WO2015140319A1

WO2015140319A1 - A process and reactor system for hydrotreatment of a gas stream

Info

Publication number: WO2015140319A1
Application number: PCT/EP2015/055988
Authority: WO
Inventors: Henrik Wolthers RASMUSSEN; Leif Storgaard; Steffen Spangsberg CHRISTENSEN; Torkil Ottesen HANSEN
Original assignee: Haldor Topsøe A/S
Priority date: 2014-03-21
Filing date: 2015-03-20
Publication date: 2015-09-24
Also published as: TW201610141A; WO2015140590A1

Abstract

A process for hydrotreatment of a gas stream to lower the total content of sulfur compounds to 10-20 ppm by weight (ppmw) or even below 10 ppmw is performed in a reactor system consisting of three fixed-bed reactors: A pre-hydrogen-ating reactor R1 for pre-treating diolefins, a hydrogenating reactor R2 and a post-treating reactor R3 used for reacting traces of carbonyl sulfide. The pre-hydrogen-ating reactor R1 can be omitted if the content of diolefins in the feed gas is sufficiently low. All reactors contain catalysts which after activation comprise mixed sulfides of Co or Ni and Mo or W supported on γ-alumina (Al₂O₃) as high- surface-area carrier.

Description

A PROCESS AND REACTOR SYSTEM FOR

HYDROTREATMENT OF A GAS STREAM

The present invention relates to a process for hydrotreat- ment of a gas stream, such as coker gas, a gasifier outlet gas or a retort gas. The invention further relates to a re^¬ actor system for carrying out the process. Said reactor system is either a once-through or a recycle reactor system comprising three fixed-bed reactors, in which a gas with the composition up to 20% H₂S

up to 35% total olefins

up to 10% di-olefins

up to 12% CO + C0₂ + COS

up to 20% H₂

up to 2% organic sulfur compounds

and balanced by saturated light hydrocarbons is treated by the process of the invention with the purpose to lower the total content of sulfur compounds to a maximum value of 10-20 ppm by weight, preferably below 10 ppm by weight . In the refining industry, sulfur removal or recovery is a very important issue that often does not get the attention it deserves. Sulfur is one of the dominant contaminants in petroleum fractions, and legislation not only limits the permissible sulfur content of finished products, but also limits refinery emissions to the atmosphere. Therefore, sulfur removal and recovery is a vital process for refiner^¬ ies and gas plant operations. In most locations, the sulfur is hydrotreated and thus converted to hydrogen sulfide, which can be scrubbed from the various liquid or gas streams. The hydrogen sulfide collected from the hydro- treaters and/or gas plants can subsequently be treated, e.g. by the Claus process.

Various gas treatment processes for sulfur removal are de^¬ scribed in the prior art. Thus, US 8,080,089 Bl describes a method and an apparatus for efficient gas treatment, where SOx compounds are removed such that the concentration of

SOx remaining in the gas is between 0 and 10 ppmv. Further, US 7,374,742 discloses a method for removing sulfur species from a gas stream without the use of a sulfur species re^¬ moval process, such as an amine scrub. The sulfur species are removed by directly subjecting the gas stream to a sul^¬ fur recovery process, such as a Claus process at high pres^¬ sure and moderate temperatures, wherein the sulfur recovery process comprises a catalyst which does not comprise acti^¬ vated carbon.

WO 2009/026090 Al discloses a process for removing sulfur from a fuel gas stream additionally containing diolefins and oxygen as well as organic sulfur compounds. In said process, the fuel gas stream is treated in a pre-treatment reactor to reduce the amount of any diolefins and oxygen contained therein prior to the hydrodesulfurization in a hydrotreater reactor wherein organic sulfur compounds are converted to hydrogen sulfide. The latter is removed from the hydrotreated gas stream by use of an absorption treat- ment method, such as amine treatment, to yield a treated fuel gas stream with a reduced concentration of hydrogen sulfide and an overall low sulfur content. It is stated in the description that it would be especially desirable for the treated fuel gas stream to have a hydrogen sulfide con^¬ centration of less than 40 ppmv and, more especially, less than 10 ppmv. However, the document contains no examples or data showing that these low concentrations are in fact ob^¬ tained .

