Oil&Gas / Oil&Gas Production
PCC Rokita SA
In future, the membrane process will gradually replace the older amalgamation process in chlorine-alkali electrolyses. The chlorine production at the Polish chemical complex PCC Rokita SA has in the meantime also been favouring the environmentally friendly and energy-saving process developed by ThyssenKrupp Uhde. A fully welded plate heat exchanger made of titanium makes an essential contribution to safe operation by bringing the chlorine gas to the right temperature so nothing can prevent safe further processing.
Chlorine chemistry is frequently referred to as the core of the chemical industry. Around 60 per cent of the turnover realized by this branch of industry is dependent on chlorine chemistry, either directly or indirectly. Chlorine chemistry products range from polyvinyl chloride (PVC) for window frames and sewer pipes up to polyurethane for thermal insulations and polycarbonate for IT applications. The base material for chlorine-alkali electrolysis is an aqueous sodium chloride solution from which chlorine, caustic soda solution and hydrogen are produced in different processes having a specific electrolysis cell design each. Today, the membrane process is used in roughly two thirds of the large-scale plants because the end products, i.e. Cl2, H2 and NaOH are very pure, but require clearly lower overall energy input. In addition, this process does completely without any mercury which is a controversial issue under environmental aspects. This is why the European Industry Association Euro Chlor has committed itself to refrain from building any new plants using the amalgamation method. All existing plants will be either shut down or converted by the end of 2020. The Polish chemical company PCC Rokita SA in the small town of Brzeg Dolny is also following this track. This location produces not only chlorine and chlorine compounds, but also polyalcohols, lyes, surface-active agents and phosphorous derivatives. In the past, a chlorine-alkali electrolysis plant based on the amalgamation process was in operation there. As the plant was modernized, a part of the plant was converted to the environmentally friendly membrane process, thus enabling an increase of production capacity to up to 120,000 tons of chlorine per year. At the same time, energy consumption (more than 20%) and CO2 emissions are reduced and waste water containing pollutants is avoided by using closed circuits.
Reliable drying of the produced chlorine gas is the decisive factor for its further processing. To achieve this, the chlorine is cooled down from 86 degrees to 40 degrees in a first cooling step, thus condensing the saturated steam contained in the chlorine gas. „In the past, we have always used a shell-and-tube heat exchanger for this process step, but at this location space was extremely limited“, says Dr. Andreas Cruse, project manager at ThyssenKrupp Uhde and responsible for setting up the new chlorine-alkali electrolysis plant based on the membrane process. PCC Rokita had already gathered experience with Kelvion plate heat exchangers in another plant at this location. This was why they were basically prepared to employ this type in the chlorine treatment section of the demanding chlorine-alkali electrolysis application.
However, to ensure safe operation, the plate heat exchanger had to meet special requirements. One of the product streams consists of corrosive, moist chlorine gas and the other of pre-treated river water used for cooling. This water still contains impurities that may quickly cause deposits preventing smooth operation. „In view of the requirements made on the material, the apparatus had to be built completely from titanium“, Dr. Cruse specifies another requirement. The corrugation of the heat exchanger plates was also an important criterion. To enable efficient heat transfer and safe operation at the same time, the flow channels must be very fine and still designed so that both deposits from the river water will not endanger operation and the units can be easily cleaned. „In the end, we needed a heat exchanger without gaskets and with an optimum plate corrugation“, as Dr. Cruse summarizes the initial requirements. As Kelvion is a company with lots of experience in welding of titanium, the choice was made in favour of the fully welded K°Bloc titanium plate heat exchanger made by Kelvion PHE GmbH. The idea behind it all is that the K°Bloc unites two different plate corrugations in an innovative way. The plates are arranged with a 90 degrees angle in relation to one another and welded together, thus forming different counterflow channels. The heat exchanger consists of four columns, a bottom and a top plate as well as four lateral thrust panels with integrated connections. All frame components are bolted and can therefore be easily dismounted for cleaning and maintenance of the plate pack. „We decided to use the Double Dimple corrugation because of the possible contaminations in the river water“, says Dr. Cruse. With a capacity of 5000 kW, the chlorine flow is cooled to 40 degrees. In another cooling step, the chlorine gas is cooled down to its final temperature. Maximum quality manufacturing The big challenge in manufacturing this heat exchanger was to weld the titanium in an atmosphere without oxygen because this is the only way to avoid temper colours. To this end, the apparatus is flushed with argon for hours in order to remove the oxygen even from the last corner. After that, the heat exchanger is treated in a sarcophagus and handled with kid gloves – which is actually true as everybody involved in manufacturing is wearing gloves. Manufacturing takes several months and many work steps are carried out by hand. In the meantime, Kelvion PHE GmbH has extended its manufacturing capacity especially for these special cases. Now automated welding of titanium is also possible. However, many manual steps will also be required in future to meet these special requirements. „The requirements regarding tightness were very high, especially because of the much-feared crevice corrosion“, says Dr. Cruse. Extended testing efforts were therefore necessary before installing this heat exchanger in order to verify the tightness of the unit. After taking this hurdle successfully, installation of the K°Bloc was without any problems. „The cooperation and project handling were very good“, Dr. Cruse summarizes. Due to its compact design – the unit itself has a size of roughly one cubic metre – it could be integrated into the plant virtually seamlessly. This is the reason why in retrofits and conversions in particular, a plate heat exchanger is frequently selected; a tube-type heat exchanger would require at least twice the space.