Overview of the case:
The Lenzing site, located in Lenzing, Austria, is a reference factory around the world for producing man-made cellulose fibers. These advanced fibers are broadly employed: since home textiles up to medical and technical applications. Large amounts of energy and chemical feed materials are required to obtain fibers with desired chemical and mechanical properties. One of the steps in the fiber production uses a product called spinbath, which is a solution of H2SO4 plus several SO4 salts. Then, this product needs to be concentrated in order to reduce the amount of water in the solution to crystallize and, thus, recover Na2SO4 for reuse. The evaporation takes place in several multi-effect evaporators with high steam consumption.
Lenzing is interested in analyzing the evaporation stage and in using the developed REIs together with decision-support techniques to reduce the energy consumption (electrical and steam) while recovering the SO4 salts from the spinbath (improving the resource efficiency). This part of the process has been selected as the case study due to the following reasons:
Evaporator plants and heat exchanging are often required in a wide class of industries. It means similar problems and, therefore, giving improved solutions for those not only will improve resource efficiency in the Lenzing case, but also results can be extended to improve other industries.
Cooling towers are also present in the process, being a sink of power consumption dependent on the weather conditions, which translate as a robust optimization problem by themselves.
Fouling dynamics appear when, after a certain time, some salts precipitate inside the heat exchangers, reducing thus the overall efficiency. So the line has to be stopped and cleaned.
Different efficiencies characterize the evaporator lines available for the different spinbath cycles and they have to be assigned in real time. So, in addition to the fouling, this translates in a scheduling problem to decide which ones are going to be working and which ones should be offline.
Expected Achievements of the case:
The main objectives of Lenzing are to optimize the specific steam consumption together with the overall cycle cost in a set of evaporator lines, in order to achieve a significant saving of energy while recovering the highest amount of raw material.