Perlite filteraid performance notes

 

Filter septums.

Clarity is considered by many engineers to be the most important measure of effiency in filteraid filtration. So many things affect clarity - both favorably and adversely - that only general principles can be stated. A high quality filteraid is most important for uniform results day after day. Selection of the particular type having the correct particle size and -distribution is a major factor. After these come many considerations such as the quantity of filteraid to be used in a fixed bed, body-feed, and rotary precoat filtrations; the flowrate needed to meet plant production schedules; the equipment and general filtration coditions; and others. All of these can best be resolved by tests using the actual liquid to be filtered.

Clarity measurements involve the most difficult aspects of filtration technology. Visual evaluation of filtration clarity is only semi-quantitative at best; critical evaluation requires instrumental measurement.

The chart shown illustrates solid removal capacities of filteraids:

Size of solid particles removed by Nordisk Perlite filteraids

Particle size in microns

Filteraid

1.1 

1.0 

0.9  

0.8  

0.7  

0.6  

0.5  

0.4  

 0.3  

*0.2  

 0.1  

Type 180                                            
Type 150                                        
Type 80                                        
Type 60                                        
Type 50                                        

*0.2 micron-Theoretical limit of resolution of optical microscope.
Few microscopes achieve it.

 

While it is possible to calculate the average pore size of a filter cake, our engineers have known for some time that particles much smaller than the calculated pore diameter and readily retained by perlite filteraids.

Flowrate, another primary measure of filtration effiency, is the fact that by reducing the specific resitance of the filtercake, filteraids vastly increase the flow, thus increasing the total throughput of the system. For example, a cake of filterable solids from raw cane sugar liquor has 10000 times the resistance of a cake of a specific filteraid.

Thus the type and grade of filteraid as well as the quantity used are the major factors in securing maximum filtration flowrates. In general, the coarsest (most permeable) filteraid that will provide satisfactory clarity is the best choice. Careful tests will determine this - an especially important point in fixed bed and rotary precoat filtrations. Adjustments can be made in body-feed filtrations by adding filteraid in increasing increments, producing very substantial flow increases up to a certain point. Further addition above this optimum amount affords only very small increases. In fact in many systems the addition of too much filteraid results in a definite decrease in throughput.

Higher flowrates may also be obtained by increasing pressure, considering the rating of the equipment and the nature of the filterable solids. Filteraids usually produce a relatively incompressible cake so that increased pressure results in increased flow. But in some systems, usually involving a high content of solids which are gelatinous in nature, the addition of filteraid suffiecient to render the cake incompressible is uneconomical. In these cases the pressure which affords the best balance between the economical use of filteraid and the desired throughput should be determined by test.

Another way of increasing flowrate is to reduce the viscosity of the liquid by heating. Heating should be analyzed as to cost vs. increased flow as well as harmful effects on the liquid and possible change in the character of the suspended solids. Another possibility is to filter at a process stage where the viscosity is lowest if such a place exists.

Another means of increasing flowrate in an emergency is effective but not often used. This is to "short-cycle" the filter by deliberately cutting off the cycle and starting another before normal top pressure is reached or before the cake space is filled.

Filter Cycle Lenght. The cycle lenght is important from the standpoint of saving time and labor. Cycle lenght is generally limited by the maximum differential pressure of the filter, assuming cake space is adequate. But aside from the rare cases where shortcycling is justified, cycle lenght in most systems can be extended to minimize labor costs.

First, select the proper type and grade of filteraid. Maximum flowrate and cycle lenght may be secured by employing a filteraid having a particle size range which is compatible with that of the filterable solids.

Second, choose a filteraid of proven high quality which can hold flowrate decrese close to the theoretical 0,5 slope on a log-log graph of time versus volume. Poor quality filteraids usually uncontrolled as to particle size distribution, sometimes give high initial flowrate but tend to "tail off" sharply as the cycle progresses.

Third, use the optimum percentage of filteaid. Cycle lenght may be limited by use of either too much or too little filteraid. All three of these points have already been discussed in some detail: they are mentiomed here because of their additional effect on cycle lenght.

 

 

Nordisk Perlite ApS © 2014