Thermal Processing Heat exchanger, chemically cleaned
Major cleaning for an oversized “immersion heater”: This is how heat exchanger cleaning ensures efficiency — When there is no more flow in the tube, it is time for maintenance. But what if apparatus cannot be bored out or mechanically cleaned? Chemical industrial cleaning can help to clear the pipe.
What shines brightly and juts metallically grey into the night sky is reminiscent of a five-metre immersion heater: the boiler coil of a waste heat exchanger. The pipe network for cooling raw gas is usually in the middle of one of the large heat exchangers at the Total refinery in Central Germany. There is, of course, a reason why the unit gets “free access” at all: acid rushes through the winding pipes for three days. In the refinery, not only fuels and combustibles are extracted from about twelve million tonnes of crude oil annually. Methanol, which is sought after as a basic raw material and solvent, can also be obtained from cracker residues via the diversions of synthesis gas. To do this, the raw gas, which has a temperature of about 1,300 °C, is first cooled — and this is where the heat exchanger comes into play. The operator usually uses the waste heat to generate steam for the various processes in the refinery — but the raw gas is not free of particles and metal particles. If these settle in the cooling coils, the efficiency of the plant gradually decreases until at some point only maintenance can help.
At what efficiency level the boillimits of economic operation are reached is clearly defined at Total. And not only that, the cleaning procedures are also specified down to the last detail. When the moment of truth came for the boiler coil of one of the heat exchangers in February 2020, the cleaning had to be done chemically. Reaming out or drilling out the tubes is not possible due to the complicated tube geometry. This required mobile equipment with an external pumping station with a closed mixing tank and high-performance gas scrubber to prevent the escape of hydrogen sulphide (produced when the residues from carbon and sulphur compounds are dissolved). The mixing tank is also designed as a separator and takes over the separation of the dissolved solids at the same time. For safety reasons, the entire equipment must be placed in a mobile catch basin.
Especially in the case of plant decontamination, it is important to use a suitable cleaning solution. Only when the right chemical composition of the cleaning solution has been found is the installation of the mobile system begun. Chemical industrial cleaning offers another decisive advantage, especially for complex plant systems: plant components do not necessarily have to be dismantled first. Without time-consuming dismantling or assembly work, the equipment is quickly available again, and downtimes are reduced to a minimum. Thanks to a closed circuit of cleaning solution and product residues, the release of gaseous or liquid pollutants can be almost completely avoided. Even if gaseous pollu- tants are produced, they are extracted directly from the circuit and cleaned from the exhaust air flow by means of gas scrubbers.
To monitor the cleaning progress, laboratory tests are necessary at close intervals. No wonder that the industrial service provider Lobbe in Leuna arrived with a mobile laboratory and shift operation. After three days, the time had come: the analysis values were in the “green zone” and the pipe coil could go back into the machine. It is not only the process that is clean: the maintenance of the plant components needed for methanol synthesis helps to turn toxic cracker residues into a valuable product: Resource conservation thanks to chemical cleaning. Sometimes it is that simple. dst