Canned Motor Pumps for TDI Applications

Phosgene: Phosgene is produced by the mixing of the gaseous substances chlorine and carbon monoxide, the activation energy of which must be reduced by the activated carbon catalyst. To avoid dissociation, i.e. separation, of the mixture, a separation temperature of well below 100 °C must be ensured. The phosgene won in this way is dissolved in cold ortho-dichlorbenzene (ODB).

TDA: The toluene substance which is formed during cracking is first nitrified by adding nitrate and then hydrogenized by adding hydrogen. The resulting TDA is then melted at 140 °C and mixed with hot ODB.

TDI: The production of TDI can be divided into 2 phases. The process begins with a cold phosgenization, and exothermic reaction, and ends with the hot phosgenization which is endothermic.

The cold phosgenization 
In the cold phosgenization, the hot TDA-ODB solution is stirred into the cold phosgene-ODB solution at low pressure (1–2 bar) or high pressure (up to 20 bar). In a reference plant, this step is performed at 300 °C which increases the energy efficiency of the plant. About 80 % less ODB and about 40 % less phosgene are needed due to the high temperature. The resulting suspension very quickly forms solids. In order to avoid this, various devices exist such as a circulating pump system, counterflow mixing chambers, centrifugal pumps, Venturi mixing nozzles and other mixing devices.

The hot phosgenization 
Then the so-called hot phosgenization takes place. Hereby, heat and phosgene are added to the suspension until a clear liquid and no further hydrogen chloride is produced. In this process step, a liquid raw material (approx. 15 % TDI, 80 % ODB, 5 % phosgene), hydrogen chloride vapour and other residues are produced. Hydrogen chloride vapour is corrosive and special alloys with a nickel content are required to protect the apparatuses. Mixing the gas with water forms hydrochloric acid. Hydrogen chloride is then mixed with 18 %-hydrochloric acid to obtain a 30 %-hydrochloric acid.

The liquid raw material is broken down into different constituent parts by various distillation steps in a rectification column. ODB and phosgene are fed back into the process. Ancillaries are cleaned from the TDI in the last column. The distillations are performed in a vacuum.

In a reference plant, a single-stage HERMETIC canned motor pump of the TCN type is used for feeding phosgene to the ODB solution. This series is particularly suitable for safe pumping of aggressive, toxic, hot, explosive, valuable and inflammable liquids and liquified gases. The pump is designed for use in fuel depots, terminals, chemical and offshore plants, gas storage caverns and industrial plants. In conjunction with different installation systems, the pumps of this series which are available both as TCN (single-stage) and TCAM (multi-stage) are the optimum pumps for these application cases.

In addition to the double safety jacket, they also achieve best marks in terms of long service life. This is because the hermetic design requires the bearings to be assembled in the operating liquid. Therefore, only hydrodynamic slide bearings are normally used. When operated correctly, these have the advantage that there is no contact between the bearing sliding surfaces. They are therefore wear and maintenance-free in continuous operation. Service lives of more than 10 years for hermetic pumps are not uncommon.

In addition, the canned motor submersible pumps offer clear advantages regarding installation space thanks to their relatively compact dimensions in comparison with conventional pumps with a face seal and magnet-coupled submersible pumps. Because the long drive shaft is omitted in canned motor submersible pumps depending on the immersion depth. The rotating parts of the pump shaft are located in the canned motor pump and are accordingly short. The pump is suspended from a static supporting tube that is mounted on the manhole cover. Apart from holding the pump, the supporting tube only has the task of feeding the cables outwards.

Medium-lubricated guide bearings or continuously greased roller bearings are unnecessary because the otherwise usual long drive shaft is omitted completely in this system. The length of the drive shaft is independent of the immersion depth in vertical submersible pumps with canned motor drive.

The application range for the single-stage TCN variants includes delivery heads up to 220 m, volumetric flow rates up to 1,800 m³/h, liquid temperatures of –160 to +250 °C and pressure stages up to 100 bar.