This also holds true for WO 2008/148077 Al disclosing a process for removing sulfur from a fuel gas stream addi- tionally containing carbon dioxide and light diolefins.

Said process includes a hydrotreating step followed by a reduction or hydrolysis step.

The process according to the present invention is a process for hydrotreatment of a gas stream to lower the total con^¬ tent of sulfur compounds to 10-20 ppm by weight (ppmw) or even below 10 ppmw. The process comprises the following steps : - mixing a feed gas with hydrogen to form a process gas,

- optionally introducing the process gas stream into a pre- hydrogenating reactor, where it is contacted with a hydro- genation catalyst under such conditions that any diolefins contained in the gas are substantially converted to mono- olefins ,

- introducing the pre-hydrogenated gas stream with a low content of diolefins into a hydrogenating reactor, where it is contacted with a hydrogenation catalyst to react sulfur- containing compounds and monoolefins, - cooling the process gas from the hydrogenating reactor, mixing it with high pressure steam and cooling the resultant gas mixture, - feeding the gas mixture to a post-treatment reactor con^¬ taining an alumina catalyst to react any traces of carbonyl sulfide (COS) , and

- subjecting the hydrotreated gas to a chemisorption treat- ment for removal of hydrogen sulfide.

The reactor system according to the present invention, also called a gas hydrotreater, comprises a catalyst technology with the capability of substantially lowering the sulfur content of mercaptan-rich refinery gases.

When a gas, such as a coker gas, is treated by the process according to the invention, it has surprisingly turned out that the content of sulfur-containing compounds can be brought down to a maximum value of 10-20 ppm and even below 10 ppm by weight after a final chemisorption step (typical^¬ ly amine-based) to remove residual ¾S.

The reactions taking place in the reactor system according to the invention are all gas phase reactions in an environ^¬ ment with ¾, CO, CO₂ and ¾S besides various hydrocarbons.

The reactor system according to the invention is a reactor system comprising three consecutive fixed-bed reactors, more specifically a once-through reactor system or a recycle reactor system, wherein - the first reactor (Rl), the use of which is optional, is a pre-hydrogenating reactor for pre-treating diolefins,

- the second reactor (R2) is a hydrogenating reactor, which is the main reactor for reacting sulfur-containing compounds and monoolefins, and

- the third reactor (R3) is a post-treating reactor used for reacting traces of carbonyl sulfide (COS) present in the outlet stream from (R2) . The composition of the reactor system is shown in the appended figure.

The first reactor Rl is required to avoid gum formation of di-olefins later in the process. In the second reactor R2, all sulfur-containing compounds and olefins are reacted, but some traces of mercaptans and COS may still be present. These compound traces are treated in the third reactor R3.

If the amount of di-olefins in the feed gas is above 1000 ppmw, then the first reactor Rl is mandatory. In case the amount is below 1000 ppmw, Rl can be omitted, and the feed gas stream is then led directly to the inlet of the second reactor R2 via a feed/effluent heat exchanger E2. The main challenge is to treat mercaptans while simultane^¬ ously handling diolefins, ¾S and COS contents. Thus, the exotherm in the first reactor Rl, which normally has to handle the conversion of diolefins to monoolefins, must be controlled carefully. Because it is possible to control the processes so that only the diolefin reactions actually take place, the system layout can be designed with just a simple low-cost fired heater. As mentioned, the reactions are all gas phase reactions, and the competing exothermic reactions are monoolefin to saturation and ¾S + alkanes to mercaptans . There is a de- sired temperature window of around 130-210°C, within which the reactions can be controlled while having a useful plant layout and catalyst system. Thus, the desired temperature window in the first reactor Rl can actually result in an economically very advantageous system layout for the treat- ment of coker gas.

