WO2019022603A1

WO2019022603A1 - Process to treat a sulphur compounds comprising gaseous feed

Info

Publication number: WO2019022603A1
Application number: PCT/NL2018/050518
Authority: WO
Inventors: Wang WEI; Johannus Petrus VAN DEN BERG
Original assignee: Wte-Tech B.V.
Priority date: 2017-07-26
Filing date: 2018-07-25
Publication date: 2019-01-31

Abstract

The invention is directed to a process to treat a sulphur compounds comprising gaseous feed having a sulphur compounds content of below 5 vol.% to obtain a gas poor in sulphur compounds comprising the following steps, (a) pre-heating a combustion space to a temperature of between 650 and 1400 °C to obtain a pre-heated combustion space, (b) discharging the sulphur compounds comprising gaseous feed into the pre-heated combustion space and separately discharging an oxygen comprising gas having a temperature of between 150 and 550 °C into the pre-heated combustion space resulting in a continuous flameless combustion at a temperature of between 800 and 1200 °C and (c) separating sulphur dioxide from the flue gas as discharged from the combustion space and (d) increasing the temperature of an oxygen comprising gas to the oxygen comprising gas used in step (b) by indirect heat exchange against the flue gas.

Description

PROCESS TO TREAT A SULPHUR COMPOU NDS COMPRISING GASEOUS FEED

The invention is directed to a process to treat a sulphur compounds comprising gaseous feed having a sulphur compounds content of below 5 vol.% to obtain a gas poor in sulphur compounds.

Such gaseous feeds comprising low contents of sulphur compounds may be byproduct or also referred to as tail gas when processing natural resources such as coal, crude oil and natural gas. Elemental sulphur and sulphur compounds are widely present in natural resources such as coal, crude oil and natural gas. When such resources are processed gaseous streams, like flue gasses, comprising high and low amounts of sulphur compounds are obtained. The sulphur compounds in these gasses are normally removed or at least largely reduced before emitting these gasses into the environment. Various processes have been developed which are aimed to isolate the sulphur compounds and to convert the sulphur compounds, like bisulphides, to elemental sulphur. Examples of process to isolate sulphur compounds are for example the well known Rectisol Process, DIPA absorption process and the SCOT process. Processes to convert sulphides to elemental sulphur are for example the Claus Process and the Superclaus Process. In a typical line-up, known for more than 40 years, an acid gas containing relatively high contents of sulphur compounds, mainly H2S, as obtained in a DIPA absorption or Rectisol Process is used as feed of a Claus Process. In the Claus Process most of the sulphides are converted to elemental sulphur. The off gas of the Claus Process which may contain 0.5 to 2 vol.% H2S is typically further processed in a

SCOT or Superclaus Process to reduce the H2S content to less than 2000 ppmv to even less than 30 ppmv. The off-gasses of these processes will be subjected to an incineration before being emitted into the environment. The above processes have proven to be very efficient in dealing with the sulphur compounds.

The Claus Process was developed by Carl Friedrich Claus in the second half of the XIX century. In this process hydrogen sulphide is converted by oxidation to a considerable extent into elemental sulphur; the sulphur thus obtained is separated from the gas by condensation. The residual gas stream (the Claus tail gas) still contains some H2S and SO2. In a SCOT process as described in GB1356289 this residual H2S may be removed. The SuperClaus process is described in US4988494 and involves a selective oxidation of H2S to sulphur using an iron-oxide-containing catalysts.

US2011/0280795 describes a flameless Claus Reactor where high temperature air is used to combust a relatively rich H2S containing gaseous feed to elemental sulphur. A tail gas is hereby obtained which still consists of COS, CS2, SO2 and S_n. These compounds may be hydrolysed to H2S and recycled to the flameless Claus Reactor.

GB14848085 describes a process wherein residual gas (tail gas) of a Claus process and a gas consisting mainly of ammonia and a fuel gas is combusted at an oxygen excess of about 1 vol.% at a temperature of 1050 °C. In this process part of the flue gas exiting the combustor is externally recycled.

A disadvantage of the process of GB14848085 is that it requires an ammonia rich co- feed to convert the tail gas of the Claus process. The object of the present is to provide a simpler process to treat a gaseous feed having a sulphur compounds content of below 5 vol.% such to obtain a gas poor in sulphur compounds.

