Geopolitical events, rising energy prices as well as directives and regulations are encouraging companies to assume social and ecological responsibility. In addition to the German energy transition or the hydrogen strategy, the EU is also massively driving forward the economic transformation through the Green Deal [1]. Energy-intensive sectors, such as the chemical and pharmaceutical industries, represent decisive levers for reducing energy consumption. Even small improvements in efficiency save large amounts of energy.

High amounts of energy are required, among other things, for the operation of chemical reactors. The main consumers are the mixing and heating technology as well as the necessary pump energy for the service media. High operating costs and ecological consequences via emissions during power generation or gas consumption are the consequences. In addition, glass-lined reactors require a lot of energy during the manufacturing process and incur high investment costs. For these reasons, glass-lined reactors must be operated as efficiently as possible. This can be achieved through short process times, which can be achieved with a process-suitable mixing technology and optimally adjusted boundary conditions to achieve an efficient heat transfer.

Use process-specific glass-lined Mixing Technology

Processes that require high speeds (gassing, suspending) or in which highly viscous media are processed have a high energy demand. In order to reduce the necessary agitator power and, in particular, the process times (associated energy quantity), a variety of glass-lined turbines and baffles are available. The targeted use of specially developed turbines significantly shortens process times, so that operating costs and CO₂ emissions can be reduced. For example, near-bottom turbines with a large diameter can be used specifically for fast and efficient whirling up of particles, or turbines with a high shear effect can be used for gassing and dispersing. Near-wall turbines, such as an anchor or cross arm turbine, are used for highly viscous media so that a good homogenisation and heat transfer are achieved. Low-shear turbines, such as the UFT (UltraFlow turbine), are used in crystallisation processes, among other things.

In addition to a process-suitable agitator, an often neglected topic is the baffle system used! Baffles generally serve to increase the specific power input (kW/m³) and to shorten blend times, as a purely tangential product flow in the reactor is disturbed/diverted, the turbulence is increased and the formation of vortices is reduced. In addition to these main effects, a baffle can be used specifically to support the process. Depending on the process, baffles allow the introduction of fluids (gases/liquids) at a suitable position, such as near the turbine (relevant for gassing) or directly behind the baffle (effective mixing). Special baffles, such as the SEGTEC (Figure 1), also significantly improve surface gassing or the entrainment of floating liquids or solids. In particular, an eccentric vortex is generated in the reactor, which guides the gases or floating substances specifically to the upper stage of the agitator system. The drawn-in substances are distributed much more effectively and finely in the reactor chamber, so that rapid reactions take place. Shorter process times or alternatively reduced speeds save energy (Figure 1).

Baffles can also be used for process monitoring (e.g. temperature measurement, sample taking etc.). In addition, THALETEC is the only manufacturer worldwide to offer a glass-lined tube bundle heat exchanger (a so-called PowerBaffle), which realises the functions of a baffle, a temperature probe and a heat exchanger implemented into an agitated vessel. Shorter process times or a reduced temperature level between product and service medium (thus less ambient losses) noticeably save energy (Figure 2)! Plant safety is also increased due to the higher available heat output in exothermic reactions. The PowerBaffle can also be used to separate cooling and heating circuits.

Another main consumer is the heating technology of glass-lined reactors. Depending on the chemical processes, the product must be tempered accordingly. Classically, a jacket or a half-pipe coil, which can also be divided into several tempering zones, is used for cooling or heating the product. Both variants have advantages and disadvantages depending on the service medium used. In most cases, however, the jacket in combination with agitating nozzles (achieving a fast tangential flow around the inner vessel) is the preferred variant for several reasons. Only at higher pressures (p > 6 bar) of the service medium can the half-pipe coil be advantageous. Typically, thermal oils, water (partly as a glycol mixture) or steam serve as heat transfer medium.

Since glass-lining is physically a poor thermal conductor, the heat transfer coefficient (k-value or U-value) depends strongly on the thickness of the glass-lining layer. This makes it even more important to adjust the other influencing variables! A suitable heat transfer medium must be selected and favourable flow conditions must be realised on the inside and outside of the inner vessel in order to achieve a high heat transfer coefficient (α) on both sides. This is achieved on the inside by turbines that have a high pumping or circulating capacity. Multi-stage agitator systems support this effect. The transferable heat flow depends on the agitator speed. However, the heat flow follows a root function depending on the agitator speed (Figure 3)! This means that above a certain speed, no appreciable increase in transferable heat flow can be achieved. On the contrary, since the input power of the agitator is directly converted into heat in the product, a further increase in speed is counterproductive in cooling processes (the speed is included in the agitator power to the 3rd power!). This extreme point of the heat balance is called "optimised agitator speed" (Figure 3).