The core concept underlying the present invention is to make a more efficient hydrodesulfurization of gas streams, such as coker gas streams. The first reactor Rl (the "hy- drotreater" or pre-hydrogenator) is situated upstream of an amine wash plant and substantially decreases the content of sulfur in the gas prior to entering the main unit R2. As mentioned earlier, the first reactor Rl is required to avoid gum formation of di-olefins later in the process. However, this has only proven necessary if the content of diolefins in the feed gas stream is above 1000 ppmw. The main unit R2 is a hydrogenating reactor, and reactor R3 is a COS post-treating hydrolysis reactor. The main technical novelty of this approach lies in a modi^¬ fication of the pre-treater catalyst to selectively treat diolefins rather than monoolefins in order to provide ap^¬ propriate temperatures in the main reactor R2 in a cost- effective way.

The coker gas, which can be treated according to the invention, is typically a coker sour gas from a coker sponge ab- sorber. In the hydrotreater plant, non-¾S sulfur compounds are converted into ¾S. Typically this coker sour gas has a composition as indicated in Table 1 below:

Table 1: Coker sour gas composition

In addition the coker sour gas may contain traces (≤ 0.01 mole percent) of carbonyl sulfide, 1 , 3-butadiene, HCN/RCN, benzene, toluene, xylene and ammonia. The catalysts present in the three reactors of the system according to the invention are catalysts generally employed in hydrodesulfurization (HDS) processes. Such HDS catalysts after activation generally contain mixed sulfides of Co or Ni and Mo or W supported on high-surface-area carriers such as γ-alumina (AI₂O3) . The main reason for their wide appli^¬ cation lies in their high tolerance to ¾S that is produced during hydrotreating reactions. The industrial application of a Co-Mo sulfide catalyst was already reported 70 years ago, and it is still the most common catalyst for HDS reac^¬ tions .

In the following the treatment process according to the in^¬ vention will be described in more detail with reference to the figure.

As mentioned above, the hydrotreater plant consists of three reactors: A pre-hydrogenator Rl, a hydrogenating reactor R2 and a COS hydrolysis reactor R3.

A feed gas (f) is mixed with hydrogen (h) . Subsequently the resulting process gas is optionally, but not necessarily (depending on the amount of diolefins in the feed gas) pre^¬ heated in a first feed/effluent heat exchanger El and passed through the pre-hydrogenator Rl . Then it is further pre-heated in a second feed/effluent heat exchanger E2 be^¬ fore entering the hydrogenation reactor R2, if necessary after passing a start-up heater (sh) . If the content of diolefins in the feed gas is sufficiently low, then the feed gas/hydrogen mixture is fed directly to the inlet of the hydrogenation reactor R2 via the second feed/effluent heat exchanger E2. From the hydrogenation reactor R2 the process gas is cooled in the second feed/effluent heat exchanger E2. A bypass on the hot side of the second feed/effluent heat exchanger is used to control the exit temperature from R2.

The cooled process gas from the second feed/effluent heat exchanger is mixed with high pressure steam (hps) and then further cooled in the process gas boiler B before being sent to the COS hydrolysis reactor R3, which is a post- treating reactor used for reacting traces of COS present in the outlet stream from R2.

The process gas from R3 is either recycled to the inlet of Rl (indicated as a dotted arrow in the figure) or cooled in the first feed/effluent heat exchanger El, heating up the gas to the diolefin pre-hydrogenator Rl . The gas is further cooled in a water cooler W, where the steam is condensing. The process condensate (c) is separated from the hydro- treated gas in a process condensate separator V.

From the process condensate separator V the hydro-treated gas is sent to an amine treatment plant A for removal of ¾S. A stream (1), lean in amine, is sent through the amine treatment plant and leaves the plant as a stream (r) , which is rich in amine. The product (p) contains 10-20 ppm by weight, preferably below 10 ppm by weight sulfur.

The system may also include means (not shown) to flush out NH₄C1 salt (formed by reaction between HC1 and N¾) with water to prevent clogging. Pre-hydrogenation reactions and catalyst

A coker sour gas feedstock contains above 1000 ppmw of di- olefins. Diolefins in a coker sour gas feed have a high tendency to gum formation due to polymerization or carbon formation at the normal operating temperature of the hydro^¬ genation reactor R2. In order to prevent these problems, the di-olefins in the coker sour gas feed are converted in the pre-hydrogenation reactor Rl containing a hydrogenation catalyst, for example applicant's nickel-molybdenum hydro^¬ genation catalyst TK-437.

