V. Prabhu, Worley India Pvt. Ltd., Navi Mumbai, India
The mixing of tank contents is required when components or additives must be blended into a product—typical examples include gasoline or lube oil blending where both components and additives must be blended to produce a uniform product. Mixing is also required for day/shift tanks prior to testing, as well as in final product tanks to prevent the layering of varying compositions due to minor changes in density.
Two types of tank mixers are in common use: a recirculation-type tank mixing eductor (FIG. 1) and a mechanical agitator, such as a propeller-type mixer. This article will provide guidance on the selection of mixing eductors for liquid storage tanks.
Advantages and applications. The use of mixing eductors is common where pumps of reasonable capacity and head are available. The advantages of using mixing eductors over propellers include:
The range of applications for mixing eductors is only limited by the viscosity of the liquid to be mixed. As a rule, mixing eductors can be used in all cases where the liquid to be mixed can still be supplied by a centrifugal pump.
Working principle. Jet mixing differs from most types of liquid/liquid and liquid/solid mixing in that the driving force is hydraulic rather than mechanical. Rather than shearing fluid and propelling it around the mixing vessel (e.g., like that of a mechanical agitator), a mixing eductor uses a centrifugal pump to force fluid through nozzles within the tank, creating high-velocity jets that entrain other fluid. The result is shear and circulation, which mix the tank contents efficiently.
The liquid jet coming out of the motive nozzle generates a partial vacuum in the inlet cone of the diffuser; therefore, a liquid flow is extracted from the tank and entrained. The motive jet mixes with the entrained liquid and accelerates its flow. The liquid mixture emerging from the mixing eductor spreads in conical form and entrains more liquid from its surroundings. Eductors can entrain up to five times the amount of pumped solution, depending on the eductor size and design. If one or several such mixing eductors are correctly arranged, a 3D flow is produced in the tank, which mixes all the contents homogeneously (FIG. 2).
Placement of the mixing eductor inside the tank. Little agitation occurs below the level of the eductor, so eductors should be positioned as close as possible to the bottom of the tank for maximum liquid turnover.1
If settling cannot be tolerated, install the eductors at 0.3 m above the bottom of the tank. In general, eductors should be placed so the flow field will reach the farthest and highest liquid level at the opposite side of the tank.
Typically, eductors are placed 0.5 m apart for even and uniform agitation.
METHODOLOGY TO CALCULATE CIRCULATION RATE AND NUMBER OF MIXING EDUCTORS
Turnover rate. The number of times per hour (hr) that the tank content is required to be circulated is known as the turnover rate. The turnover rate is based on the viscosity of the solution and the number of particulates. A general rule of thumb is 20 turnovers/hr.
Some typical guidelines on turnover rate are given below:
The required circulation rate is calculated by multiplying the turnover rate by tank volume. Considering a tank volume of 40 m3 (using the tank dimension indicated in FIG. 2) and 20 turnovers/hr gives a circulation rate of 800 m3/hr.
Eductors normally mix at a 5:1 ratio. This equates to a flowrate at the inlet of the eductor of 160 m3/hr. The inlet flowrate through a mixing eductor depends on its size and pressure at the eductor inlet. The optimum pressure range at the eductor inlet varies from 1 barg–4 barg. An inlet pressure of 3 barg is common.
Installation of the mixing eductor. Mixing eductors are installed at an upward angle for greater coverage. For large tanks, multiple eductors are installed on a circular manifold. The angle of inclination varies with the position inside the tank and the application. The specific number of mixing eductors resulting from the tank mixing system dimensions will be placed on two pipes close to the tank bottom and the tank wall. These two pipes follow the shape of the tank, as shown in FIG. 3.
For a round tank, the pipes are semicircular; for a rectangular tank, the pipes are straight. The required motive flow is supplied to the liquid jet-mixing nozzles via these pipes. The motive flow pipes are situated opposite to each other at two sides of the tank. Supply pipes can either be fixed on the tank wall or on the tank bottom. Pipe dimensions will be determined by normal flow velocities to keep the friction losses inside the pipes low.2
Construction materials. Mixing eductors can be made of a variety of materials ranging from cast iron and stainless steel to plastics such as polypropylene (PP), polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). The inlet connection can be a threaded or flanged type.
The efficiency of a mixing eductor. A mechanical agitator requires about 25% less energy for blending liquids in tanks smaller than 3 m in diameter. In larger tanks, mixing eductors and mechanical agitators use about the same amount of energy. In applications involving solids suspension and for gas/liquid contacting, a mixing eductor uses less energy (typically 20%–40% less) than a mechanical agitator. HP
LITERATURE CITED
VINAYAK PRABHU is a Deputy General Manager, Process, at Worley India Pvt. Ltd. in Navi Mumbai, India. He has more than 28 yr of industry experience as a process engineer for engineering, procurement and construction (EPC) Companies. Prabhu earned a B.Tech degree in chemical engineering from Nagpur University, India, and is a Chartered Engineer (CEng MIChemE) in the UK. The author can be reached at Vinayak.Prabhu@worley.com.