How to Design and Operate a PXŽ System Part 2

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ABSTRACT

The new pressure exchanger (PX) device transfers the energy from the concentrate stream directly to the feed stream. This direct, positive displacement approach results in a net transfer efficiency of over 95%. Although application of the PX technology is simple in both theory and practice, in order to get the most benefit from this technology it is important to reconsider the SWRO design and operation approach. Pertinent design considerations include pre-filtration, conversion rate optimization; pump selection, and operating pressures. Some important operating procedures and characteristics are start up, high pressure regulation, conversion rate optimization, flow balancing, and shutdown. Furthermore, it will be shown that these considerations affect the design and operation of SWRO systems counter-intuitively and may possibly reverse given standards that have been developed over the past 20 years of SWRO design.

There has been a recent proliferation of commercially available energy recovery devices based on the positive displacement direct pressure exchange approach. This increased interest is driven by the fact that the technology can reduce the energy consumption of an SWRO system by as much as 60%. Since energy costs are rising and can consume as much as 75% of the total operating costs of an SWRO plant, it is important that the technology be encouraged and disseminated throughout the industry. Although the authors of this paper are directly associated with Energy Recovery, Inc., a leading company in pressure exchanger technology, the principles and theories presented in this paper will be applicable to all devices based on the positive displacement, direct pressure exchange approach.

4.0 START AND STOP PROCEDURES

Starting and stopping an SWRO system designed around the PX pressure exchanger device is actually simpler than with systems designed around other technologies such as regulating valves, turbos and Pelton wheels. This is because of the self-balancing nature of SWRO systems designed around pressure exchanger technologies.

4.2 System start up sequence

Start up of an SWRO plant designed around the PX device is very simple. The first step is to start the raw water supply pump. At this point the system will begin to fill with water and the PX may or may not start to spin. Next, start the high-pressure boost pump. The associated pressure drops through the RO membranes and PX combine with the high-pressure boost pump curve to dictate the reject flow rate into and seawater flow rate out of the PX unit. This means that once the high-pressure boost pump is running and has stabilized the reject flow is now running at or very near the normal flow rate for the plant. Now it is time to start the main high-pressure pump, which will pressurize the RO system. The system will reach the exact pressure required to produce the amount of product water being injected to the RO system by the main high-pressure pump. The membranes create the back-pressure in the system and now act like the pressure-regulating valve. It will take 5-10 seconds for a typical system to pressurize once the main high-pressure pump is started. It may be advisable to install a high-pressure bypass valve at the outlet of the RO membranes that can be closed slowly at start up. This will allow the operator to control of the rate at which the RO system reaches full operating pressure.

4.3 SYSTEM SHUT DOWN SEQUENCE

First stop the main high-pressure pump. After approximately 30 seconds the pressure in the RO system will drop to around 27 bar. At this point it is proper to stop the high-pressure booster pump and raw water supply pump. It should be noted that because the high-pressure side of the PX is sealed from the low pressure side of the PX the high pressure RO portion of the plant can maintain significant pressure for an extended period of time.

4.4 Fresh wate flush

If the SWRO system is going to be shut down for an extended period of time it is required to fresh water flush the RO membranes and pressure exchanger in order to inhibit biological growth and fouling. Start by supply the RO system with un-chlorinated fresh water at the normal system feed pressure. Next run the high-pressure booster pump until all of the seawater has been purged from the RO membranes. It may also be desirable to also run the high-pressure pump for a few seconds during this process to ensure that it gets a complete flush as well.

5.0 FLOW CONTROL AND BALANCING THE SYSTEM

Flow rates and pressures in a SWRO plant will vary slightly over the life of a plant. Variations may be due to temperature, membrane fouling, seasonal feed salinity variations, etc. The following designs and procedures should be used to control these variables.

5.1 High Pressure Reject and Seawater Feed Flow Control

In order to control the high-pressure reject and feed flow rates, adjustment of the pressure and flow supplied by the high-pressure booster pump is typically required. Recommended practice is to use a high pressure booster pump with some additional capacity, and control its flow and pressure with a variable frequency drive or control valve. A high-pressure flow meter can be used to determine the amount of reject and feed water flowing through the high-pressure side of the pressure exchanger device. Remember that the high-pressure reject water and feed water are hydraulically connected and are separated only by a water barrier/piston. It is also possible to infer the high-pressure flow rates from the pressure drops across the pressure exchanger and/or high-pressure boost pump. Increasing and decreasing these flow rates is how we decrease and increase respectively the conversion rate of the RO system independently of the product water being produced.

5.2 Low Pressure Reject and Seawater Feed Flow Control

In order to control the flow rates of the low-pressure feed and reject water, adjustments to the low pressure seawater inlet pressure to the PX should be made. Recommended practice is to install a valve at the low-pressure seawater inlet of the PX unit(s). Remember that the low-pressure seawater and reject water are hydraulically connected and are separated only by a water barrier/piston. The low pressure seawater and reject flow rates can be determined by installing a flow meter at the low-pressure seawater inlet of the PX.

5.3 Balancing the pressure exchanger using flow meters

All flows in and out of the Pressure Exchanger must be approximately balanced. The following equation applies to this process:

High pressure seawater outlet flow = Low pressure seawater inlet flow

Determine the desired amount of high-pressure seawater outlet flow, which is approximately equal to the reject flow for your system. Adjust the variable frequency drive or control valve on the high pressure booster pump until that flow rate is achieved as seen at the high-pressure flow meter. In the absence of a high-pressure flow meter it is also possible to infer the flow rates from the pressure drops across the boost pump and/or pressure exchanger(s).

Adjust the low-pressure seawater inlet valve until the low-pressure seawater inlet flow equals the high pressure seawater outlet flow.

If the low pressure seawater inlet flow is less than the high pressure seawater outlet flow excessive intermixing of reject with the feed will occur which will result in lower quality permeate, increased feed pressure and higher energy consumption. If the low-pressure seawater inlet flow is greater than the high-pressure seawater outlet flow, treated feed water is being wasted and dumped to the low-pressure reject drain.

6.0 CONCLUSION

The PX is a new pressure exchanger device that promises to revolutionize SWRO design. The device affects the design and operation of SWRO systems in several counter-intuitive ways. As we have discussed, lower conversions rates in the order of 30-40% actually yield lower energy profiles than higher conversion rates. Furthermore, these systems operate at lower pressures and produce better water quality.

The fact that the main high pressure pump flow equals the permeate flow means that we can now achieve SWRO systems with nearly 100% conversion rates when considering the pumping power that is being applied. This fact also means that for any given high pressure pump, systems that are 2-3 times larger can be achieved with that same pump.

The PX device now makes it possible to adjust the conversion rate of an SWRO plant independently of product water production. This means that the conversion rate can now be simply and directly used to optimize the efficiency of the RO membranes rather than having to balance it against energy consumption, product quality, membrane pressure limits, and so on.

It is clear that this device is an extremely efficient approach to energy recovery, and that the SWRO systems of today and the future will consume far less energy than those of yesterday. However, the impact that this device will have on design concerns such as the conversion rate, water quality, and operating pressures will surely surprise us all.

About the Author

John P. MacHarg of the American Water Works Assoc writes articles relating to Energy Recovery Inc., a Clean Technology Company that provides affordable Seawater Desalination with the PX Pressure Exchanger which reduces energy costs up to 98%.