In areas with little or only contaminated ground and surface water, the desalination of sea water to obtain drinking water plays a major role. To ensure quality and process safety, important parameters such as pressure and conductivity must be recorded, monitored and controlled continuously.
The authors: Dr. Öznur Brandt Product Manager for Analytical Measurement Technology, Jumo Sarah Weinmeister Product Manager Level and Pressure, Jumo
Water is the most important life-sustaining resource and needs to be available in sufficient quantity and quality for every human being. Water is also an important raw and aux-iliary material for the chemical and pharmaceutical industries, where process water is mostly required in drinking water quality or better. Especially in regions where potable water is only available in limited quantities, it is important to develop sea water as an impor-tant source of drinking water. Drinking and process water can be obtained from sea water by reducing its salinity. Various thermal or membrane processes are used for desalination: membrane distillation, reverse osmosis, evaporation and electrodialysis. The most widely used method today is reverse osmosis. Desalination is achieved using a special form of membrane filtration. Sea water is forced through a semipermeable membrane under high pressure. Salt is retained and only water is allowed to pass through. The result of this process is fresh water. To prevent the sensitive membrane from being clogged or destroyed by particles or layers of algae or bacteria, the sea water must be pretreated prior to reverse osmosis. Ensuring quality in reverse osmosis plants calls for a number of measures including continuous monitoring of the pressure and conductivity parameters. Jumo offers a wide assortment of process sensors for analysis and pressure measurement technology as well as compatible transmitters and controllers.
Pressure measurement at high salinity
In the reverse osmosis process, the sea water being treated is forced through a synthetic membrane at more than twice the osmotic pressure (in other words, at 60 to 80 bar) using a high-pressure pump. Apart from a few minor exceptions, only water molecules are small enough to pass through the fine pores while particles, microorganisms and assorted other contaminants are removed with a partial flow. The sea water used for reverse osmosis contains salts in concentrations of up to 35 g/l. When the salinity is this high, conventional pressure transmitters are prone to measurement errors or sensor failure due to deposits and corrosion. The Jumo Midas C18 SW transmitter meets these specific demands on account of its case, the pressure connection made of sturdy, corrosion-proof titanium and the chemically impervious measuring cell without oil filling. The crucial element here is a hysteresis-free, piezoresistive, ceramics based pressure sensor featuring thick-film technology. In addition to excellent long-term stability, it provides overload resistance three times the full scale value. The pressure transmitter can record relative pressures from 1.6 to 100 bar and export them as standard signals with 4 to 20 mA (two wires) or 0 to 10 VDC (three wires). The high system availability required for reverse osmosis and the resistance of the selected materials are achieved with a 100 % final inspection of each individual transmitter using a fully automated measuring and calibrating system.
Inductive conductivity measurement
The Jumo ecoLine Ci inductive conductivity sensor is ideal for conductivity measurements of sea water. Compared to the conductive 2-pole or 4-pole measuring method, this inductive sensor is virtually maintenance-free. Deposits, grease or oil film on the surface of the sensor have practically no effect on its measuring accuracy. The built-in Pt1000 temperature sensor records the process temperature while ensuring fast and precise temperature compensation – something that is particularly important for measuring conductivity. It can either be free-standing in a stainless steel pocket (ultra-fast response) or horizontal and completely enclosed inside the plastic body. Internal installation is advisable if the stainless steel pocket is vulnerable to chemical corrosion from the process media (e. g. sea water). The standard material used for the sensor body is polypropylene (PP), which is especially suitable in sea water. All materials that come into contact with the process medium are approved for food use, physiologically safe and FDA listed.
User-friendly operation
The Aquis 500 Ci transmitter/controller was explicitly designed for inductive conductivity sensors such as the ecoLine Ci. A membrane keypad with a plain text graphic display (and selectable interface language) or a special PC software program guarantees user-friendly operation. All parameters are shown in plain text to simplify configuring, support programming and make operating the device as intuitive and uncomplicated as possible. The unit is designed for on-site use with an illuminated LC graphic display. The modular layout makes it easy to adapt to the requirements of specific applications. Up to two analogue outputs are available. The start and end values of the measurement range are freely adjustable for the analogue actual value outputs. The response of these outputs to overrange or underrange measurements, alarms and calibration is freely configurable. P, PI, PD and PID structures can be freely programmed as control functions. A robust case protects the electronics and the electrical connections against aggressive environmental conditions (IP67). Panel mounting is also possible as an alternative.
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Four tasks combined in a single device
Liquid analysis
Jumo developed a modular, multichannel measuring instrument for liquid analysis which has been presented at Achema 2012. From pH and redox through electrolytic conductivity, ultrapure water resistance, temperature, disinfecting parameters (such as free chlorine, chlorine dioxide, ozone, hydrogen peroxide and peracetic acid) to flow rates, the Aquis touch S provides a central platform for displaying and processing the corresponding sensor signals. Up to four analysis parameters can be connected directly while others can be linked up using standard signals. The device can measure and manage up to ten parameters simultaneously. Frequency inputs (counters) are available for flow rate measurements. The measured values can also be converted into various measurement parameters that can be used for extensive calculations as part of the integrated logic and math modules. The results of these calculations can in turn be displayed or taken as a starting point for switching tasks.
A 5.7” colour screen with a touch screen function is used to show all parameters as well as to operate and configure the device. In addition to razor-sharp representation of measured values and device states, up to four independent control circuits can be defined for each analysis parameter. The built-in paperless recorder is another highlight. Four analogue and three binary signals can be recorded simultaneously in two groups and output on the screen as a graph. Data is stored internally and protected in the event of a power supply failure. An integrated USB device interface enables a PC or laptop to be connected for parameterisation or data interchange using a PC setup program. The device can be integrated in a LAN via an Ethernet interface. Aquis touch S can also export alarms and other messages as text messages or e-mails. A Profibus-DP interface or up to two RS422/485 interfaces with the Modbus protocol are provided for incorporating the device into more complex controllers and process systems.
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