| f Reverse Osmosis (RO) in Food and Beverage Plants | | | | PV = nRT = (g/m)RT or |
| In a Food or Beverage plant, Reverse Osmosis (RO) | | | | P = (g/m)RT/V, where |
| is often used for plant service water and boiler water | | | | R = universal gas constant, 0.0821 Litre•atm |
| pre-treatment. More recently, RO is finding increasing | | | | (mol•K) |
| use in the processing of food and beverage | | | | T = absolute temperature, K (degrees Kelvin)g = |
| products, for example concentrating fruit juices. | | | | solute weight, grams |
| The end use of the permeate (or reject water) will | | | | V = volume of solution, Litresm = molecular weight |
| generally dictate the design of the Reverse Osmosis | | | | of solute, if non-ionicn = moles |
| (RO) system. Since most boilers in a food plant tend | | | | P = osmotic pressure, atmospheres |
| to require low hardness and solids feed water, RO | | | | Using this equation, and applying it to an aqueous |
| systems in this application are invariably followed by | | | | solution of 1,000 mg/L. of dissolved ionic solids, as |
| some type of further purification treatment, such as | | | | CaCO3, we arrive at an osmotic pressure of 7.2 psi |
| softening (if low pressure boilers are present), or | | | | [50 kPa] at 77° F. |
| demineralization (if higher pressure boilers are | | | | In general terms, the osmotic pressure averages |
| present). Reverse Osmosis equipment, by itself, is | | | | about 1 psi [6.9 kPa] for every 100 mg/L. of |
| incapable of providing the boiler feed water quality | | | | dissolved solids. |
| demanded even by lower pressure boilers. | | | | By applying a pressure on the concentrated side of |
| If Reverse Osmosis (RO) water (either permeate or | | | | this membrane, we can cause this process to |
| reject) is used in other than boiler feed water | | | | reverse. Pure water molecules (and dissolved gas |
| applications, further purification of the fluid is generally | | | | molecules) can be forced to flow from the |
| not required. | | | | concentrated side to the dilute side. |
| If an RO system is directly involved in the processing | | | | This is the entire Reverse Osmosis or |
| of foods or beverages, RO performance (permeate | | | | “RO” process in a nutshell. Water |
| or reject quality and flow) must be maintained at | | | | purification occurs when water molecules are forced |
| expected levels. If issues occur with the RO | | | | to flow from a concentrated solution through a |
| equipment, such as fouling, then the plant | | | | semipermeable membrane to the dilute side. |
| products’ quality or quantity is directly | | | | To overcome the osmotic pressure, and force water |
| affected. This can have a drastic effect on plant | | | | molecules to reverse flow, one must apply a |
| profitability. | | | | pressure. The Net Driving Pressure needed is defined |
| Understanding How Reverse Osmosis Works | | | | as: |
| In order to understand how RO works, one must | | | | NDP = Feed Pressure + Permeate O. P. (usually |
| look into the physics of osmotic pressure and | | | | negligible) – Permeate Pressure – Feed |
| semipermeable membranes. | | | | O. P. |
| A semipermeable membrane allows the passage of | | | | O. P. = Osmotic Pressure |
| specific molecules through it. If a concentrated | | | | The flow through an RO membrane is proportional to |
| aqueous solution exists on one side of a | | | | the NDP. |
| semipermeable membrane, pure water molecules | | | | In order to obtain reasonable permeate flow rates, |
| tend to spontaneously diffuse from the more dilute | | | | and to minimize membrane fouling, the applied feed |
| side of the membrane to the more concentrated | | | | pressure must be very much greater than the |
| side. This is called Osmosis. | | | | calculated P. It is generally in the range of 200 |
| As water molecules continue to flow across the | | | | – 450 psi [1.4 – 3.2 MPa]. This high |
| membrane, the amount of water increases on the | | | | pressure requires specific design considerations of RO |
| concentrated side of the membrane, as does its | | | | trains, and elements. |
| pressure, called the head pressure. Once this head | | | | An in-depth analysis of Reverse Osmosis design for |
| pressure increases to a given level such that further | | | | the Food and Beverage Industry, including tables and |
| water flow can no longer occur across the | | | | drawings can be downloaded in the free Layne |
| membrane, the system is said to be in equilibrium. | | | | Christensen white paper titled REVERSE OSMOSIS |
| The pressure at this point is called the Osmotic | | | | DESIGN FOR THE FOOD AND BEVERAGE |
| Pressure. It is proportional to the dissolved solids | | | | INDUSTRY- WHAT YOU NEED TO KNOW. |
| concentration in the more concentrated solution. | | | | As a leader in the development of reverse osmosis |
| According to the Van’t Hoff equation for the | | | | (RO) systems, Layne Christensen Company has the |
| calculation of osmotic pressure | | | | technical expertise to design and build reverse |
| (symbol P)... | | | | osmosis systems for all of your plant water needs. |