Page 28 - Impiantistica Italiana

RENEWABLES

sign and optimize the integration system
in order to satisfy the client’s needs. The
final number of methanators is optimized
balancing CAPEX and OPEX and conside-
ring the provided syngas and the required
SNG quality. As previously explained, re-
action runaway conditions are the major
concern in the methanation process. Sin-
ce in VESTA process this risk is avoided
thanks to the presence of CO that acts as
2
a thermal flywheel and moderates tempe-
ratures, the CO is chosen to be removed
2
downstream of the methanation section
rather than upstream. This strategy eli-
Figure 1 – The concept of available methanation processes. The main sections are in order: gasification, minates the need for reaction gas recycle
sour gas shift, acid gas removal, and methanation block. Two main features are highlighted: the and its associated compressor and costs
need of recycle compressor to handle the exothermic reactions and the complex adjustment and in terms of consumptions and installation.
continuous control of the feed gas composition to achieve on-spec SNG. In addition, this choice permits to use a
simple CO removal system to produce
2
high purity CO (e.g., amine, hot carbona-
2
(Ruggeri and Romano, 2014). In order to such risk is avoided due to the presence tes) because at that point the gas does not
moderate the exothermic methanation re- of CO and water, which act as temperatu- contain any other contaminant and easily
2
action temperature and avoid reaction ru- re dampers. In contrast to other available produce high purity CO . Since sulphur
2
naway, several techniques can be emplo- technologies, CO removal in the VESTA compounds such as H S, mercaptans,
2
2
yed, such as the recycling of products, the process is performed downstream of the COS, thiophenes, etc., are poisons for the
dilution of process gas with inert or steam, methanation reactors rather than before downstream catalyst, the syngas can be
or the installation of isothermal reactors. the methanation section, and there is no conditioned upstream by means of several
The most common choice of standard need of product recycle compressors. For techniques, e.g. AGR, catalytic removal.
technologies (Figure 1) to moderate the the sake of clarity, Figure 2 shows sche-
heat of reaction is the recycle of reacted matically the concept of the once-through VESTA technology in
gas which has, as the main drawback, VESTA process.
the need for a recycle compressor which In the VESTA SNG process, sulphur-free Wood portfolio has great
complicates the scheme of the plant and syngas (e.g., coming from an acid gas “flexibility in terms
represents a significant part of the asso- removal (AGR) unit) is routed to a sweet of gas quality and adapts
ciated investment cost and of the overall shift reactor in which the high temperature
power consumption. water-gas shift reaction (3) happens: to renewable resources
For the sake of clarity, Figure 1 shows a CO+H O↔ CO + H 2 (3)
2
2
typical conceptual scheme of the standard At the beginning, Foster Wheeler suc-
methanation process based on recycle of The stream leaving the shift reactor is sent cessfully developed a plant design with
converted gases. Moreover, in order to to the methanation reactors where the re- a very simple process scheme that le-
obtain the desired SNG quality, a complex action (2) takes place: ads to excellent performance. The plant
adjustment of the syngas composition CO+3H ↔ CH + H O (2) performances were modelled with com-
4
2
2
has to be performed across the Sour Shift mercially available process simulators,
block, as well as a compositional control This highly exothermic reaction is carried with the reactor performances regres-
on a continuous basis in order to achie- out across a series of reactors with inter- sed on the basis of reserved laboratory
ve the SNG of the correct quality without stage heat recovery. Wood is able to de- data measured by Clariant. Then, Foster
downstream functionality issues.
VESTA process
In this field, Foster Wheeler developed a
simple methanation process, called VE-
STA in 2006, using catalyst provided by
Clariant, formerly Foster Wheeler’s and
now Wood’s partner in SNG technology.
The technology is based only on well-pro-
ven equipment, such as fixed bed reactors
and shell and tubes heat exchangers, and
the catalyst has already been extensively
tested by Clariant (Ruggeri and Romano,
2014).
In conventional SNG processes, the risk Figure 2 – The concept of VESTA process. The main sections are in order: gasification, sulphur
of runaway of the methanation reaction removal, methanation block, and CO removal. CO and water in syngas control the exothermic heat
2
is very high, while in the VESTA process of reaction with no need of product recycle. 2

26 Impiantistica Italiana - Novembre-Dicembre 2018