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Challenges and Solutions in Industrial Mixing Technology
The current trend in industrial practice is going away from standardised solutions to wards individual solutions. One no longer talks about the agitator alone but about the complete mixing system. As a result , production and investment costs can be lowered, and as this article shows productivity increased.

Mixing technology in the past was characterised by the desire to use simple, universally standard impellers that could be operated in standard vessels with standardised vessel internals. Blade impellers, pitched blade turbines, flat blade disk turbines or anchor impellers for viscous applications all belonged to this category. The reason for their popularity was partially due to the fact that characteristic flow values were well studied and published.

On the other hand, the simple geometries were beneficial to grid generation for flow simulations, which explains their popularity with users of numerical simulations. The prevailing common opinion was that such mixers were reasonably priced. However, when one considers the specific hydraulic values of the blade impellers and flat blade disk turbines - particularly the power number and radial force coefficients - then it becomes quite obvious that the high forces and torques will lead to an overall more expensive mixing solution.

In industrial practice, however , a reverse trend from standardised to more individualised solutions is beginning to assert itself. This can influence impellers that have been developed for specific mixing tasks such as suspension or gas dispersion. In addition, adapting a mixer to an individual process or process step is no longer unusual. All functional components such as the vessel itself, baffles, feed points or heat exchangers are all taken into account with respect to their interaction with the impellers. One no longer talks about an agitator alone but of a mixing system. The overriding aim of all these measures is to reduce production and investment expenditures while at the same time integrating all factors such as the effectiveness and profitability of a production process thus leading to an increase in profit.

Economy of Scale in "World Scale" Plants At the same time, current development is moving towards ever larger plant facilities to benefit from the so-called economy of scale. With respect to investment and fixed operating costs, one large unit is always less expensive than several smaller units that result in the same total output. This trend places new challenges on the design engineers; traditional scale-up rules have not yet been proven with these sizes. Economy of scale is being applied to all branches in the process industry, especially impressive are examples in the production of bulk chemicals.



Figure 1, on page, shows an oxidation reactor in which paraxylene in a continuous reaction with atmospheric oxygen is transformed to terephthalic acid as a precursor to polyester. Today, it is possible to produce up to 1 million tonnes of terephthalic acid per year in one single vessel; the agitator power approaches 2 MW. The impellers in this case are no longer the combination of pitched blade and flat blade disk impellers that were used for decades. They were replaced by the so-called concave impellers that reveal a number of advantages as shown in Figure 2 on page. The systems approach led to a complete reworking of the reactor concept by taking into account reactant feed points and product discharge as related to the kinetic and thermodynamic reaction parameters.



This not only enables a satisfactory fulfillment of the requirement for further performance consolidation and larger reactors but revamps of reactors in older units amortised themselves within less than half a year. Higher productivity and lower raw material consumption thanks to fewer byproducts are the factors leading to profitability.

New Concepts for Specialty Chemicals
Investment and production costs can also be lowered for specialty chemicals with smaller production amounts using the system approach. One example is a reactor concept that is based on combined gassing (Figure 3 on page). In the meantime, this system has gained a firm position when it comes to reactions using purified gases such as Hydrogen (H2), Carbon Monoxide (CO), Ethylene Oxide (EO), etc.



With the standard design, the fresh gas is finely dispersed using a primary disperser but is only partially dissolved. The non-reacted gas enters the headspace and is permanently recirculated into the fluid by a self-aspirating impeller. This leads to high mass transfer rates and a fast and complete conversion of the gaseous reactant. This concept can be extended to include an external gas loop driven by the agitator.

There are reactions where gas or vaporous by-products are created, for example, when producing tertiary fatty amines - first of all water (vapor), and then ammonia in a later process step. These gases can accumulate in the headspace and bring the reaction to a standstill due to pressure build up and in case of an equilibrium reaction, lead to an incomplete conversion.

In such cases, the traditional technology of external separation and gas recirculation with an additional blower must be applied or the discharged gas must be disposed of. But it is also possible to use the agitator as a drive for external circulation of the gas through a gas cleaning device. This cleaning device is usually a separator or condenser that removes the non-desirable components out of the gas. The purified gas is then returned to the reactor. To avoid a remixing with the contaminated gas in the headroom, the purified gas is fed into a dip pipe that surrounds the agitator shaft located below the agitator lantern.

In this way, the purified gas is kept separate from the contaminated headroom. The gas proceeds into the hollow shaft from the space inside the dip pipe and on to the self aspirating impeller and back into the liquid. Instead of a condenser another separating option can be used for cleaning non-condensable gases such as an acid or alkaline washer.

Flexibility for Commercial Products
Another broad field for special impeller types or system solutions is in the area of formulated products. This includes ready-to-use commercial products for commercial or private use, for example, adhesives, sealing compounds, insulations, foods, food supplements or care products and pharmaceuticals.

All fields have in common the value (price per ton) as well as high requirements for a reproducible quality and therefore production process. They almost always pass through phases of high viscosity, most often with non-Newtonian flow behavior and yield stress.

The universal anchor impeller has been superseded by an axial forced flow impeller with markedly improved mixing behavior in the laminar flow area in this case as well. This impeller that is suited for viscous media is now the starting point for a whole range of individual system solutions. The functions of viscous mixing and dispersion have been separated using multi-shaft systems.

Individually and adjusted to the mixing task, high-speed impellers can be combined with forced flow impellers. The high speed impellers can also fulfill an axial pumping function; often a dissolver disk or rotor-stator system is used. Independent of one another, the rotational direction can be adjusted and herewith the pumping direction or the power input depending on the speed. Cooling viscous media is often the rate-determining step within a batch; fluids with a yield stress often create a layer on the vessel wall through which heat is transferred by thermal conduction.

The solution for slow-running impellers are scrapers that continually renew the wall layer and can increase the heat transfer coefficient by up to 10 times. It is possible to cover a multitude of process requirements using this described system. This could include blending water-like to highly viscous media, processing in multi-phase systems with surface entrainment, dispersion of solids or liquids in the high viscous phase, degassing steps and parallel to this an efficient heating and cooling process. The combinatorial analysis of multi-shaft systems leads to the so-called 'single-pot-process': The production of formulated products does not always have to be carried out in several different units connected serially. With the modern mixing systems they can be sequentially processed in one and the same vessel. What this innovative technology means for investment and operating costs is quite obvious (Figure 4).



Outlook
Economic production of chemicals can no longer be achieved with standard solutions. Mixing technology encounters this challenge with individually adapted mixing systems. The impellers are planned according to the mixing task and all functional components within the vessel are coordinated to fit respectively.

The design of such complex systems is still impacted by the theoretical fundamentals of flow and process technology, by the empirical knowledge of the equipment manufacturer as well as laboratory and pilot tests.

With increasing computational power in the coming decade that allows numerical full span models of reactors while including the hydrodynamic, kinetic and thermodynamic conditions, it will be possible to deliver valuable information and herewith contribute to the operational efficiency in production processes.