Pump Selection For Filter Presses y839

Selection of feed pumps for filter press Sai Durga Prasad V.S., Sasikumar D., Sriram Subramanian, & Sangeetha Subramanian Introduction Pump can be defined as a device used for transferring fluid or as feeders for unit processes. When using pumps as feeders for equipments such as filters that develop head loss, positive displacement pumps are a better choice compared to centrifugal pumps. Fluid control equipments are leading manufacturers of filtration systems for more than 3 decades and share their experience in pump selection for filter presses in this article. Filter press is a pressure filtration device that has found a niche place in various industries. Commonly used applications include liquid solid phase separation in chemical and pharma industries, powdered activated carbon filtration from sugar syrup, ceramic industry, Surface treatment and phosphating industry, Water, effluent and sewage sludge dewatering etc. Depending on the application some customers require filtrate, while others require the separated cake as their end product. For customers who need the filtrate as their product, throughput filtrate flow rate from the filter press is dependent on three basic factors – 1. Filtration area provided 2. Feed pump flow rate and 3. Maximum Operating pressure possible by feed pump. For those who need the dewatered cake as their end product, cake compression is an important performance feature of filter press which will depend on the total pressure head capacity of the feed pump. Whatever be the size of the filter press, the throughput of filtrate obtained depends on the feed pump. Batch frequency and operation and maintenance constraints decide to choose the size of the filter press. This article focuses on the selection of appropriate feed pumps for pressure filtration in filter press. Typical filter press operation Filters are batch operations by nature. They accumulate unwanted particles on one side of the membrane, allowing remaining fluid to through. As this process happens over a period of time, head loss (back pressure ed on pressure gage) develops across the filter, resulting in decreasing flow rate. When the flow rate falls below required or permissible range, the filter is shut down and the accumulated solids are removed from the filter and the process is restarted. The below graph shows typical filtrate flow rate versus head loss in a filter press. The data was collected during filtration of anaerobic digested sludge.

Figure 1: Throughput and head loss progression over time in filter press Typical head loss permitted for filter press is about 3.5 to 4.0 kg/ sq cm. Feed pump should be capable of developing at least 1 kg/sq cm higher pressure than the maximum permissible head loss. Only then, the filter press will be able to function at optimal throughput rates. Pump selection for filter press Pump selection is based on a number of criteria and is critical to ensure optimal functioning of the machine. For a given pressure of filtration, filtrate through put rate will vary depending on sludge properties such as solids concentration, viscosity, temperature as well as pump capacity and filtration area being provided by the filtration equipment. The factors to be considered while choosing a pump are Flow rate The pump flow rate selected has to take into consideration the filtrate throughput rate required, by or return flow if any, and optimal filtration rate for available filtration area. Table below lists typical thumb rules for selecting pump flow rates for some industries. S.No . 1. 2. 3. 4.

Industry Sugar syrup filtration

Pump capacity. (x expected flowrate) 1.25 to 1.5 x

Activated carbon filtration ETP/ STP

1.25 to 1.5 x

Online filtration in electroplating and

2.0 to 2.5 x

2.0 to 2.5 x

5.

phosphating Textiles and dyestuf

2.0 to 2.5 x

6.

Pulp and paper industries

2.0 to 2.5 x

7.

Oil, resins, plasticisers

2.5 to 3.0 x

8.

Biotech and pharma

2.0 to 2.5 x

Machinery filters (CNC cutting machines, EDM etc) 10. Other industries

1.5 to 2.0 x

9.

1.25 to 3.0 x

It should be noted that the hydraulic flux rate through the filter should not be exceeded. Pump flow rate should be selected proportional to the Filtration area also. If filtration rate has to increase, size of the filter press will also increase after a given range. Flux rates for filtration using filter press range between 150 lph/ m2 to 600 lph/ m2 depending on sludge viscosity, solids percentage, temperature of filtration, filter medium being used, level of clarity required and pumping rates. For better estimation of filtration and pumping rates for a particular slurry, pilot trials can be conducted.

Selecting type of pump and MOC The most commonly encountered pump in the market is the centrifugal pump. However, these are not the best suited pumps for coupling with the filter press. The impeller of a centrifugal pump rotates to move liquid through the pump. The impeller’s velocity imparts energy on the fluid. The resulting rise in pressure, or head, is proportional to the velocity of the liquid. In contrast, a positive displacement pump moves a set volume of liquid. The delivered capacity is nearly constant throughout the discharge pressure range. Positive displacement pumps are capable of pumping at the same rate against any obstruction or raise in pressure in the system. For selecting a feed pump for filter press, it is preferred to use a positive displacement pump; it can be classified into two types, i. ii.

Rotary type pumps such as Screw Pumps, Gear Pumps, Vane Pumps and Reciprocating type such as Air Operated Diaphragm Pump and Piston Pumps

Figure 2: Filter press coupled with screw pump; mounted on trolley for multiple location usage

Figure 3: Filter press coupled with Air Operated Double Diaphragm pump

Figure 4: Filter press coupled with centrifugal hygienic pump. Depending on the type of industry, the pump is selected. Table below lists common applications and the type of pump chosen. S.No. Industry/ Process parameters Type of pump application taken into consideration 1. Oil and resins Highly viscous – high Helical Gear pumps temperature 2. Sugar syrup Low viscosity – high Centrifugal hygienic temperature- low solids pumps content 3. STP/ ETP Low viscosity – ambient Screw pumps/ multi temperature – good stage centrifugal pump solids compacting required 4. Electroplating High temperature, easily AODD pump and phosphating precipitated, settling slurry - high solids content 5. Textile and High temperature, easily AODD pump/ centrifugal

dyestuf 6.

Pulp and Paper

7.

Biotech and pharma

8.

Machinery filters

precipitated, settling slurry Ambient temperature, high volume Temperature, presence of solvents, clean room operation Ambient temperature

slurry pump Centrifugal slurry pump/ AODD pump Sanitary pump/ AODD pump Screw pump/ multi stage centrifugal pump

Centrifugal pumps are generally not coupled with filter presses due to the head loss generated across the equipment. If the pump has to be selected such that a reasonable flow is obtained till cake compression is achieved, the pump will be very oversized in of power and initial throughput rate. So, in order to avoid this, only P.D. pumps are suggested to be coupled with filter press. In certain extreme or critical pH/ temperature applications like hot dip zinc galvanizing, where acid baths are set up, it is easier to adopt existing pumping practices instead of trying to introduce a new pump along with the filter press. If centrifugal pump chosen is oversized in such situations, it is suggested to allow a bigger by line, as compared to the inlet line. Using single inlet and multiple outlet configurations along with suitable by will help to regulate flow without damage to the filter media. Conclusion When using pumps as feeders for equipments that develop head loss, positive displacement pumps can serve as efective solutions. Proper precautions in the form of pressure relief valve/ pressure switches should be incorporated into the line in order to prevent extreme build-up of pressure in the process line. 90% of filter press operational difficulties arise only due to feed pump failure. Filter presses can be operated to their maximum potential only when coupled with suitable feed pump. A suitable trade of between power consumed, type of pump, capital investment has to be made and pump selected accordingly. References 1. Characteristics of Screw pumps – R.K.Annis Ohio state engineer 2. Consider positive displacement pumps – McCandless and Richard Meighann, Chemical processing, July 2012.

Ms. Sangeetha Subramanian Head - technology Fluid control equipments A-5, nathan’s complex,

69, ECR, Thiruvanmiyur, Chennai – 600041. INDIA Tel: +91 44 2448 1130 E-mail: [email protected]