This object is provided by the following process. Process to treat a sulphur compounds comprising gaseous feed having a sulphur compounds content of below 5 vol.% to obtain a gas poor in sulphur compounds comprising the following steps,

(a) pre-heating a combustion space to a temperature of between 650 and 1400 °C to obtain a pre-heated combustion space,

(b) continuously discharging the sulphur compounds comprising gaseous feed into the pre-heated combustion space and separately continuously discharging an oxygen comprising gas having a temperature of between 150 and 550 °C into the pre-heated combustion space resulting in a continuous flameless combustion at a temperature of between 800 and 1200 °C wherein the sulphur compounds are oxidized to sulphur dioxide and in that any hydrocarbons are combusted thereby obtaining a flue gas comprising sulphur dioxide, carbon dioxide, water and between 1 and 10 vol% non-reacted oxygen, which flue gas is continuously discharged from the combustion space, and

(c) separating sulphur dioxide from the flue gas as discharged from the combustion space to obtain the gas poor in sulphur compounds, and (d) increasing the temperature of an oxygen comprising gas having a temperature of below 100 °C to the oxygen comprising gas used in step (b) by indirect heat exchange against the flue gas as discharged from the combustion space.

Applicants found that when step (b) is performed when starting with a pre-heated combustion space obtained in step (a) a stable and full conversion of the sulphur compounds to sulphur dioxide is possible even when the composition of the feed varies in time. Furthermore, the process is simple and robust in that it may be continuously performed in only two steps. Finally, almost no formation of NOx to even no formation of NOx is achieved with this process.

In step (a) the combustion space is pre-heated to a temperature of between 650 and

1400 °C to obtain a pre-heated combustion space. This may be performed by combustion of a suitable hydrocarbon comprising gaseous having a lower heating value (LHV) of between

2-40 MJ/Nm^. Such a gas may comprise natural gas and/or an off-gas as obtained in for example a refinery process. Once the combustion space reaches the desired temperature the hydrocarbon comprising gas may be replaced by the sulphur compounds comprising gaseous feed to perform step (b).

The sulphur compounds comprising gaseous feed will have a sulphur compounds content of below 5 vol.% and preferably below .2 vol.%. The sulphur compounds content may be above 30 ppmv. The gaseous feed may also comprise of carbon dioxide and/or gaseous hydrocarbons, such as for example methane, ethane and propane. Such a gas may comprise the off gas of a Claus Process, a SCOT Process or a Superclaus Process. The sulphur compound contents of the off -gas of a SCOT Process or a Superclaus Process is typically lower than the off-gas of a Claus process which may comprise for example between 0.5 and 1.8 vol% hydrogen sulphide. The sulphur compounds comprising gaseous feed may be the gas as obtained when degassing liquid sulphur as obtained in a Claus Process, the SCOT Process and/or the Super Claus Process. Such a gas may contain elemental sulphur and it has been found that the present process is suited to also combust such streams. Although the process is directed to treat a gaseous feed having below 5 vol.% sulphur compounds it may incidentally also process higher sulphur containing feeds like the feed of a Claus Process. This is advantageous in case the Claus Process is temporarily out of service. Suitably the lower heating value of the sulphur comprising gaseous feed is between 1 and 40 MJ/Nm³ and preferably between 1.5 and 20 MJ/Nm³. In situations where a sulphur compounds comprising gaseous feed has a lower heating value of less than 1 MJ/Nm³ it is preferred to increase the lower heating value of the total gaseous feed to step (a). This may be effected by co-feeding a hydrocarbon comprising co-feed having a higher lower heating value and preferably having a lower heating value of between 2-40 MJ/Nm³. The above described lower heating values will ensure that the combustion in step (b) is maintained, especially when the sulphur content in the gaseous feed varies in time.

An example of a suitable hydrocarbon comprising gaseous co-feed is a natural gas comprising co-feed. Applicants found that when the sulphur processing is performed in a refinery environment it is advantageous to use as the hydrocarbon comprising gaseous co- feed a gas comprising an off-gas as obtained in the refinery process. Especially the refinery off-gas which would otherwise be used as fuel in a flame combustion furnace before being flared. The conditions of step (b) have been found more favourable as compared to a furnace because large variations in quality and quantity of the off gas can be processed together with the sulphur comprising gaseous feed without the fear for incomplete combustion, excessive NOx formation and flame extinction. Thus in this embodiment not only can a SCOT and Superclaus tail gas incinerator be omitted but also the refinery off gas furnace can be omitted.

