as not it is simply a recirculation system (and probably not ideally designed for that process).
The two liquids are typified by:
90% is a 400-1000cst fluid at approx 50deg C
10% is a 1-5cst fluid at ambient
The two components often have significantly different densities.
Using air is a problem for many of the operations because (a) they don't have available adequate compressors
(b) because the air gives rise to what they call the "cappucino effect" and as the product is sold, as soon as mixed, and the volume determined by tank dipping, this leads to disputes over the amounts delivered
(c) density and viscosity are key target qualities measured at completion and entrained air gives false low density and false high viscosity (they do not have the facilities to centrifuge the samples nor an ultrasonic bath. Paradoxically, most quality disputes arise from false high density (around 6%; the product is sold by mass) and false low viscosity (around 7%; a better product but more expensive). Most quality disputes are not identified as a problem from the mixing process but from the pre-determined ratio of the components.
Hence many such "sparge mixing" operations are now based on pumped recirculation.
In these cases it is the discharge pumps that are used for recirculation. Pump capcities are usually sufficient to discharge the entire tank contents in around an hour. So even if the pumps are adequately sized, the time available and the pipe layout may not be. So far as I know most such installations do not have any special pump inlet piping arrangement, they just connect to a hole in the side/bottom of the tank.
The return might be via a sparge pipe in the bottom of the tank, it may be a stilling pipe returning the liquid back below the surface or it might be a "sparge" pipe (so called because the return pipe opening is above the liquid level in the mixing tank). One operator admitted they have problems because the return pipe is very close to the pump inlet.
One last feature is that time is often limited.
One of the major concerns would be that the mix is not homogenous especially as the mixing time is often not the time needed for mixing but simply the time available which can vary from one operation to another. It would be OK if the time available was always greater than the time needed but this is notthe case.
In almost all cases there is a reluctance to invest in special equipment and the systems that have evolved have exploited existing needed equipment as much as possible.
Any thoughts?
JMW
So of all the operations described as "sparge mixing" the some may more accurately be described as " recirculation mixing."
As I am not overly familiar with mixing technology or processes I need some help with basic concepts in order to sort out what are the key problems in these applications.
I will be more than content with some definitions, basic pointers or guidelines, or references to some good web sites. I need a basic understanding to apreciate the problem areas, not to be able to design a system. A sort of "identikit" approach to appreciating what is good and bad about any system I come across would be helpful.
The process is commonly performed around the world but the equipment used appears to vary significantly.
Two liquids are being mixed by a process that is often described as "sparge mixing".
In this industry and in some plants this involves, as my limited knowledge would lead me to expect, using a sparge pipe at the bottom of the mixing tank to blow air through the liquids until the resultant mix is homogenous.
Undoubtedly this is how the process originally was used (even though my own concern would be the significant density and viscosity differences between the components i.e. is it the most suitable anyway?) but the term now seems to be applied to any arrangement of pipe work and as often as not it is simply a recirculation system (and probably not ideally designed for that process).
The two liquids are typified by:
90% is a 400-1000cst fluid at approx 50deg C
10% is a 1-5cst fluid at ambient
The two components often have significantly different densities.
Using air is a problem for many of the operations because (a) they don't have available adequate compressors
(b) because the air gives rise to what they call the "cappucino effect" and as the product is sold, as soon as mixed, and the volume determined by tank dipping, this leads to disputes over the amounts delivered
(c) density and viscosity are key target qualities measured at completion and entrained air gives false low density and false high viscosity (they do not have the facilities to centrifuge the samples nor an ultrasonic bath. Paradoxically, most quality disputes arise from false high density (around 6%; the product is sold by mass) and false low viscosity (around 7%; a better product but more expensive). Most quality disputes are not identified as a problem from the mixing process but from the pre-determined ratio of the components.
Hence many such "sparge mixing" operations are now based on pumped recirculation.
In these cases it is the discharge pumps that are used for recirculation. Pump capcities are usually sufficient to discharge the entire tank contents in around an hour. So even if the pumps are adequately sized, the time available and the pipe layout may not be. So far as I know most such installations do not have any special pump inlet piping arrangement, they just connect to a hole in the side/bottom of the tank.
The return might be via a sparge pipe in the bottom of the tank, it may be a stilling pipe returning the liquid back below the surface or it might be a "sparge" pipe (so called because the return pipe opening is above the liquid level in the mixing tank). One operator admitted they have problems because the return pipe is very close to the pump inlet.
One last feature is that time is often limited.
One of the major concerns would be that the mix is not homogenous especially as the mixing time is often not the time needed for mixing but simply the time available which can vary from one operation to another. It would be OK if the time available was always greater than the time needed but this is notthe case.
In almost all cases there is a reluctance to invest in special equipment and the systems that have evolved have exploited existing needed equipment as much as possible.
Any thoughts?
JMW