WO2017093012A1

WO2017093012A1 - Regenerable sulfur adsorption and removal

Info

Publication number: WO2017093012A1
Application number: PCT/EP2016/077689
Authority: WO
Inventors: Poul Erik Højlund NIELSEN; Birgitte HAMMERSHØI; Burcin TEMEL MCKENNA
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2015-12-02
Filing date: 2016-11-15
Publication date: 2017-06-08
Anticipated expiration: 2018-06-02

Abstract

A method for the removal of sulfur from a sulfide adsorbent comprises contacting the sulfide-containing gas stream with a zinc oxide-based adsorbent, desulfurizing the gas stream while sulfiding the adsorbent and regenerating the sulfided adsorbent by treatment with steam or steam enriched with oxygen at an elevated temperature. This way the adsorbent is converted back to zinc oxide.

Description

Title: Regenerable sulfur adsorption and removal

The present invention relates to regenerable zinc oxide based sorbents. More specifically, the invention concerns a method for the removal of sulfur from a sulfide adsorbent, said method comprising the following steps:

- contacting the sulfide-containing gas stream with a zinc oxide-based adsorbent,

- desulfurizing the gas stream while sulfiding the adsorbent and

- regenerating the sulfided adsorbent by treating it with steam or with steam enriched with oxygen, thereby converting the adsorbent back to zinc oxide.

It is well-known that the removal of sulfur from gas and liquid streams is an important part of many industrial pro^¬ cesses, including those utilized in refinery operations. Thus, sulfur removal is desirable as well as necessary for a variety of reasons. If a sulfur-containing fluid stream is to be released as a waste stream, then removal of the sulfur will be necessary to meet certain, still more stringent, environmental regulations. And if a sulfur-containing fluid stream is to be used in a catalyzed process, removal of the sulfur is often necessary to prevent poisoning of the catalyst.

Sulfur is both an environmental hazard when it is present as a contaminant in fuel for combustion and, as mentioned, a poison for several catalytic materials when it is present in electrochemical systems such as fuel cells. Supported metal catalysts are widely used to produce high purity hy^¬ drogen in fuel processing systems via processes such as catalytic reforming, water gas shift (WGS) and oxidation of carbon monoxide, and they are also used as electrode mate^¬ rials in fuel cells. Generally, the metals in supported metal catalysts have a very low sulfur tolerance (of the order 0.1 to 10 ppmv) , but unfortunately typical sulfur concentrations in fuels may be as high as 3000 ppmw.

It is anticipated that it will soon be desirable to remove rather low concentrations of sulfur (i.e. around 100 ppm) in processes involving thermal biomass gasification. In this case, the traditional way of removing sulfur by using an acid gas removal (AGR) unit will become a rather expen^¬ sive option.

Methods and means for the removal of sulfur from fluid streams is the subject of a number of prior art publica^¬ tions. Thus, US 6.350.422 Bl discloses sorbent composi^¬ tions comprising zinc oxide, at least one silica component, at least one colloidal metal oxide and optionally at least one pore generating component. The sorbent composition can be regenerated with oxygen or an oxygen-containing gas.

US 6.951.635 B2 describes zinc oxide-based sorbents and processes for making and using them. The active zinc compo^¬ nent is a two-phase material consisting essentially of a zinc oxide phase and a zinc aluminate phase, each charac- terized by a small crystallite size of typically less than 500 A (Angstrom) . Regenerable sorbents for removal of sulfur from hydrocarbon feedstocks are disclosed in US 2008/0251423 Al . These sorbents contain zinc aluminate, free alumina as well as iron oxide.

US 5.703.003 Al describes sorbent pellets for removing sul^¬ fide from a coal gasification stream at an elevated temperature, comprising contacting a zinc oxide sorbent with a ¾S containing gas, thereby desulfurizing the gas and sul- fiding the sorbent. The sulfide sorbent can be regenerated. Durable zinc oxide containing sorbents for coal gas desul- furization are also described in CA 2.145.250 Al .

US 2008/0271602 Al describes doped supported zinc oxide sorbents for regenerable desulfurization applications, said sorbents being impregnated with a mixture comprising zinc oxide and a copper material.

In US 2010/0098618, a process is disclosed for removing or recovering sulfur from a gas stream using a Claus-type reactor followed by contact with a regenerable sorbent and recycle of SO₂ from the sorbent regeneration to the Claus- type reactor feed. The sorbent comprises zinc oxide, ex^¬ panded perlite, alumina and a promoter metal, said promoter metal being at least one metal selected from Ni, Co, Fe, Mn, W, Ag, Au, Cu, Pt, Zn, Sn, Ru, Mo, Sb, V, Ir, Cr and Pd.

US 2010/0170394 Al describes a silicate-resistant desulfu- rization sorbent composition comprising a support component in the form of one or more silicate-resistant silica-con^¬ taining that have been treated with one or more silicate- inhibiting metals selected from the group consisting of Sr, Ba, La and combinations thereof. The sorbent compositions exhibit a surprisingly low in-situ silicate generation rate when exposed to oxidative regeneration conditions.