The refinery off gas as described in this application typically does not contain sulphur compounds in any significant amounts. The refinery off gas as here described is suitably any one or any combinations of the following refinery streams, namely FCC/coker off gas, crude off gas, stabilizers off gas, fractionator off gas, naphtha flash off gas, cat reformer off gas, hydrotreaters off gas, hydrocrackers off gas and isomerization off gas. Such off gasses will mainly be comprised of gaseous hydrocarbons and will typically have a lower heating value of more than 20 MJ/Nm³.

Step (b) is performed such that no visible flame is observed, also known as flameless combustion. This is achieved by on the one hand pre-heating the combustion space in a step (a), by maintaining a temperature in the combustion space of above the auto ignition temperature of the feed and optional co-feed and thirdly by using an oxygen comprising gas having a temperature of between 150 and 550 °C and preferably between 300 and 550 °C. The gaseous feed and optional gaseous co-feed into and the oxygen comprising gas may be discharged into the combustion space from a co-axial burner. The feed and optional co-feed may be discharged from a central channel of the burner and the oxygen comprising gas may be discharged from a co-axial annular channel of the burner or wherein the feed is discharged from the annular channel and the oxygen comprising gas is discharged from a central channel. The central channel may also be an annular channel positioned within the other annular channel. No pre-mixing of the oxygen comprising gas and the gaseous feed optionally in combination with the co-feed takes place up-stream or in the co axial burner. Such a co-axial burner may be used in case a single burner or a small number of burners is used. The gaseous feed optionally in admixture with the optional gaseous co-feed may alternatively be discharged in the combustion space via numerous channels which channels are separate from numerous channels via which the oxygen comprising gas is discharged into the combustion space. Such an arrangement is preferably used when a large combustion space is used. These channels, suitably present as nozzles, will be arranged in one or more walls of the combustion space in a specific pattern resulting in a good mixture of both reactants.

In a preferred combustion space the flameless combustion will take place while a large part of the resulting flue-gasses internally recycle to where the feed and oxygen comprising gas is discharged from the burner at its burner front. This will result in that the feed and oxygen comprising gas is mixed with recycling flue gas such that the local oxygen content will be reduced. This reduced oxygen content will avoid that a flame is formed and that the temperature in the combustion space remains with the temperature range according to this invention. Part of the flue gas is discharged from the combustion space. The oxygen content in the flue gas and thus in the combustion space will depend on, for example, the oxygen content in the oxygen comprising gas, the volume of this gas and the volume and composition of the gaseous feed or feeds as supplied to the combustion space and the recycle ratio of the flue gas to the burner front. The oxygen content is between 1 and 10 vol.% and preferably between 1 and 6 vol.%.

In step (b) the combustion of the earlier referred to sulphur compounds to sulphur dioxide may have a yield of more than 95 wt%, preferably more than 99 wt% and even more preferably above 99.5 wt% as calculated on the weight of atomic sulphur being present as sulphur dioxide in the flue gas obtained in step (b) relative to the sulphur in the feed. The oxygen comprising gas may be any oxygen containing gas, preferably having an oxygen content of above 10 vol.% and preferably above 20 vol.%. A suitable gas is air, in oxygen enriched air up to pure oxygen.

The temperature in the combustion space may initially be below 1400 °C and preferably below 1200 °C. In step (b) when the combustion continuously takes place the temperature is suitably between 600 and 1200 °C and preferably between 800 and 1200 °C. At these temperature conditions, low formation of NOx is observed which is an additional advantage of this process. In a situation wherein a co-feed is required to increase the lower heating value of the gaseous feed it may be advantageous to limit the use of such a co-feed as much as possible. This may be achieved by performing step (b) at the lower possible temperature in this range. Preferably the gaseous feed and optional co-feed have a temperature of between 10 and 500 °C when discharged into the combustion space. The pressure in the combustion space may range from 50 mbar to 10 bar.

The combustion space having an internal flue gas recirculation may have any cross- sectional shape, like for example circular or rectangular and provided with one or more discharge channels for the gaseous feed and oxygen containing gas present as separate nozzles or as co-axial burners. The number of discharge channels may range from one to 200. There is no practical upper limit and for large combustion spaces these numbers may even be higher. The co-axial burners or discharge nozzles may be arranged at one end of the combustion space or positioned opposite each other.

