(429c) Some Studies on the Inherent Resilience for a Gas Sweetening Unit

ABSTRACT

Mitchell
and Mannan (2007) evaluated resilience
of simple process systems like steam pipe, water pipe, water pump and heat
exchanger etc., and considered physical failures of these systems. However,
they did not provide any mathematical background and their approach was too
simplistic. Hence, it could not be directly applied to complex process systems
such as absorber, regenerator and distillation columns.
Linh
et al. (2012) reported resilience analysis considering physical
failure of the process systems to improve the safety of process plants.

In contrast to physical
failure of process systems, quantification of system resilience has been
carried out considering performance failure of process systems under this work.
The inherent resilience for process systems can be
conceptualized from the perspectives of material resilience.

Correlations
have been developed to assess resilience properties of constituent process
systems pertaining to a Gas Sweetening Unit (GSU) as a case study. Fig.1 shows the schematic flow diagram of the Gas
Sweetening Unit (GSU).

A steady state condition
has been considered and system stress and system strain equations have been
developed to quantify the system resilience.

This study has enabled developing a
new method for identification of thermodynamically efficient operating capacity
regime for optimization of energy usage in process systems. Optimum energy
usage for a process system is achieved when it is operated at the most
thermodynamically efficient capacity regime and this is evaluated by estimation
of newly conceived Thermodynamic Coefficient of Performance (THCOP) data at
various capacities.

This
work provides general formulae for quantification of system resilience
properties of one absorber column, one regenerator column, one shell and tube
heat exchanger and associated one process as well as one utility pipeline
system pertaining to a GSU (Fig.1).

It
is found that the GSU (Fig.1) operating capacity range of 80%-100% is the most
thermodynamically efficient operating capacity regime. The minimum operating
capacity for the GSU is considered as 1% whereas the maximum operating capacity
is 150%. The design capacity (i.e., 100%) of the GSU is 27814 kg/hr. The GSU is designed to sweeten sour natural gas containing 1.28 wt%
of H₂S to sweet natural gas containing less than or equal to 1 ppmw
H₂S.

It
is found that absorber column and regenerator column systems under study
possesses inherent resilience of around 5% and 15% respectively with regard to
variation in upstream feed sour gas flow rate beyond 100% design flow rate,
i.e., 27814 kg/h.

It
is  also found that the lean-rich
exchanger system under study possesses inherent resilience of around 10% with
regard to variation in upstream feed sour gas flow rate beyond 100% design flow
rate.

It
is noticed that the inlet feed sour gas pipeline system (process pipeline)
indicates drastic fall in modulus of resilience value (Ur) and great
increase in modulus of elasticity (Esys) value above feed gas mass flow
rate of 100% indicating attainment of allowable pressure drop and limiting or
yielding stress.

It
is also found that the utility pipeline system (which carries superheated
steam) shows a plateau at very high steam flow rate (57600 kg/hr-64800 kg/hr)
indicating attainment of allowable pressure drop and limiting or yielding
stress region for the system in case of flow variation.

Results also indicate that similar
to a material, all the process systems under study (i.e., absorber,
regenerator, lean-rich exchanger, process pipeline and utility pipeline) of Gas
Sweetening Unit.(GSU, Fig.1) demonstrate inverse relationship of modulus of
resilience (Ur) with modulus of elasticity (Esys) in all
applicable operating variable deviation regimes.

Computer
simulation using a process simulator SIMULATION SCIENCES INC, Pro/II has been
utilized for this study.

Finally,
one example is given regarding design
procedure in relation to incorporation of 50% over capacity factor or
inherent resiliency in the absorber column by augmentation of number of column
trays.

Literature Cited

            [1] Linh T.T.
Dinh, Hans Pasman, XiaodanGao, M.SamMannan (2012), “Resilience engineering of
industrial processes: Principles and contributing factors”, Journal of Loss Prevention in the Process
Industries, 25,233-241

[2] Mitchell S M. and M. Sam Mannan (2007),
Ph.D Thesis, “Resilient Engineered System: The Development of an Inherent
System Property”, Texas A&M University, Texas, U.S.A.

Nomenclature

Ur = Modulus of Resilience for system (kJ/m³-sec),
Esys = System Modulus of elasticity, (kJ/m³-sec)