cover story

Natural Gas Unit Operations Engineering

by

Engr. Razmahwata bin Mohamad Razalli, MIEM, P. Eng.

In a previous article published in Jurutera back in 2007,

the author had provided the beginning for the thought
processes involved in designing a gas processing facility.
The article started off with an introduction to pertinent gas
properties, and then moved on to the overall concepts that
had to be considered when putting together the specifications and concept selection of the facility. Then some insight into what goes on in the upfront modelling process
and the tools available for use were provided.
This article continues on the same topic. It will focus
more on describing at a high level the functions of various
unit operations in the facility, and what to think about
when putting the whole scheme together. As before, the
thinking that goes into the process is provided from a
chemical engineering standpoint,
which corresponds to the designation
of process engineer in the oil, gas and
petrochemical industry.

constraints. For example, the water content in condensate

may have to be removed if a downstream process has a
maximum allowable limit, or if the condensate is to be
sent directly from separation to a customer who requires
it to be below a certain level. The same thought process
might apply if the hydrocarbon content of produced
water needs to be reduced to regulatory or acceptable
levels prior to disposal.
Note that to reduce the amount of water vapour in the gas
phase, physical separation is not a practical process. Other
means are required to achieve this task. The most common
form of equipment used in separation is a vessel. It will have
the following features and is illustrated by Figure 2:

Separation

Separation, in its basic form, is to

physically separate the different
phases in a multi-phase process
stream. At the high school level, the
phases of matter are solid, gas and
liquid (and plasma for those who attended the advanced courses.) In the
industry, we talk about two phases
(gas/liquid and liquid/liquid) and
three phases (gas, liquid hydrocarbons and liquid water) separation,
depending on the demands of the
process. The illustration in Figure 1
relates to the following text:
i) If the intent is to separate out
liquids, to either prevent damage
to the equipment that will process
the gas or degrade the efficiency of
such equipment, then two phase
- separation might be sufficient.
Examples of such a separation may
be upstream of a compressor, or
upstream of a glycol dehydration
contact tower.
ii) Three-phase separation might
be required if the liquid phase
has to be treated to meet certain

Figure 1: Block diagram of gas processing

Figure 2: Separation vessel

(To be continued on page 8)

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i) Initial bulk separation is where the bulk of gas/liquid

separation occurs.
ii) Compartments where the liquid phases are allowed to be
relatively still to allow for further separation. Sufficient
time is given to allow gas bubbles to rise to the top of
the liquid space, condensate droplets in water to rise to
the bottom of the condensate/water interface, or water
droplets in condensate to drop to the same interface.
Various equations that assist in the design of separators
are available in the industry. These equations attempt to
provide minimum sizing dimensions using physical first
principles that are functions of droplet size, settling velocity
and residence time. These equations are then tuned based
on the separation requirements and operating experience to
determine vessel sizes. However, the dimensions that result
from such equations may be reduced if the efficiency of the
separation process is increased. This can be achieved via
two approaches:
i) Take the normal separator design and add internals to
improve the quality of the separation. This might be in
the form of devices to improve gas and liquid flow (e.g.
Schoepentoeter to improve inlet flow, or straightening
vanes to make the best use of the separator width) as
illustrated in Figure 3, or devices that assist in increasing
entrained liquid droplets so as to improve the process
of separation by gravity (coalescer devices.)
ii) Separation of phases using forces other than gravity. An
example as illustrated in Figure 4 is to use centripetal
force for either gas-liquid separation, or the separation
of different liquid phases. These designs are more
vendor dependent, as the sizing equations tend to be
proprietary, with the objective of delivering a more
compact device of given process requirement, which is a
plus on both technical and commercial basis.
The following items should be considered when designing
a separation system:
i) The design flow rates throughout the life of the facility.
There are different demands on the separation vessel
during the stages of initial facility start up, production,
rejuvenation and tail end. These different periods
would entail different flow rates of the various phases.
The vessel should be designed to either cater for all
conditions, or designed in such a way that the vessel
can be later adopted to suit the production conditions.
ii) Non-steady conditions: Upstream facilities generally
do not have steady process conditions at the inlet of
the facility. An example of such an unsteady flow is the
phenomena of slugging. This is where liquid and gas
may come in batches to the facility due to the dynamic
interplay between the gas and liquid phases and the
physical layout of the pipeline. The flow rates of phases
will fluctuate around an average value. The profile
of these fluctuations can be predicted using various
modelling software. The production separator, being the

first unit operation that faces these conditions, should be

designed to handle this phenomenon.
iii) Non-steady conditions: Another phenomenon that
might need to be catered for is pigging. This occurs
when a pig is sent down the pipeline upstream of the
facility. Typically, liquid builds up ahead of the pig, and
acts as a large slug.

Solids handling: a full wellstream might not only
consist of gas and liquid, but might contain a significant amount of solids, usually in the form of sand. The
sand might be the cause of erosion of piping, heavy
wear of valve and pump internals. Therefore, there is
a good reason to remove sand from the process at the
separation vessel, which is usually the first unit operation. Such solid handling methods could be an online
continuous removal system, transportation of solids to
a process downstream of the vessel, or something as
basic as a scheduled shutdown with big, burly men,
shovels and a lot of bags.