TK-437 catalyses the following reactions: Ri=R₂-R₃=R4 + H₂ → R!=R₂-R₃H-R₄H

Ri=R₂-R₃=R4 + 2H₂ → HR1-HR2-R3H-R4H

Ri=R₂ + H₂ → HRi-HR₂ where R is a hydrocarbon radical.

If the operating temperature is sufficiently low, the last reaction is unlikely to occur.

For any given feedstock a certain hydrogen flow is required for the hydrogenation reactions. A sufficient amount of hy^¬ drogen must always be added in order to minimize the risk of polymerization or carbon formation.

The TK-437 catalyst is pre-sulfided and does not need to be sulfided prior to operation.

Another useful catalyst is applicant's molybdenum-based catalyst TK-719, which is especially suitable for olefin containing feeds, where activity grading is needed to pre^¬ vent formation of gum.

Hydrogenation reactions and catalyst

The hydrogenation reactor R2 is loaded with a nickel- molybdenum hydrogenation catalyst, preferably applicant's TK-261 catalyst, placed in a single bed in the reactor. TK-261 catalyses the following reactions:

RSH + H₂ RH + H₂S

RiSSR₂ + 3H₂ RiH + R₂H + 2H₂S

R1SR2 + 2H₂ RiH + R₂H + H₂S

(CH)₄S + 4H₂ C₄H₁0 + H₂S

COS + H₂ CO + H₂S

C0₂ + H₂S COS + H₂0

Ri=R₂ + H₂ HRi-R₂H

The conversion of olefins to saturated hydrocarbons is a strongly exothermic reaction. The temperature rise will ap^¬ proximately be between 50 and 90 °C depending on the content of olefins in the feedstock. An excess of minimum 10% hydrogen must be present at the outlet of the hydrogenation reactor R2 to prevent carbon formation or polymerization of the olefins. If the hydrogen flow is insufficient, this can also result in a poor con^¬ version of organic sulfur compounds and increase the slip of organic sulfur through the unit. The gas exiting the hydrogenation reactor may contain up to 150 ppm of olefins. This residual content of olefins may recombine with the H₂S present in the gas at a concentra^¬ tion around 14 %. The general reaction schemes are:

C_nH_2n + H₂S ~ C_nH_2n+iSH (n = 1-4)

C_nH_2n + H₂S ~ 2C_nH_2n+1SH (n = 3, 4)

C_nH_2n + H₂S ~ (CH₃)₃CSH The maximum activity of the hydrogenation catalyst depends on the concentration of hydrogen and the temperature at the inlet to the reactor. The recommended outlet temperature of the reactor is 400°C. At temperatures above 400°C, coke can be formed on the catalyst surface, thereby decreasing the activity of the catalyst.

Two locations are of importance here: the piping and heat exchangers that take the gas from the exit of the hydro^¬ genation reactor (R2) to the inlet of the COS hydrolysis reactor (R3) , and the CKA catalyst in R3, which provides a large contact surface area that might enhance the recombi^¬ nation reaction.

The gas leaving the hydrogenation reactor R2 will be at or very close to equilibrium at 400°C with respect to olefin hydrogenation, organic sulfur hydrogenation, COS hydrolysis and water-gas shift. Equilibrium of the first two would im^¬ ply that the recombination reactions are in equilibrium, too, and equilibrium of the last two would imply that the COS hydrogenolysis is in equilibrium. During start-up, the catalyst is heated with once-through natural gas and hydrogen. The catalyst must not be operated above 300 °C with hydrocarbons without hydrogen, because a carbon laydown may otherwise take place and thereby block the catalyst surface. As a result of this, the hydrogena- tion will be insufficient.

The TK-261 catalyst is available as a pre-sulfided or as an oxidized product. A pre-sulfided catalyst does not need to be sulfided before being taken into operation. An oxidized catalyst must be sulfided in situ to obtain its activity.

Operation on a non-sulfided catalyst will increase the risk of hydrocracking, resulting in severe temperature fluctua- tions. Olefins have a marked tendency to effect carbon for^¬ mation on the non-sulfided catalyst. The affinity for car^¬ bon formation is higher at low hydrogen partial pressures and high temperatures. The affinity for carbon formation also depends on the type of olefins.

