OXIDATIVE COUPLING COMBINED WITH DISTILLATION TO REMOVE MERCAPTAN SULFUR FROM NGLS

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GBH Enterprises, Ltd.
Oxidative Coupling Combined with
Distillation to Remove Mercaptan Sulfur
from NGLs
Case Study: #0184157GB/H
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CONTENTS
Background
Figure 1. Typical LPG Fractionation line-up and Sulfur Compound
Distribution
Potential Market for low Sulfur LPGs
Sulfur removal from LPGs
Oxidative coupling combined with distillation
Figure 2. Debutanizer with Catalytic Oxidative Coupling
Aspen Simulation Results
Integration of reaction and distillation
Technical Issues
Practical Issues
Future Considerations
Market Issues
Technical Issues
Additional Technical Requirements
References
Appendix
Kerosene Sweetening

PROPOSED USE OF OXIDATIVE COUPLING COMBINED WITH DISTILLATION TO REMOVE
MERCAPTAN SULFUR FROM NGLS
Background
The production of natural gas liquids from natural gas generally involves liquefaction followed by
fractionation to give the individual products – ethane, propane, butane (sometimes separated into
iso and normal) and natural gasoline.
The sulfur content of the liquids depends on the initial content of the associated gas, the
processing method and the treatment of the liquids themselves.
Once the liquids have been condensed out of associated gas they are fractionated – typically
following a scheme as in fig.1. This may or may not be followed by a pre-treatment step such as
an amine/ caustic wash to partially or fully remove H2S / COS. The typical sulfur compounds
present and their split between the different products is shown. More details in refs 1,2,3.
As can be seen, the propane and butane streams (LPGs) are contaminated with mostly
mercaptan sulfur, often at levels of several 100 wt ppm S.
Fig 1 - Typical LPG fractionation line-up
and sulfur compound distribution
De-ethanizer De-propanizer De-butanizer Butane splitter
Ethane Propane Butanes i-butane
C5+ n-butane
H2S
COS
C1SH
C2SH
DMS
C3SH
++
H2S
COS
C1SH
H2S
COS
C1SH
C2SH
DMS
C3SH
++
H2S
COS
C1SH
C1SH
C2SH
DMS
C3SH
++
C1SH
C2SH
DMS
C3SH
++
C1SH
C2SH
DMS
C1SH
C2SH
DMS

Potential Market for Low Sulfur LPGs
At present there are no real sulfur limits on LPGs. So, typically they contain several 100 ppm of
sulfur. There are many small plants in North America producing LPGs. There are a number of
markets – auto fuel, cooking fuel, refinery/chemicals feed or fuel, aerosols and others. I believe
that in N America in most situations the sulfur is not removed. However, there are probably
caustic (UOP Merox or Merichem) sweetening units on many facilities, turning the mercaptans
into disulfides and thus reducing corrosivity.
Similarly in Europe and the ROW, sulfur is not usually extracted at the moment. In Europe there
is probably even less processing with more situations of ‘smelly’ LPGs.
Going forward, it seems likely that there will be more restrictions imposed, especially in the
market for auto-gas as it will have to conform to the same standards as other fuels.
Sulfur Removal from LPGs
As stated above, the main sulfur compounds, which get into propane and butane, are determined
by boiling point due to the production by fractionation (shown below in order of increasing bpt.):
Hydrocarbons bpt C Sulfur compounds bpt C
Ethane -90
H2S -60
COS -50
Propane -40
i-butane -10
n-butane 0
CH3SH 10
Pentanes 10-
35
C2H5SH 35
DMS (CH3SCH3) 40
Other S compounds >50
The main contaminants of propane are H2S, COS and CH3SH, the first two of which can mainly
be removed by pre-processing. Those in butanes are CH3SH, C2H5SH and very low levels of
DMS.
These could be removed by extractive caustic-based oxidation, adsorption onto mol. sieves or by
hydrotreating and subsequent removal of the H2S (e.g. there is a low temperature HDS to purify
LPG for aerosols). These would all involve addition of an extra processing step and significant
extra number of equipment items.

Case Study: #0184157GB/H: Use oxidative coupling combined with distillation
I believe that it is common to run the depropanizer and debutanizer on these trains at high
enough pressures, which enable overhead distillate cooling to be carried out by using cooling
water or with air coolers. If they run at pressures of about 20 bara and 10 bara respectively, the
overhead condensing temperatures are about 70
o
C in each.
The concept would be to put a catalyst in the upper section of the debutanizer, which could
continuously oxidize the mercaptan species to disulfides. These would then separate out by
fractionation and go down the column into the debutanizer bottom fraction (natural gasoline) – i.e.
catalytic distillation (figure 2)
The advantage would be that a pure butanes stream can be obtained in a single step, rather than
having to use an additional step (such as caustic extraction, mol. sieve or hydrogenation). This
could also be applied to the depropanizer.
GBH Enterprises have a suitable catalyst, at least in terms of chemistry – VULCAN Series VGP
Xc 300. This is a CuCl2, based catalyst, which performs the overall reaction, e.g.:
2CH3–S–H + 0.5 O2 = CH3-S-S-CH3 + H2O
(See Appendix: VULCAN Series VGP Xc 300 Technical Overview)
The supposed mechanism for this is via Cu (1) mercaptide formation initially, which is then
oxidized to give the disulfide. The Cu catalyst (VULCAN Series VGP Xc 300) works in kerosene
and has also been used on NGLs (butane/pentane). Furthermore a temperature of 70-80
o
C is
within its operating range.
So it seems that there is a fortunate combination – the operating temperature of the catalyst is
close to the ideal temperature for fractionation. Some water and air would need to be added –
these could be added to the feed just before the column.

