Breaking up an emulsion of compressor condensate into oil and water can take place using cross-flow filtration. This purely physical separation process takes place selectively without additives. A semi-permeable membrane retains the larger oil molecules, thus separating them from the smaller water molecules which can pass through. Through varied and successful applications, membrane technology occupies a secure place in waste water treatment. Extreme applications show the robustness of silicon carbide membranes. The emulsion under pressure flows at high speed parallel to the filtration surface of the membrane, largely preventing a rapid contamination of the fine membrane pores.
Figure 11.4.1
The treatment of condensate emulsions from air compressors therefore poses no unusual challenge to membrane technology. It is the task of the plant designer to adapt the peripherals of the design to match the harsh conditions of the application and requirements. These include:
Ceramic membranes
Silicon carbide guarantees high filtration performance with constant quality of filtration and long service life of the membrane.
Cyclic back flushing with compressed air
Through cyclic reversal of the direction of flow of the purified condensate through the membranes, the filtration surface is effectively cleaned and the filtration performance kept at a constant level for a long period of time.
Cyclic membrane cleaning
Membranes automatically regenerate themselves after treating a quantity of condensate which has been specified beforehand.
Integrated oil separator with coalescing stage
The free oils carried from the compressor are safely and reliably separated. This greatly reduces the load on the membrane, and increases the filtration capacity.
Fully automatic programmed control
All necessary functions of the installation are performed automatically with the most important parameters monitored. This can only be achieved by the use of programmable logic controllers. The pore diameter of the ceramic membrane used is such that even the finest emulsified oil droplets are retained, whereas water permeates the membrane and flows away as pure water.
Figure 11.4.2
The service life of the membrane is an essential factor. The filtration membrane consists of the inorganic material silicon carbide. This extremely robust material is characterised by high thermal and chemical resistance as well as extreme longevity. A number of successful applications such as, the thickening of biomass or the pre-cleaning of cesspool water seeping through the ground are proof that even biologically highly loaded waste water can be treated reliably without the membranes blocking up. To achieve correct sizing of emulsion separating installations, the compressor capacity, the periods of use and, possibly, installed dryers and their parameters are of significance. On this basis, the annual quantity of condensate can be determined with a reasonable degree of accuracy. Differing conditions should, however, be taken into account. The module contains up to five silicon carbide membranes. Filtration takes place automatically via programmed control. The sequence of separation is made clear by Fig. 11.4.3. In the integrated oil separator (item 1) with dirt trap, oil floats to the top of the emulsion and is separated. A two-stage coalescing filter element improves the efficiency of separation. Overflowing emulsion is stored in the buffer container (item 2) and is pumped, into the process container (item 3). As soon as the process container is filled up to the maximum level, the concentration cycle commences.
Figure 11.4.3
In a closed loop operation, the emulsion is pumped through the filtration module (item 4). A flow meter (item 5) with a minimum flow switch monitors the quantity of permeate (quantity of purified water) flowing to drain. For cyclical membrane cleaning, the direction of flow of the permeate is reversed for a brief time by the back flushing unit (item 6). After the content of the process container has been treated, the float controlled process pump reverses.
Photo 11.4.1
The emulsion collected in the buffer container during the processing of the process tank is now pumped back into the process vessel, and the next concentration cycle commences. A pre-selected number of cycles specifies the degree of concentration. When this number of cycles has been carried out, the concentrate is pumped into a collecting vessel. After this, the cleaning cycle starts again automatically. From a fresh mains water connection, the process container is filled up with the required quantity of rinsing water. From the cleaning solution dispensing device (item 7), the cleaner is fed in. The rinsing water cleans the membrane. After the rinsing time has elapsed, the charge concentration process commences again.
Figure 11.4.4
Condensate drain lines from the individual drainage points to the emulsion separator operate under the following conditions, depending on the particular application.
Under pressure:
Mounted inside compressors (Fig. 11.4.4), where the condensate is fed in by means of level controlled condensate drains via flexible connections.
Through pump pressure:
Through use of a condensate collecting tank, a submerged pump in the collecting tank fills up the emulsion separator tank batch by batch.
Without pressure:
From a condensate collecting tank, the outlet of which is higher than the inlet of the emulsion separator. The iinlet quantity of emulsion must be controlled to match the outlet quantity of clean water. A concentrated oil/water emulsion remains as residue. Compressor condensates can reach an oil content of approximately 500 mg/L. The oil content within the concentrate from an emulsion separator can reach about 30% or 300 000 mg/L. Volume reduction by a factor of 600 means that, theoretically, 1000 L of condensate emulsion results in a residual concentrate of only 1.6 L which has to be disposed of responsibly. There can be no doubt that emulsion separating plants represent a future trend. The further development of ceramic membranes will make new applications possible in the future. In direct comparison to flocullating equipment which uses powder or liquid means for splitting emulsions, emulsion separators offer a considerably higher operational reliability. The difference in pore size enables the membrane to retain oil particles while permitting water particles to pass through. This means that, even with high fluctuations of oil content in the condensate, there will be no deterioration in the quality of the purified water. Drain water treatment systems operating on the basis of the filtration principle are the only ones, from the process technological point of view, which do not use any chemicals, bentonite, etc. This means that the condensate to be treated remains in its original state, and is not chemically modified or irreversibly occluded through an additive. This largely eliminates the problems of residue disposal. The condensate remains in a fluid state and is therefore convenient to handle during intermediate storage and for later disposal through a specialist company. Moreover, when concentrating through filtration equipment, the disposal company has the possibility of splitting the concentrate into water and oil to a degree of nearly 100%, thus utilising the oil for recycling. One should note that, under normal circumstances, 1m3 of condensate leads to maximum 10 L of concentrate. The possibility of recycling the residue resulting from condensate treatment will exist in the near future.