After sulfiding the catalyst is pyrophoric and thus it should not be exposed to air at temperatures above 70 °C.

COS hydrolysis reaction and catalyst

The COS hydrolysis reactor R3 is loaded with an activated alumina catalyst, preferably applicant's CKA-3 catalyst, placed in a single bed in the reactor. The CKA catalyst is selectively active for the COS hydrolysis reaction:

COS + H₂0 «■ C0₂ + H₂S The CKA catalyst does not require any activation in connec^¬ tion with start-up. It is heated in natural gas to a tem^¬ perature at least 50°C above the dew point of the process gas .

During operation the gas must stay around 50°C or more above the dew point to prevent condensation in the pores of the catalyst. Such condensation may damage the catalyst. The COS slip-out from the COS hydrolysis reactor is deter^¬ mined by equilibrium, and a low COS leakage is favoured by a high steam content and a low temperature.

High pressure steam is added to the process gas stream to the COS hydrolysis reactor in order to reduce the slip of COS out of the reactor. Loss of steam will result in a breakthrough of COS through the reactor. A high CO₂ content in the feed gas will also result in a higher COS slip due to a shift in the equilibrium reaction.

The invention is further illustrated by the examples which follow. The invention is however not in any way limited to these examples. Example 1

Treatment of a sour semi-coker gas

A sour semi-coker gas from a retort gas plant has the over- all composition as indicated in the following Table 2: Table 2 : Sour semi-coker gas composition

The above gas is mixed with hydrogen. Subsequently the re^¬ sulting process gas is pre-heated to 1 7 5 ° C in a first feed/effluent heat exchanger. The pre-heated gas is passed through the pre-hydrogenator and then it is further pre- heated in a second feed/effluent heat exchanger before en^¬ tering the hydrogenation reactor at around 290-400°C.

From the hydrogenation reactor the process gas is cooled in the second feed/effluent heat exchanger. A bypass on the hot side of the second feed/effluent heat exchanger is used to control the exit temperature from the hydrogenation re^¬ actor to 400°C. The process gas from the hydrogenation reactor is mixed with high pressure steam, and then it is further cooled in a process gas boiler before being sent to the COS hydroly^¬ sis reactor. The process gas from the COS hydrolysis reactor is cooled in the first feed/effluent heat exchanger, heating up the gas to the diolefin pre-hydrogenator . The gas is further cooled in a water cooler, where the steam is condensing. The process condensate is separated from the hydro-treated gas in the process condensate separator.

From the process condensate separator the hydro-treated gas is sent to an amine treatment plant for removal of ¾S. A stream lean in amine is sent through the amine treatment plant and leaves the plant as a stream rich in amine. The product of the treatment contains less than 10 ppm by weight sulfur. Example 2

Treatment of a coker sour gas A coker gas with a composition as indicated in Table 1 is subjected to the same treatment as in Example 1. Also in this case the final product of the treatment contains less than 10 ppm by weight sulfur. Example 3

Gas analysis following plant start-up

This example shows the results of analysis of gas samples taken after start-up of the plant. The samples were taken at the following sites:

Sample 1 (SI) : the gas inlet (f)

Sample 2 (S2) : the outlet of Rl

Sample 3 (S3) : the outlet of R2

The last sample, Sample 4 (S4) was taken from the product stream (p) . The results, given as mole ppm compound, are indicated in Table 3 below. Results below 0.4 mole ppm compound (for sulfur dioxide, dimethyl sulfide, thiophene, isobutyl mer- captan, dimethyl disulfide, 2-ethyl thiophene, 2,5-dimethyl thiophene, ethyl methyl disulfide, tetrahydrothiophene, 2- methylthiophene, 3-methylthiophene, 2-methyltetrahydro- thiophene and diethyl disulfide) are not indicated in the table . For gases, mole ppm equals ppmv.

