Process for removing HCl from hydrocarbon streams

The invention relates to a process for removing hydrogen halides from hydrocarbon-containing streams. More particularly, the invention relates to a process and an HCl sorbent for the removal of HCl and other hydrogen halides from hydrocarbon streams to prevent the formation of green oils. The sorbent has an increased capacity for the sorption of HCl and a reduced catalytic activity for the formation of green oils which, surprisingly, results from the pre-loading of water on a sodium promoted alumina adsorbent. The pre-loading of water on the sodium promoted alumina adsorbent in the range of about 5 to about 11 percent of the essentially water-free adsorbent increases the HCl sorption capacities by about 25% with a corresponding decrease in catalytic reactivity.

FIELD OF THE INVENTION 
The present invention relates to a method for treating hydrocarbon streams 
to remove acid gases. More particularly, the present invention relates to 
a process using a catalytically inert sorbent for removing HCl from 
hydrocarbon-containing streams. 
BACKGROUND OF THE INVENTION 
Acid gases are present as impurities in numerous industrial fluids, i.e., 
liquid and gas streams. These acid gases include hydrogen halides such as 
HCl, HF, HBr, HI and mixtures thereof. For example, one of the key 
processes in refining petroleum is catalytic reforming. In the catalytic 
reforming process, a light petroleum distillate or naphtha range material 
is passed over a noble metal catalyst to produce a high octane product. 
Hydrogen is a by-product of the catalytic reforming process, and a portion 
of the by-product hydrogen is recycled to the reaction zone to maintain 
catalyst stability. Typically, the noble metal reforming catalyst is 
promoted with chloride which, in the presence of hydrogen, results in the 
production of a small amount of hydrogen chloride. Thus, the net 
by-product hydrogen withdrawn from the catalytic reforming process 
generally contains a small amount of hydrogen chloride. Similarly, in a 
process for the dehydrogenation of light isoparaffins to produce 
isoolefins, the promoting of the noble metal catalyst with chloride will 
produce a net hydrogen stream containing small amounts of HCl. The net 
hydrogen produced in the catalytic reforming process and the 
dehydrogenation process is generally used in sensitive downstream 
catalytic processes. In addition, there are other hydrocarbon and chemical 
processes in which small amounts of HCl are generated and carried away in 
gas or liquid streams. Even small amounts of gaseous HCl present in the 
net hydrogen can seriously interfere with the operation of downstream 
processes which use the hydrogen and can cause corrosion problems in the 
equipment such as pipes, valves, and compressors which convey hydrogen. 
Generally, HCl in gas or liquid hydrocarbon streams must be removed from 
such streams to prevent unwanted catalytic reactions and corrosion to 
process equipment. Furthermore, HCl is considered a hazardous material and 
releasing the HCl to the environment must be avoided. 
Currently, activated alumina is the most widely used sorbent in the 
petroleum refining and chemical industries. Activated alumina is employed 
as a scavenger for the removal of small quantities of HCl from fluid 
streams. Significant developments to improve the performance of alumina to 
remove HCl from hydrocarbon streams are disclosed in U.S. Pat. Nos. 
4,639,259 and 4,762,537 which relate to the use of alumina-based sorbents 
for removing HCl from gas streams. U.S. Pat. Nos. 5,505,926 and 5,316,998 
disclose a promoted alumina sorbent for removing HCl from liquid streams 
by incorporating an alkali metal oxide such as sodium in excess of 5% by 
weight on to an activated alumina base. It is also known that alumina can 
be promoted to sorb more HCl by impregnating the alumina with sodium 
carbonate or sodium hydroxide or calcium hydroxide. U.S. Pat. No. 
4,639,259 discloses the use of calcium acetate to improve the dispersion 
of the calcium oxide on the alumina to achieve higher sorption capacity. 
The use of promoted alumina compared to other alumina-based sorbents can 
extend the length of time a fixed amount of sorbent will sorb HCl. By 
increasing the content of promoters such as sodium carbonate or sodium 
hydroxide, the HCl sorption capacity of the scavenger can be increased. 
