To address the increasing global demand for fresh water, seawater reverse osmosis (SWRO) desalination has emerged as a long-term, sustainable solution. SWRO desalination is only economically viable, however, if the energy consumed during the process can be minimized. Using advanced ceramics techniques and in-house green manufacturing processes, isobaric energy recovery devices machined to precise micron tolerances can be fabricated to comprise almost frictionless water-lubricated hydrodynamic bearings. This new family of devices can operate at up to 98% efficiency, thus reducing the energy required in SWRO desalination by as much as 60%. The dramatic improvement in process efficiency results in more economical production of drinking water and a diminished overall carbon footprint for the desalination plant.

These advanced energy recovery devices contain only one moving part—ideally, a high-purity sintered aluminum oxide rotor turning at up to 1000 revolutions per minute (RPMs) in an almost frictionless hydrodynamic bearing. If properly formulated and manufactured, this ceramic material should be unaffected by chemicals or corrosion when operating in a hot brine environment. The material is also three times harder than steel, providing unmatched durability for SWRO desalination operations.

Manufacturing ceramics, however, is an inherently labor-intensive and dirty process. When making energy-recovery devices for SWRO desalination thatare intended to reduce energy consumption, it is critical that the ceramics manufacturing facility and process be as environmentally friendly as possible to minimize the carbon footprint and local environmental impact.

Plant Layout

Labor-efficient and environmentally benign ceramic manufacturing for energy recovery devices starts with having a properly designed facility for vertically integrated ceramics manufacturing. The facility floor plan and equipment layout should allow for product to flow in a commonsense manner through the factory. Automation should be used as much as possible to reduce non-value-added product handling, especially when manufacturing heavy ceramic components such as those used inside the isobaric energy recovery devices.

Efficient product travel also allows for quality control checkpoints to be placed appropriately along the product flow path. An advanced ceramics analysis and quality control lab should serve as the central point in this facility so that data material complex analysis can be completed to insure product purity and physical properties in the critical formulation, compaction and sintering steps. Many existing vertically integrated ceramic manufacturing facilities are inefficient because they were not designed with basic product flow and integrated quality in mind. These types of factories can suffer quality control and efficiency issues, as products often need to travel to the quality and lab area many times during production.

Spray Drying and Mixing

The overall ceramic production process begins in the spray dry and mixing room. Most alumina ceramic formulations use no toxic chemicals, dyes or powders, and raw powders are processed under custom-designed dust extraction systems to eliminate fugitive air pollution and provide a comfortable and clean working environment. To avoid worker strain or injuries, all in-process powders should be moved within the milling area using a custom radio-controlled bridge crane system.

Controlled and managed by a computer, the spray dryer should use exhaust heat recovery to reduce clean-burning natural gas consumption. Better heat recovery systems can reduce CO₂ emissions and fuel consumption by more than 20%. State-of-the-art dust extraction systems should be installed to ensure that no particulates are discharged into the atmosphere during the spray dry process. Water cooling for milling systems and spray dryers should be supplied using roof-mounted, energy-efficient heat exchangers.

In addition, all non-toxic water within the spray dry and mixing room can be filtered, recycled and stored for less- critical applications such as cleaning. Finally, all powder products—prior to compaction—should be stored and transported in reusable, waterproof, color-coded bins to reduce waste and prevent contamination. Though inexpensive, fiber drums and supersacks are often difficult to handle safely and can contaminate products. They also cannot be reused and therefore become solid waste.

Pressing and Machining

Figure 1. Near-net-shaped blocks from a PLC-controlled iso-press reduce waste.

Green processes can be applied in the pressing room as well—dust collection and powder dispensing can be integrated directly into an iso-press bag vibrating and loading station. To reduce process errors, enable traceability and eliminate NVA labor, the press can be designed with an integrated bar code reading system that logs operator and production job information, as well as the associated pressing recipes, to part numbers printed on job packets. Though also capable of manual operation, the press can be semi-automatically loaded and operated by scanning just four bar codes. In addition, it is preferred to press green block to near net shape to therefore reduce material waste during subsequent CNC machining.

Green machining can then be completed in a large climate-controlled ballroom environment. Green machining operations are highly automated using a variety of CNC machines. Specially designed CNCs protect key internal components from the abrasive ceramic dust generated during the milling process.

Purchasing tools with large turret tool changing systems can also reduce NVA process time by reducing tool change-out frequency. In this setup, a single worker can easily program and operate many machines simultaneously, thus reducing labor costs. Dust collection for this area can be done with a central unit located in a sound-proof room, while scrubbed and filtered air can then be returned into the room via a ducting system to reduce air conditioning energy costs.

Firing

Enlarge this picture Figure 2. CO2 pollution avoided through the use of kilns with exhaust heat recovery and advanced process controls, compared to conventional updraft gas kilns.

After green machining, near-net-shaped components should be gently moved to the kiln room for firing. Kiln loading and unloading can be consolidated to a single point within the room through the use of a computer-controlled kiln transfer car system. A large elevator, capable of raising and lowering the kiln cars, enables ergonomic loading and unloading of the relatively heavy components. After loading a kiln car, the system can automatically take the car off the elevator and place it into a waiting gas-fired kiln (1500°C+).

In an ideal green environment, all gas kilns should utilize exhaust heat recovery, oxygen monitoring systems and advanced controllers capable of stoichiometric or non-stoichometric operation (see Figure 2). For alumina ceramics, the partial pressure of oxygen in the kiln during sintering can affect properties of the ceramic. These combined systems reduce natural gas consumption by an estimated 30%, thus reducing CO₂ emissions from the facility by more than 100,000 lbs/kiln per year in comparison to updraft kilns of the same shape, capability and size.

Figure 3. A kiln control screen.

Intake and exhaust fans for the kilns should be located within a fire- and sound-proof structure with separate roof-mounted air intakes and a pressure control system. This feature significantly reduces kiln room noise pollution and increases worker safety while also keeping extraneous heat outside of the work area.

A supervisory control and data acquisition system (SCADA) can also be implemented to automatically track and manage the kilns at all times (see Figure 3). This system reduces process control labor, improves factory safety, and provides redundancy for the kiln control systems. From a green manufacturing perspective, this system eliminates the need for paper chart recorder records. Lastly, all gas kilns should be supported by a battery backup system that enables a smooth transition to a generator in the event of a power failure.

Finishing

After firing, components should undergo final finishing using automated CNC and modern universal grinders. By maintaining tight shrinkage control of spray-dried material, stock removal in the hard state is minimized. Minimal stock removal using the proper diamond grinding machinery can reduce the time to complete certain grinding steps by more than 500%. Final quality screening for cracks should be completed using specialized equipment and non-toxic dye penetrants to avoid generating toxic waste.

Minimized Footprint

When producing advanced technical ceramics, implementing technologies and best-practice processes to reduce the environmental impact is often overlooked. However, it is critical when designing energy-efficient solutions to minimize the customer's carbon footprint. This is especially true for isobaric energy recovery devices used in SWRO desalination applications, because reducing the energy consumption of the desalination process is essential for long-term economic viability.

For more information, contact Energy Recovery, Inc. at 1717 Doolittle Dr., San Leandro, CA 94577; (510) 483-7370; fax (510) 483-7371; or visit www.energyrecovery.com.