Homogeneity and Blending
The more advanced your side entry mixer, the more traditional flow patterns it will incorporate into its mixing actions. Multi-directional and dimensional flow patterns have been shown to achieve the most homogeneous solution in the most timely fashion. Here are a few of the traditional patterns that should be combined in a modern side entry mixer:
- High velocity flow - Increasing the speed of mixing in a side entry mixer creates the energy needed for many of the other types of flows that create homogeneity. Materials with a lower viscosity such as light oils, gasoline and a variety of aqueous solutions require higher velocities than other substances. High velocity is also imprint when mixing substances of vastly different densities.
- Cross tank flow - Creating a flow that is contrary to the pull of gravity and the resistance held by substances of higher densities creating a floor under low density materials is important for both homogeneity and speed of process. The cross tank flow helps to quickly make substances of highly variant densities, breaking the barriers these substances tend to form against each other if they are allowed to settle into layers.
- Bottom to top/top to bottom flow - A flow that works with gravity can be used to speed up the other flows that occur within your side entry mixer. The more tank terms that you can create in total, the more effective your mixing efforts will be.
- Running mixer during pump or filling - If you have materials of extremely variant densities, you may benefit from adding the heavier materials during the operation of the side mixer instead of starting the process with all materials already inside. You help to eliminate the traditional hurdle of stratification, and you have the ability to mix materials of heavier densities at lower levels of power.
Running a high velocity flow, a bottom to top flow and a cross tank flow gives a three dimensional flow state that is optimal for extremely fast, efficient mixing. If you are looking for the most cost-efficient way to quickly blend materials that may give traditional mixers problems, ensuring that your side mixer has this capacity is a great way to improve productivity.
The angles that are used in your side entry mixer also help to optimize the process. If you have the ability to operate your mixer during its pump time, having a mixer cluster that is 22.5° left of your tank inlet is the best position. The angling becomes less important if you are not planning on utilizing the so-called "pump up" time of your mixer.
If you are looking to prevent BS&W buildups, putting a 60° rotation on your swivel angle option will give you optimal performance in this area. Make sure that your swill ball is protected from leakage with a seal that can handle the job on hand.
Selecting The Proper Impeller
Choosing the right impeller for the job ensures that you will will achieve the highest efficiency during your mix. The fluid foil impeller has been used by many processes because of its incredibly high thrust to power ratio. This ratio ensures the maximum flow efficiency of the side entry mixer. The fluid foil impeller can also be easily used in parallel to supply the needed flow to larger tanks.
However, you may need to choose an impeller based on an imbalance of characteristics depending on the job that is to be completed. When this is the case, here are the most important aspects of an impeller that you should consider before committing your project..
- Viscosity – This is always the primary factor that you must consider when you are choosing your impeller, because viscosity is the factor that affects many other aspects of the impeller design. For instance, a hydrofoil impeller is usually best for jobs with lower viscosities. Substances that are more highly viscous may actually benefit from a turbine or axial flow blade. Viscosity also tends to affect the density of substances that are being mixed, which creates further considerations for a job.
- Tank placement and design - The scale of your mixing tank is an incredibly important consideration when choosing your impeller. First, consider the aspect ratio of the tank. The most efficient mixing usually occurs when this ratio is at unity, or as close as possible to unity. You must also consider where in the design the impeller may be placed. If the impeller is placed improperly, you are sure to get a nonuniform result along with staged flow patterns. Do not mistake the phenomenon of "vortexing" with creating homogeneity in a mix - the wrong impeller will only rotate the substances in your tank rather than really mixing them. The size of your tank should also determine the number of impellers that you use. Lastly, the size of each individual impeller should be considered based upon the intensity that you want to apply to your mix.
- Impeller construction - Stainless steel is perhaps the most common and popular material used to construct impellers today. Stainless steel resists corrosion, which will naturally decrease the depreciation of your impellers. It is also relatively simple to clean. However, depending on the needs of the job, you may also want to look at titanium, nickel alloys and carbon steel. These metals have properties that are more suited to mixes with certain characteristics, and they may also be coated with different finishes depending on their application.
Bottom Sedimentation and Sludge Control
If density and gravity are allowed to take their toll on a mix, the resulting sedimentation and/or sludge will create numerous problems, which, even if addressed, will drastically slow the efficiency of the mixing process. This is usually an issue when mixing crude oil, a substance that will almost always bring along with it a certain percentage of bottom sludge and water. The natural concentration of this BS&W is usually around 0.5 wt %.