However, the addition of promoters to alumina to improve the capacity of 
the sorbent for HCL appears to have a point of diminishing returns. 
Despite the type and amount of promoter incorporated into the 
alumina-based and promoted alumina materials, commercial experience shows 
that alumina-based and promoted alumina sorbents have a relatively low 
capacity for the sorption of HCl, often limited to levels less than 10-16 
wt-% HCl. 
Existing sorption processes for removing HCl from hydrocarbon-containing 
streams typically involve passing the hydrocarbon-containing fluid stream 
over the sorbent, which is disposed in a fixed bed. Conventionally, these 
fixed beds contain alumina-based sorbents wherein sodium or calcium is 
doped or coated on the alumina. Typically, the alumina-based and promoted 
alumina materials are formed into nodules or spheres. As the alumina-based 
sorbents pick up HCl, the sodium or calcium promotor, as well as aluminum, 
reacts with HCl to form chloride salts. Because HCl molecules are able to 
form hydrogen bonds with chloride ions, a limited amount of HCl can become 
physically sorbed on the surface of the salt molecules. However, the 
alumina sorbent in this service is known to have the undesirable property 
of converting certain hydrocarbons in the streams into a substance often 
called "green oil" which often collects in the fixed sorbent bed. 
Typically, these green oils are green or red in color. They generally 
contain chlorinated C.sub.6 -C.sub.18 hydrocarbons and are believed to be 
oligomers of light olefinic hydrocarbons. The presence of green oils in 
the fixed sorbent bed fouls the sorbent bed and results in the premature 
failure of the sorbent. When this fouling occurs, often costly measures 
are required to remove the spent sorbent from the bed. Furthermore, the 
chloride content of the green oils on the spent sorbent makes disposal of 
the spent sorbent an environmental problem. While the exact mechanism of 
green oil formation is unknown, it is believed that green oils are formed 
by catalytic reaction of aluminum chloride or HCl with the hydrocarbon 
resulting in a chlorinated hydrocarbon. Since both aluminum chloride and 
free HCl are known to be acidic and present on the surface of the sorbent, 
they are able to catalyze the polymerization of reactive hydrocarbons. 
Since it is very difficult to avoid the physical sorption of HCl and the 
formation of chloride salts on alumina-based and promoted alumina 
sorbents, the catalyzed polymerization reaction of hydrocarbon and the 
formation of green oil is not easily avoided. Green oil formation remains 
an unresolved industry problem during the removal of HCl from hydrocarbon 
streams. 
When unsaturated hydrocarbons such as butadiene or other olefinic compounds 
are present in a hydrocarbon-containing stream, these compounds can be 
polymerized on acidic surfaces. Alumina based sorbents and promoted 
alumina sorbents, once they adsorb HCl, become acidic during the sorption 
process, and thus, acquire catalytic activity for the polymerization of 
the reactive hydrocarbons in the stream. When green oils are produced 
during the HCl sorption process, the spent sorbent represents a costly 
disposal problem. The formation of these polymers fouls the sorbers, 
shortens sorbent life, and creates a problem for the disposal of the solid 
adsorbents now containing chlorinated hydrocarbons. Since an HCl sorbent 
is not regenerable, the treatment of streams with even moderate to high 
HCl content, such as an HCl sorbent with a capacity of 10-16 wt-%, 
requires the fixed bed of sorbent to be changed frequently and imposes a 
downtime on the upstream process. Because the change of sorbent beds 
containing polymerized hydrocarbons requires costly measures to dig the 
sorbent out of the sorbent bed, the loss of production time and the 
maintenance costs are especially significant. The polymerization or acidic 
reactivity of the Cl loaded adsorbents must be reduced to avoid these 
problems. 
