The following are some issues surrounding the operation of portable exchange deionization (PEDI), beginning with an explanation of what PEDI is through plant logistics and maintenance considerations.
What is PEDI, and What are PEDI Plants?
PEDI (a.k.a. service deionization) is a service business whereby tanks known as two bed and/or mixed bed regenerated bottles of resin are placed in service to produce a desired water quality at a customer location and when exhausted are exchanged (replaced) with fresh regenerated bottles. The exhausted bottles are returned to the portable exchange plant for regeneration. Then the cycles of service flow, resin exhaustion and resin regeneration begins again. PEDI plants are processing plants that accept spent (exhausted) resins for reconditioning (regeneration) to allow repeated re-use. This type of service offers many advantages.
Customer benefits include
- Installation ease—Units consist of portable bottles connected by hoses or pre-assembled plastic plumbing assemblies. Tailored to space limitations.
- No capitol cost for equipment—rental or lease (customer owns nothing) payments are monthly.
- Little or no customer maintenance.
- Ease of expansion by increasing the size or number of bottles.
- No regenerate chemical handling at customer location easing safety concerns of employees.
- Fixed cost takes the guess work out of operation cost.
- No regenerate waste disposal concerns
- Skilled operators are not required at the customers location.
- Flexibility in available water qualities.
- Back-up equipment to fixed in-house deionizers.
- Short high volume use for brief periods or continual use applications.
- Ease of providing pilot demos for various processes.
Dealer Benefits include
- Source of repetitive income.
- Good probability for high profit margin.
- Because you regenerate the resin,the skilled operator is on your team.
- Door opener to other equipment applications, services and/or consumable.
Types of Plants
There are two basic types, commonly referred to as batch and funnel. Each offers two-bed, mixed bed or a combination of each.
Batch plants are multifunctional and designed to regenerate large quantities of resin within one cycle. These are available in several sizes. Common sizes are 10 ft3, 20 ft3 and 40 ft3 plants. This style plant dependent on size accepts many of the small PE tanks previously exhausted by customers for regeneration.
Two-bed batch DI plants permit the resins to be taken directly from the exchange bottle and loaded into the respective cation or anion regeneration vessel. The resin is regenerated, quality tested and loaded down into numerous smaller portable bottles.
Mixed-bed batch plants permit the resin to be loaded directly from the exchange bottles into a separation vessel. Once separated the resins are moved into the respective cation and anion regeneration vessels. The resin is regenerated, quality tested and moved into a re-mix vessel. The resin is mixed and placed back into exchange bottles with a final quality test.
Two-bed batch plants include separate cation and anion regeneration vessels with either pumped or educted chemical systems and a test station. Associated controls, waste neutralization and other related accessories.
Mixed-bed batch plants include a two-bed plant and a separation, ,re-mix or single unit designed for both separation and re-mix. Figure 1 provides an illustration of a two-bed DI and a mixed-bed DI batch-style plant.
In comparison a funnel plant as the name implies is designed around the use of a large clear funnel. This plant type regenerates several small batches of resin at a given time. Common sizes are of single-, three- and five-station plants. With a base design each station regenerates a single cation and anion bottle.
When regenerating two-bed DI trains, this type of plant works on the basis of using the funnel for the purpose of resin backwash. The resin is backwashed out of the bottle into the funnel. When backwash is complete the resin is dropped back into the bottle. The bottle is moved to a regeneration station and regenerated.
Mixed-bed bottles use the funnel for backwash, separation and re-mix. Bottles are brought to the funnel where resins are lifted from the spent bottle into the funnel where the resins are backwashed and separated. Once separated, the resins are downloaded from the funnel into separate cation and anion bottles, which are moved to a regeneration station, regenerated and quality tested. The bottles are moved back to and lifted into the funnel. The cation resin and anion resin is mixed together and loaded back into PE tanks as mixed beds and tested.
Funnel plants in addition to the funnel include cation and anion regeneration stations with either pumped or educted chemical systems, a test station and waste neutralization. Usually one backwash funnel is provided for each regeneration station. The funnels and components are usually sized to handle 9-, 12- and 14-inch diameter bottles. Figure 2 provides a simple illustration of funnel-style plants.
Advantages of Batch Plant.
- Operator efficiency—this plant allows the highest volume of daily resin regeneration. It permits an operator to be separating a mixed batch while regenerating a separated batch and re-mixing or reloading a regenerated batch. This multifunctional nature is the most efficient.
- High quantity of resin regenerated at a given time.
- Greater number of automated steps yielding lower labor.
- Allows minimal equipment for two-bed bottle design with ability to add on the additional components for mixed-bed bottles, as the market needs or customer base increase.
Advantages of Funnel Plant.
The resin is segregated to each bottle allowing assurance that each customer receives the same resin time and again.
Disadvantage of Batch Plant.
All resins are mixed together. Resin/customer segregation requires the individual customer to have a resin demand equal to the size of one regenerated batch.
