The manufacture of sulphuric acid from solid sulphur starts with producing molten sulphur. The granules are melted in specifically designed equipment. The molten sulphur is then filtrated and conveyed to the sulphur burner. It is highly important to work with reliable equipment from the very beginning of the process. Any issue in the very first step of the process may have significant consequences on the whole fertilizer complex. Proper melting of sulphur requires well-performing agitating equipment.
A client had an expansion project which included two sulphur melters. The heat transfer through heating coils was extremely important to manage the process. In addition, cold winters and hot summers caused challenging ambient conditions. It was specified that the agitator should have a 200kW motor to drive pitch blade turbines.
Rather than just following the specification, Sulzer wanted to fully understand the process and the conditions to propose the best technical solution. The Scaba SHP hydrofoil propeller was calculated to provide more agitation than a pitch blade turbine and would generate higher velocities and better heat transfer. Sulzer concluded that a 132kW motor was enough, thanks to the highly efficient hydrofoil impellers.
During the engineering phase, Sulzer evaluated two options, 132 and 200kW. The velocities inside the tank and locally at the heating coils were examined in a CFD (computational fluid dynamics) study. The average flow velocity in the tank would increase by 11% with 200kW, while the local velocity at the heating coils would be only 4% higher versus the 132kW. This illustrates how much more motor capacity is needed to achieve a higher velocity.
Sulzer supplied two agitators of type Scaba 240FVPT-Sff with hydrofoil SHP impellers to the customer. The agitators were equipped with variable frequency drives for lower operating cost. The agitators are running at the rate of 132kW or lower, while the actual installed motor is 200kW, only as capacity reserve for extremely cold days.
Due to the critical application and the demanding site conditions, these agitators are of heavy-duty design. The ambient temperature spans from -47 to +40°C, which required special precautions for the drive unit and the motor. The agitators are equipped with an industrial gearbox with both heaters and coolers. The wetted parts are made of standard stainless steel, 316L or duplex stainless steel for critical parts where higher strength is required. The gas phase inside the tank can be aggressive, and the parts exposed to gas are made in 904L with additional rubber lining on top for extra security.
Agitators in the chemical industry - advice from Patrik Kolmodin, agitators product line manager at Sulzer
Agitators need to have a flexible design to suit various tank sizes and shapes. It is also important to adapt the agitation intensity to the process needs. Unlike pumps, it is not possible to select an agitator based on flow and head. Instead, the agitation must be quantified in other ways to manage the process. This often leads to misunderstandings, oversizing and costly investments. When designing an agitator, the most common mistakes are:
Quantifying agitation
Most people have some intuitive idea of mixing and blending processes. For example, the infusion from tea leaves will remain near the bottom of a cup of hot water until it is stirred. Just a small amount of stirring will blend the mixture. Continued stirring will not improve the blend.
There are many processes in the chemical industries that require agitators. Contrary to the way in which we blend tea, many agitators mix too much and at too high intensity, without achieving a better result.
For quantifying, Sulzer uses methods such as the degree of agitation, which is based on the liquid velocity at the surface of the tank. This is a very powerful design method for many types of processes, different rheologies, and different tank shapes. It also takes more efficient hydraulics into account. Other less good dimensioning tools are for example power per volume, using W/m3. This is a poor dimensioning principle which is neither good for scaling, nor does it take more efficient hydraulics into account.
Another method is to use the impeller pumping capacity. This method can be difficult as well because suppliers have different measuring methods and – not least – it depends on the impeller diameter and rotational speed. A large, slow-running impeller will generate a higher pumping capacity at the impeller than a smaller impeller running at a higher speed, while the overall agitation level may still be the same. In addition, the impeller pumping capacity does not consider the pumping direction.
When targeting a certain agitation level, it is important to understand that a small increase in the agitation level may result in a much higher power consumption, because the power need increases exponentially with the degree of agitation. An increase in the degree of agitation from 8 up to 10 may double the power consumption, but the agitation intensity only increases by 25%.
How much agitation is needed?
Good enough would be fine! The challenge in designing agitators is that what is “good enough” for one may be different for another. There are basically three things we must know to make a good agitator design:
These three simple rules will allow good selections, even though not necessarily according to the original expectations.
Not getting the right agitation intensity Replicating existing agitator design Poor input data Poor evaluation of the energy costs Mechanical design – heavy-duty or light-duty design? The tank shape. This also includes the agitator mounting height. The process. What is the application or how much intensity is needed? The media. For example, for mineral suspensions, it is important to specify parameters of the solids, such as the particle density and the size distribution, the liquid properties, and a process description.