Page 175 - Fundamentals of Enhanced Oil and Gas Recovery

163
Thermal Recovery Processes

coefficient for the condensed steam is higher than hot water, its sweeping efficiency
becomes lower. There is always a vertical temperature gradient in formation which is
normally ignored; this is called infinite vertical thermal conductivity. Although there
is temperature gradient in direction of the heat transfer injection, this also is ignored
and the steam temperature (T S ) is assumed to suddenly change into reservoir tempera-
ture (T R ). This approximation using the step function may be useful, as the injected
fluid in the porous media leads to conductive heat transfer from the sand to overbur-
den and underburden. Steam would also lead to modification of temperature distribu-
tion along the formation. This is emphasized that the fluid front moves more rapidly
in comparison to heat front [70].

5.2.8 Steam Generation
To generate the steam, usually flue gas is used as the source of energy in generation
plants. Furnaces consist of boilers, soot blowers to transfer the mixture of steam water
to furnace, burners and combustion air systems, a pressure system for emission of
flue gases, and pressurized air system for sealing the system to avoid flue gas emission.
The boiler tubes are placed between steam distribution drums and water collectors at
the bottom of boiler. A super heater is placed before steam distribution system.

5.2.8.1 Heater Fuel
A combination of natural and refinery gas, coal, and fuel oil are applied as heater fuel.
A combination of LPG, natural gas, and off-gas from the process units make the refin-
ery off-gas stream. Fuel oil consisting of a mixture of straight-run and residuals pro-
vides the fuel for the system at the required pressure and temperature. The duties of
balance drum include providing fuel with stable heat content, constant pressure, and
recovery of gas vapors from liquid. It also prevents carrying condensates through the
system. The fuel is heated under control to flow within the unit. It is filtered before
burning. Sometimes these fuels are applied in various units. For instance, heat recov-
ery from catalytic cracking unit is provided in carbon monoxide boiler. It is then con-
verted to carbon dioxide via combustion. The units of waste heat recovery provide
steam from the flue gas.

5.2.8.2 Steam Distribution
In this system, a number of fittings, pipes, valves, and connections are provided. The
steam-required pressure is determined by the process units or electrical generators that
use it. This pressure is declined as steam enters turbines for driving the compressors
and pumps. The steam is condensed to water by traveling through the heat exchan-
gers. The condensates are then recycled to boiler or are transported to waste water
treatment unit. The steam which is applied in generators must be produced at pres-
sures higher than the steam pressure required for the process.