PROCESS DESCRIPTION AND OPERATIONAL MODE OF CHLORINATED SOLVENTS RECOVERY UNITS DR SERIES

 

The chemical process employed by OMNIATEX for the plant of the DR series for the abatement and recovery of chlorinated solvents, is the adsorption on activated carbon.

The adsorption Process by solids like activated carbon or zeolites is a simple method, but with a high efficiency and very fast pay-back, for the abatement and recovery of organic solvents.

The adsorber medium is activated carbon, derived from vegetal raw material, thermically treated to obtain a porous material with a very large activated surface. The vegetal carbon is the best adsorption and economical  medium, higher than a mineral one.

 

 

The chemical process applied for treating solvent laden air is the adsorption on an activated carbon bed. When passing through the  activated carbon bed, the solvent contained in the air is removed and  retained by the carbon. The solvent is stored inside the carbon pores, while the air, in this way purified, is driven back to the atmosphere. This phase is named "ADSORPTION".

 

The adsorption is also an exothermic process due to the reason that the solvent vapours, present in the air, condense on the activated surface of the carbon and they are retained in the carbon pores in a liquid phase.

 

The solvent retention capacity of the activated carbon at equilibrium and at a given temperature and pressure is shown in a special diagram called ISOTHERM, in which the adsorption capacity expressed in percentage of solvent retained by a given type of carbon is plotted for each solvent concentration (or partial  pressure). When reaching the isotherm curve, the same quantity of solvent entering the carbon bed comes out of it, just because equilibrium has been reached, i.e. the maximum retention capacity of a specific carbon referred to a specific solvent.

 

As soon as the carbon has reached the saturation level foreseen for the  solvent (the adsorption capacity of the carbon is temporarily exhausted), it is necessary to arrange for the carbon desorption. This  operation is performed through a "stripping" of the carbon by means of  saturated steam at low pressure. This phase is named "REGENERATION".  The chlorinated solvents are submitted, during the regeneration phase,  to a chemical decomposition and to a loss of stabilizers; therefore it  is necessary to utilize, for the recovery, solvents originally well  stabilized.

 

Before starting a new phase of adsorption, it is necessary to equalize  the inside pressure of the unit with the outside one. This phase is  named "PAUSE". Once the pause phase is completed, the activated carbon  bed will be cooled, so as to reach a low temperature and obtain in this  way a better adsorption efficiency. This phase is named "CARBON COOLING".

 

 

Usually for air pollution treatment is used an high activity carbon with more than 1100 m²/g of B.E.T. and Omniatex uses a very good carbon, with at least 1250 square meters per gram of active surface.

 

 

 

When Breakthrough is reached, according to the preset emission limit, the carbon must be regenerated to be put again in the initial adsorption condition.

 

The carbon regeneration is carried out by mediums that can loose the links between the liquid solvent and the activated surface in the pores. The most employed medium is heat used directly or indirectly.

 

Heat, supplied by steam is the cheapest and most effective regenerating medium used for activated carbon regeneration.

 

The heat evaporates the solvent in the inner side of the pores, empties them almost completely and leaves a free activated surface of the pore. The same effect can be achieved with less efficiency by a hot gas stream that lowers the partial pressure of the solvent. Usually the efficiency of the carbon regeneration by means of live steam, hot gas or vacuum can be assumed as follows:

 

 

The regeneration medium usually does not clean completely the inside of the pores in which the solvent has liquefied. This because the quantity of energy needed, in the narrow part of the pores, is very high and the steaming temperature is not enough to loose the ties between the solvent and the active surface. This means that the regeneration medium only empties more or less only half of the pore, the largest part on which the forces are weaker and this is the explication why in the beginning the first solvent sent into the carbon is never recovered, it is used to fill the inner part of the pores in which will remains forever. Due to the fact the only an inner part of the pores is emptied by the regeneration medium, this gives the necessity to use the carbon not at full retention value give by the laboratory isotherm but at a value that usually does not exceed the 50 % of the isotherm’s value. The values obtained with this rule are called “working isotherm”.