There are many compounds that are reactive to acid gases such as hydrogen 
halides which can be employed as a scavenger sorbent to remove trace 
amounts of acid gases from fluid streams. However, for a compound to 
function in a fluid stream from a process plant where hydrocarbons are 
present, the material must have good acid gas sorption capacity, have 
sufficient physical strength, and be catalytically inert in the presence 
of reactive hydrocarbons. That is, the compound should have a reduced 
catalytic activity. By a reduced catalytic activity, it is meant that the 
catalytic activity is about 1/3 to 1/2 that of the catalytic activity of 
the conventional sodium promoted alumina. 
It is an objective of the present invention to provide a sorbent which is 
effective for removing HCl from hydrocarbon streams and which is 
catalytically inert to reaction of those hydrocarbons to form the green 
oils. 
It is an objective of the present invention to provide a sorbent for 
removing HCl from a hydrocarbon stream with an improved capacity for 
sorption of HCl with a minimum requirement for maintenance costs. 
It is objective of this present invention to provide a sorbent which avoids 
the production of potentially hazardous chlorinated hydrocarbons. 
SUMMARY OF THE INVENTION 
Surprisingly, it was discovered that pre-loading water on the sodium 
promoted alumina adsorbent increased the HCl adsorption capacity by 
approximately 25% and that the reactivity for polymerization was 
surprisingly decreased by about 80% (as measured by the 1,3-butadiene 
reactivity test.) In actual commercial applications, this significant 
decrease of reactivity may translate to an even higher Cl loading capacity 
at a lower unit cost for the adsorbent, thereby extending adsorbent life 
and minimizing emergency shutdowns from increased pressure drop or 
adsorbent bed plugging. 
The sorbents of the present invention exhibit a low catalytic activity for 
hydrocarbon polymerization and a high HCl sorption capacity. Since the 
physical sorption of HCl is generally thought to be a surface phenomenon 
and the presence of HCl on the surface of the sorbent would tend to 
catalyze the polymerization of hydrocarbon, it is surprising that the HCl 
sorbents of the present invention show such a low catalytic activity. 
Surprisingly, it was found that the alumina promoted adsorbents--when 
pre-loaded with a critical amount of water--exhibited an improved HCl 
adsorption capacity and reduced catalytic activity for the formation of 
polymers or green oils and displayed a much higher overall sorption 
capacity than the adsorbents of the prior art. According to the present 
invention, a process is provided for treating a hydrocarbon stream 
comprising hydrocarbons and a hydrogen halide. In order to remove the 
hydrogen halide from the hydrocarbon stream, the hydrocarbon stream is 
contacted with a sorbent comprising an alumina-containing material 
impregnated with at least 3% by weight of an alkali metal oxide based on 
the weight of alumina present and wherein the sorbent comprises from about 
5 to about 11 percent water based on the weight of essentially water-free 
sorbent to provide a sorbent having a reduced catalytic activity to form 
green oils from said hydrocarbons. Preferably, the essentially water-free 
basis of the sorbent corresponds to an LOI at 950.degree. C. of less than 
about 5 wt %. 
DETAILED DESCRIPTION OF THE INVENTION 
To measure the catalytic reactivity of the sorbent for hydrocarbons, 
1,3-butadiene was used as the reactant. The sorbent was first loaded with 
HCl by exposing the sorbent to HCl gases. The unsorbed HCl was removed and 
the now HCl loaded sorbent was exposed to 1,3-butadiene. For those 
sorbents that are not catalytically active, 1,3-butadiene was only 
physically sorbed. For catalytically active sorbents, the sorption of 
1,3-butadiene resulted in the production of C.sub.12 or larger molecules. 
These heavy molecules formed a liquid phase (not sorbed) on the surface of 
solid sorbent. Surprisingly, the water pre-loaded sorbents had much lower 
catalytic activity than the alumina promoted sorbents which were not 
pre-loaded. To put the degree of pre-loading on a definite basis, we 
introduce the term "loss on ignition". The term loss on ignition (LOI) 
means the loss which results from heating a sample of adsorbent using an 
ignition temperature of 950.degree. C. Typically, water and other volatile 
components such as chlorine or fluorine, which are generally found in the 
adsorbent, are driven off at this temperature and are included in the LOI. 
