Page 289 - Hydrocarbon Exploration and Production Second Edition

276 Oil and Gas Processing

11.1.2.4. Dehydration and water treatment
Produced water has to be separated from oil for two main reasons: firstly, because
the customer is buying oil not water, and secondly, to minimise costs associated
with evacuation (e.g. volume pumped, corrosion protection for pipelines). A water
content of less than 0.5% is a typical specification for sales crude.
Water separated from oil usually contains small amounts of oil which have to be
removed before the water can be released to the environment. Specifications are
getting tighter but standards ranging from 10 to 100 ppm (parts per million) oil in
water before disposal are currently common. In most areas, 40 ppm of oil in water is
the legal requirement, that is 40 mg/l.
The simplest way to dehydrate or de-oil an oil–water mixture is to use settling or
skimming tanks, respectively. Over time the relative density differences will separate
the two liquids. On land in situations where weight and space considerations are not
an issue, this can often be accomplished by using combined settling/storage tanks in
a tank farm, either within a field or at an export facility. Unfortunately, this process
takes time and space, both of which are often a constraint in offshore operations.
Highly efficient equipment which combines both mechanical and chemical
separation processes are now more commonly used.

11.1.2.5. Dehydration
The choice of dehydration equipment is primarily a function of how much water
the well stream contains. Where water cut is high it is common to find a ‘free water
knockout vessel’ (FWKO) employed for primary separation. The vessel is similar to
the separators described earlier for oil and gas. It is used where large quantities of
water need to be separated from the oil/oil emulsion. The incoming fluid flows
against a diverter plate which causes an initial separation of gas and liquid. Within
the liquid column an oil layer will form at the top, a water layer at the bottom and
dispersed oil and oil emulsion in the middle. With time, coalescence will occur and
the amount of emulsion will reduce. If considerable amounts of gas are present in
the incoming stream, three-phase separators serve as FWKO vessels. Some FWKOs
have heating elements built in, their main purpose being the efficient treatment of
emulsions. In some operations, a FWKO will produce oil of a quality adequate for
subsequent transport.
A knockout vessel may, on the other hand, be followed by a variety of
dehydrating systems depending upon the space available and the characteristics of
the mixture. On land a continuous dehydration tank such as a wash tank may be
employed. In this type of vessel, crude oil enters the tank via an inlet spreader and
water droplets fall out of the oil as it rises to the top of the tank. Such devices can
reduce the water content to less than 2% (Figure 11.11).
Where space and weight are considerations (such as offshore) plate separators may
be used to dehydrate crude to evacuation specification. Packs of plates are used to
accelerate extraction of the water phase by intercepting water droplets with a
coalescing surface. An electric charge between plates can also be employed to
further promote the capture of water droplets, such a vessel is called an electrostatic
coalescer. Plate separators have space requirements similar to that of knockout vessels.