Process Description
Overview of Reverse Osmosis Process
Simtronics’ Reverse Osmosis (RO) simulator consists of the following equipment:
- Sea Water Filters
- Sea Water Pumps
- Reverse Osmosis Uni
- Permeate Tank
- Permeate Pumps
- Limestone Filters
- Product Filters
- Cooling Water System
Sea Water Filters, F-101A/B, filter water from the ocean to remove suspended solids from the feed water prior to entering the unit. Filtered feed water from F-101A/B is pumped to high pressure by Sea Water Pumps, P-101A/B, where it is fed to the Reverse Osmosis (RO) unit, X-101.
The RO unit consists of 3 racks of 6 permeable membrane units. Each rack of permeable membranes operate in parallel. The combination of high membrane cross-sectional area combined with a high differential pressure across the membrane results in water diffusion/transport through the membrane, creating purified water. Most of the non-water molecules remain on the high pressure side of the membrane where excess water and impurities (brine) are collected and routed for disposal.
The purified water (called permeate) from the Reverse Osmosis unit collects in Permeate Tank, T-101. Permeate is pumped from T-101 to Limestone Filters, F201A/B, by Permeate Pumps, P-102A/B. The Limestone Filters contain limestone (calcium carbonate with small amounts of other natural minerals) in fixed beds. The permeate flows through the bed and dissolves some of the limestone, adding minerals to the water and reducing the pH. After combining with the balance of permeate bypassing the Limestone Filters, it is suitable for potable (drinkable) water.
Prior to distribution, the water from the Limestone Filter is filtered through Cartridge Filter, F-102, to remove any solid particles and in Activated Carbon Filter, F-103, to remove any trace organic compounds that may have entered the process. Potable water is sent to users at the battery limits.
The RO simulator includes a Cooling Water System which provides circulating cooling water for protection of the Sea Water Pumps, P-101A/B, and cooling equipment outside of the Reverse Osmosis Unit. Warm cooling water is collected from users via the Cooling Water Return Header into two elevated Expansion Tanks TK-603A/B
When potable water is used to fill the Expansion Tanks, corrosion inhibitor must be added from the Corrosion Inhibitor Tanks, TK-601A/B. Small motor-driven Corrosion Inhibitor Injection Pumps, P-606A/B, transfer corrosion inhibitor from the Corrosion Inhibitor Tanks to the Expansion Tanks. Four motor-driven, centrifugal Cooling Water Pumps, P-602A/B/C/D, pump cooling water from the Return Header, through the Cooling Water Coolers HX-601A/B/C/D and into the two Cooling Water Supply Headers (A & B).
Sea Water Filters
Sea Water Filters, F-101A/B
Sea water from the ocean is filtered in Sea Water Filters F-101A/B. These are cartridge filters designed to remove any suspended solids from the feed water. The sea water supply to the unit has had debris and large particles removed prior to entering the unit. Remaining suspended solids in the sea water can easily plug the permeable membranes in the Reverse Osmosis Unit. Either cartridge filter can be taken out of service for maintenance.
Sea Water Pumps
Sea Water Pumps, P-101A/B
Filtered feed water from F-101A/B is pumped to high pressure by sea water pumps P101A/B. Normally only one pump is in operation. The high pressure developed by the pumps generates a lot of force on the pump bearings which can result in excessive bearing temperatures. Therefore, the bearings are cooled with circulating cooling water to preserve their mechanical integrity.
The design flow of sea water to the unit is 200 GPM at a temperature of 68 DEG F. The salinity of sea water is 3.5 weight %.
Reverse Osmosis Unit
Reverse Osmosis Unit, X-101
The high pressure sea water is fed to the Reverse Osmosis (RO) unit X-101. The RO unit consists of 3 racks of 6 permeable membrane units. Each rack of permeable membranes operate in parallel. Each membrane unit consists of 20-foot long spiral wound membrane sheets within an 8-inch diameter housing. The spirally-wound configuration permits a large cross-sectional area of the membrane sheets to be installed in a relatively small volume.
The membrane sheets are specially fitted on each end of the housing to segregate the feed side from the permeate (purified water) side. A feed manifold routes the sea water to the inlet sides of the membrane units and two outlet manifolds collect the permeate and the brine reject.
