Accidentally oily water

Overall blowdown is very different depending on the relative size of atmospheric cooling. The CONCAWE 5/77 report forecast cooling system water flow rates in 1980 for a typical 10 Mt/year refinery at 2500 to 10 500 m h , i.e. 2 to 8.4 m t of crude, depending on the complexity of the plant. Relatively old refineries (see Chapter 6) circulate from 10 to 24 m t M Here deconcentration blowdown varies from 50 to 500 and can make up a considerable proportion of their discharge, ranging from 0.2 to 1 m -r excluding rainwater. 

These figures should be compared to French requirements (see Section 2.1)on discharge volumes (excluding rainwater and deballasting water). 

In both cases, the amount of rainfall and the areas involved (12 to 24 acres) need to be known. 

In France, depending on the location, the volume of rain falling for 12 to 30 min on a refinery can vary between 2000 and 8000 m. Rainfall can amount to around 5 to 20 mm in 15 min. 

During storms, the first water to fall washes the ground and is the most polluted. The flow may be from 500 to 3000 m -h Since it is not feasible to treat this amount at its instantaneous flow rate, the water must be stored in a storm basin designed on the basis of historical meteorological statistics. 

---

The main effluent itself, oily and accidentally oily water, undergoes the now routine series of three basic purifications which constitute the three stages of general treatment ... 

Accidentally oily water is much more variable and accounts for the main HC input in sewers. In a refinery with 8 to 10 Mt/year throughput, HC losses of 20 to 100, even 200 per day may occur depending on the number of accidents and how well it is managed. Of this amount, 80 to 95% is reclaimed in gravity separators, a fact which justifies good separator design and operation. 

The accidentally oily water sewer which is characterized by the addition of a storm basin. 

Tliey deal with effluents that arrive directly without prior storage and that ate loaded with HC according to malfunctions due to manufacturing and maintenance. The effluents constantly exhibit strong, mainly mechanical emulsions and unexpected slicks of oil. The effluent categories involved here are oily water and accidentally oily water whose average daily HC concentration can vary between 0.1 and 2 g l , or more. 

HC density poses a problem. It is well known during production which involves one or more well-defined crudes and is still more or less predictable during tanker transportation. However, it can no longer be assessed in oily water and accidentally oily water effluents where the nature of the HC, crude, intermediate cut or finished product varies according to malfunctions and routine operations. The choice of a maximum density for use in designing an oil separator facility would hardly be reasonable if the range of specific gravities is recalled ... 

A second limit on their use comes from their modular construction. With four-centimeter spacing and a nominal capacity of 30 m h per assembly, around ten prefabricated packages are used to treat a maximum flow rate of 300 m h of accidentally oily water. When so many modules are lined up parallel fashion, there are problems ... 

Scraped systems are suitable for accidentally oily water that is irregular, cold and loaded with SS. 

Softer recyclable water topping condensates, accidentally oily and nonoily water, HP boiler blowdown. 

Filtration with coalescence by upflow filters can be contemplated when there is no interference by an overly-high SS concentration and when the HC present do not have too high a viscosity (hot water) or too high a pour point (paraffin risk). The required conditions are fulfilled by many types of produced water and all heating condensates, but less often by oily or accidentally oily refinery water. 

With saline or accidentally salty and oily water, there is a risk of rapid electrode polarization. An inductive measurement, without electrodes, is required (deconcentration of cooling systems). 

---