Heat recovery & saving

For evaporator and rectification plants on an industrial scale, consideration of the possibilities for minimizing energy consumption is of central importance. corosys has many years of experience and know-how, which we are applying specifically for your process.

This includes common methods for the use of hot and cold material flows, which can be investigated, for example, in a pinch analysis, as well as the optimisation of the rectification column with regard to separation stages and required reflux ratio.

Mainly for pure evaporation processes, but also to some extent for rectification processes such as bioethanol rectification, the following energy saving concepts can be realized – depending on the size of the plant, specific energy costs, etc. The concepts are illustrated using the example of falling film evaporator plants, for which the energy savings are most frequently used.

In mechanical vapor recompression, the vapours are compressed by an electrically operated compressor and reused to heat the evaporator. Depending on the application (boiling point increase, heat transfer), one- or two-stage turbo fans or turbo compressors are used as heat pumps.

The process steam (steam) produced in industrial processes is raised to a higher temperature and thus to a higher energy level by a radial fan (mechanical steam compressor, MBV) and then fed back into the process as fresh heating steam. The energy contained in the steam is not lost, only the energy required to raise the temperature must be additionally applied in the form of electricity. In addition to the reduced consumption of fossil fuels and thus heating steam, CO2 emissions are also significantly reduced.

The specific energy consumption reduction with mechanical vapour recompression is very economical and reliable, especially for very large quantities to be evaporated.

Thermal Vapour Compression (TVR) is based on the same principle as the mechanical alternative, but uses only part of the steam produced to heat the system. The compression of the steam for heat recovery takes place in a steam jet pump. This is usually designed for a specific operating point and works according to the jet pump principle. For the operation of a thermal steam compressor a certain amount of steam, the so-called motive steam, is required. In many cases, the resulting energy saving is roughly equivalent to an additional evaporator stage.

The main advantage of thermal vapour recompression is the reduced steam consumption at moderate investment costs, which are lower than those of an additional evaporator stage.

In multi-stage evaporation plants, the vapours from the product are used to heat the downstream evaporation effect, to reduce the steam consumption accordingly.

Taking into account the heat balance of a single-stage evaporator, the heat content (enthalpy) of the evaporated steam is approximately equal to the heat input on the heating side. In the usual case of water evaporation, about 1 kg/h of live steam is produced, since the specific evaporation heat values on the heating and product side are about the same. If the amount of steam generated by primary energy is used as heating steam in a second evaporator, the energy consumption of the entire system is reduced by about 50%. This principle can be continued via further effects to save even more energy.

The maximum permissible heating temperature of the first effect and the lowest boiling temperature of the last effect form a total temperature difference which can be divided among the individual evaporators. Consequently, the temperature difference per effect decreases as the number of evaporators increase

For this reason, the heating surfaces of the individual evaporators must be dimensioned suitably larger in order to achieve the required evaporation rate, but with a lower temperature difference (∆T). A first approximation shows that the total heating surface of all evaporators increases in proportion to the number of evaporators. Consequently, the investment costs increase while the amount of energy saved becomes smaller and smaller. An optimum for this has to be worked out with the customer.