Reverse Osmosis Clean in Place Handbook

Design tool for Reverse Osmosis and Nanofiltration CIP systems

Introduction

Reverse Osmosis projection and design tools are limited to the filtration equipment itself, leaving the auxiliary (but important) CIP devices out off the calculations. This tool aims to facilitate and standardize the cleaning system design for virtually any type of spiral membrane, manufacturer agnostic. The computation model will make sure the end user designed an appropriate pump and the adequate piping and cartridge filters for the task.

The diagram for this model is a P&ID, it lists the recommended instruments, piping routing, valves and devices for a very efficient Clean in Place system. It was carefully conceived to prevent common mistakes that cause problems for the operators and, sometimes, damage permanently the expensive membranes. The optional features like the concentrate-to-drain line and the flush intake can save minutes to hours of cleaning.

Design tool

Web Based Excel Interface

Diagram

Diagram

Quick calculation instructions

The settings listed below are the minimum required for an adequate design, the defaults for the rest of the parameters should be fine for most of the designs.

  1. System inputs
    • Set the number of pressure vessels that will be cleaned in parallel simultaneously (if you plan to clean two stages at once, adjust the volume safety factor to 1.5)
    • Set the amount of membranes per vessel.
    • Set the makeup (dilution) water temperature.
    • Set the maximum heating time for the cleaning solution.
    • Set the distance from the CIP system to the membrane bank, this will help to estimate the piping length.
  2. Membrane and vessel properties inputs
    • Select the appropriate membrane type from the membranes database and enter the index (number) in the MIndex field. If you prefer you can enter all properties manually, in this case, replace the index number by the word 'false'.
    • The remaining fields will be filled automatically by the database data if you entered an index.
  3. Cartridge filter properties inputs
    • Select the appropriate cartridge filter type from the cartridge filters database and enter the index (number) in the CFIndex field. If you prefer you can enter all properties manually, in this case, replace the index number by the word 'false'. Note: The default index is for the most common type of filter.
    • The remaining fields will be filled automatically by the database data if you entered an index.
  4. Acid cleaning solution inputs
    • Specify the index (number) of the solution in the AcIndex field according to the acid cleaning solutions database. If you want to fill the solution properties manually, type the word false in the index field. Note: The default index is for the most common type of solution.
    • The remaining fields will be filled automatically by the database data if you entered an index.
  5. Alkaline cleaning solution inputs
    • Specify the index (number) of the solution in the AkIndex field according to the alkaline cleaning solutions database. If you want to fill the solution properties manually, type the word false in the index field. Note: The default index is for the most common type of solution.
    • The remaining fields will be filled automatically by the database data if you entered an index.

Calculation model description

    1. Vessel volumes are calculated using the external membrane dimensions without considering the space occupied by the membrane internals.This slightly overestimates the water volumes and account for the pressure vessel volumes in the feed/concentrate chambers.
    2. The cleaning flows are based on the recommendations of the membrane manufacturers for each type of membrane [1] and [2]
    3. The permeate flow during cleaning is set by default to 20% of the feed flow (can be changed later). This is a conservative approach and will give comfortable pipe sizes to prevent any problems with back-pressure or excessive head loss in this line.
    4. The bypass flow is determined by subtracting the slow circulation flow from the maximum feed flow [1] and [2]
    5. Piping diameters are defined based on the water speed setting and the internal piping database library, based on the ANSI B36.10 and B36.19 standards [6]
    6. Piping pressure drop is calculated based on the Darcy–Weisbach formula with a roughness of 0.0015mm (Stainless Steel or Plastic Pipes) and using the makeup water temperature for the viscosity.
    7. Pipe fittings pressure drop is calculated using the Darby 3K [7] method and assume:
      • 3 gate valves
      • 1 swing check valve
      • 1 diaphragm valve
      • 1 tee (for the bypass)
      • 8 long radius 90° elbows (4 for the feed and 4 for the concentrate)
    8. Cartridge pressure dirty drop is based on the manufacturer's data for 20°C and is set in the database [3],[4] and [5]. Note that some filters can handle much higher differential pressures. The designer can manually adjust this value by setting the CFIndex variable to 'false'.
    9. Vessel/membranes pressure drop are assumed to be the maximum allowed by the membranes according to the manufacturer manuals. For pressure vessels with less than 4 elements, the maximum individual membrane pressure drop is assumed [1] and [2].
    10. CIP volume is based on the the pressure vessels volume + cartridge filters volume + feed line volume + concentrate line volume.
    11. The heater design is based on the maximum temperature for the acid or alkaline solutions and the user defined time to heat the solution.
    12. The energy consumption for each cleaning solution always assume the maximum power and reduces the time according to the solution temperature.
    13. If the cleaning temperature is lower than the makeup water temperature, the heater power will be assumed as cooling power (adjust the efficiency because cooling is less efficient)
    14. Volume per flushing is the total CIP volume. The model allows using decimal numbers for the flushes/clean setting so the user can reduce the required water volume (set 0.5 for 50% reduction).
    15. Energy consumption per cleaning solution assumes the pump will run at 100% and the bypass will be used to moderate the flow in lower circulation velocities. The consumption will be overestimated if the pump is using a variable frequency drive (VFD).
    16. Cleaning time considers the circulation, soaking and heating times.

Known limitations and important notes

  • Cleaning pHs and concentrations are based on stoichiometry. When using demineralized makeup water with pH close to 7 (recommended), the quantities should be enough to reach the desired pH. If the water has a higher acidity or alkalinity, the amounts might not be enough. It is advised to add some safety factors for the quantities when ordering the chemicals.
  • Cleaning times do not consider the time required to prepare the solution, fill the CIP tank or position the valves. Increase the soaking time to account for that activities.
  • Chemical concentrations (mg/L or ppm) are always based in 100% pure substances, not the stock concentrations.
  • Pressure drop in the cartridge filters assume the elements are dirty. Consider a valve after the pump to limit the initial flow with clean cartridges.
  • Pressure switch at the membrane vessels entrance must be set to the maximum allowable differential pressure for the vessel (PVdp variable)
  • Solution mixing can be performed using the bypass line.
  • Tank should have round or conical bottom for full drainage.
  • Flow meter is not necessary if the pump is correctly sized. Differential pressures should be the main process parameters to monitor.
  • pH meters should not be installed inline because they will be damaged after months dry (interval between CIPs). Use hand-held devices instead.
  • Permeate, bypass and concentrate return lines should be submerged to reduce foaming.

Literature references

[1] FilmTec FT30 cleaning procedures, Form No 45-D01696-en Rev 4, April 2020.
[2] Hydranautics Foulants and Cleaning procedures for composite polyamide RO membrane elements, TSB107.24, December 2014.
[3] Technical sheets for Pentair Europe Polyval (MKT-TS-064-EN-D), Polydepth (MKT-TS-052-EN-C), P-Series (MKT-TS-049-EN-D), DGD (MKT-TS-132-EN-A), SWP (MKT-TS-099-EN-B) and Wound Depth (MKT-TS-099-EN-D) cartridge filters.
[4] Technical sheets for Parker DuraBond cartridges, 2017
[5] Parker Industrial Process Filtration Catalog, 2017
[6] American National Standards Institute - ANSI B36.10M and B36.19M.
[7] Ron Darby, Chemical Engineering Fluid Mechanics, 2nd edition, 2001.