Aria™ Microfiltration system

Challenges

A major North American utility station deploys conventional lime softening and demineralizers to produce high-quality water for boiler makeup. The utility station is a large coal-fired power plant that operates three coal-fired units. The units are about 600 MW each and were originally designed to handle high sulfur bituminous coal. The conventional system uses coagulant, lime, acid and caustic in large quantities to produce demineralized water. Pall Corporation and plant personnel worked together to develop an integrated membrane-based system to produce high-quality permeate water to maximize ion exchange run times.

Conventional clarifier/multimedia filters for the treatment of incoming fresh water into plants suffer from several drawbacks. The primary one is the inability of these systems to cope with sudden upset conditions that could result in increases in total suspended solids in the feed water. This is also reflected in an increase in Silt Density Index (SDI), or in turbidity (NTU values) in the feed water to the unit, as well as in the permeate (filtrate).

Raw surface water from an adjoining lake is the fresh water source for the power plant. In the original treatment scheme, about 1500 gpm of this water was being treated with a conventional clarification and cold lime softening process followed by sand/gravel bed filtration. Demineralization units were subsequently used to produce the required water quality for the HP boilers. These units consisted of a conventional cation bed using sulfuric acid for resin bed regeneration, and weak and strong base anion beds as well as a mixed-bed demineralizer using caustic soda for resin regeneration. Downstream of the demineralized units, the condensate water for the boilers was sent to storage tanks. A Graver Powdex^® precoat filtration system was used for polishing condensate before the low-pressure heaters.

There were several driving forces that led the utility station to consider alternate treatment schemes in lieu of the conventional clarifier/sand bed. The first was the chemical costs required to regenerate the demineralizer beds. These costs were extremely high since regeneration of the resin was carried out once a day or even more frequently. The ion exchange run time needed to be improved significantly.

Secondly, they were looking for operational simplicity. Frequent upset in the clarifiers would result in frequent regeneration. Although the plant was designed with a three-bed demineralizer followed by mixed-bed polishers to meet boiler feed water quality, silica breakthrough in the strong base bed was frequent and beds were regenerated more often. And finally, the condensate polishing system had to be precoated frequently due to the poor quality of the condensate water. An improvement in the frequency of precoating the resin would provide an indirect benefit to the plant. With these changes in mind, the utility station enlisted Pall to help drive the much-needed changes.

Solution

Pall water treatment experts performed a detailed analysis of the plant conditions and decided to recommend an integrated membrane (Microfiltration/ Reverse Osmosis) system (IMS) to replace the cold lime softening clarifiers and sand/gravel filter beds. Since the power plant was online, an additional challenge was to install the systems without shutting down the demineralizer trains or negatively impacting the amount of treated water while the new IMS plant was brought on line. Pall recommended installing the IMS in parallel with the clarifiers and sand/gravel filters and reusing the existing filtered water tanks. One filtered water tank was used as an MF/RO break (filtrate tank), and the other as a Reverse Osmosis (RO) permeate water storage tank (the new demineralizer train feed tank).

After commissioning the IMS, the plant personnel bypassed the clarifier/sand bed system and fed the incoming water from the feed system directly into the MF system.

Pall’s integrated system consisted of a Pall Aria™ Microfiltration system for Treatment of Plant Make-Up Water  using Microza microfiltration modules. The system comprised of two independent treatment trains of 42 modules each. This system is 2 x 100% capacity (770 gpm maximum each) and allows for an average production of 1400 gpm (input of 1540 gpm with 95% recovery) with both trains in service. Since the IMS system was installed along with the existing system, the space available for the RO skid was limited. Therefore, the RO skid had to be custom designed to fit in the available space. To accomplish this, the RO system consisted of three stages (single train), arranged in a 16:8:4 array with five membrane elements each. The inlet flow to the RO system was 625 gpm. The system was designed for a totalpermeate production of 500 gpm, the average capacity of the boiler.

