A-value
The A-value is a standard way of describing the flux of water through a membrane.
It is also a most peculiar and strange unit, making it possible to make a comparison between membranes from various producers and between various types of membranes.
The program can be used to calculate:
1. Flux in an element when A-value, element area and operating pressure is known.
2. A-value when flux, element area and operating pressure is known.
Diafiltration
Calculation of flows when concentrations are known and calculation of concentrations when flows are known
Diafiltration is a method used to wash out low molecular solutes (LMWC) while
retaining the high molecular solutes (HMWC). In this way it is possible to purify the HMWC
more than straight forward concentration can do. The process is very simple. The best
and simplest way to diafilter is to run a batch process and add water in the same rate as
permeate is taken out. This is also referred to as constant volume diafiltration.
There are several ways to make the diafiltration inefficient. Just follow the above
simple rule and you can hardly go wrong.
Batch diafiltration is in reality a rare thing. It is much more common to diafilter in
multistage recirculation systems. That makes it much more complicated to do. The rule is still:
add the same (or a little less water) then the permeate volume taken out in a loop.
Diafiltration in a multistage recirculation system is less efficient than batch. Nevertheless, it is in
many cases the only way to run an industrial diafiltration process.
You can make two types of calculation.
Calculate the volume of water needed for diafiltration as function of the other
variables, primarily feed concentration and final concentration.
Calculate the final concentration as function of the other variables, primarily start
concentration and diafiltration volume.
Depending on how the process is operated you need a varying number of parameters to be known.
1. Volume to be treated per time unit (Often liter per hour)
2. Feed concentration of LMWC (unit of no importance for the calculation)
3. Demand to final concentration of LMWC (unit of no importance for the calculation)
4. Permeability (or rejection) of the LMWC. In real life the permeability can vary as
function of concentration.
In this program it is assumed that permeability is aconstant.
5. Number of recirculation stages in a multistage system.
The program can also be used to find the ratio between the volume treated and the
diafiltration volume. That number will give a hind about the economic consequences and
will also tell whether diafiltration is deemed realistic.
Osmotic pressure
Osmotic pressure is estimated based on MW and the number of ions.
Example 1 : Glucose. MW = 180. Number of "ions" =1. (It does not dissociate).
Example 2 : NaCl. MW 23 + 35 = 58. Number of ions is 2.
Example 3 : Na2SO4. MW = 2*23+96 = 142. Number of ions = 3.
Example 4 : Na3PO4. MW = 3*23+ 96 = 165. Number of ions = 4.
Notice that the calculation does not take the ionic strength in account. That can give
a result which is significantly different (higher) than the real value. From that point of view
the program is giving a conservative value.
Apparent permeability
Papp is an abbreviation for "Apparent Permeability". Multiply the feed TDS with Papp to
calculate the concentration in the permeate.
This nomenclature comes from the days before the electronic calculator.
The apparent permeability is a function of the membrane salt
permeability, the flux and the flow conditions. The math behind it is somewhat complicated
- unless you like so solve differential equations.
Today (2012) the EXE program has been discontinued and "only" the spreadsheet is available.
Pressure drop in spiral wound elemetns
Pressure drop in a spiral wound element seems to be one of the best kept secrets in
the world. This program estimate the flow drop when several parameters are known.
- Product spacer height
- Product spacer type
- Bypass around the element
- Pressure drop
Viscosity is a difficult issue. There are so many opinions about the effect of viscosity, that
at this moment the real effect can not be calculated with any accuracy.
Definitions Handbook
There are a lot of buzzwords in the membrane industry. This is an attempt to
describe most of the words used now and in the past.
Pilot testing Handbook
Have you ever wondered how to make a good pilot test? Here are some hints, which
should enable you to avoid the most obvious errors.
Disinfection
All membrane systems needs disinfection.
Some several times per day, some a few times in the element life time.
In this paper you will find a collection of possibilities discussed.
Remember: disinfection means killing microorganisms. Consequently it is to be expected
that the products used are toxic and aggressive and sometimes decidedly bad for humans.
Pretreatment
It is always necessary to pretreat a product before it enters a spiral wound membrane system.
There is long debate about how to do it and what is needed.
In this paper there are a number of controversial (?) suggestions.
Viscosity of water
Water is believed to have the viscosity 1 cP (= mPaS). That is far from true.
Close to freezing point viscosity is close to 2.
Close to boiling point viscosity is close to 0,2.
This has a big effect on the layout of a plant.
It is not commonly known that the reciprocal value of viscosity and water flux is proportional.
TDS and degree Brix
Degree Brix is a difinition used in the sugar industry. 1 degree Brix equals 1 W/W% sucrose at 25 degC. Degree Brix is sometimes used as synonymous with TDS = Total Dissolved Solds. That is rarely true.
The paper discussed briefly what degree Brix is and its relation to the refractive index of a liquid.
Evaporator condensate and RO permeate
Recovery of water is done increasingly for many good reasons.
Be aware that evaporator condensate and RO permeate contains enough BOD to make it quite difficult to keep a system in operation, unless you follow a few simple rules. A standard water plant is doomed to fail.
Oxidizers and extreme pH
Oxidizers and extreme pH can damage thin film membranes.
This paper presents an overview of the oxidizers commonly seen. The list is longer than you may expect.
An example of the effect of extreme pH is presented.
Membrane Filtration Handbook. 1995
The book has the subtitle Practical Tips and Hints. The book is a collection of misc. information about how to build systems and how to automate systems. There is only a little membrane theory.
The book has for yeas been available from GE. Since it has disappeared from their home page it is now available here.
The book was published 1995 and a new version is available. See front page.
Wagner units.
The temperature and pressure limits written on official membrane specification sheets can be misleading. The specifications for spiral wound elements treating water were established early 1980 and they have not been changed since then. Today (after 2000) membrane elements are operated under conditions which were impossible earlier. The Wagner units is used to take pressure AND temperature into consideration and in this way to better define what the real operating limits are.
Curve fitting.
Curve fitting (regression analysis) is often done on data from membraen filtration systems. The hyperbolic curve fitting is especially useful, since many natural data fits very well. It is non-linear and it is very rarely mentioned. The formulae comes from the magazin BYTE and was publised more than 40 years ago. The formulae are a bit weird.
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