The polluted effluent, cooled in E-01, is fed to the plant by the blower V-01.ï¿½The VOCs are adsorbed by the macroporous resins contained in a system of multiple fixed beds. The beds alternatively operate according to a sequential cycle which includes the following main steps:
â€“ adsorption phase (A);
â€“ desorption phase (D);
â€“ finishing desorption phase (DF);
â€“ cooling phase (C).
The flow distribution for establishing and varying the different steps of the plant operation is carried out by means of an unique multi-port distribution system (T-01 and T-02), that switches the beds operation at regular intervals of time, and diverts the flows accordingly.ï¿½The hourly cycles of the process phases are adjustable depending on the VOCï¿½s quality and concentration in the effluent.ï¿½The primary effluent at the outlet of the adsorption phase meets the law prescriptions, or other possible requirement, and can enter the atmosphere from the top of T-02.
The VOCs desorption phase is obtained by heating the adsorbent beds with an inert flow in closed loop, heated in E-02 with steam or hot oil. The desorption circuit is continuously fed with a nitrogen stream, small flow-rate (hundred ï¿½ two hundred times smaller than the primary polluted effluent), heated in E-04, previously passed through the beds in finishing desorption phase, thus carrying only the last residues of organics. Therefore from the desorption circuit a stream, having the same small flow-rate, but carrying all the desorbed compounds, must be purged out.
The purge passes through E-05 (cooling water heat exchanger), where the most of VOC are condensed, and through E-06 (liquid nitrogen condenser), where the least quantities are separated. The liquid mixture is stored in a collection tank. In such way the desorbed compounds are quantitatively recovered by using the minimum quantity of liquid nitrogen, which, evaporated under pressure into E-06, can be used as inert gas for purging in the desorption phases.ï¿½The secondary effluent from E-06, inert, with only traces of organics, can be recycled to the primary effluent. In alternative, and more fruitfully, whenever possible it can be partially added to the inert stream entering the finishing desorption phase to improve the desorption phases efficiency. Furthermore, it helps to maintain inert conditions with redundancy in the desorption loop, where the organics concentration is high and any possible process risk must be avoided.ï¿½The inert conditions in the plant can be monitored by the oxygen analyser AI installed in the circuit downstream E-06.ï¿½The regenerated bed after desorption must be cooled before the next adsorption phase, by circulating a cool inert flow (cooling phase), in closed loop, passing through the heat exchanger E-03.
The main benefits achievable with the new Polaris technology have been already synthetically mentioned. In particular, the use of macroporous resins presents many peculiar advantages.ï¿½The first is the absence of active sites of catalysis, which are present in the activated carbons in form of metal oxides. Therefore the process is recommended for the adsorption of compounds like several ketones and esters, sensitive to the redox catalytic action by oxides.ï¿½The resins adsorb little quantities of water, also ten times less than the activated carbons. In fact, they are hydrophobic. It means that the recovered VOCs contain much less moisture, therefore a possible hydrolysis is avoided or reduced, their recovery is made easier and more profitable, or their disposal becomes less expensive.ï¿½The adsorption and desorption energies are lower (one half) if compared with the activated carbons, and such feature has a positive impact on the energy consumption of the whole process, in particular as far as heating and cooling are concerned.
The resins can be easily regenerated at a lower temperature. The desorption temperature is in the range 120 ï¿½C ï¿½ 140 ï¿½C, whilst for activated carbons the operating temperature is usually higher, 160 ï¿½C ï¿½ 190 ï¿½C and more.ï¿½As far as the safety aspects are concerned, very important in the adsorption processes, the use of resins, and the feeding of nitrogen during their thermal regeneration, allow the highest safety conditions for the plant running.ï¿½A significant feature of the new technology, independently from the adsorbent used, consists on the reduction of the desorption stream, from which the VOC have to be finally separated, to a small inert flow purged from the desorption loop.ï¿½In fact, this secondary stream, hundred and more times smaller than the incoming polluted effluent, but containing all the adsorbed VOC at extremely high concentration, can be efficiently cooled to recover the same compounds in liquid form.
The separated VOC can be directly reused, if possible, in the production processes, or they can be submitted to further separation processes, for example by distillation, before reuse. If their recovery for reuse or sell is not convenient or possible, their disposal is easier.ï¿½In fact, if their destruction is the only way of solution, inside or outside the factory, their incineration is more economic and safer, in a very compact unit sized for a volumetric capacity 30-50 times lower, if compared with the direct incineration of the huge flow-rate of the polluted effluent, very diluted, not supporting the flame, thus requiring additional fuel.
Another advantage consists on the fact that the utilities requirements (heating and cooling media, electric energy, nitrogen) do not present abnormal peaks as typically occurs in all other batch-wise adsorption technologies. Therefore, in the most of cases, it is not necessary to face further investments to provide an adequate capacity of supply.ï¿½The unit can include were necessary and/or convenient some pre-treatment steps, e.g. filtering, precooling, drying etcâ€¦ï¿½The units are fully automatic, thanks to a control system (PLC based) that monitors and adjusts the process parameters in order to get the required operating temperatures and a reliable working cycle. The operator interface is user friendly and includes useful features like automatic trouble-shooting guide, records of events and process data, etc.
The system performances during the operative cycle are ensured by controlling and monitoring the process parameters. The reliability of the process is ensured by the plant control system, that constantly checks the correct process conditions, detects and solves any malfunction.