The RO system capacity was designed to meet an average demineralized water demand of 500 GPM. However, during boiler chemical cleans or tube leaks, the demineralized water demand could be as high as 900 GPM. During high demand conditions, the RO permeate will be blended with the MF filtrate. The demineralizer feed pump has the capability to draw from both MF filtrate and the RO permeate, thus blending the two streams before being fed to the demineralizer trains.

The average MF filtrate required for the RO is 625 GPM and during the peak demand, the MF filtrate required would be 1,025 GPM to make the blended feed of 900 GPM to the demineralizer system. Flux Maintenance (FM) is being performed to lower Trans Membrane Pressure (TMP) across the MF membrane. There are three FM methods used in this system.

The first FM method is air scrub/reverse filtration (AS/RF), which involves injection of air at low pressure into the feed side of the module approximately every 20 minutes. Clean filtrate is also pumped in a reverse direction through the hollow fibers to dislodge foulants and deposits. After the AS/RF, the MF unit will ramp up to the instantaneous peak flow to compensate for the loss in filtered water, thus maintaining the constant average filtrate output.

The second FM method is Enhanced Flux Maintenance (EFM), which is being performed based on an increase in the TMP. This fluctuates between once per day and once per week to remove microbial fouling, thus lowering TMP values. During EFM, a hot caustic chlorine solution or a hot chlorine solution is circulated through the feed side of the membrane. During the EFM the MF unit will be off line for 30 – 60 minutes.

Normally, as TMP approaches 25-30 psig, a chemical clean-in-place (CIP) is performed -- the third FM method. This is a two-step protocol, first using hot caustic/chlorine, and then an acidic solution to return the modules to “nearly new” conditions. This is carried out hundreds of times over the lifetime of the modules. A CIP can also be performed at periodic intervals (once every 60 days, for example) as a precautionary step to protect the membrane even if TMP does not rise significantly during that interval.

Considering the FM requirements for the MF system, two MF units were selected. Since the average demineralizer feed rate is 500 GPM (RO feed 625 GPM), each MF unit was sized for 700 GPM average filtrate production.

Results

The utility power plant deploys a Powdex condensate polishing system (three vessels per unit) for handling the condensate. These systems are installed to meet stringent boiler feed water requirements, to improve the reliability of production, and to increase the efficiency of the power plant. Prior to installation of Pall’s integrated system, the conductivity from the condensate system to the boilers was elevated, possibly due to high total organic carbon content. These precoatable filters were precoated at a very high frequency – once a day. Since it costs approximately $850/precoat, the plant was spending $1,100/day for precoating work for the two polishing units during operating times and start-ups.

Since the operation of the integrated system commenced, the frequency of the precoating has decreased from once a day to once a week. The double-membrane (MF/RO) IMS system was effective in reducing total organic carbon content to very low levels in the condensate, and this had an immediate positive impact on the performance of the condensate polishing system. The total precoat cost decreased, contributing to substantial savings of approximately $250,000/yr. These savings were in addition to the $1.2 million/yr saved in chemical costs, as described earlier.

The cost of the Pall Integrated System was approximately $1.2 million. Savings from chemical costs from installation of system: $1.2 million/yr. savings from improved performance of polishing system: $250,000/yr. Hence, total savings to the plant: $1.2 +$0.25 = $1.45 million/yr. The return on investment was achieved in less than 10 months of operation.

Pall’s integrated system resulted in considerable direct savings on chemical costs and improved ion exchange unit run times. Considerable indirect savings were also achieved by reducing the frequency of precoat operation in the condensate polishing system. Additionally, included in these costs was the average lifespan of the precoatable elements. Changing elements every two years may extend to five years. Challenges associated with the plugging of the RO pre-filters in summer were overcome with an innovative technical solution.

Partnering with Pall allowed this successful North American utility station to avoid major financial losses by way of an integrated high-purity water solution. If you are looking for industrial manufacturing solutions, contact your Pall representative today.