Hi all,
I want to simulate a large capacity (>40 m3/h permeate production) seawater desalination reverse osmosis plant. So, I need to find the perfect RO membranes configuration. My main question is the optimum number of elements at each pressure vessel for a given seawater feed. According to the design guidelines, for large scale plant 6-8 elements per pressure vessel are adopted. However, I need to know the procedure to find the optimum element number (6, 7 or 8 ?!). Would you please let me know the specifying procedure of the optimum number of each pressure vessel elements?
Thanks in advance
There are Several designs of RO plants, one of the best practices to calculate a specific design is knowing the feed water quality of your system (if you'll treat wastewater, sea water or sour water) and the amount of water that you wanna produce.
To calculate the number of elements that you will use, exist some softwares available in internet for free like ROSA or NanoH2O and then you input the water quality and amount of permeate water that you wanna produce.
best regards.
Dear Hossein
To answer your question, 6 elements per pressure vessel is the standard number while PV's which take 7 to 8 elements are special ones. Therefore, i recommend that you design your plant based on 6 elements per PV.
Otherwise, if you dont care about that, my advice is to keep playing with the plant configuration including the number of elements per PV to balance the hydraulic load on each element as much as possible (which also minimizes the specific energy consumption) and of course getting a design operating within the recommended limit by the manufacturer for the expected feed water temperatures and the guideline flow factor
The optimization will greatly depend on the selected recovery rate and feed flow rate
Good luck
To determine the number of elements to rely on within a pressure vessel requires calculation of the critical (minimum) brine velocity within the feed channel and permeate recovery is of course related to this value. For most systems 6-element vessels are standard, but vessels with up to 8 elements are available for use in large installations - for example Tampa Bay Water relies on 8 element PVs; however, if left to redesign this plant I believe element vessels would have been used. The main advantage of 7 or 8 elements per pressure vessel would be cost (since fewer pressure vessels needed), but a disadvantage would be a greater chance of scaling.
Most manufacturer software can help you determine this ...suggested steps to follow:
Step 1: Consider Feed Source, Feed Quality, Feed / Product Flow, and Required Product Quality
The membrane system design depends on the available feed water and the application. The system design information and the feed water analysis should, therefore, be collected first.
+Step 2: Select the Flow Configuration and Number of Passes
The standard flow configuration for a membrane system is plug flow, where the feed volume is passed once through the system.
+Step 3: Select the Membrane and Element Type.
Elements are selected according to feed water salinity, feed water fouling tendency, required rejection and energy requirements.
+Step 4: Select Average Membrane Flux (Design Flux)
RO systems are usually designed for a specific permeate flow rate (GPD or l/h) and a specific system recovery. These numbers, and the specific feed water source, are the information required to estimate the number of membrane elements, pressure vessels and stages as flows: Select the design flux (l/m2h) based on pilot data, experience or the typical design fluxes published by manufacturers.
+Step 5: Calculate the Number of Elements Needed
Total number of elements needed = (design permeate flow rate) / (design flux) / (active membrane surface area of selected element)
+Step 6: Calculate the Number of Pressure Vessels Needed
A) Total number of pressure vessels needed = (total number of elements) / (number of elements in pressure vessel)
B) Round up to the nearest integer.
C) For large systems, 6-element vessels are standard, but vessels with up to 8 elements are available. For larger systems longer PVs are typically evaluated.
+Step 7: Select the Array (One pass for your design ??)
The number of passes defines how many pressure vessels you will need - each pass consists of a certain number of pressure vessels in parallel. The number of passes depends on your permeate boron, bromide and chloride targets (typically). Also, the number of element positions is linked with the system recovery .... not knowing your design you most likely are looking at 6 or 7 element PVs....the problem with 8 element PVs is the loss of concentrate minimum brine velocity so your last element contributes nothing to permeate flow.
+Step 8: Balance the Permeate Flow Rate
I missed adding a value towards the end of the second sentence....that sentence should have read: For most systems 6-element vessels are standard, but vessels with up to 8 elements are available for use in large installations - for example Tampa Bay Water relies on 8 element PVs; however, if left to redesign this plant I believe 7 element vessels would have been used..
Eight, most RO commercial plants use 8 element/vessel. In case of two stages then 8 in the first and 7 imn the second
Dear All,
Thank you.
I select 7 element for each pressure vessel because modeling of each pressure vessel shows that the recovery and rejection of final element in 8 element pressure vessel is very low and dosnt work efficient.
In SWRO plant 8 element per vessel will be more economic since the value of CFV (Cross Flow Velocity) is maximum at that arrangement.
in full scale RO plant for SW desalination there are 8 elements of 8 inch in each PV
in full scale RO plant for SW desalination there are 8 elements of 8 inch in each PV
I think the best suggestion is the choice of 6 elements for each PV and increasing the number of stages.
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