Examples of suitable combustion spaces and burners are known from the MILD combustor (M.N. Noor et al., MILD Combustion: The Future for Lean and Clean Combustion Technology), International Review of Mechanical Engineering (IREME), Vol. 8(1), ISSN 1970- 8734, pages 251-257 illustrating an external recycle of flue gasses. Examples of suitable burners and/or combustion spaces related to the so-called FLOX process are described in US2010119983, US8622736, EP1995515, EP1497589, EP1355111 and EP1248931 and US5154599. These publications further provide additional background information on how the preferred flameless combustion in step (b) may be performed.

In step (c) sulphur dioxide is separated from the flue gas as discharged from the combustion space to obtain the gas poor in sulphur compounds. The separation of the sulphur dioxide may be performed by well-known processes, such as a caustic wash, salt water wash, or plain water wash and active carbon absorption. A suited technology for step (c) is absorbing and oxidation of the sulphur dioxide with lime-stone to gypsum. Another suited process is wherein the sulphur dioxide is separated from the flue gas in step (c) by contacting the flue gas with a calcium hydroxide comprising solid compound. More preferably step (c) is performed by the calcium hydroxide contacting processes as described in US5277837, US6939523, WO16207159 and CN105642103. A suitable calcium hydroxide is for example described in EP2200940. Preferably step (c) is performed such that the SO2 content in the gasses as treated is lower than 100 ppmv, preferably lower than 50 ppmv.

In step (d) the temperature of an oxygen comprising gas having a temperature of below 100 °C is increased such to obtain the oxygen comprising gas used in step (b). This increase in temperature is performed by indirect heat exchange against the flue gas as discharged from the combustion space. Such a heat exchange limits the energy loss from the combustion space and enhances the continuous flameless combustion in step (b). The remaining energy as contained in the flue gas may be used to prepare steam or for power generation. Preferably the flue gas is indirectly cooled in the combustion space or after it is discharged from the combustion space against evaporating water in for example a steam boiler or waste heat boiler.

The invention shall be illustrated by making use of the following Figures. Figure 1 shows a prior art scheme for treating sulphur compound containing gasses and the off gasses of a refinery. Figure 1 shows an acid gas 1, for example obtained in a DIPA absorption unit, which acid gas 1 may contain 80 vol.% H2S being treated in Claus Process Unit 2 to yield a Claus off gas 3,4 which may contain between 0,5 and 1,6 vol.% H2S. The Claus off gas may be treated in a SCOT Process unit 5 to yield a SCOT off gas 6 having a H2S content of less than 30 ppmv, which off gas 6 is further treated in an incinerator 7 before being sent to the stack. In parallel or alternatively the Claus off gas 4 is treated in a Super Claus Process unit 8 to yield a SuperClaus off gas 9 having a H2S content of between 800 and 2000 ppmv, which gas 9 is further treated in an incinerator 10. In parallel a mixture of refinery off gasses 11, 12, 13 are used as fuel in a furnace 14 to obtain a flue gas 15 which is flared in flare 16. These off gasses 11,12 and 13 represent one or any combination of refinery off gasses as described earlier in this description.

Figure 2 shows a scheme according to the present invention showing the acid gas 1 and Claus Process unit 2 as in Figure 1. Furthermore Figure 2 also shows the refinery off gasses 11, 12, 13 which represent one or any combination of refinery off gasses as described earlier in this description. Figure 2 illustrates that the SCOT process unit 5, Superclaus Process unit 8, incinerators 7,10, the off gas furnace 14 and the flare 16 may be

advantageously be replaced by a combustion space 22 and a sulphur dioxide separating unit 24. The combustion space 22 is fed by a combination of the sulphur compounds comprising gas 20 as obtained in the Claus Process unit 2 and any combination of refinery off gasses are indicated by streams 11,12,13. In case the Claus Process unit 2 would temporally be out of service it is possible to direct acid gas 1 via stream 26 direct to combustion space 22. The sulphur dioxide containing gas 23 is contacted with calcium hydroxide in sulphur dioxide separating unit 24 to obtain a gas 25 poor in sulphur dioxide. This gas 25 may be emitted to the environment.

The invention shall be illustrated by the following calculated examples.

