Printing processes and air handling systems
The customer company commissioned BROFIND to supply the plant to treat the emissions of the rotogravure printing machine (150,000 Nm3/h), performing solvent recovery used in inks (toluol) during the magazine printing process, at a rate of approximately 1,500 kg/h.
The solvent laden air, coming from the 8-colour rotogravure printing machine at a temperature close to 55°C, is extracted by two centrifugal fans and sent to two cooling coils, where it drops to approximately 35°C, and to two filter mats which remove any impurities present.
The extracted flow is regulated by inverters, in turn managed by an appropriate negative pressure control system applied on the main intake pipe. A sophisticated, specifically designed control/regulation system optimises the ejected air from each printing element, through modulating dampers that determine the amount of air to extract from each semi-hood and consequently required to recirculate within the element itself, according to the concentration value of the outgoing solvent.
The stream is sent following the adsorbers containing activated carbon: of the five present, four are simultaneously kept in adsorption while one is involved in the regeneration phase. The purified air leaving the adsorbers is sent into the atmosphere through a single stack, while the solvent/steam mixture obtained in regeneration is sent to a phase separator where, thanks to the different specific weight of the two substances, it is physically separated by decanting.
The recovered solvent is sent to storage tanks, while the steam condensate is sent to a final stripping treatment to eliminate residual traces of toluol. Any non-condensable gases present in the desorbate stream are taken by the main fans and sent back to the plant.
Plant operation is fully automatic: an electric control panel includes, in addition to the auxiliary equipment interfacing with the field, an operating logic management PLC. The PLC is in turn connected to the operator station supplied with supervision system with graphical screens, where the plant can be controlled remotely.
The operator can set the regeneration cycles of the adsorbers in different modes as required:
- in manual mode, the opening/closing sequence of the valves is managed by the operator by enabling the relative symbols represented on the graphical screens of the supervision system;
- in semi-automatic mode, only the start control of the regeneration cycles is managed manually, while the valve opening/closing sequence is fully automatic;
- lastly in automatic mode, the regeneration controls are managed without any intervention by the operators depending on the preset variables such as instantaneous incoming and outgoing solvent concentration to the plant and the mass stream of said solvent adsorbed by each bed.
Specific objectives of the order, specific issues addressed
The solvent recovery plant installed by the customer recovers over 99% of the solvent used in printing, so as to be reused in the production cycle, thus reducing purchases of new solvent.
Intervention and proposal
To considerably reduce steam consumption, a system capable of recovering the latent heat still present in the steam/solvent exiting the adsorber during the regeneration phase has been provided. The previously stored condensate can be re-evaporated by installing a vertical film evaporator/condenser upstream of the refrigerant condenser and applying a slight degree of vacuum produced by an ejector fed with steam at 8-10 bar. The condensate is added to the fresh feed steam, reducing the actual steam consumption by about 40%.
To complete the supply of the entire system, BROFIND has at the same time extended the toluol storage tank fleet both regarding sale and its distribution to the different utilities.