The LOI is determined by placing a weighed sample of the adsorbent in a 
crucible and heating the crucible to a temperature of about 950.degree. C. 
for about 1 hour. The material evolved during the heating of the sample is 
analyzed by conventional methods to determine a base water content and the 
content of the other volatile components. After cooling, the adsorbent 
sample is weighed again and the mass loss is calculated as a mass-percent 
loss on ignition. Thus, the essentially water-free basis of the adsorbent 
is the weight of the sample, less the water portion of the LOI at 
950.degree. C. For example, a typical sample of promoted alumina was found 
to have an LOI of about 3.4 wt %, wherein 1.9 wt-% was water and 1.5 wt-% 
was determined to be other volatile components. For the purposes of this 
application, water contents are expressed in terms relative to an 
essentially water-free basis. 
A commercial version of 8% Na.sub.2 O promoted alumina has an HCl capacity 
of 12.9% and 1,3-butadiene reactivity of 5.7%. By gradually increasing the 
amount of water pre-loaded on the adsorbent, a steady increase in HCl 
adsorption capacity and a steady decrease in 1,3-butadiene reactivity was 
observed with a water pre-loading of from about 5 to about 11 weight 
percent of the adsorbent relative to the adsorbent on an essentially 
water-free basis. At a water loading above about 11%, the reactivity and 
HCl capacity began to decrease, indicating the critical range over which 
this surprising advantage results. More preferably, the water loading 
comprises from about 7 to about 8.5 weight percent of the adsorbent on a 
dry basis. At this water loading level, the HCl capacity increased about 
25% and the 1,3-butadiene reactivity decreased about 80% compared to the 
commercial 8% Na.sub.2 O promoted adsorbent. The HCl loading was also 
verified by chloride chemical analysis, and the results matched well with 
McBain results as indicated in the following table by corrected Cl wt %. 
Experimental results showed the effect of pre-loading the promoted alumina 
adsorbent. It is believed that the water possibly hydrates the Na.sub.2 O 
or NaAlO.sub.2 for NaOH on the surface of the adsorbent which is a 
stronger base. The stronger base appears to explain the increase in HCl 
capacity. It is believed that the decrease in 1,3-butadiene reactivity 
results from better NaOH re-dispersion on the surface of the adsorbent or 
the blocking of alumina sites. It appears that exposed alumina surface 
sites contribute to the olefin reactivity when these sites are loaded with 
HCl. By improving the disbursement of the Na.sub.2 O on the surface and by 
covering the alumina sites on the surface of the adsorbent, an increase in 
the HCl capacity and a reduction in catalytic reactivity results.

EXAMPLES 
To more fully illustrate the invention, the following examples are 
presented. 
The equilibrium HCl adsorption capacity of promoted alumina was evaluated 
in a conventional McBain Bakker Balance. A detailed description of this 
device, in general, can be found in text books such as "Physical 
Adsorption of Gases" by D. M. Young and A. D. Crowell, Butterworths, 1962, 
hereby incorporated by reference. A series of approximately 1 gram each of 
samples a-f adsorbents were pre-loaded as described hereinbelow with water 
at the level shown in Table 1 and activated through vacuum evacuation at 
room temperature for a period of about 12 hours until the vacuum reached 
at least 1.3.times.10.sup.-3 kPa (10.sup.-2 torr). HCl adsorption was 
carried out at an HCl partial pressure of about 0.665 kPa (5 torr) by 
exposing all of the samples to a gas containing HCL at 24.degree. C. for 
about 24 hours. The samples were maintained at this HCl partial pressure 
for the duration of the procedure. HCl adsorption was monitored by 
recording the weight of each sample at different time intervals until the 
weight gains had stabilized. The final HCl loadings were verified by 
conventional chemical analysis for chloride content of the adsorbent 
samples. The chloride content reported in Table 1 is shown corrected by 
the residual chloride content in the fresh base. The chloride contents 
ranged from about 11.9 wt % for the "as received" a to about 17.2 wt % for 
the "as received" a with an additional 8.2 wt % pre-loaded water. 