Figure 2 shows the concept. Looking at a feed to the debutanizer of the following molar
composition :
Propane 1.3%
n-butane 33.0%
i-butane 30.3%
n-pentane 35.3%
Methyl mercaptan 124 ppmv
Ethyl mercaptan 141 ppmv
DMS 71 ppmv
Total S 170 ppmwt

The debutanizer is running at 10 bara, a reflux ratio of 2.0, an overhead temperature of 70
o
C., a
bottoms temperature of 120
o
C, with 20 stages below the feed and 10 stages above it. The
ASPEN simulation gives the results in the following table – the oxidation case has been modeled
as a single reaction stage above the feed for now – basis feed rate about 60 bpd :
Comparison table of debutanizer operation with and without oxidative coupling:
Base case With
No oxidation Oxidation
Water added kgmols/hr 0 0.025
Air added kgmols/hr 0 0.022
Butane rate kgmols/hr 43.2 43.2
CH3SH Ppmv 194 0
C2H5SH Ppmv 8 0
DMS Ppmv 2 2
Total S Ppmwt 160 1
Water Ppmwt 0 160
O2 Ppmwt 0 8
N2 Ppmwt 0 200
Gasoline rate kgmols/hr 26.8 26.8
CH3SH Ppmv 10 3
C2H5SH Ppmv 356 125
DMS Ppmv 180 180
DMDS Ppmv 0 160
DEDS Ppmv 0 122
Total S Ppmwt 247 392
Water Ppmwt 0 100
The following comments can be made –
1. The concept works well in simulation
2. There is no difficulty with water balance being an issue as at 80
o
C the capacity of
butane/ pentane to carry away water is about 1500 wt pm in solution. This will far exceed
water generation from any possible level of mercaptan in the hydrocarbon stream. It will
however mean that the product butane is not bone dry.
3. The oxidation balance shows a 15% excess of O2 used. There is a significant margin
above this, while still being able to keep all of the O2/N2 soluble in the butane product.

Integration of reaction and distillation
There are 3 technical issues that I can think of relating to whether the catalyst could be used
inside a debutanizer column.
1. Do required LHSV requirements match with available column volume? Based on the
debutanizer column design for a North American LPG recovery plant train 1 – 1100 bpd
butane - which is 48” diameter with a typical tray spacing of 18”, 6 trays depth
corresponds to a LHSV of around 1.0. We know that the catalyst has a very satisfactory
performance, especially at 70-80
o
C and should perform in much less volume / much
higher space velocity.
2. Will mass transfer of oxygen be adequate? The conventional sweetening process is
carried out all in the liquid phase. Hence all the stoichiometric reaction oxygen and the
excess is in solution. In the distillation case, there are 2 phases and most of the oxygen
will be in the vapor phase. As the reaction proceeds at the catalyst surface, oxygen will
have to dissolve in the liquid to replenish what has reacted and diffuse through the liquid
to the catalyst surface. The feeling is that this should be ok, because the rate determining
step is likely to be the initial mercaptide adsorption. It could mean that higher air injection
rates are required.
3. What kind of particle size / structure does the catalyst need to have to satisfy
hydrodynamic considerations, how does this impact on method of manufacture and how
will it be supported in the column? The present generation of VULCAN Series VGP Xc
300 has a very broad size range distribution. This has not been looked at so far.
More detailed calculation work could be done in the area of 2 and 3 to understand these points.
Other practical issues
1. Safety.
Clearly any process in which air is added to a hydrocarbon stream needs to be
considered from the safety point of view very carefully. However, it should be possible to
reach a satisfactory solution as this is not dissimilar to MEROX sweetening.
2. Loading the catalyst.
Consideration will need to be given to the loading of the catalyst system into the column.
3. Materials of construction.
Due to the background levels of HCl around the catalyst, the column would need to be
epoxy lined as in kerosene sweetening. It is assumed that there is no significant loss of
HCl up or down the column as in normal sweetening.
Points 2 and 3 taken together probably mean that it would be difficult to consider this as a revamp
option for existing debutanizers.

4. There is information in the references that DMDS partially dissociates at reboiler
temperatures exceeding 120
o
C or so. This might mean that the bed is challenged with
some H2S. It might need to have a small bed of ZnO underneath it.
5. Is there any problem with water / dissolved gases in the products? This should be no
different from if the liquids are caustic treated. What about the fact that the gasoline
fraction has a higher sulfur level ?
Future Considerations
Market Issues
A key question is whether there is any market for low sulfur LPGs or in the next few years in new
build projects. We also need to establish better what is the current extent of purification facilities
and whether there is any drive to achieve better purification.
Technical Issues
We need to do some more limited work around mass transfer / catalyst particle size and packing
etc, to understand what would be needed to put the catalyst in a column, to optimize the
performance of VULCAN Series VGP Xc 300.
Note: Testing of the catalytic distillation is not easy due to the high pressures involved.
Additional Technical Requirements:
We need some basic data on reaction rates of CH3SH coupling vs. temperature and dissolved O2
level. This could probably be done with the mercaptan in a heavier carrier at atmospheric
pressure.
In terms of further work on the distillation, we need to consider collaboration with identified
experts in catalytic distillation, or LPG technology providers.
References :
1. Gas conditioning and processing – vol 4 – chapter 8 (Liquid sweetening)
2. Sulfur compound distribution in NGL: plant test data – Harryman and Smith – 73
rd
GPA
annual convention 1994
3. Update on sulfur compound distribution in NGL: plant test data – Harryman and Smith –
75
th
GPA annual convention 1996

APPENDIX
VULCAN Series VGP Xc 300 Technical Overview

OXIDATIVE COUPLING COMBINED WITH DISTILLATION TO REMOVE MERCAPTAN SULFUR FROM NGLS

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