The sludge that crude oil brings with it is a dense collection of mud, sand, organic acids, salt, polymers, waxes, asphaltenes and corrosion products. Sludge will settle at the bottom of your tanks, as it is always heavier than the crude oil that brings it. Sludge becomes excessive if a mixer is not operated properly or if it is underpowered. If your mix does not retain the appropriate ambient temperature, sludge may also disperse more freely. The heavier that a crude oil is, the higher that its percentage of sludge will be. For instance, high sulfur crude oil comes with a wt % of sludge that is significantly higher than 0.5. No matter what kind of sludge you have in your mix, once it is fully settled on the floor of your tank, it will harden and refuse to be removed except by a special process.
The side entry mixer is known to prevent the settling of sludge on the tank floor as well as helping to suspend hard and sludge that has accumulated over time. Below are just a few of the problems that your mix may incur if you allow sludge to fully settle on the floor of your tank.
- Loss of storage capacity - Sludge takes up very real space in your tank, and the way that it throws off measurements can also cause you to incorrectly assume the capacity of your tank.
- Entrapment – Sludge also tends to cause other materials to become caught in your tank, including settled water and oil. These problems will not only showcase themselves in your mixing tank, but also in your downstream equipment. You may have to deal with increased water carryover, desalter overload, oil carryunder, pressure build up in the pipestill and crude preheat train fouling.
- Environmental and safety concerns - There is a possibility of tank boilover if a fire occurs in the vicinity of the tank because of the trapped water. Floating roofs may also land on uneven layers of sludge.
- A need for more frequent cleaning - Every off stream or unexpected tank cleaning that occurs introduces hazards and new expenses into the process. The process of cleaning a tank is also incredibly time-consuming, and will require extra resources to dispose of the sludge.
- Corrosion of the tank wall - Sludge eats away at the walls of your tank, shortening its lifespan.
If you are operating your side entry mixer to get rid of hardened sludge, the mixer shaft should be placed at an optimal angle of 10 degrees left of the diameter of the tank when the mixer rotates clockwise from the side that the motor is on. The mixer should not be installed facing the center of the tank; otherwise, you risk forming a vortex that will only build sludge in the center rather than on the bottom of the tank.
Side Entry Mixer Positioning In Oil Storage Tanks
Although the architecture of a side entry mixer automatically improves the architecture of most projects, the positioning of that mixer is also very important. Proper positioning of the mixer keeps it from having to overextend its resources to create homogeneity. The total number of mixers used in a process may also be reduced if the positioning of each mixer is optimal.
The use of either gear driven or belt driven systems also affects the mixer drives that are used. In general, mixer drives should only be 60 hp if a system is gear driven, with an increase to 100 hp if a system is belt driven. In both types of systems, when multiple mixers are used, they are usually of the same size.
There are two types of mixer distribution patterns that side entry mixer projects employ – clustered or distributed configurations. These two configurations are generally equal as long as the total energy that is generated from all mixers is adequate for the project. One configuration may outperform the other if it allows for a more optimal use of individual mixers in the process, creating more room for additional mixers of the same size to be employed.
Although the configuration and positioning of side entry mixers is contingent upon each individual job, there are some general guidelines that you can follow for common mixing projects.
- Light or medium crude oils - If light or medium crude oils comprise the majority of substances to be mixed, side entry mixers should have a minimum of 0.4 hp/kbbl.
- Heavy crude oils - Heavy crude oils are more likely to contain high density sludge. They require a more robust mixing energy in order to keep that sludge and suspension. A mixer design of 0.4 hp/kbbl is the absolute minimum to keep sludge from accumulating, although the outside temperature and the other substances in the mix will have an effect on the optimal number. In short, the design should be above 0.4 hp/kbbl for best results.
There are also certain swivel programs that may be used as a guideline for optimal efficiency in mixing. Depending on the cleanliness of the tank and the need of the project to blend homogeneous solutions, the following configurations may be followed.
- Solids suspension for clean tanks - 30° left, 10° right, 20° left, 20° right, 10° left, 30° right, 30° left, etc., with revisions to be made every 2 months at a minimum.
- Solids suspension for dirty tanks - Use the same program as for clean tanks. However, the initial operation of each mixer should be for 8 hours on one angle per batch. This process should be continued until the unwanted deposits have been reduced to a manageable level. After this, revert to the previous program of changing the swivel angle every 2 months.
- Maintaining a homogenous mix – In order to maintain a mix that has already been homogenized and vetted for uniformity and temperature, position your mixer at the 10° left angle. This position will help to ensure a full top to bottom turnover and keep your mix homogenous for longer periods of time.