The water content of each adsorbent sample was determined via a standard 
Karl Fisher amperometry using a 701 Metrohm titrator unit. In this 
apparatus the titration compartment is attached to a glass tube which is 
heated by a horizontal tube furnace. Approximately 1 gram of adsorbent is 
used for each test. The test sample was heated to 950.degree. C. in the 
presence of a nitrogen purge. The purge gas was bubbled through a solvent 
mixture (such as sold under the trademark "Hydranal"-Solvent, manufactured 
by Riedel-de-Haen) comprising methanol, imidazol, and sulfur dioxide which 
scrubbed and reacted with the moisture present. The water amount was 
determined by titration with a titrant (such as sold under the trademark 
"Hydranal"-Titrant/5, manufactured by Riedel-de-Haen) comprising methanol 
and iodine. LOI (Loss on Ignition) was obtained by the weight difference 
before and after the heating using an analytical balance. The difference 
between LOI and the determined water gives the non-aqueous volatile 
content of the sample. The level of water pre-loading was attained by 
exposing approximately the 1 gram samples of the adsorbent to a partial 
pressure of H.sub.2 O vapor which would result in a particular water 
pre-loading prior to starting the HCl capacity testing. In the McBain 
measurement, 1 g of sodium promoted alumina is used for the testing. After 
loading the samples into the McBain, the adsorbents were activated through 
vacuum evacuation at room temperature for a period of about 12 hours. The 
samples were exposed to a specific water partial pressure by adjusting the 
temperature of liquid water between a temperature of from 0.degree. C. to 
24.degree. C. to obtain the required partial pressure of water vapor. When 
the water loading for each sample reached equilibrium, the McBain tubes 
containing sample buckets were sealed in preparation for the HCl capacity 
test. The water pre-loading wt % was reported relative to the water 
content of the "as received" a adsorbent. The McBain manifold was 
evacuated and HCl gas was introduced as described herein above. HCl 
adsorption and 1,3-butadiene reactivity tests were then carried out with 
the pre-loaded adsorbents. 
To further verify the HCl loading, conventional chloride chemical analysis 
of the adsorbent samples was conducted and the results showed that 
chloride loading in the McBain test correlates well with the analysis of 
chloride on the adsorbent. The "as received" sodium promoted alumina was 
found to have an LOI at 950.degree. C. of about 3.4 wt % with a water 
content of about 1.9 wt-%. 
TABLE 1 
__________________________________________________________________________ 
CHLORIDE CAITY AND REACTIVITY OF WATER 
LOADED ADSORBENT 
Cl 
H.sub.2 O 
H.sub.2 O Water- 
HCl 1,3 BUTADIENE 
Chemical 
Cl 
Pre-Loading 
free Basis 
Capacity, 
REACTIVITY 
Analysis. 
Corrected 
WT % .degree. C. 
WT % WT % WT % WT % 
__________________________________________________________________________ 
a As rec'd 
1.9 12.9 5.7 9.7 11.9 
b 2.6 4.5 11.0 3.0 11.1 13.3 
c 5.3 7.2 14.1 2.2 10.3 13.1 
d 7.0 8.9 16.1 1.0 12.1 15.9 
e 7.6 9.5 15.9 0.9 12.0 15.9 
f 8.2 10.1 17.2 1.5 10.6 14.5 
__________________________________________________________________________ 
EXAMPLE II 
Catalytic reactivity of the HCl loaded sodium promoted alumina samples from 
Example I was evaluated as follows. Following the HCl adsorption of 
Example I, the McBain system was evacuated briefly to remove essentially 
all residual HCl gas. Each of the samples was then exposed to 
1,3-butadiene at a partial pressure of 13.3 kPa (100 torr) for 48 hours. 
For the catalytically active sorbent materials, the 1,3-butadiene reacted 
continually to form oligomers, which was reflected by the continued weight 
gain over exposure time. It was found that as the water content increased, 
the 1,3-butadiene reactivity decreased continuously. At 7.6% water 
loading, the reactivity had decreased about 80% compared to the "as 
received" a sodium promoted alumina.