Table 3: Gas analysis after plant start-up

Sulfur compound SI S2 S3 S4 carbonyl sulfide 39.4 32.1 62.3 5.6 carbon disulfide 1.0 1.2 1.0 0.8 methyl mercaptan 9.0 10.7 0.7 0.7 ethyl mercaptan 5.7 15.5 0.4 0.0 isopropyl mercaptan 1.0 17.2 0.0 0.0

N-propyl mercaptan 0.0 1.3 0.0 0.0 tert-butyl mercaptan 0.0 1.6 0.0 0.0

N-butyl mercaptan 0.0 0.8 0.0 0.0

C5-mereaptans 0.0 0.4 0.0 0.0

Claims

Claims :

1. A process for hydrotreatment of a gas stream to lower the total content of sulfur compounds to 10-20 ppm by weight (ppmw) or even below 10 ppmw, said process compris^¬ ing the following steps:

- mixing a feed gas with hydrogen to form a process gas, - optionally introducing the process gas stream into a pre- hydrogenating reactor, where it is contacted with a hydro- genation catalyst under such conditions that any diolefins contained in the gas are substantially converted to mono- olefins ,

- introducing the pre-hydrogenated gas stream with a low content of diolefins into a hydrogenating reactor, where it is contacted with a hydrogenation catalyst to react sulfur- containing compounds and monoolefins,

- cooling the process gas from the hydrogenating reactor, mixing it with high pressure steam and cooling the resultant gas mixture, - feeding the gas mixture to a post-treatment reactor con^¬ taining an alumina catalyst to react any traces of carbonyl sulfide (COS) , and

2. The process according to claim 1, wherein the feed gas has the following composition: up to 20% H₂S

up to 35% total olefins

up to 10% di-olefins

up to 12% CO + C0₂ + COS

up to 20% H₂

up to 2% organic sulfur compounds the balance consisting of saturated light hydrocarbons.

3. The process according to claim 1 or 2, wherein the catalysts after activation contain mixed sulfides of Co or Ni and Mo or W supported on γ-alumina (AI₂O3) .

4. The process according to claim 1 or 2, wherein the process gas is pre-heated to around 175°C before entering the pre-hydrogenation reactor.

5. The process according to any of the claims 1-3, where^¬ in the entry temperature to the hydrogenating reactor is around 290-400°C.

6. The process according to any of the claims 1-4, where^¬ in the process gas from the hydrogenating reactor after mixing with high pressure steam is cooled to 160-220°C.

7. The process according to any of the claims 1-6, where- in the catalyst in the pre-hydrogenating reactor is the nickel-molybdenum hydrogenation catalyst TK-437.

8. The process according to any of the claims 1-6, where^¬ in the catalyst in the pre-hydrogenating reactor is the molybdenum oxide catalyst TK-719.

9. The process according to any of the claims 1-6, where^¬ in the catalyst in the hydrogenating reactor is the nickel- molybdenum hydrogenation catalyst TK-261.

10. The process according to any of the claims 1-6, where- in the catalyst in the post-treatment reactor is the alumi^¬ na catalyst CKA-3.

11. The process according to any of the preceding claims, wherein the exotherm in the first reactor, which handles the conversion of diolefins to monoolefins, is controlled so that only diolefin reactions take place.

12. The process according to claim 11 comprising a desired temperature window in the first reactor of around 130- 210°C, within which the reactions can be controlled.

13. A reactor system for the hydrotreatment of a gas stream to lower the total content of sulfur compounds by the process according to any of the claims 1-12, more spe- cifically a once-through reactor system or a recycle reactor system comprising three fixed-bed reactors, wherein

- the first reactor (Rl), the use of which is optional, is a pre-hydrogenating reactor for pre-treating diolefins, - the second reactor (R2) is a hydrogenating reactor, which is the main reactor for reacting sulphur-containing compounds and monoolefins, and

- the third reactor (R3) is a post-treating reactor used for reacting traces of carbonyl sulphide (COS) present in the outlet stream from (R2) .

14. Reactor system according to claim 13, said system further comprising feed/effluent heat exchangers (El and E2), a process gas boiler (B) , a water cooler (W) , a separator (V) , a start-up heater (sh) and means to flush out NH₄C1 salt with water to prevent clogging.