GB2084480A - Treatment of steam condensate - Google Patents

Abstract

Oil-contaminated steam condensate is de-oiled by passing it through a coalescer 3 to coalesce small droplets of oil into larger droplets which settle out and removing the recovered oil at 4. The condensate of reduced oil content is then passed through an absorbent bed 7 to absorb further quantities of oil. The recovered condensate is suitable for return to the boiler as feedwater without further treatment other than normal de-aeration. <IMAGE>

Description

SPECIFICATION Treatment of steam condensate This invention relates to a process for the de-oiling of return steam condensate.

Many industries require a supply of steam for process heating. Such steam is usually generated in large scale industrial boilers from boiler feed water which has been extensively purified to remove dissolved solids and gases. After use, the steam is condensed and because the condensate is hot water free from mineral contaminants it is desirable to re-use it for further steam generation in order to reduce expenditure on water, water treatment and energy.

In many cases, however, re-use is not possible without further treatment because of oil contamination. Despite high standards of maintenance and operation, traces of oil will almost certainly leak into steam or condensate lines. Condensate usually contains about 1-100 ppm oil in the form of droplets of insoluble oil, insoluble oil so finely dispersed in water that the resulting emulsion behaves like a solution of oil in water and dissolved organic substances. These materials, particularly the soluble and quasi-soluble oils are difficult to remove.

It not removed, however, oil can give rise to trouble during steam raising by coking in the boiler tubes leading to the formation of hot spots and eventually to tube failure.

For the separation of certain oil and water mixtures, separators known as coalescers are commercially available. These generally contain a cartridge or packed bed through which the liquid flows in such a manner that droplets of the dispersed phase collide with a film of the dispersed phase on the cartridge or bed material and with each other and grow in size to such an extent that they settle out.

Such coalescers, although effective in dealing with insoluble oil, cannot resolve the problem of soluble oil.

We have now discovered a two-stage process from the treatment of steam condensate which minimises the residual oil content. This can be reduced to such a low level, e.g. 1 mg/litre or less, that its harmful effects are eliminated.

Thus according to the present invention there is provided a process for the de-oiling of oilcontaminated steam condensate which process comprises passing the condensate through a coalescer to coalesce small droplets of oil into larger droplets which settle out and form recovered oil, removing the recovered oil, passing the condensate of reduced oil content through an adsorbent bed to adsorb further quantities of oil and recovering the de-oiled condensate.

Preferably the condensate is passed through a coarse filter or strainer before the coalescer.

The recovered condensate is suitable for return to the boiler as feedwater without further treatment other than normal de-aeration.

The coalescer is preferably of the cartridge type in which an element is used within a container.

However, packed bed coalescers such as anthracite coalescers are also suitable.

The element may comprise, for example, pleated paper, glass fibre or other fibrous material, conveniently in the form of an annuius through which the mixture flows radially.

The pore size of the element will be chosen so that it is fine enough to effect coalescence. Usually the pore size will be in the range 1 to 30 micron.

Suitable materials for use in the adsorbent bed include charcoal, ion exchange resins and, preferably, activated carbon.

Optionally, a heat exchanger may be interposed between the coalescer and the adsorbent bed to reduce the temperature of the condensate before adsorption to increase the efficiency of the adsorption stage.

The flow rate of the steam condensate through the cartridge coalescer should be related to its geometry so that some turbulence is created in the liquid as it flows through the cartridge but not sufficient turbulence to prevent the deposition of a film of oil on the element nor to strip a deposited film off. This favours the movement of oil droplets across the stream lines in the winding passages in the network of the cartridge so that they will collide with the oil film on the element and with each other.

Droplets colliding with the oil film trap oil which can be collected. Colliding with each other leads to growth in the size of droplets. If the water velocity is too great, however, the resulting increased turbulence strips the oil film from the element and breaks up oil droplets, thereby reversing the above effects.

In general the hydraulic loading of liquid flowing through the equipment is in the range 1 5-1 50 m3/m2/h.

The invention is illustrated with reference to the accompanying drawing which is a schematic diagram.

Recovered steam condensate at a temperature of 60-900C is fed by line 1 to a cartridge type coalescer 2. The cartridge 3 is of radial flow design and contains a number of concentric glass fibre elements of progressively finer construction. Dispersed free oil is coalesced in the coalescer 2 and settles out to be removed as recovered oil through the line 4.

The partially treated condensate then flows through the line 5 and optionally through a heat exchanger 6 to a column 7 containing a bed of activated carbon. The condensate percolates down through the bed and emulsified and soluble oils are adsorbed. Any residual droplets of insoluble oil are retained by a filtration effect.

De-oiled condensate having a total oil content usually below 1 ppm then leaves by line 8.

EXAMPLE The coalescer cartridge 3 was a 1 micron coalescer sold by Alan Cobham Limited under the name P3M.

The column 7 had a bed volume of 500 ml and contained 196 g Anthrasorb CC1236, a coal based activated carbon sold by Thomas Ness Limited.

Simulated condensate at 650C contaminated with a nominal 100 mg/l Forties crude oil was used as the feed.

The results set out in the following Table were obtained.

The mean oil content of water passing from the cartridge coalescer was found to be 5.7 mg/l total oil. It was assumed that most of this oil was soluble hydrocarbon as no free oil droplets were visible when the cartridge effluent was examined under the microscope (1000 X).

At flow rates of between 2.8 and 5.6 bed volumes (BV)/hour, water passing from the activated carbon bed contained 0 to 1.9 mg/l total oil. The mean purity of the treated water was found to be 0.6 mg/l total oil during 107 hours on stream. The treated water quality had not deteriorated at the end of the run. The oil retained by the adsorbent was calculated to be 0.54% w/w.

TABLE

Feed-Water Flow Total Oil (mgtl) Run Time Cartridge | Carbon Filter Cartridge Carbon Filter (hr) (I /min) (BV/hr) Effluent Effluent 4.5 18.2 4.7 5.1 0.1 8 18.0 4.4 6.8 0.3 12 19.0 5.6 8.4 0.2 19 18.3 5.6 5.4 0 26 18.3 4.8 5.1 1.9 32 18.3 4.2 5.0 0 39 18.3 4.6 5.1 49 19.0 3.1 6.1 0.2 55 18.0 3.6 6.3 0.2 61 16.0 2.8 6.4 0.8 69 19.0 2.9 5.0 1.9 76 18.5 3.5 5.5 0.9 83 19.0 3.4 5.6 0.7 91 17.0 3.1 5.6 0.7 99 18.0 3.1 4.8 0.5 107 18.0 3.2 5.1 0.5

Claims (6)

1. A process for the de-oiling of oil-contaminated steam condensate which process comprises passing the condensate through a coalescer to coalesce small droplets of oil into larger droplets which settle out and form recovered oil, passing the condensate of reduced oil content through an adsorbent bed to adsorb further quantities of oil and recovering the de-oiled condensate.

2. A process according to claim 1 wherein the coalescer is of the cartridge type in which an element is used within a container.

3. A process according to either of the preceding claims wherein the adsorbent in the adsorbent bed is activated carbon.

4. A process according to any of the preceding claims wherein the de-oiled condensate has a residual oil content of 1 mg/litre or less.

5. A process as hereinbefore described with reference to the accompanying drawing.

6. Products whenever obtained by a process according to any of the preceding claims.