The process is based on cooling of the effluent to very low temperatures, using liquid nitrogen or other cryogenic fluid as source of cold, separating the pollutants by decreasing the vapour pressure as function of vapour/liquid and vapour/solid equilibrium.

If the residual concentration of the pollutants in the effluent at condensation conditions still results more than the allowed emission limits, the cooling is carried out to lower temperature as necessary, in order to get vapour pressure reduction decrease based on sublimation equilibrium.

The check of the system performances during the operative cycle, and consequent the effectiveness of the treatment, is ensured by the control and monitoring of the effluent temperature in condensation phase, which is verified during commissioning and validation of the system, and then kept at reasonably lower levels. The reliability of the process is ensured by the plant control system, that can constantly check the correct process conditions, detect any malfunction and eventually promptly put in service the stand-by condensation line.

For easier separation cases, where the condensation temperature required for complying with separation prescriptions is higher, it can be sufficient the use or other cooling fluids, like refrigerated solutions or refrigerated organic liquids, or directly expanded refrigerants.

Polaris, in the course of years, has developed and optimised the cryogenic treatment of polluted emissions in all details, solving a number of technical, construction and control problems, by facing several cases with growing complexity, and making the technology effective and reliable, for fairly all types of pollutants, in particular for VOC.

Polaris has the experience of more than fifty plants built, installed and running successfully in Europe and extra-Europe countries, in full respect of the more severe norms in force or in phase of approval, as well as of the more strict quality requirements imposed by customers for company ecological policy.

Each system is supplied as a compact unit, skid mounted, complete and ready to run just after installation and external connections are completed and checked.

ENERGY CONSUMPTION

The cooling energy is the only significant consumption in the operation of the cryogenic treatment plant, with more relevant specific costs for lower condensation temperatures.

Besides a minor contribution due to insulation losses and to transitory start-up phases, the energy consumption is basically due to:

a) cooling of the effluent, between inlet and outlet temperature in the unit (sensible heat to be removed)
b) change of phase of the separated compounds (latent heat)

While it is practically not possible to save on the second consumption (excluding some processes where the pollutant is re-evaporated and separated in vapour phase), it is possible to carry out an efficient recovery on the first one, by cooling the inlet effluent with the same cold treated effluent coming from the cryogenic condenser in a precooler, and by using the inlet polluted effluent for defrost purposes.
Also it is possible to use cheaper cooling systems for preliminary cooling steps, in particular for process with high water humidity content, due to significant latent heat due to water condensation.
The process is normally optimised and configured taking into account the net consumption of the system, considering for liquid nitrogen the quantity of gas recovered in the factory network for other uses.

In fact it is a common practice of most chemical industry, and in general where solvents or other VOC are used, to use nitrogen as inert gas for blanketing or purging in production and storage systems. Most frequently the production of the gaseous effluent is associated to a purge or to a product transfer, and consequently to a gas nitrogen consumption, so that the used nitrogen corresponds to the carrier gas of the effluent itself. For such cases the liquid nitrogen required for the cryogenic treatment of the effluent is fairly always less than the flowrate of the effluent itself, and consequently less than the quantity of recoverable nitrogen. In these cases the operating cost of the cryogenic treatment is very close to zero.

COMPARISON WITH OTHER PURIFICATION TECHNIQUES

The cryogenic treatment of vent gases is a recuperative technique, that allows the recovery or other way to increase the value of the separated compounds.

Compared to other recuperative techniques, based on activated carbons or resins adsorption, or sometimes on absorption with liquids, the cryogenic treatment results economically convenient, for investment and operation costs, when the concentration of the pollutants exceeds values between 1.000 to 3.000 mg/m3, and depending on the type of pollutant, and for flowrate up to 1.000 to 3.000 Nm3/h.
Anyway, by integrating the cryogenic treatment with adsorption, the combined method results optimal also for lower pollutant concentration, and particularly when the compounds to be separated are in form of gas.
Also, compared to common activated carbon adsorption systems with steam regeneration, with the cryogenic treatment the production of significant quantities of polluted water, that must be than purified internally or by third party, is avoided, as well as any other negative phenomenon resulting in other recuperative systems in phase of regeneration.

If the separated pollutants cannot be recovered anyway, but they are to be transferred to third party for disposal, the cryogenic technique is anyway competitive as it reduces the quantity, and consequent costs, of wastes. It must be underlined that the use of non-recuperative techniques for vent gas treatment, like catalytic or regenerative combustion, to solve the same environmental problem, involves locally the problem of significant environmental impact, associated to risk situations to be evaluated with attention.

The combustion originates large volumes of flue gases, even often increased by the preliminary dilution of the effluent for safety or functional reasons, that contain, even if within the law limits, the new products of combustion (carbon oxides, nitrogen oxides, organic compounds originated by uncompleted combustion of VOC, often even very toxic) and the residual quantities of the original VOC present in the effluent. In particular, as the cryogenic treatment does not require any dilution, the final concentration refers normally to lower flowrate than for such systems, so that the mass-flow rate is also lower.
Even not taking into account the operating costs for support fuel and for purification of flue gas where necessary, there is an intrinsic difficulty in ensuring in all moments a perfect performance of combustion. In some industrial contexts, in particular near civil installations or inside chemical poles where the environmental conditions are already jeopardised by diffused emissions, the impact of a combustion plant for vent gases can result non tolerable. It is then much more appropriate to separate the pollutants in liquid form and to transfer the same to an adequate disposal centre, or to burn the wastes in a captive incinerator for liquids, as this kind of system does not present particular safety problems and produces much lower quantities of flue gases, for same quantity of VOC to be incinerated.

As far as safety aspects are concerned, the cryogenic treatment technique is incomparably safer than any other, and while frequent accidents occur related to incinerators and conventional activated carbon adsorption systems, it has been never occurred an accident related to cryogenic abatement system. In particular it is not required to make dilution of the effluent with excess air or inert gas to avoid explosivity problems of the organic mixtures or overheating of adsorption or catalytic beds: during the process path the effluent only meet surfaces at lower temperature than lighting levels, and where it is not possible to generate an ignition for friction or electrostatic charge accumulation.

For the above described reasons, the cryogenic treatment appears as the ideal solution from the point of view of ecology and safety. The best application field is for medium and high VOC concentrations, where the economical convenience is more evident, but it can also be economically suitable for lower concentrations, as combined method. To support the application of cryogenic treatment, it is recommendable that the emission collection points at source (reactors, mixers, centrifuges, dryers, tanks, and all other process and accessory equipment) are made in order to avoid any unnecessary dilution with ambient air. In this way the size of the treatment unit is reduced and the efficiency is increased, and consequently the investment and operating costs are reduced.