الفهرس 
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Abstract
Abstract
Natural gas treatment and production plants often choke the natural gas coming from high
pressure wells or compression station in order to meet pipelines or plant pressure. Recovery
of the wasted energy in choking process has the potential to makes the natural gas production
more energy-efficient. In addition, low pressure gas from the later stages of the process system
is to be flared for being uneconomical to recycle. The flaring of low-pressure gas is an energy
inefficient process and detrimental to the environment. The high-pressure gas coming from the
high-pressure wells or compression station could be used to boost gas production from aging
wells or to recompress and recycle the low-pressure gas from the later process stages. Because
of ejector advantages, many researches have been conducted on the application of ejectors in
natural gas production industry. Ejectors have no moving parts, which subsequently increase
its reliability and reduce maintenance cost significantly. Moreover, ejectors are distinguished
by construction and operation simplicity. Despite the advantages of ejectors, they are still not
widely used in natural gas production industry due to their low performance specially at a
relatively high back pressure and low induced flow pressure.
In the present study, the effect of nine ejector geometrical factors on the entrainment
ratio is investigated and the optimum ejector design is obtained at 12 MPa motive pressure, 2
MPa induced pressure, and 5.2 MPa discharge pressure using CFD technique, and surrogate
based optimization approach. The numerical simulations of natural gas ejector flow is carried
out by solving the compressible steady-state axisymmetric turbulent form of the fluid flow
conservation equations. RNG k model is adopted to simulate the turbulence phenomena
inside the ejector. Methane is used as the working fluid which represents more than 85 %
of natural gas volume. The density is modeled using the real gas equation of Soave Redlich
Kwong. Validation results of the presented CFD model gave a good agreement with the
experimental data from literature with an average error of 0.6 % in the critical mode.
The validated CFD model along with regression Kriging surrogate model are used to
utilize the factorial approach in natural gas ejector geometrical optimization process. The
optimization process passes through four main steps: defining the optimization goal and factor
ranges, constructing the sampling plan, building the surrogate model and finally defining the
infill process and convergency criterion.
The optimization goal is to maximize the entrainment ratio. In order to achieve such
objective, the geometrical factors of natural gas ejector such as the primary nozzle convergent
angle pc, primary nozzle divergent angle pd, primary nozzle exit diameter Dp, primary nozzle
I
exit position NXP, secondary nozzle inclination angle s, mixing tube diameter Dmt, diffuser
inclination angle D, mixing tube length Lmt, and primary nozzle throat length Lt have been
varied. All lengths and diameters are normalized by the primary nozzle throat diameter (Dt)
and expressed as dimensionless geometric ratios. Regression Kriging surrogate model is used
to fit the objective function. Genetic algorithm is used as a global search algorithm to optimize
Kriging model parameters, and to search the constructed model for the optimum design.
Genetic algorithm properties have been optimized to give the best optimization result for the
same initial population for both model construction, and infill process.