A high differential pressure across the RO membrane results in a large osmotic pressure differential of water across the membrane. The membrane is fabricated with material that will mainly permit smaller molecules to diffuse through it. In sea water, the smallest molecule is water. The combination of high membrane cross-sectional area combined with a high differential pressure across the membrane results in commercially economic rates of water diffusion/transport through the membrane. Most of the non-water molecules remain on the high pressure side of the membrane, resulting in purification of the sea water.
By design, a large fraction of the feed water flow does not permeate the membrane to keep the impurity concentrations fairly low at the outlet end of the high pressure side of the membrane. Otherwise, high concentrations of impurities would increase their osmotic pressure and force their way through the membrane. The excess water and impurities (brine) are collected and routed to disposal facilities.
The Reverse Osmosis Unit normally operates at 1,038 PSIG on the sea water (brine) side and at 20 PSIG on the permeate side. The unit normally produces 80 GPM of permeate.
Permeate Tanks and Pumps
Permeate Tank, T-101
The purified water (permeate) from the Reverse Osmosis unit, X-101, is collected in the Permeate Tank, T-101. Permeate is highly corrosive to steel piping and equipment over time because it lacks the minerals naturally found in ordinary surface and well waters as a result of passing through the RO Unit. As a result, its pH is somewhat acidic (in the range of 5.0 to 7.0).
Permeate Pumps, P-102A/B
The permeate is pumped from T-101 to the Limestone Filters by Permeate Pumps, P-102A/B. Normally, only one pump is in service. The design flow rate of permeate taken from the tank is the same as the permeate flow rate produced by the RO Unit (80 GPM)
Limestone Filters
Limestone Filters, F-201A/B
Normally, a large portion (63%) of the permeate from the Permeate Pumps flows to one of the Limestone Filters, F-201A/B. These filters contain limestone (mainly calcium carbonate with smaller amounts of other natural minerals) in fixed beds. The permeate flows downward through the bed and dissolves some of the limestone in the bed of the filter. This results in neutralization of the acidic permeate and adds minerals to the
water. The concentration of the calcium oxide in water is commonly referred to as hardness’. The hardness of the water leaving the Limestone Filter is 140 milligrams per litre (mg/l). After combining with the balance of permeate bypassing the limestone filters the final hardness of the permeate is 90 mg/l which is suitable for potable (drinkable) water standards.
Product Filters
Cartridge Filter, F-102 and Activated Carbon Filter, F-103
Prior to distribution, the water from the Limestone Filter section is further filtered in Cartridge Filter, F-102 to remove any solid particles and in Activated Carbon Filter, F-103 to remove any trace organic compounds that may have entered the process.
Either product filter can be bypassed for maintenance. Potable water is sent to users at the battery limits. A line is provided from the outlet of F-103 to fill the Expansion Tanks TK-603A/B as needed during operation. Normally there is no flow of potable water to the Expansion Tanks.
Cooling Water System
Cooling Water System
The Reverse Osmosis simulator also includes a Cooling Water System which provides circulating cooling water for protection of the Sea Water Pumps, P-101A/B as well as for cooling equipment outside of the Reverse Osmosis Unit.
Expansion Tanks, TK-603A/B
Warm cooling water is collected from users via the Cooling Water Return Header. Two elevated Expansion Tanks, TK-603A/B are connected to the header to allow for expansion of the water in the system as the density of the water changes with temperature. Water is denser at lower temperatures, so for a given fixed mass of water in the system, the volume of water will be lower and the expansion tanks will transfer some of its water to the Return Header by gravity. Conversely, when the cooling water becomes hotter it will occupy more volume and water from the Return Header will be pushed up into the tanks as the water expands.
Corrosion Inhibitor Tanks, TK-601A/B
In case of leaks or maintenance purges of cooling water from the system, potable water from the Activated Carbon Filter can be used to fill the Expansion Tanks back to their normal operating levels. When this is needed, corrosion inhibitor should be added from the Corrosion Inhibitor Tanks, TK-601A/B. Small motor-driven Corrosion Inhibitor Injection Pumps, P-606A/B are used to transfer the corrosion inhibitor from the Corrosion Inhibitor Tanks to the Expansion Tanks.
Cooling Water Pumps, P-602A/B/C/D
Four motor-driven, centrifugal Cooling Water Pumps P-602A/B/C/D pump cooling water from the Return Header, through the Cooling Water Coolers HX-601A/B/C/D and into the two Cooling Water Supply Headers (A & B).
