Page 57 - Synthetic Fuels Handbook

NATURAL GAS 45

near the wellhead that serve primarily to remove sand and other large-particle impurities.
The heaters ensure that the temperature of the natural gas does not drop too low and form
a hydrate (Chap. 1) with the water vapor content of the gas stream.
Many chemical processes are available for processing or refining natural gas. However,
there are many variables in the choice of process of the choice of reining sequence that dic-
tate the choice of process or processes to be employed. In this choice, several factors must
be considered: (a) the types and concentrations of contaminants in the gas; (b) the degree
of contaminant removal desired; (c) the selectivity of acid gas removal required; (d) the
temperature, pressure, volume, and composition of the gas to be processed; (e) the carbon
dioxide–hydrogen sulfide ratio in the gas; and (f) the desirability of sulfur recovery due to
process economics or environmental issues.
In addition to hydrogen sulfide and carbon dioxide, gas may contain other contaminants,
such as mercaptans (also called thiols, R-SH) and carbonyl sulfide (COS). The presence
of these impurities may eliminate some of the sweetening processes since some processes
remove large amounts of acid gas but not to a sufficiently low concentration. On the
other hand, there are those processes that are not designed to remove (or are incapable
of removing) large amounts of acid gases. However, these processes are also capable of
removing the acid gas impurities to very low levels when the acid gases are there in low to
medium concentrations in the gas.
Process selectivity indicates the preference with which the process removes one acid gas
component relative to (or in preference to) another. For example, some processes remove
both hydrogen sulfide and carbon dioxide; other processes are designed to remove hydro-
gen sulfide only. It is very important to consider the process selectivity for, say, hydrogen
sulfide removal compared to carbon dioxide removal that ensures minimal concentrations
of these components in the product, thus the need for consideration of the carbon dioxide
to hydrogen sulfide in the gas stream.
To include a description of all of the possible process for gas cleaning is beyond the
scope of this book. Therefore, the focus of this chapter is a selection of the processes that
are an integral part within the concept of production of a product (methane) for sale to the
consumer.

2.7.1 Olamine Processes
The most commonly used technique for gas processing is to first direct the gas flow through
a tower containing an amine (olamine) solution. Amines absorb sulfur compounds from
natural gas and can be reused repeatedly. After desulfurization, the gas flow is directed to
the next section, which contains a series of filter tubes. As the velocity of the stream reduces
in the unit, primary separation of remaining contaminants occurs due to gravity. Separation
of smaller particles occurs as gas flows through the tubes, where they combine into larger
particles which flow to the lower section of the unit. Further, as the gas stream continues
through the series of tubes, a centrifugal force is generated which further removes any
remaining water and small solid particulate matter.
The processes that have been developed to accomplish gas purification vary from a
simple one—wash operation—to complex multi-step recycling systems (Speight, 1993;
Mokhatab et al., 2006; Speight, 2007b). In many cases, the process complexities arise
because of the need for recovery of the materials used to remove the contaminants or even
recovery of the contaminants in the original, or altered, form (Kohl and Riesenfeld, 1985;
Newman, 1985).
As currently practiced, acid gas removal processes involve the chemical reaction of the
acid gases with a solid oxide (such as iron oxide) or selective absorption of the contami-
nants into a liquid (such as ethanolamine) that is passed countercurrent to the gas. Then the