The type of equipment used to measure the process
conditions. Some fluids are dirty and quickly block
the small nozzles used by the displacer type level
measurement devices. In this case, a more suitable
device might be a capillary type device, or one that uses
radar to directly detect the liquid surface. Note that this
plethora of choices does not apply to the measurement
of gas properties.

Figure 3: Flow enhancement device

Figure 4: Separation via centripetal force

(To be continued on page 10)

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Figure 5: Dehydration of the gas stream

Figure 6: Glycol contactor and regeneration system

Dehydration

Dehydration of the gas stream means to remove water vapour in the gas to meet
the maximum water content. Note that this is different from the removal of water
in a liquid state. The reasons for drying a gas stream have been mentioned in the
previous article.
The most common method of dehydrating a gas stream is using glycol, an organic liquid that has an affinity for water vapour. Glycol, which has low water content (dry glycol), is put into contact with a gas stream. The water vapour is then
absorbed by the glycol and removed from the process stream (wet glycol.)
(To be continued on page 12)

10 Jurutera March 2009

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The glycol-gas interface happens inside a contactor, which has internals

designed to maximise the amount of surface area between the two phases
to allow for the fastest amount of mass transfer between the two. A lot of
computation power and effort has been put into the design of the internals,
matching available information and construction capabilities. The earliest
method of creating the largest amount of surface area per volume of contactor
was using oddly shaped discrete objects and poured inside a container vessel.
An obvious advantage is that the packing was simple to install, and
results could be analysed and predicted with the technologies of the day.
The disadvantage of this system was that it was not optimised. Channelling
(preferential, non-distributed flow) of the gas and liquid could occur, thus
reducing the contact area per contactor volume, and requiring additional
contactor volume to compensate for this effect.
With better tools and understanding of the process, structured packing was
more likely to be used and installed. This packing comes prefabricated, and
bears some resemblance to a Weetabix wafer. These components are systematically installed, and provide a larger interface area per volume available, reducing the amount of volume required to meet the necessary phase contact area.
Efficient designs are under the purview of specialist companies.
However, the purchasing engineer needs to provide correct data to the vendor, for example, expected flow rates, available utilities and potentially provide corrective systems around the contactor (Does one need a recovery vessel
downstream for glycol that is entrained in the gas stream? This point has to be
confirmed.)
Wet glycol is regenerated by heating the glycol and evaporating the water
in a reboiler. The temperature chosen has to be as high as possible such that
the maximum water boil off rate may be achieved without decomposing the
glycol.
Although functionally simple, various optimisation steps have been
incorporated into the design. For example, the relatively cold and rich glycol
stream from the process is used as a reflux coil in the glycol reboiler. This
has the effect of recovering heat from the water boiled off the glycol. Heat
exchangers provide the further recovery of heat. Vessels as illustrated in Figure
6 are installed to recover any hydrocarbons that may have been entrained in the
glycol. As contaminants affect the efficiency of the system, systems have to be
installed to remove them (via filters) or reduce the effects (via neutralisers.)
(To be continued on page 15)

12 Jurutera March 2009

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Another method of drying gas would be through the

use of solids. The solid used has a structure such that water
molecules are trapped and held within the solid while gas
molecules are not. Hence, the solid works as a desiccant. An
example, as illustrated in Figure 7, of such a material is the
molecular sieve.
One advantage of using such materials is that a much
lower dew point can be achieved, between -40C to -50C.
Another is that the material may be regenerated using
either a heat source (heated dried gas) or by lowering the
system pressure. This eliminates the need for a reboiler and
its various accessories. A common example where solid
dehydration is used is to dry out air for use in pneumatic
instrumentation.
A third advantage is that dehydration by solids can
remove more than just water molecules. This conditions
the gas for further processing downstream. For example,
molecular sieves might remove H2S from a gas stream in
addition to water, sweetening the gas in preparation for
further processing downstream.
How dry does the gas need to be? This is where
engineering number crunching and a laymans intuition
come together. The dryer the gas needs to be, the lower the
water content of the drying medium or, in countercurrent

systems, the lower the water content of the wet glycol

exiting the contactor.
This, in turn, affects the design of the regeneration
system. One could lower the exit water content of the wet
glycol either by reducing the moisture content of the dry
glycol entering the contactor, or by increasing the glycol flow
rate. This eventually turns out to be an economic decision
where operating costs versus capital costs are considered.
Another consideration might be two different processes
in series to achieve the required effect, for example, using
glycol to perform bulk dehydration, and then using solid
absorption to meet the required dewpoint.

Summary

The preceding text discusses two unit operations that

would typically be found in an upstream gas processing
plant. It is hoped that the reader will appreciate the high
level thought processes invoked in a design, and provide
prompts when considering other designs. n
Note: The author intends to continue this series with
discussions of other unit operations.

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