 

Regeneration is greatly affected by dust, solid or high boilers entrained in the air stream that is treated. These components are remaining inside the carbon pores and cannot be taken away by the regeneration steam, and, for consequence, the retentivity of the carbon slowly decreases, and the carbon bed has to be changed with a new-one.

 

 

Unit with two vessels filled activated carbon, self steam  production (Patented) contact water recovery (Patented)  and inlet air cooling.	Unit with two vessels filled activated carbon, self steam production (Patented) contact water recovery (Patented), inlet air cooling, carbon bed cooling and steam plume abatement (Patented)

 

The efficiency of the unit is conditioned by the features of the air to  be treated. The air temperature must                never exceed, for the main part of  the solvents, the ambient one. In case it were higher, a suitable device,              called RA, for the cooling of the air to be treated has to be provided. This cooler can be supplied on                        request.      The units can be supplied vith theCRA option that is a Patented system for the recycling of the contact                   water drained by the decanter. It uses, to produce the steam of both the Electric and V Steam versions,            the condensed water coming out from the decanter. The decanter contact water cannot be sent to the                    swage system.      The plants are also supplied in  RC version, that means with the cooling of the carbon bed before the                      re-enter in Adsorption, or the AV version that prevents the emission of the steam plume at the end of the                regeneration.      These option are compulsory when the emission limit must be respected for all the working time of the unit,            not only during the adsorption cycle.

 

The DR series is manufactured on standard production by OMNIATEX, large units are only custom made.   OMNIATEX standard production of this kind of equipments consists of more than 96 base models, allowing to extend the range offered to 3840 final versions. These can be supplied, on request, in Eex d version. The DR series is covered by four patents concerning the energy consumption and the lack of produced of wastes.   DR series solvent recovery units allow to treat solvent laden air volumes from 320 to 6000 m3/h, at room temperature. They can be used for abating and recovering a wide range of solvents, with high efficiencies, as results in the certificates of the tests performed by the Faculty of Engineering of the University of Bologna in 1975, by "Geotechnisches Institut" of BVFA Arsenal (Research Institute) in Vienna, Austria, in 1976 and by TÜV, Germany, in 1990.   For wider air volumes, OMNIATEX has a Design. Department being able to face whatever abatement and solvents recovery problems, both in gaseous and in liquid phases. The plants realized have allowed to recover a wide variety of organics and inorganic and to treat air volumes up to 250.000 m3/h.

 

 

All plants are respecting the last emission adopted limit of 2 mg/m3. Most recovered solvents are TETRACHLOROETHYLENE (Perchloroéthylène) and TRICHLOROETHYLENE (trichloroethylene). CARBON TETRACHLORIDE can be recovered with a specially glazed unit. OMNIATEX has manufactured more than 2400 units of the DR series, supplying them to the most of the biggest European Companies of the mechanical, metallurgical, automotive, electronic industries.

1800 m3/h

 

 

 The DR unit are working on steam for the carbon bed regeneration, so are manufactured both for the connection to a boiler room and with a self steam producer.

 

 All electrical and electronic components are made by Siemens, and the execution complying with CEI, DIN, SA, APAVE, ATEX norms is approved everywhere. All  units with more than one vessel are commanded by a PLC Siemens SIMATIC and have a LCD display, or, on demand, a Touchscreen panel. The plant can be cabled  to send, far away, all operational data.

 

 Several manufacturer of metal degrasing machines and plants have, on board, an OMNIATEX solvent abatement and recovery plant.

 

 Must be pointed out that all chlorinated solvents that have to be recovered, must be stabilized and neutralized before their use. The neutralization made by the end  user, must bring to the elimination of all chlorides. Be aware that in some case, a solvent with a PH 8 can contains chlorides corresponding to a PH 3. The  stabilization must re stabilize the right “acid acceptance factor” as indicated by the solvent supplier and always must be remembered that solvent with a lower acid  acceptance factor cannot be used in production.

 

2000 m3/h	3600 m3/h with solvent treatment	1800 m3/h

 

 Also the atmospheric humidity is affecting the unit abatement efficiency. For the most of the solvent the relative humidity of the air cannot be higher of the 60%. For  some solvent cannot pass the 50%.