Hydrocarbon Processing - August 2022 - 24

Refining Technology
or byproducts, so no additional treatments are required. Because
the reaction occurs in the aqueous phase at near-ambient temperatures,
the process is inherently safe. The removal of sulfur
is not affected by turndown conditions, and the process can be
applied to almost any vapor stream containing H2
Liquid redox is an economical process for H2
S.
S removal and
FIG. 1. Anadarko Petroleum Corp.'s Pinnacle Bethel Gas Plant,
near Bethel, Texas, with the addition of the proprietary sulfide
treatment systema
.
SRU process selection. In the European refinery example, the
existing higher capacity Claus SRU had been shut down when
oil processing ceased. The typical minimum H2
S content for a
feed to a Claus unit is 35 mol%, but that condition cannot be met
when the HVO process is operating. Modified Claus units can
operate with less H2
S in the feed stream, but to achieve >99.9%
sulfur recovery, a tail gas treating unit is required downstream of
any Claus SRU, which increases cost. Turndown conditions can
also negatively affect Claus unit sulfur recovery efficiency.
A highly flexible sulfur recovery process was needed to account
for the range of feed gas conditions. The selected process
also needed to achieve high recovery percentages over a wide
range of operating flowrates and H2
finer required:
* Maintaining sulfur removal efficiency at turndown
conditions
* No minimum H2S concentration in the feed acid gas
stream
* A fully guaranteed product to meet customer
requirements.
The sulfur recovery processa
processa is a liquid redox system that converts H2
. The author's company's
S to solid elemental
sulfur (FIG. 1). The process utilizes an aqueous iron
solution with a catalytic performance that is enhanced by a proprietary
blend of chemicals. The H2
S is converted to elemental
sulfur by redox chemistry according to the following overall reaction
(Eq. 1):
Direct oxidation reaction: H2S + 1/2 O2 j H2
process, as summarized in Eqs. 2 and 3:
Absorber: H2
Oxidizer: ½ O2
of H2
O + S°
S + 2Fe+++ j S° + 2Fe++ + 2H+
+ H2O + 2Fe++ j 2OH- + 2Fe+++
(1)
The redox reaction is conducted in separate sections of the
(2)
(3)
The exothermic absorber reaction represents the oxidation
S to elemental sulfur and the accompanying reduction of
the ferric iron state (active) to the ferrous iron state (inactive).
This reaction is irreversible and not equilibrium dependent.
The oxidizer reaction (also exothermic) represents the oxidation
of the ferrous iron back to the ferric iron state.
Liquid redox technology produces a sulfur " cake " that contains
approximately 65% sulfur, 35% moisture and trace amounts
of proprietary chemicals. It does not produce any other products
24 AUGUST 2022 | HydrocarbonProcessing.com
S concentrations. The resulfur
recovery within a given range. On the low end of sulfur
recovery, non-regenerative chemical absorbents (scavengers)
are likely to be more economical due to the low capital cost of
a two-vessel design. As the sulfur removal requirement increases,
operating costs become prohibitive using scavengers. This
is when a liquid redox process is advantageous due to lower
operating costs. The higher capital cost of a liquid redox unit
is quickly paid out in operating savings vs. replacement of the
non-regenerative scavenger. An amine unit + Claus SRU is advantageous
for high-end sulfur recovery. Excessive loss of sulfur
recoverya
chemicals when removing more than 20 tpd of sulfur
cake, however, drives up costs.
The first milestone. A proprietary sulfur recovery unita
went
into commission at the European refinery in August 2019. The
refinery installed a rectangular autocirculation unit enabled by
the dilute concentration of H2
S in the feed to the SRU. In the
summer of 2020, the liquid redox unit underwent its first turnaround.
The working solution was drained and stored for further
use. The autocirculation vessel was opened, cleaned and inspected.
A small amount of sulfur buildup was detected on the vessel's
floor. The unit was cleaned with a warm water wash and no corrosion
was noted. The shutdown for maintenance took 1 wk-
the equivalent to 98% availability in the first year of operation.
In 2020, the biorefinery reached full operation with a fivefold
increase in biofuels production compared to 2019.
Takeaway. The rising demand for sustainable and reliable energy
is driving biofuel production around the world. Analyst
firm Precedence Research predicts the global biofuels market
will exceed $200 B by 2030.
Slight reconfigurations of existing petroleum refineries enable
a seamless and lower-cost entry into the biofuels production
business at scale. Refineries that will continue to process
crude oil will typically utilize existing Claus units to handle
any increased demand for sulfur recovery. In facilities without a
working Claus unit or where the Claus unit is overloaded, biorefiners
can manage the low-tonnage sulfur recovery with the
proprietary liquid redox technologya
nage sulfur recovery and achieves very low SO2
, which addresses low-tonemissions.
The
systems are reliable and have high availability, the operating cost
is low, and bio-refiners can consistently achieve sulfur removal
guarantees while taking a solid stance on sustainability.
NOTES
a Merichem's LO-CAT® process (Liquid Oxidation CATalyst)
b Merichem's FIBER FILM®
WILLIAM ECHT has worked in gas conditioning for more than 40 yr. He serves as
Technology Licensing Director for Merichem Co., representing the technologya
for sulfur recovery from gas streams and its technologyb
for sulfur removal from
liquid hydrocarbons. After receiving a BS degree in chemical engineering from the
University of Texas at Austin, Mr. Echt joined Phillips Petroleum in Oklahoma for
8 yr as Staff Engineer for gas gathering and natural gas liquids (NGL) extraction
plants. He then worked for UOP for 25 yr as a Process Engineer and Manager for
gas treating technologies based on adsorbents, solvents and membranes.
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - August 2022

Table of Contents for the Digital Edition of Hydrocarbon Processing - August 2022
