February 2024Tanks, Terminals and Storage—Advani (L&T Energy Hydrocarbon)

HP Tagline--Tanks, Terminals and StorageTank side-entry jet mixer design, arrangement and applications L. Advani, L&T Energy Hydrocarbon, Mumbai, India Side entry mixers, or jet mixers, are commonly used to achieve mixing in storage tanks and reactors. Impellers are another conventional device used for mixing in industry, but they are expensive for large storage tanks and underground tanks, so jet mixers have proven to be a better alternative. As opposed to mechanically agitated mixers, these mixers use jet technology to create high turbulence, high shear rates and vortex motion that offer several advantages in achieving rapid mixing over a short period of time without any moving parts. Jet mixing has become an alternative to conventional impeller mixing for various applications in numerous process industries. To ensure optimal mixing performance, it is important to understand the basis of jet mixer design, the impact of different parameters in jet mixer design and the preferred installation of mixers. Jet mixers can be used as a substitute for agitators in almost all cases where the liquids to be mixed are pumpable. Typical applications of jet mixers include mixing/blending, solid suspension, electro coating/pretreatment tanks, lube oil blending, neutralization tanks, storage tanks homogenization, temperature homogenization, lube oil blending, etc.Working principle. Jet mixers are primarily used in unit processes where liquid blending, solids suspension, flow generation or chemical reactions are key process parameters. The mixing of reactants, catalysts, etc., in a chemical and bio-chemical reactor can be achieved using a jet mixer, which offers the advantages of having no moving parts inside the reactor.A liquid flow is taken from the tank and supplied to the liquid jet mixing nozzles via a pump. Inside the motive nozzle pressure, energy is converted into kinetic energy. Negative pressure is generated at the motive nozzle outlet and the ambient liquid is sucked in. The suction flow is strongly intermixed with the motive flow in the adjoining mixing section and accelerated by impulse exchange. The drag effect of the exiting mixed flow increases the mixing effect significantly. Parts of a jet mixer assembly, including the nozzle, pipe, elbow, cone, jet nozzle, etc., are shown in FIG. 1.Advani Fig 01A mixing nozzle is defined as plain pipe or tube where the pumped liquid is accelerated into the tank, as illustrated in FIG. 2. Tank-mixing eductors, like that shown in FIG. 3, are used to agitate liquid, dissolve powdered solids in liquid, and mix two or more liquids intimately within a tank or other vessel without the use of baffles or moving parts inside the tank. Advani Fig 02Advani Fig 03Jet mixers work best with Newtonian fluids with low viscosities (< 0.01 Pa-s). Viscous fluids require more invasive mixing to achieve a uniform consistency, and jet mixers will probably not provide satisfactory results.Typical arrangement. The jet is usually positioned near the bottom of a large storage tank, angled toward the opposite liquid surface, as shown in FIG. 4.The length of the path from the nozzle to the tank wall should be as long as practicable. This will incorporate more of the contents compared to a shorter path. For large storage tanks, jet mixers are usually installed near the bottom of the side wall and aimed at the far wall just below the liquid surface, providing a diagonal path across the center of the tank. However, if the level drops below the target, the flow stream will hit the surface rather than the tank wall, reducing the mixing effectiveness.Small tanks (< 4-m diameter) can also be mixed with a jet mixer. The jet can be directed straight down if it is submerged and is not aimed at the tank outlet.Advani Fig 04Computational fluid dynamics (CFD). To determine the optimum configuration of any tank, CFD analysis of the tank mixing system must be performed based on the specific frame conditions. This analysis enables the definition of the exact performance data as well as the best possible installation position to avoid any dead zones inside the tank. By using CFD, it is possible to deliver a “perfectly” designed tank mixing system to decrease energy input and to deliver clear installation instructions that enable a quick system startup.The four steps for the proper design of mixers are:

Step 1: Determine the required turn time, or the time required to turn the tank contents over once. For a jet mixer, the turn time is simply the liquid volume divided by the pumpage—pumpage is typically five times the pump output, due to entrainment. The engineer estimates turn time based on the intensity of mixing required:

For mild agitation, typically 3 min–60 min. Applications include dye-blending, neutralization and storage-tank agitation.

For medium agitation, typically 30 sec–3 min. Applications include pH control and batch mixing (as well as solids suspension and heat transfer).

For violent agitation, typically 10 sec–30 sec. Applications include flash mixing, disinfection and pigment blending (as well as gas/liquid contacting).

Detailed estimations for mixing time/turn time can be estimated using correlations in the following section.

Step 2: Calculate the pumpage requirement. Dividing the maximum liquid volume by the required turn time provides the required pumpage [i.e., pumpage = (maximum volume)/(turn time)].

