Hydrocarbon Processing - October 2021 - 54

Hydrogen
The molar H/C = 12.01/(C/H weight), which equals
1.9944. The empirical formula of naphtha is CH1.9944
. The empirical
molecular weight is shown in Eq. 5:
1 × 12 + 1.9944 × 1.008 = 14
The carbon number of naphtha is found by using Eq. 6:
Actual molecular weight/empirical molecular weight =
78.2/14 = 5.586
(5)
(6)
Feed purification, Section 1: Hydrogenator. The feed,
along with recycled H2
, first enters the hydrogenator reactor.
One of the primary functions of the hydrogenator is to convert
organic sulfur (S) compounds (e.g., mercaptans, sulfides, disulfides
and thiophenes) to H2
to convert organic chlorides (if present) to hydrochloric acid
(HCl) by reacting with H2
be present in the feed.
Key parameters to be monitored include:
* H2
level for each type of feed or combination of feeds.
This level is generally suggested by the catalyst supplier.
The process engineer should confirm this level with
the catalyst supplier in case of any deviation in feed
specifications. The recommended H2
level in the outlet
stream is normally 2% for NG feeds and about 26% for
highly aromatic naphtha feeds.
* The optimum operating temperature is 350°C-400°C
(662°F-752°F). For the LPG feed, the maximum
temperature limit may be lower due to susceptibility to
form carbon. Operating higher than the recommended
maximum temperature may cause carbon deposition
on the catalyst and upstream pre-heat coil, leading to
high pressure drop issues. Operating lower than the
recommended minimum temperature may cause
organic sulfur or chloride to slip through and poison
the downstream reforming catalysts.
typical H2
Among the KPIs is pressure drop. Being the first reactor in a
flow sheet, the hydrogenator is vulnerable to pressure
drop issues. Some general reasons for increases in pressure drop
across any fixed bed include:
* Breakage or erosion of catalyst particles, primarily
due to poor handling and loading, is one of the causes
of high pressure drop.
* Disintegration of catalyst pellets-primarily the
top layer-is another cause of pressure drop due
to poor inlet gas distribution and/or an inadequate
hold-down layer on the top. In some cases, the
disintegration of poor-quality support balls in the
hold-down layer contributes to high pressure drop.
* Carryover on the catalyst bed is one of the most
likely reasons for high pressure drop across the
hydrogenator. Any debris upstream of the reactor,
if not properly removed, can get carried over to
the top of the catalyst, leading to a high pressure
drop. Specialized foulant trapping materials are
commercially available to address this issue.
* Deformation of catalyst pellets due to accidental wetting
of the catalyst, causing a decrease in catalyst strength, can
lead to deformation and, in the worst case, disintegration.
54 OCTOBER 2021 | HydrocarbonProcessing.com
S. Other primary functions are
and to hydrogenate olefins that may
This issue can be encountered in high-temperature or
medium-temperature shift reactors where there is a
likelihood of upstream boiler water leaks.
* The collapse of the bed support grid or any damage
to the outlet collector can result in a significant
pressure drop.
* Operating the reactor (hydrogenator and pre-reformer)
at higher than recommended temperatures can cause
thermal cracking of the hydrocarbon feed, thereby
depositing carbon over the catalyst, leading to a high
pressure drop. In addition, for pre-reformers processing
naphtha feeds, there is a minimum bed temperature
below, which could have issues of polymeric carbon
formation that can lead to pressure drop increases.
It is important to monitor the pressure drop trend closely
from the time the catalyst has been put in operation. If the pressure
drop increases suddenly, then that time (before and after)
should be isolated and investigated in detail. This generally applies
to all reactors in the flowsheet.
Inlet and outlet chlorides and sulfur. It is important to
monitor the inlet and outlet levels of chlorides and sulfur. In
most H2
rides and sulfur are known. The outlet H2
S.
S measurement
will help determine if the hydrogenator is converting all
organic sulfur to H2
For example, if the feed (NG + recycle H2
taining 10 ppmv of H2
) in Case 1-conS
and 5 ppmv each of methyl mercaptan
S value in Eq. 7:
S
(7)
and DMDS-is passing through the hydrogenator, then the
outlet should measure the calculated H2
10 ppmv H2S + 5 ppmv × 1 (methyl mercaptan) +
5 ppmv × 2 (DMDS) = 25 ppmv of H2
Feed purification, Section 2: Chloride absorber. This
section is not common and is only required when the feed contains
chlorides. The primary function of the chloride absorber
is the absorption of HCl, which is either present in the feed or
formed across the hydrogenator. In most cases, this catalyst is
installed as a small layer below the hydrogenator. In some flowsheets,
it is also installed above the H2
S absorber. KPIs include
the following:
* Outlet chloride should be less than 0.1 ppmv vs.
inlet chloride.
* The catalyst life depends on pickup/capacity.
The process engineer should have the information
of expected pickup from the catalyst supplier.
Feed purification, Section 3: H2S absorbers (zinc oxide).
The primary function of the H2
S absorbers is to absorb H2
In addition to pressure drop, KPIs include the following:
* Outlet H2
* The catalyst life depends on pickup/capacity.
The process engineer should have the information
of expected pickup from the catalyst supplier.
S,
which is present either in the feed or formed across the hydrogenator.
This section is usually seen as two vessels in a lead-lag
arrangement or as a single vessel when sulfur levels are very low.
S should be less than 0.1 ppmv vs. inlet H2
S.
plants, the feed is analyzed daily, so the total inlet chloS
and HCl can be routinely
measured by process engineers using detector tubes, or
they can be analyzed in a laboratory. The outlet H2
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - October 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - October 2021