Similar conditions are present on the outside of the inner vessel. Here again, the influence of the flow velocity in the jacket or in the half-pipe coil follows a root function, i.e. above a certain volume flow, an increase in the same causes only a marginal increase in the transmittable heat flow (Figure 3). The motto "more the better" does not apply here! For energy-efficient operation, the material data of the heat transfer medium must be favourable and the flow conditions (agitator speed, volume flow) must be set sufficiently.

All these aspects flow into the heat transfer coefficient. In addition, the heat exchange surface and the temperature difference between the service medium and the product are also decisive. However, to minimise ambient losses, the temperature difference should be kept at a low level. Insulation material, such as rock wool, mineral wool or foam glass, reduce ambient losses.

The most energy-saving heat technology is thus achieved by means of a high heat transfer coefficient, a low temperature level between product and service medium with the largest possible heat exchange surface! The heat exchange surface is specified and limited by the reactor size. Thanks to THALETEC, this limit is broken using one or more PowerBaffle, which significantly increase the heat exchange surface! This Unique Selling Point also provides particularly high-quality heat exchange surfaces, as the glass-lining technology allows for a thinner glass-lining layer thickness with the same chemical resistance (Figure 4). THALETEC also offers a thinner and controlled glass-lined layer thickness for the inner boiler within the permissible standard range (DIN-EN-ISO 28721-1) to further increase the U value.

Further operating costs and CO₂ emissions result from the provision of service media via pumps. Large volume flows are usually necessary for the operation of the reactors. Nevertheless, to keep the pumping power at a low level, hydraulic pressure drops in a system must be minimised. Large-diameter piping systems are to be preferred, and the choice of heating or cooling chambers for the reactors also has a decisive influence. Half-pipe coils have the greatest pressure drop in comparison due to their long spiral shape and the secondary vortex that forms inside the coil. The greatest pressure drop in jackets results at the parallel-connected agitating nozzles. A PowerBaffle has the lowest operating and energy costs. This also makes sense, as several pipes connected in parallel generate a low pressure drop in a PowerBaffle (Figure 5).

Reglassing prevents CO₂ emissions!

Glass-lined reactors or storage vessels have to withstand extreme chemical-corrosive and partly abrasive conditions. When the service life of an apparatus is reached, a replacement must be procured. The production of glass-lined apparatus is an energy-intensive process due to the use of steel and the multiple firing processes. To reduce or even prevent CO₂ emissions, there is the option of Reglassing instead of manufacturing a new apparatus. For Reglassing, a used reactor or storage vessel and all built-in parts are decontaminated and the worn glass-lining is blasted off. Necessary repairs or modifications can now be carried out. After that, the new glass-lining is applied. The result is an as-new glass-lined apparatus with a significantly lower CO₂ footprint compared to a new apparatus. For this purpose, THALETEC has recalculated an exemplary glass-lined reactor of the size CE16000 (17.95 t incl. drive, agitator, baffle etc.). For this purpose, the energy requirement for steel production [2], the internal energy expenditure of the manufacturing steps and the firing processes were considered. Internal calculations showed that about 131,300 kWh are required for a new production and about 58,400 kWh for a Reglassing of the same apparatus (Figure 6). This corresponds to an energy saving of 55 % and, according to the German electricity mix of 2019 [3], a CO2 saving of around 29.8 t! This is mainly due to the reuse of the steel body and thus the energy savings in steel production.

All listed aspects can be evaluated by THALETEC and thus an assessment of the energy saving potential is made possible. Operating costs and CO₂ emissions can thus be significantly reduced depending on the application.

References

[1]  E. Kommission, „Der europäische Grüne Deal,“ Amtsblatt der Europäischen Union, Brüssel, 12/2019.

[2]  M. Schimmel, C. Achtelik, Jannik Schlemme, „Energiewende in der Industrie – Branchensteckbrief der Eisen- und Stahlindustrie,“ Navigant Energy Germany GmbH, 2020.

[3]  Umweltbundesamt, „Bilanz 2019: CO₂-Emissionen pro Kilowattstunde Strom sinken weiter,“ [Online]. Available: umweltbundesamt.de. [Zugriff am 10 2021].