Chemical Engineering February 2022 - 36

Feature Report
Optimizing Vapor Recovery
from Storage Tanks
Light hydrocarbons in storage tanks can vaporize and vent to the atmosphere, creating
harmful emissions. An optimized vapor-recovery unit can effectively and economically
reduce such emissions
Yik Fu Lim
Taner Industrial
Technology Sdn Bhd
Dominic C. Y. Foo
University of Nottingham
Malaysia
Mike Boon Lee
Ooi
NGLTech Sdn Bhd
IN BRIEF
BASE-CASE SIMULATION
PARAMETRIC STUDIES
HYDRATE FORMATION
STUDY
OPTIMIZATION MODEL
COMPARISON OF
RESULTS
SENSITIVITY ANALYSIS
I
n the chemicals, petroleum refining and
natural gas industries, storage vessels
are used to contain various liquids,
such as condensates, crude oil and
produced water. Condensate and crude oil
are usually kept in fixed-roof, atmosphericpressure
tanks between production wells
and pipelines or truck transportation. In
offshore fields, the storage vessels usually
contain crude oil and condensate
produced from connected wells, or from
nearby platforms [1].
In most cases, light hydrocarbons, such
as methane, volatile organic compounds
(VOCs), natural gas liquids (NGLs) and hazardous
air pollutants (HAPs), in the crude
oil tend to vaporize and collect within the
space between the fixed roof and liquid
level of the tank [2]. Ambient temperature
changes cause the fluctuation of liquid level
in the tank, leading to the escape of vapors
into the atmosphere. These escaped vapors
cause income losses due to reduction
in hydrocarbon volume and changes in the
American Petroleum Institute (API) gravity
measure of the oil. Apart from potential fire
hazards, they also contribute to environmental
pollution, because methane (C1)
and carbon dioxide (CO2) are greenhouse
gases that contribute to global warming [3].
Flash gases may be flared or vented directly
to the atmosphere - the latter results
in an environmental emissions impact [4].
Therefore, a commonly accepted option
to simultaneously reduce light hydrocarbon
emissions and realize significant economic
savings is to install vapor-recovery
units (VRUs) on storage vessels. VRUs are
relatively simple systems that can capture
approximately 95% of the light hydrocarbon
vapors for sale, or for onsite usage
- for instance, as fuel. Ref. 2 reported
the generation of savings from recovering
light hydrocarbons, while at the same
time reducing the volume of HAPs and
36
TABLE 1. FEED COMPOSITION FOR STUDY [5]
Components
Oxygen (O2)
Water (H2O)
Hydrogen sulfide (H2S)
Carbon dioxide (CO2)
Nitrogen (N2)
Methane (C1)
Ethane (C2)
Propane (C3)
i-Butane (i-C4)
n-Butane (n-C4)
i-Pentane (i-C5)
n-Pentane (n-C5)
Hexanes (C6)
Heptanes (C7)
Octanes (C8)
Nonanes (C9)
Decanes (C10)
Undecanes (C11)
Dodecanes (C12)
Totals
methane emissions.
For this article, simulation and optimization
were performed on a VRU for the recovery
of light hydrocarbons. The process
parameters that affect profitability were
identified and optimized in order to achieve
higher profitability for the VRU.
Base-case simulation
A base-case simulation model (Figure 1)
was developed using the commercial process
simulation software Aspen Hysys v8.8,
using a thermodynamic package employing
the Peng-Robinson equation of state,
which is frequently used to evaluate natural
gas systems in industry. The feedstream
composition is taken from a literature case
study reported for a floating production
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2022
Composition, mol %
0.80
9.28
0.00
6.69
13.80
31.68
6.73
13.41
3.29
7.41
2.38
1.76
1.20
0.91
0.47
0.12
0.04
0.01
0.01
100
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Chemical Engineering February 2022

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