Disadvantage of Funnel Plant.
- Operating efficiency—typical funnel plants regenerate small individual resin quantities simultaneously.
- The quantity of regenerated resin per cycle is small.
- There are fewer automated steps.
Determining Plant Size
A study of the local market is necessary to determine the amount of available business within your area. The distributor or service provider should perform this study to accomplish two major goals: the amount and the type of expected DI business.
The amount of business begins to identify the daily quantity requirements for regenerated resin. The type of business defines through customer water quality needs the type of needed bottles. This being either two-bed and/or mixed-bed units. Customer types and customer requirements should be defined.
This study also should include an in-depth look at area waters. The quality of the feed water determines the level of pretreatment needed to enable the IX resins to produce water qualities. It also serves as an indicator of whether resin fouling will occur. Customer feed water quality and required treated quality ultimately determines cost and charges.
There are three basic bottle setups of which two belong to the two-bed family and the third is a mixed bed. These setups are described as
- Two Bed DI Weak Base—Utilizes strong acid cation resin followed by weak base anion resin. These are used when customer quality needs are roughly 20,000 ohms. This system type requires two cation bottles for each anion bottle. This system will not split salts nor remove SiO2 and CO2. Treated water quality will primarily consist of SiO2, CO2, NaCl and water with a pH less than 7.
- Two Bed DI Strong Base—Utilizes strong acid cation resin followed by strong base anion resin. These are used when customer quality needs are 50,000 to 500K ohms. This system reduces all cations and anions including SiO2 and CO2. Treated water quality will primarily consist of Na, OH and water, with an average pH range from a high of 9 to
a low of 6.
- Mixed Bed DI—Utilizes strong acid cation mixed together with strong base anion. These bottles are used when customer quality exceeds 1 Meg-ohm and higher. Treated effluent will primarily consists of Na, OH and water with a neutral pH of 7. Figure 3 illustrates these bottle designs.
PEDI Plant Requirements
Regardless of the type of plant used, each requires additional treatment equipment. The equipment ensures proper resin regeneration, handling of the waste regenerate chemicals and treatment of the waste regenerate waters. Additional equipment needs include
- Softened water for backwashing and/or separation of spent resins.
- Two-bed DI water for regeneration steps of chemical introduction, chemical rinse and quality test of regenerated resins.
- Heated water is recommended for anion chemical introduction and chemical rinse cycles.
- Air supply for pneumatic valve operation and resin mixing.
- Waste Neutralization is used to trap and hold regenerate wastewater for neutralization prior to discharge. Make certain to obtain any needed discharge permits.
- Safety equipment is required as the plant will be using aggressive acids and caustics. Consider the use of eye wash stations, chemical containment in event of spills, atmospheric venting, eye protection, protective clothing, local codes, etc.
- The facility must include floor drains connected to a central sump where a chemically rated pump will transfer drain, spilled liquids, etc., to plant neutralization. Be sure to include proper floor coatings and drain linings.
- Resin transfer is generally handled through hydraulic operated eductors or pumps. Batch operated systems require resin transfer from the small exchange bottles into a separator
when mixed bed resins are used or directly into the regeneration vessels when two beds are used. Funnel plants transfer resin from the spent bottles into a funnel.
Other important ingredients are water, pressure, gas and electric. These utilities are the lifline of the system. (See Table 2 for more details.)
Regenerate and Other Chemicals
- Cation regenerate is HCL generally provided at 30 percent concentration from the supplier. Dosage is normally 8 lbs. to 10 lbs. of 100 percent HCl for each regenerated cubic foot. This roughly equals 3.5 gallons of 30 percent HCl/ft3 of resin. Be aware that HCl acid has aggressive fumes when exposed to any moisture, including moisture in the air. Proper ventilation to the outdoors is required.
- Anion regenerate requires rayon grade or mercury cell grade NaOH. It is commonly supplied as 50 percent, but is available in lower concentrations. Dosage is normally 8 lbs. to 10 lbs. of 100 percent NaOH for each regenerated cubic foot. This roughly equals 1.6 gallons of 50 percent NaOH/ft3 of resin. When using 50 percent NaOH you must provide for heating of the chemical container to prevent freezing (at room temperature) and to allow consistent chemical usage each and every time you regenerate. When heating isn’t possible use lower solution concentrations. When used you must increase the dosage amount for correct resin regeneration.
- Other Chemicals—this includes salt for water softening and/or resin separation. Sanitizing chemicals such as chlorine should be used periodically to guard against organic and/or bacterial growth within your plant components.
Part two of this article covering plant accessories, market considerations and troubleshooting ran in the December issue.
About the Author
Nevin Rudie is senior application engineer at Ecodyne Water Treatment, Inc. in Naperville, Ill. He has been in water treatment since 1976, having been involved in the design and operation of PEDI plants for 16 years. He can be reached by phone at 630-637-2116; e-mail [email protected].