Energy costs have now become a long-term issue in the chemicals sector and are becoming an important efficiency factor. Process versions which only a few years ago laid abandoned and disregarded in the drawer due to their high investment costs are benefiting from this sea change. This also includes mechanical compression of vapors in evaporators which remained in the long grass for a long time and is now enjoying a renaissance.
The reason for the lack of credibility given to some processes were the considerably higher investment costs which operators shied away from in times of lower energy costs. However, the situation is completely different now. Options that initially seem more expensive often turn out to be the considerably cheaper ones after only a few months. The mechanical compression of vapors in evaporators for upgrading a product is a clear example of this phenomenon.
Common plant connections for evaporators in the past were multi-stage connections or thermal vapor compression. In the first variant, the live steam heats not only one evaporator stage. The more stages an evaporator has, the lower its primary energy requirement in the form of steam. As the number of stages increases, the temperature difference between the heating medium and product automatically diminishes, so that the size of the effective heating surface increases at an ever faster rate. The investment costs therefore rise as the number of stages increases and the saving in steam per additional stage diminishes.
The second common method is the thermal compression of the vapors of a boiling chamber to the higher pressure of a combustion chamber according to the injector principle. Motive steam compresses the vapors and these are fed back into the process as the heating medium. However, the energy supplied for the motive steam part goes into the next evaporator stage or to the condenser as excess vapors. In both cases, the volume of energy to be dissipated is still extremely high.
Mechanical compression of vapors can reduce the volume of heat to be dissipated considerably, however. For the operator this means a low specific energy requirement, ergo lower energy costs. Wolfgang Hansen, Head of the Marketing/Sales Division at GEA Wiegand adds: “Customers usually only look at the efficiency of a plant at the time of investment. At a crude oil price of 30 US dollars a barrel, the amortization period for a plant with mechanical vapor compression is still two to three years due to their high investment costs. At a price of 60 US dollars this is reduced to one year. In addition, mechanical vapor compression also helps to cut CO2 emissions due to the considerably lower energy requirement.”
What does mechanical vapor compression involve? A mechanical compressor compresses the vapors to the higher pressure required for the combustion chamber. The use of a relatively small amount of energy improves the energy flow involved in the process heat and it is fed back into the process as heating medium. For cost reasons, radial compressors and high-performance fans are predominantly used.
The type of compressor which is appropriate depends on the operating conditions. The crucial parameters are the requisite pressure increase and the volume flow of the vapor to be compressed. A maximum temperature difference of 6 - 8 K can be achieved with a fan. In a plant with two fans in a tandem system, the temperature difference can be doubled to 12 - 14 K. If higher temperature differences are required, the turbocompressor which can attain temperature differences of up to 20 K is the best option.
A vapor compressor is usually driven using an electric motor. The advantages of this drive system. Standardization of sizes and degrees of protection, low power-weight ratio and volume, minimal maintenance costs and a good price-performance ratio.
Low susceptibility to faults
The growing importance of mechanical vapor compression is also attributable to the diminishing susceptibility of the compressors to faults. Improved materials and sophisticated monitoring and safety installations ensure reliable and fault-free operation. Hansen explains: “Customers used to be wary of introducing yet another moving part into their plant. However, these concerns have been allayed thanks to the progress in the development of the fans and compressors and in I&C.
Mechanical damage to their plant is the nightmare of all operators. So that this scenario does not become a reality, GEA 16 monitors parameters in order to detect irregularities in the compressor operation and any signs of wear at an early stage. A speed recorder and vibration monitoring device monitor the impeller continually. Another important aspect is the temperature of the fan housing.
The compressor housing is also heated through the pumping medium as a result of the compression work. Constant injection of condensate at the impeller inlet saturates the vapors and prevents the housing temperature becoming too high. The monitoring equipment also checks a number of parameters in the oil circuit, motor, shaft and pump.
The operator does not always want to operate the plant in the same stationary condition. Partial load operation is often unavoidable in every day operating scenarios. In addition, the evaporator must be able to react to changes to product properties in the supply and to the gradual collection of dirt on its effective heating surfaces. This all happens by means of the compressor control. Speed control and suction pressure control in particular are common methods here. Controlling the impeller speed and in turn the circumferential speed of the fan affects the volume flow and compression ratio. This generally happens by means of a three-phase induction motor with frequency converter.
If the plant is not connected to the power supply of other plants and the process is to be operated at different temperatures, suction pressure control is the easiest option to consider. In many cases, it offers a sufficiently large tolerance range without high machinery costs. The combination of evaporation with mechanical vapor compression with a pre-concentration of the product by means of membrane filtration offers additional energy savings.
This allows the option of separating substances as low temperatures without a phase change. Depending on the particle size, particles smaller than 0.001 micrometers through to coarse particles of 1000 micrometers can be separated using this separation process.
It is often advisable to perform the pre-concentration by means of a membrane filtration process and only the high concentration by means of evaporation. This often makes sense for customers expanding their capacity: instead of retrofitting a second evaporator, a membrane filtration system can be added upstream which is more economical in its energy consumption than an evaporator.
Hansen describes the response to membrane filtration as follows: “Customers are often worried about replacing the membrane and the cost this involves. However, the materials have now improved considerably and the service lives are longer, with the result that membrane filtration will become increasingly more important as energy costs rise.”
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