WO 03/053579 Al describes how a deactivated sorbent compo^¬ sition is reactivated by contacting the deactivated

sorbent, which comprises a promoter metal and at least about 2 wt% sulfur as sulfates, with a hydrogen-containing stream under activation conditions sufficient to reduce the valence of the promoter metal and reduce the amount of sul^¬ fates associated with said deactivated sorbent, thereby providing an activated sorbent. Finally, applicant's older PCT application WO 90/14876 dis^¬ closes the removal of sulfides from gas streams while using a solid adsorbent containing Sn, Sn-oxides or mixtures thereof and optionally a stabilizing component consisting of Ni, Cu, Co, Fe or oxides thereof, said removal taking place by contacting the sulfide-containing gas stream with the solid adsorbent, which is expediently in the form of a carrier with the Sn-component and the stabilizing compo^¬ nent. This way the adsorbent is sulfided and the gas stream is desulfurized. Subsequently the sulfided adsorbents are regenerated by contact with steam. The regeneration process employed is substantially thermoneutral , so that superheat^¬ ing and consequently sintering of the adsorbent is avoided. Moreover, it is not necessary to operate at low tempera^¬ tures, whereby the formation of sulfates as by-products is avoided. The feed gas typically contains 600-800 ppmv ¾S which is a much higher concentration than that foreseen in the present invention. The goal underlying the present invention has been to demonstrate that it is possible to strip off sulfur from zinc sulfide (ZnS) by treating it with steam or steam en- riched with oxygen, thereby regenerating the sulfided ad^¬ sorbent by converting it back to ZnO. The ZnO can then be used to adsorb sulfur again.

Thus, in the method of the present invention, the adsorbent is used to capture sulfur in the feed stream for a certain period of time. Then the sulfidized adsorbent is regener^¬ ated with steam and minor amounts of oxygen in the feed, especially in a swing operation. After the regeneration, the adsorbent can be turned in for the sulfur adsorption process again.

Since, as already mentioned, an increased demand for sulfur removal can be expected within the field of thermal gasifi^¬ cation of biomass, the method according to the invention for the removal of sulfides from a gas stream may be a de^¬ sirable option. This is because the sulfur concentrations in connection with thermal gasification of biomass are in the 100 ppm range, which means that the traditional way of removing sulfur by using an acid gas removal (AGR) unit will become a rather expensive and thus not attractive pos^¬ sibility.

Another situation that would benefit from a regeneration scheme is the disposal of spent ZnS adsorbents in industry. This creates an additional burden for the plant owners, and using a regenerative scheme would remove the disposal prob- lem as well as open up new revenues based on sulfur down^¬ stream technologies, such as making elemental sulfur or sulfuric acid. Therefore, the idea of the present invention has been to develop regenerable zinc oxide based sorbents along with a regenerating scheme. Various experiments in this respect have been made : Below, SK-501 and HTZ-5 refer to the names of products sold by Haldor Topsoe. HTZ-5 is used in cleaning sulfur from various feeds. SK-501 is used for high temperature water gas shift operations. It contains a surplus of ZnO and may be regarded as a prototype of a supported ZnO guard.

Thus, applicant's SK-501 adsorbent (supported ZnO) has been sulfided and then regenerated under different conditions, more specifically in steam alone and in O₂ alone. Regenera^¬ tion in steam alone is thermodynamically very slow, espe- cially at temperatures below 600°C, while regeneration in O₂ alone leads to ZnS0₄ which is very stable against decom^¬ position. So the idea has been to regenerate in steam and O₂, using minor amounts of O₂ to drive the ¾ away, thereby being able to shift the equilibrium

ZnS + 3H₂0 <=> ZnO + 3H₂ + S0₂ towards right. H₂O₂ in water plus ₂ was used, which corre^¬ sponds to 50% steam and 0.5-1% O₂ from H₂O₂ decomposition, the balance being N₂, at various temperatures, i.e. 500 to 600°C. At a temperature of 600°C with addition of 1 vol% 0₂, the sulfur content decreased from 3.8 wt% to 0.3 wt%.

Experiments have also been made with an already sulfided HTZ-5 (>99 wt% ZnO) adsorbent, which was regenerated at

600°C, first with steam only and afterwards with steam and 0.5 vol% 0₂. The sulfur content decreased from 13.7 wt% to 4.81 wt% with steam only and further to 0.3 wt% with steam and O2 following the steam only treatment.

The results of the experiments show that it is possible to remove sulfur from an adsorbent with steam or with steam and 0₂, depending on the regeneration conditions. The re^¬ generation method of the invention has the advantage that the same batch of adsorbent can be used many times with this method.

The invention is illustrated further in the examples which follow .