Step 3: Determine pump capacity, nominal pump horsepower (hp) and other parameters. Convert the pumpage to m³/min, then use FIG. 5 to find the pumpage figure that equals or exceeds what is required. In the same row, read the pump capacity, nominal pump hp, total primary nozzle area, number of primary nozzles and area per nozzle. Note that FIG. 5 assumes that the pumpage is five times the pump capacity; this is based on 1-cP fluid viscosity, but is fairly accurate to 30 cP.

Step 4: Calculate actual motor hp. As for solids suspension, the nominal pump hp must be corrected for fluid specific gravity.

Advani Fig 05Experimental correlations (and their limitations) for mixing time estimation based on literature.^1,2,3 Mixing time is an important design parameter in jet mixing. Many experiments have been conducted in literature to determine mixing time. In this section, various methods for mixing time determination and experimental correlations are explained in FIG. 5.The impact of the parameters affecting mixing time include:

Tank height—From the experimental correlations for mixing time, it can be observed that the mixing time is directly proportional to tank height. So, with an increase in tank height, the mixing time increases, while all other parameters remain constant.

Tank diameter—It is observed that the mixing time increases with an increase in tank diameter, and vice versa for a constant set of other parameters.

Jet velocity—From all correlations given in FIG. 5, it is observed that the mixing time decreases with an increase in velocity, and increases with a decrease in velocity, when all other parameters are kept constant.

Jet diameter—From most of the correlations, it can be observed that under the same tank dimensions and jet velocity, the mixing time is inversely proportional to the jet diameter.

Jet location—When the liquid depth is equal to the tank diameter, the optimum nozzle depth ranges from the liquid surface level to ¾ of the liquid depth; and when the liquid depth is smaller than the tank diameter, the optimum nozzle depth is the mid depth of the liquid.

Jet angle—As per experimental reports, inclined jets at the bottom provide better mixing times. It was found that horizontal jets provide longer mixing times than inclined jets at the bottom, and an angle of 45° provides shorter mixing times than angles of 30° and 60°.

Jet configuration—Many experiments have been conducted with different jet configurations, including inclined jets, vertical jets from the bottom or top of the liquid surface, downward pointing jets and upward pointing jets. By using the best configuration to eliminate poorly mixed regions, mixing times can be reduced.

Reynolds number—Experimental correlations depict mixing time as a strong function of the Reynolds number in the laminar regime and as a weak function in the turbulent regime. For example, the ranges of the jet Reynolds number should be considered while designing the jet mixer.

Density (stratification)—When the liquids to be mixed have comparable different densities or (Eq. 1):

(ρ₂ – ρ₁) / ρ₂ > 0.05 (1)

between the jet liquid and the bulk liquid; if the jet velocity is less than the critical velocity, then layers of high- and low-density liquids form and mixing will not occur. This is called stratification.

Viscosity—Different fluids can have different viscosities, which can affect mixing time. Higher viscosity fluids are more difficult to mix, so any change in viscosity must be considered while calculating mixing time.

A basic limitation of the correlations presented in FIG. 5 is that they predict well only over the range of parameters (i.e., correlations are case specific). Available literature is concerned with liquid-liquid jet mixing, and very few authors have considered solid sludge suspension using jet mixers. Mixer installation guidelines.

The jet nozzle should always be submerged during the actual mixing operation.

Side-entry jets should be installed along a radius to the tank wall and should protrude no more than 5 dj from the tank wall. The nozzle should be no more than 5 dj from the tank floor or liquid surface. Axial jets should be installed perpendicular to and no more than 5 dj from the tank floor or liquid surface.

CFD simulations can be extended to indicate the proper position of the jet, which can also affect mixing time. This must be considered during jet mixing design.

The optimum angle of injection for mixers is 30°–45°, which lowers mixing times.

The tank outlet location should not be near the jet location or feed from the jet will be sucked directly to the outlet system.

Multiple jets: Single-axial jets should only be used when the ratio of the tank height (H) to diameter (D) lies in the range of (Eq. 2):

0.75 ≤ (H / D) ≤ 3 (2)

Single-side entry jets should be used only when the ratio of H:D lies in the range of (Eq. 3):

0.25 ≤ (H / D) ≤ 1 (3)

Multiple side entry jets should be used only when (Eq. 4):

(H / D) ≤ 0.25 or if (H / D) > 3.0 (4)

Therefore, multiple jets should be used when designing a large-diameter shallow tank or a tall tank.

O-ring setup: Jet mixers should be installed at the deepest possible point so that good operation and effective mixing are obtained even with low liquid levels. A level of 1 m–2 m above the jet mixer is sufficient to avoid foaming. FIG. 6 shows an installation example in a tank and mixer arrangement in a neutralization basin.

