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Safe production of polyaluminum chloride thanks to double hose membrane


The selection of suitable pumping technology, as well as the energy-efficient use of raw materials, is especially challenging if the manufacturing plant operator handles corrosive or toxic media. When designing a pump specifically for the production of polyaluminium chloride, special materials were used to avoid any pump corrosion by the aggressive medium. Furthermore, the double hose-diaphragm pump meets the optimum technical requirements for this challenging process and ensures low-pulsation operation, while simultaneously providing favorable procurement and operating costs.

At the end of the process chain in many industrial manufacturing processes, a by-product is created in addition to the desired commodity. This by-product often has a commercial use. An example of this comes from an Indian production unit for caustic soda. When producing caustic soda from sodium chloride, large amounts of chloride slurry are created, an acidic and therefore particularly aggressive medium. Over the course of the process, the chloride is made to react with aluminium hydroxide in order to produce polyaluminium chloride (PAC), which is an extremely corrosive solution with a pH value of between 1 and 3. A suitable high pressure pump feeds the PAC at high temperature and a pressure of 70 to 100 bar into a spray dryer, where PAC powder in micro-fine parts is produced as an intermediate product for further use. PAC is used in water treatment and also in the cosmetic industry for making antiperspirants (deodorants).

 

The PAC production process presents several challenges which must be considered when selecting the suitable pump. The criteria include:

-    pressure and corrosion-resistant materials
-    hermetically sealed process
-    pressure of 70 to 100 bar
-    temperature up to 120 °C
-    low pulsation
-    optimum efficiency
-    minimum costs (capex and opex)

 

Titanium as material

The high pressure has to be maintained in order to achieve even granulation of the PAC powder. With pressures of up to 100 bar, metals are primarily considered as the material for piping and valves. Both Hastelloy C and titanium resist corrosion. Based on specific experiences and tests, the operator decided to have medium-wetted and pressurized parts made of titanium, whereas PTFE (polytetrafluoroethylene) was to be used for seals and to separate the wet end from the drive end.
Due to their special design, displacement pumps have proven themselves for such demanding processes at high temperatures. Screw spindle pumps and eccentric screw pumps do have low-pulsation properties, however the special requirements, such as hermetic seals and material resistances, are difficult to align with reasonable overall costs. Centrifugal pumps are not applicable because of the high operating pressure.

 

Undesirable pulsations

Taking the process-related requirements into account, only a plunger or piston pump can be used. In this case, the decision was made in favor of a double hose-diaphragm pump. Spray dryers are generally very sensitive to pulsations, but a triple acting pump with offset arrangement of the crank shaft/connecting rod assembly by 120° has proven successful in ensuring operation at low pulsation. Through use of an appropriately sized pulsation dampener, the residual pulsation can be reduced to less than 1.5%. Furthermore, the use of a triplex pump ensures that low pulsation is even maintained at the very low maximum flow rate of 2m³/h required for this specific spray dryer.

Schematic diagram of a triplex pump with integrated pulsation dampener
Fig. 1: Schematic diagram of a triplex pump with integrated pulsation dampener

 

Reduce slurry-wetted parts

The slurry-wetted parts of the feed line and connecting pipe, as well as the check valves and valve seats, are made of titanium. While the inner parts of traditional diaphragm piston pumps, e.g. diaphragm casing and pump cover, are in direct contact with the pumped fluid, the slurry chamber of the chosen double hose-diaphragm pump is sealed from the environment by double seals. Fabricating the pump heads from titanium was therefore not required, which contributed significantly to cost reduction.
By using a flexible double hose-diaphragm, the medium is fed through the pump in a favorable way without contact to any parts subject to pressure. The pressure within the pump casing and pump head caused by the displacement of the plungers is the same in the hydraulic fluid and in the medium, which means that the only function of the diaphragm is to separate the critical medium from the hydraulic fluid. Almost identical pressure is maintained inside and outside the diaphragm, so that the diaphragm is only subject to deformation energy and no other stress. This design helps noticeably reduce costs for maintenance and wearing parts.

 

difference of double hose diaphragm pumps to normal diaphragm pumps
Fig. 2: Traditional diaphragm piston pump (left) & double hose-diaphragm pump with hermetical sealing of the conveyed fluid and sensors (right)

 

Material with special properties

While the hose-diaphragm for many standard applications is typically made from NBR (nitrile rubber), this particular case of PAC application only allowed for PTFE as the diaphragm material. With this special material, which is resistant to the corrosive properties of PAC, a service life of more than two years is achieved. Valve balls and valve seats are therefore regarded as the sole real wearing parts of the chosen pump design.

 

Safe operation thanks to proven technology

In addition to the special features, proven serial parts offer the ideal solution regarding initial and operating costs. Additional features guarantee safe and low-wear operation. When exchanging wearing parts, in this case only the internal parts of the check valves, the pump remains integrated in the piping. Specially developed sound sensors allow condition monitoring of every individual valve. Thanks to these sensors, only the actually affected valve is subject to maintenance, which in turn reduces the operating costs.
The pump is usually driven by a variable speed motor. If two or more units are being operated in parallel, electronic crankshaft coupling is applied, which further reduces pulsation in the system, resulting in a uniform volume flow. By means of crank shaft coupling, an appropriate offsetting of the crank angle is maintained over the whole duration of the operating time, which means that two triplex pumps are working at the uniformity of a six-head pump.
Thus the used double hose-diaphragm pump combines the operator’s special requirements with the proven and tested properties of a serial product.

 

FELUWA Triplex pump with double hose diaphragm

Fig. 3: Double hose-diaphragm pump for polyaluminium chloride production

 

Source & Pictures: Mr. Gänsel / FELUWA

 

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