Examples

The examples are divided into two parts, dealing with the SK-501 (>30 wt% ZnO, supported) adsorbent and an HTZ-5 (un- supported) adsorbent, respectively.

Part I: SK-501 (Examples 1-5)

Sulfidation: 60 g of SK-501 pellets were sulfided for 92 hours. The sulfided sample contained 3.77 wt% sulfur. Regeneration: 1 barg pressure, a total gas hourly space ve^¬ locity (GHSV) of 5000 Nl/h/kg catalyst (steam + N₂ + 0₂) , 50 vol% steam, 0.5-1 vol% O2 and balance N₂. Example 1

10.3 g of sulfided SK-501 was loaded into a Berty reactor (a chemical gradient-free reactor for kinetic investiga^¬ tions of heterogeneous catalytic reactions) . The regenera- tion took place with 0.5 vol% H2O2 in steam + 2 (50:50) for 72 hours at 500°C. The spent catalyst had 3.6 wt% sulfur left on it.

Example 2

10 g of sulfided SK-501 was loaded as halved pellets into a tubular reactor. The regeneration took place first with only steam + 2 at 550°C. After 24 hours, the regeneration condition was switched to 1 vol% H2O2 in steam + N₂. After 48 hours, 1 g of spent catalyst was taken out from the top of the catalyst bed, and it was found to contain 0.8 wt% sulfur. The regeneration test was continued as Example 3 below . Example 3

With the remaining amount of catalyst from Example 2, the test was continued by increasing the temperature to 600°C for 24 hours using steam + N₂, and afterwards 1 vol% H2O2 in steam + 2 was introduced for another 24 hours. When the test was concluded, the catalyst was taken out in three fractions, where the top, middle and bottom fractions con^¬ tained 0.3, 1.0 and 1.2 wt% sulfur, respectively.

Part II: HTZ-5

Example 4

5.4 g of sulfided HTZ-5 in small extrudates was loaded to the tubular reactor. The sample, which had 13.7 wt% sulfur on it, was exposed to steam + ₂ (50:50) for 48 hours at 600°C. Then 0.6 g of spent catalyst sample was taken from the top of the bed for chemical analysis, and it was found to have 4.8 wt% sulfur on it. Afterwards, 0.5 vol% H₂O₂ in steam + ₂ feed was used for 48 hours at 600°C, and the top and bottom fractions of spent catalyst were found to have 0.3 wt% and 0.4 wt% sulfur on them, respectively.

The SO₂ that was formed during regeneration was partly cap^¬ tured in the exit condensate and partly lost to the vapor phase over the low pressure separator. The condensate anal^¬ ysis involved oxidizing SO₂ further to SO₄ ions by H₂O₂ and analyzing SO₄ by ion chromatography.

The results are summarized in the following table:

In all experiments, a total GHSV of 5000 Nl/h/kg cat. was used.

The experiments described in the examples were aimed at identifying the approximate parameters for ZnO generation studies. It had turned out earlier that either steam only, i.e.

ZnS + H₂0 (g) <=> ZnO + H₂S (g) or oxygen only, i.e.

ZnS + 1.5 0₂ (g) <=> ZnO + S0₂ (g) and

ZnS + 2 0₂ (g) <=> ZnS0₄ for regeneration are not effective, since steam only has a very small equilibrium constant and regeneration with oxygen only will create ZnS0₄, and this compound is extremely stable. However, it has turned out that combining steam and very low levels of oxygen can help to speed up the steam regeneration reaction as the accompanying oxygen will increase the hydrogen removal further and hence shift the equilibrium towards right.

The amount of oxygen in the steam mixture is a critical pa^¬ rameter. In the experiments, 0.5 to 1 vol% O₂ was used, and the amount of sulfur remaining on the catalyst was 0.3 wt% which probably is in the form of ZnSC^.

A higher temperature will favor steam regeneration and also decrease ZnSC^ formation. The results regarding ZnS to ZnO regeneration show that it is necessary to use a temperature around 600°C and 1-5 bar pressure of 0.5 vol% O₂ in steam with or without ₂ dilution to remove sulfur successfully.

Claims

Claims :

1. A method for the removal of sulfur from a sulfide adsorbent, said method comprising the following steps:

- desulfurizing the gas stream while sulfiding the adsor- bent and

- regenerating the sulfided adsorbent by treatment with steam or steam enriched with oxygen at an elevated tempera^¬ ture, thereby converting the adsorbent back to zinc oxide.

2. Method according to claim 1, wherein the sulfidized adsorbent is regenerated in a swing operation.

3. Method according to claim 1 or 2, wherein the regenerating treatment is carried out at a temperature of around 600°C.

4. Method according to any of the claims 1-3, wherein the adsorbent comprises ZnO.

5. Method according to any of the claims 1-4, wherein the regenerating treatment comprises use of 1-5 bar pres- sure of 0.5 vol% O₂ in steam with or without ₂ dilution.