Selection Criteria for Membranes used in Filter Presses

By Dr. Hans Valerius
October 2005

The Author is Director at Enigma Process Technologies Corp in Muntinlupa, Philippines. His company provides engineered solutions for laundry waste, hospital waste, animal blood, animal waste, palm oil mill waste etc. → See also:

Membranes represent quite an investment and therefore prior to taking any decision certain criteria should be taken into consideration well balancing the pros and cons of the individual designs that are available.

I. Application Details

The application in which membranes are supposed to be used should be clearly described and in case experience is already available from any past operation of membranes they should be mentioned.

The main topics are:

Based on the above the design, construction, material of membrane and application data can be evaluated and prepared for a first decision.

It goes without saying that any experience with membranes in the same application before is of great help since the fact and figures based on the records during the previous operation of membranes is of utmost importance and shortens the time that is required to take a decision tremendously. Additionally the gained experience can trigger a further in-depth study of possible design and construction changes, if not to say improvements, special material selection or new design/construction in general.

Experience should by no means be underestimated in such a decision finding mission.

II. Monetary Aspects

As mentioned, membranes represent quite an investment. Thus the decision to use membranes for a specific application must consider many aspects which are related with the filtration result, the operation and the work force that is involved as well as maintenance and replacement costs and additional equipment required for the operation of membranes.

The monetary value of a membrane can easily exceed the value of a standard filter element three to six times. There are applications where the filtration result achieved with membranes does not exceed the dry solid content achieved with conventional filter elements and the savings is only to find in the range of filtration time which in some cases might not be of any importance at all. It also has to be considered that in some areas the issue labor costs does not play an important role at all or that the operation skill required does not exist and therefore not recommendable to use membranes at all, unless proper steps are taken to find a remedy for the existing obstacle.

Some decisions are merely based on a non-monetary aspect others strictly on monetary issues. As such an evaluation can be objective or subjective either on the manufacturers or on the end user´s side. Nevertheless a clear understanding should be found between the two parties to avoid any disappointment at a later stage.

A. Monetary Evaluation

The objective part deals with the fact and figures of the process.

Still easy to calculate are the costs for the investment, the installation and the operation. Rather difficult is the calculation of the savings or the monetary benefits. They are as follows:

Details

Nevertheless many aspects have to be taken into consideration, the main issues were listed above but additionally floor space, feed pumps, plumbing, conveyors, labor etc are further topics that need to be watched.
Lastly the combination of a mixed pack, i.e. chamber plate/membrane plate combination should be investigated and might be in some cases the better choice with savings of approximately 25 % in comparison with a pure full membrane pack.
Special attention however is here required when the type of membrane is selected.

B. Non-monetary Evaluation

The non-monetary evaluation is not objective, calculations of advantages versus disadvantages; savings etc. are not possible to the extent as they are possible for the monetary evaluation.

The following are to be considered as subjective:

Higher quality in the end product can lead to advantages over the competitors and can also lead to increasing sales price. For example the volume of wash media to remove soluble salts in the product during filtration might be above permitted level. By designing the right membrane system the limited volume could be accommodated which would result in a better quality of product.

Possibility to meet increasing environmental control refers to the fact that existing laws from the individual governments re disposal of hazardous waste, regulations for landfill etc are still based on other systems than filter press - membrane configurations. Under the existing rules and regulations are many other systems to find which judged by the achievement of dry solids are far exceeded by the application of membranes. The investment in membranes would therefore not only reduce the possibility of an installation of a new system in case the rules and regulations are changed again it would also give ample safety in terms of forthcoming changes of rules and regulations since the indicated standards by the law are more than met.

Flexibility within the process is important in case more than one product or within one product various parameters have to be covered which a well-designed membrane, which meets all the requirements, can easily do.

III. Construction Evaluation – IV. Application Evaluation

III and IV are summarized in the following breakdown. For construction evaluation the explanations are categorized with III (n) and for application evaluation with IV (n).
In a further table the results out of the evaluation are summarized and form the basis for the later decision on the type of membrane to be selected.
The indicated figures III (n) and IV (n) can refer to more than one category.

Membrane PP PP EP EP  
Web Design PP One-piece PP Detachable PP One-piece PP Detachable  
Criteria         Comments
III-1 Chemical      
III-2 Physical      
III-3 Experience      
III-4 Service Conditions      
III-5 Leakage     No contamination
III-6 Metals     Chemical resistance
III-7 Economics     Cost effective
III-8 Special Notes      
III-9 Special Notes      
III-10 Filter Press     Mobile, fixed
Overall      
Membrane Rubber Rubber Rubber PVDF  
Web Design PP Detachable AL Detachable ST Detachable PVDF One-piece  
Criteria         Comments
III-1 Chemical      
III-2 Physical      
III-3 Experience      
III-4 Service Conditions      
III-5 Leakage     No contamination
III-6 Metals     Chemical resistance
III-7 Economics     Cost effective
III-8 Special Notes      
III-9 Special Notes      
III-10 Filter Press     Mobile, fixed
Overall      
  Corner Feed Center Feed Air Squeezing Water Squeezing Product Squeezing Comments
Criteria Data Mixed Pack Full Pack Mixed Pack Full Pack Int. Ext Int. Ext. Int. Ext.
IV-1 Solids %            
IV-2 Solids nature            
IV-3 Flow rate            
IV-4 Cake Thickness            
IV-5 Washing            
IV-6 Washing Experience            
IV-7 Application Experience            
IV-8 Squeeze Pressure            
IV-9 Air Supply Pressure            
IV-10 Economics            
IV-11 Special Conditions            
IV-12 Special Conditions            
Overall            
Abbreviation Keys
  NA   Not Applicable
NRNot Requested
RRequested
AApplicable
PPreferred

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