DM Plants | Demineralizers

Technology and Process Description

The demineralization process is accomplished by removing the ions present in water using ion- exchange resins in multiple stages. Deionization or demineralization process removes up to 99% of the dissolved salts (minerals) from the water.

The feed water is first passed through pre-treatment modules (generally multimedia filters and activated carbon filters) for the reduction of excess turbidity, suspended solids, bad colour, odour, residual chlorine and volatile organic compounds. This water is subsequently sent into a two bed Demineralization (DM) plant.

A DM plant comprises of two separate vessels internally connected in series. Two distinctly different types of resins are utilized for this process. The first vessel is filled with strongly acidic cation (SAC) exchange resins and the second one is filled with strongly basic anion (SBA) exchange resins.

When water containing cations (positively charged ions such as sodium (Na+), calcium (Ca2+), magnesium (Mg2+), potassium (K+) etc.) passes through the cation resin bed, the cation resin exchanges the positive ions for hydrogen ions (H+). To keep the water electrically balanced, for every monovalent cation (e.g. Na+), one hydrogen ion is exchanged and for every divalent cation (eg., Ca2+ or Mg2+), two hydrogen ions are exchanged.

Similarly, when the cation-free water (negatively charged ions such as chlorides (Cl-), sulphates (SO42-), carbonates (CO32-), Bicarbonates (HCO3-), Nitrate (NO3-) etc flows through the anion resin bed, the anion resin exchanges the negative ions for hydroxide ions (OH-).

Twin-Bed DM Plants generally produce water quality in the range of 50,000 ohms up to 200,000 ohms resistivity, which is between 8.5 and 2.0 ppm TDS as CaCO3.

Quality of the demineralized water can be analysed by measuring the ability of the water to conduct or resist electricity. Demineralized water is a poor conductor and therefore will have high resistivity and low conductivity.

Resistivity of treated water is measured in MΩ-cm (18.2 at 25°C indicating highly purified water) and Conductivity of treated water is measured in µS/cm (0.055 at 25°C indicating highly purified water). MΩ-cm and µS/cm are linked. One is the inverse of the other. It is therefore very simple to convert from micro siemens to megohms and vice-versa.

Regeneration Process

The resin beds need to be periodically regenerated to restore the exhausted resin back to its proper ionic form for service. The resins are considered exhausted when the active ion concentration reaches a low level and effluent has a high leakage of unexchanged ions. This is known as endpoint leakage and is usually indicated by a conductivity setpoint.

First step in the regeneration of DM plant is to backwash the plant by sending water in reverse direction from the bottom of the bed. This helps in dislodging dirt, suspended solids and debris present in the resin bed besides removing air pockets. While regular backwash is done in co-flow systems, it is done in counter-flow systems only when required.

After backwash, regeneration is achieved by passing a base and / or acid solution through the resin beds. This removes the ions (cations and anions respectively) trapped by the resins in order to allow a new cycle of demineralization process.

Cation resins are regenerated with hydrochloric acid (HCl). HCl during regeneration splits into H+ ions and Cl- ions. The H+ ions react with the cationic resins. Anion resins are regenerated with sodium hydroxide-caustic soda (NaOH). NaOH splits into Na+ and OH- ions. The OH- ions react with the anionic resins.

The DM plant can be regenerated in series (cation followed by anion) or in parallel (cation and anion simultaneously). After regeneration, the resins will have to be rinsed (slow-fast) to remove the excess regenerants.

Mixed Bed Demineralizer (MB) Plants

MBD plant intimately combines strongly acidic cation and strongly basic anion resins within a single pressure vessel. Mixed bed demineralizer plants are also called as polishing units and are used for applications that require a higher level of water quality.

Thorough mixture of cation and anion exchangers in a single column makes an MB plant very effective as post-treatment to either a twin-bed DM plant or Reverse Osmosis Plant. These units are used downstream of the DM unit to achieve very high purity levels in the treated water. Standard units are mounted on self-supporting, fully assembled skids with necessary piping / valves and include the necessary chemical regeneration systems, piping and valves.

Mixed bed resins are regenerated with acid and alkali but the resins must be separated before this can be done. Bed separation is done by backwashing. The backwashing carries the lighter anion resins to top and the cation resins sink to bottom. Thereafter, regenerant acid is sent upwards through the bottom distributor and caustic soda is sent through distributors above the resin bed. Once the regeneration is completed, the resins are rinsed and made ready for the next service cycle.