Normally, only pumps P-601A and P-601B are in operation. The flow of each pump is normally 35 GPM. The returned, warm cooling water is cooled with sea water from a utility supply header at battery limit. The warmed sea water is returned to the ocean via a discharge header at battery limit.
At design, the cooling water supply temperature is 74 DEG F. The cooling water return temperature is 120 DEG F.
Cooled water from Cooling Water Supply Header A is routed to Sea Water Pump P-101A. Cooled water from Cooling Water Supply Header B is routed to Sea Water Pump P-101B and returned by a common line to the Cooling Water Return Header.
Cooling water is also supplied to equipment external to the Reverse Osmosis Unit (not shown on Schematic #9) and returned to the Cooling Water Return Header.
Instrumentation
Feed and RO Unit
The temperature of the brackish feed water is indicated on TI-101, the pressure is indicated on PI-101 and the flow rate is indicated on FI-101. AI-101 indicates the salinity of the brackish water in weight percent. AI-104 indicates the conductivity of the brackish water in S/m units (siemens per meter). The conductivity depends on the salt concentration of the brackish water. AI-102 indicates the turbidity (cloudiness) of the water entering the Brackish Water Pumps and is measured in NTU (Nephelometric Turbidity Units). An NTU value less than 5 represents a very clear fluid, which is normally produced by the Brackish Water Filters. Visually discernible cloudiness starts between an NTU of 10 to 20. Values above 100 are obviously cloudy. AI-103 is the pH of the brackish water. Values between 6.5 and 8.5 are acceptable for the Reverse Osmosis Unit.
PDI-101A and PDI-101B indicate the pressure drop across the brackish water filters F-101A/B. HIC-101A and HIC-101C control the block valves of the brackish water filters F-101A/B. HIC-101B and HIC-101D control the bypass valves of the brackish water filters F-101A/B.
The status of the brackish water pumps P-101A/B is indicated on switches HS-101A/B, same switches can be used to start/stop the pumps. The discharge pressure of brackish water pumps P-101A/B is indicated on PI-102.
The brackish water flow from battery limits to the Reverse Osmosis Unit X-101 is controlled by FIC-102. TI-102 indicates the brackish water temperature at the entry of the Reverse Osmosis Unit X-101 while PDI-110 indicates the pressure drop across the Reverse Osmosis Unit X-101.
The flow rate of the reject brine is controlled by PIC-111 and is indicated on FI-111.
The flow rate of the pressure of the purified water are indicated on FI-110 and PI-110 respectively while HIC-110 controls the block valve at the entry of the purified tank T-101. Normally HIC-110 is wide open to keep a low product pressure and high differential pressure across the unit. HIC-110 can be used to reduce the purified water production rate at startup.
Purified Water and Product Filters
The water level in the purified tank T-101 is indicated on LI-120, LAH-120 and LAL-120. LAH-120 and LAL-120 are also connected to interlocks which are activated in case of high or low level, respectively, in T-101.
AI-120 indicates the conductivity of the purified water in S/m units.
The status of the purified water pumps P-102A/B is indicated on switches HS-102A/B, same switches can be used to start/stop the pumps. The discharge pressure of purified water pumps P-102A/B is indicated on PI-120.
PDI-120 and PDI-121 indicate the pressure drop across the cartridge filter F-102 and the activated carbon filter F-103, respectively. HIC-120A controls the block valve of the cartridge filter F-102 while HIC-120C controls the block valve of the activated carbon filter F-103. HIC-120B controls the bypass valve of the cartridge filter F-102 and HIC-120D controls the bypass valve of the activated carbon filter F-103.
FIC-120 controls the flow rate of the produced purified water to the users. The temperature of the produced purified water is indicated on TI-120.
Interlock I-101
Interlock I-101 protects the purified water tank T-101 from overflow. I-101 is activated when the level of LAH-120 is more than 90%. I-101 will remain active anytime LAH-120 is more than 90% and will stop the brackish water pumps P-101A/B. I-101 will automatically reset when LAH-120 indicates lower than 90%. However, the brackish water pump P-101A/B must be manually started after the interlock resets.
Interlock I-102
Interlock I-102 protects the purified water pumps P-102A/B from cavitation. I-102 is activated when the level of LAL-120 is less than 10%. I-102 will remain active anytime LAL-120 is less than 10% and will stop the purified water pumps P-102A/B. I-102 will automatically reset when LAL-120 indicates higher than 10%. However, the purified water pump P-102A/B must be manually started after the interlock resets.