Advani Fig 06The 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 in the tank. Supply pipes can either be fixed on the tank wall or on the tank bottom. Pipe dimensions depend on normal flow velocities to keep friction losses inside the pipes low. The size of each mixing nozzle and its alignment (e.g., the installation angle as well as the distance from one nozzle to another) are further results of the dimensions.To determine the number of mixers, the following criteria—explained in the section below—are decisive: mixer geometry, size of the tank, basin liquid to be mixed, mixing time, maximal and minimal tank liquid level, etc.Jet mixers vs. mechanical agitators. FIG. 7 shows a jet mixer and a top-entering turbine mixer in typical mixing vessels. Advani Fig 07A comparison of the following factors can be helpful in deciding between mixers and mechanical agitators for applications:

Metal fatigue—Rotating parts in a mechanical agitator are subject to reversing stresses that cause metal fatigue and, often, failure of shafts, seals and agitator blades, especially in certain corrosion/temperature environments. A jet-mixing system is not subject to reversing stresses.

Mechanical components—A mechanical agitator has a shaft and gears (and potentially immersed bearings if the shaft is very long), but a jet mixer has no such parts. A jet-mixing system does have a centrifugal pump with a motor, while a mechanical agitator has only a motor. However, a mechanical-agitator system may have a feed pump.

Structural supports—A jet mixer is usually anchored to and supported from the bottom of the tank but may be supported from the walls or top (if the tank is very deep). A top-entering agitator requires support at the top of the tank, which may mean specifying thicker walls or stronger materials.

Location in the tank—A jet mixer is typically located about 0.5 m above the bottom of a tank, which saves energy in achieving off-bottom solids suspension because the mixing energy is provided where it is needed. A top-entering agitator typically requires about one impeller diameter of clearance at the bottom.

Utility holes—A jet mixer may require a larger utility hole in the top of the tank because it may have a greater diameter than a typical mechanical agitator for the same application.

Liquid/liquid blending—Experience shows that a mechanical agitator requires about 25% less energy for blending liquids in tanks with a diameter of < 3 m. In larger tanks, jet mixers and mechanical agitators use about the same amount of energy. HP

LITERATURE CITED

Wasewar, K. L., “A design of jet mixed tank,” Chemical and Biochemical Engineering Quarterly, January 2006.

Zughbi, H. D. and M. A. Rakib, “Mixing in a fluid jet agitated tank: Effect of jet angle and elevation and number of jets,” Chemical Engineering Science, February 2004.

Bathija, P. R., “Jet mixing design and applications,” Chemical Engineering, December 1982.

First Author Rule LineAuthor pic AdvaniLAXMI ADVANI is a chemical engineer with more than 13 yr of experience in process engineering within the chemical process industry. She has worked on numerous greenfield and brownfield projects/proposals in oil and gas, LNG regassification terminals, petrochemicals, offsite/utilities and specialty chemicals facilities during her tenure with companies like Petrofac, Larsen & Toubro and Toyo Engineering. In an ongoing project, she has utilized learnings for multiple side-entry jet mixers, the design aspects of these mixers, mixing time evaluation criteria and their installation. Advani earned a B.Tech degree in chemical engineering from the Institute of Chemical Technology (formerly known as UDCT) in Mumbai. Additionally, she is a TUV certified Functional Safety Professional (Level 1). She intends to appear for PMP certifications that will enhance her capabilities in the project management domain.CoverAd—Shell revampContentsAd—Honeywell UOPMastheadAd—MerichemEditorial CommentAd—AFPM Annual MeetingGlobal Project DataAd—HPI Market Data 2024Viewpoint—Tanoguchi (Yokogawa Electric Corp.)Special Focus OpeningSPECIAL FOCUS—Gardner (KBC)SPECIAL FOCUS—McEleney (Emerson)Ad—PEMN CCS StrategySPECIAL FOCUS—McBul (OQ Specialty Chemicals)SPECIAL FOCUS—Burton (AMETEK Process Instruments)Ad—MEPECSPECIAL FOCUS—Bramson (ABS Group)Petrochemical Technologies—Delhomme (Linde Engineering)Maintenance and Reliability—AnwerTurnaround/Project Mgmt—BenomarAd—AFPM IPCEnvironment and Safety—Rodrigues (Saudi Aramco)Environment and Safety—Manelick (aeSolutions)Environment and Safety—Bowling (Seeq Corporation)Ad—PE Outlook 2024Carbon Capture—Carugo (Emerson)Ad—H2T CircTanks, Terminals and Storage—Advani (L&T Energy Hydrocarbon)Tanks, Terminals and Storage—Safamirzaei (Pasargad Energy Development Co.)Sales OfficeAd—HP Circ (Back Cover)