Where ultrapure water is required, two mixed beds are provided. The first mixed bed is known as primary mixed bed and the second as secondary mixed bed.

Mixed-bed demineralizers produce higher quality of water. They are often used to polish the output water from twin-bed demineralizers to produce water of up to 18 MΩ-cm.

Degasifiers

KORGEN also offers a full range of degassers as a post-fit to the cation unit of the DM plant. The cation exchangers convert alkalinity present in raw water to its equivalent acid. ie., carbonic acid. A degasser also known as de-carbonator is used in conjunction with ion exchange units for mechanical removal of CO2. Degassers are installed downstream of the cation units and thereby reduces the acidic load on the anion unit. This results in cost savings by way of reduced chemical consumption.

A degasser unit essentially consists of a tower (MSRL / FRP) with integral tank, an air blower and a water pump with necessary piping and valves to connect this system to the CA unit. The tower portion is packed with PVC Pall Rings. The air blower is connected by air ducting to the tower just above the tank portion. Additional pumps and blowers are offered as options depending on the applications.

Product Features and Technical Information of DM and MB Plants

Unless warranted under specific conditions, DM Plants and Demineralizers generally comprise the following major parts

• iVessels
• iiValves
• iiiResins (Cationic-Anionic)
• iv Regeneration Tanks
• vDegasser Tower

In addition, packaged DM Plants come with mounting skids, intermediary piping, valves and other monitoring instruments.

The basic design and dimensions of a DM Plant are highly formula-driven and should be primarily based on the following factors

• Total Dissolved Solids (TDS) / Ionic Load in the water
• Other related feed water characteristics
• Hourly Flow Rate (LPH) required
• Total water volume to be treated per day
• Desired OBR (Output between Regeneration)
• Site Conditions and end use applications

General Applications of Demineralized Water

There are primarily three ways in which ion-exchange technology can be applied in water treatment and purification. Firstly, cation-exchange resins alone can be employed to soften water by base exchange. Secondly, anion-exchange resins alone can be used for organic scavenging or nitrate removal in the water and thirdly, cation-exchange and anion-exchange resins can be combined and used to remove virtually all the ionic impurities present in the feed water. Demineralized Water is used in a variety of applications detailed here below.

Electrodeionization Systems (EDI)

Electrodeionization (EDI) is a time-tested electrically driven membrane-separation technology that provides high-efficiency demineralization used for industrial process water and applications that require ultrapure water.

This technology provides key advantages over traditional ion-exchange processes. Electrodeionization Systems remove ions from aqueous streams typically as a post-fit to Reverse Osmosis (RO) and DM Plants. Our high-quality deionization modules continually produce ultrapure water up to 18.2 mW/cm. EDI can be run continuously or intermittently.

EDI operates on the principle of migration of charged ions across a semi-permeable membrane which contains charged resin beds. The migration of membranes is initiated by applying current, by means of cathode and anode. Feed water flows in a thin sheet between ion exchange membranes through the stack where a DC electrical field is present. This generates the migration of ions across the oppositely charged membranes. The oppositely charged ions permeate through the charged membranes and collect in the concentrate chamber. A constant flow is maintained in the concentrate chamber to continuously evacuate the ions into the reject stream.

The ultrapure water from the pure water chamber flows out continuously. The reject water is not wasted and is recycled back to the pre-treatment sections.

EDI uses high-capacity ion exchange membranes which not only removes residual salts but also ionizable aqueous substances like carbon dioxide, silica, ammonia and boron.

EDI units are used in power generation, microelectronics, food and beverage production, chemical and pharmaceutical factories. EDI helps in eliminating expensive and hazardous chemicals used in ion-exchange resin regeneration. EDI increases the product recovery rates, reduces operating expenses and gives consistent treated water quality. With a smaller foot print, EDI helps companies meet their ISO 14000 requirements.

Correctly configured and properly operated EDI plants produce water which contains

less than 5 ppb silica and measures 16-18 megohm-cm

Advantages of EDI

• Superior quality of ultrapure water
• Eliminates expensive and hazardous regeneration chemicals
• Reduction in hazardous waste
• Stable and continuous performance
• Reduced energy and operating expenses
• Non-polluting, safe and reliable
• Modular designs requiring little space (plant footprint) and minimal supervision
• Reduces the facility size requirement
• Helps plants meet ISO 14000 requirements