Example 1

A sulphur compound containing gaseous having the composition as listed in Table 1 and having a Lower Heating Value (LHV) of 0.47 MJ/Nm^, was combined with a fuel gas co- feed having the composition as listed in Table 1 and having a Lower Heating Value 33,7 MJ/Nm3. The fuel gas is a typical mixture of a crude distiller off-gas, an isomerization unit off-gas and a hydrocracker off-gas. This combined feed was discharged via 15 nozzles together with a flow of air via 25 nozzles as positioned in a pattern of feed and air nozzles at one end of a square shaped combustion space.

A combustion air blower and a fuel gas blower is used to increase the pressure to the 0.2 Mpa in the combustion chamber. The temperature in the combustion space is below 930 °C. From the combustion space a flue gas is discharged having the properties as listed in Table 1. 28.5 tons/hr of 1 MPa steam is produced in the combustion space by indirect heat exchange.

The calculated results show that the hydrocarbon compounds and the sulphur compounds are fully combusted to carbon dioxide, sulphur dioxide and water. Table 1

The flue gas as obtained is contacted with calcium hydroxide in a sulphur dioxide separating unit to obtain a treated gas having a sulphur dioxide content of well below 50 ppmv.

Example 2

Example 1 is repeated except that a gaseous feed poor in sulphur compounds having the composition as listed in Table 2 and having a Lower Heating Value (LHV) of 2.9 MJ/m3 is combusted. Further conditions may be found in Table 2. The gaseous feed is an off gas of a coal gasification process. A combustion air blower and a fuel gas blower is used to increase the pressure to the 0.2 Mpa in the combustion chamber. The temperature in the combustion space is below 930 °C. From the combustion space a flue gas is discharged having the properties as listed in Table 1. 24 tons/hr of 1 MPa steam is produced in the combustion space by indirect heat exchange.

This example illustrates how steam may be generated making use of the combustion space of Example 1 starting with a gas poor in sulphur compounds.

Table 2

Gaseous feed

poor in sulphur

Flow compounds air

Flow

Nm3/hr 27.094 22.000

LHV (dry)

MJ/Nm3 2.93

T (C) 100 240

Composition in

vol.%

H2 0.3702

N2 0.0091 78.09

CO 1.3790

Ar 0.0124

CH4 5.5560

C2H4 0.0737

C2H6 1.0460

C3H6 0.0093

C3H8 0.0065

C4H10 0.0000

C5H12 0.0002

C02 91.4780 0.03

H2S 0.0000

COS 0.0001

CH30H 0.0354

H20 0.0000

02 0.0238 21.88

Total 100 100 Example 3

Example 1 is repeated for a situation wherein the sulphur compound containing feed is a mixture of a Scott Process tail gas (Feed A) and a gas as obtained when degassing liquid sulphur (Feed B) as listed in Table 3. Ambient air having a starting temperature of 20 °C is increased in two heat exchange steps against the flue gas to 130 in the first heat exchange step and 500 °C in the second heat exchange step. The flue gas is thereby decreased in temperature from 933 C to 816 °C in the second heat exchange step. The flue gas having a temperature of 816 °C is subsequently used to make low pressure steam from boiler feed water in a boiler. The flue gas used in the boiler having a temperature of 200 °C is used in the first heat exchange step to heat up the ambient air resulting in a reduced flue gas temperature of 166 °C.

The flue gas having the properties listed in Table 3 as obtained is contacted with calcium hydroxide in a sulphur dioxide separating unit to obtain a treated gas having a sulphur dioxide content of well below 50 ppmv.

This example illustrates how these sulphur gaseous feeds can be used without having to add an additional fuel gas to increase the LHV.

Table 3

Feed A Feed B mixed gas Air Flue gas

Vol.% Vol.% Vol.% Vol.% Vol.%

H2 19,9130 0,0000 18,4000 0,0000

CO 0,7640 0,0000 0,7060 0,0000

C02 63,9890 0,0200 59,1286 0,0300 36,9076

N2 7,9870 51,5450 11,2966 78,0800 41,5309

Ar 0,0180 0,6140 0,0633 0,9400 0,4552

CH4 0,0050 0,0046

C2H4 0,0000

C2H6 0,0000

C3H6 0,0000

C3H8 0,0000

C4H10 0,0000

C5H12 0,0000

C6H14 0,0000

H2S 0,0000 0,0330 0,0025

CS2 0,0000 0,0000 0,0000

COS 0,0240 0,0000 0,0222

S02 0,0000 0,0310 0,0024 0,0188

S6 0,0000 0,0010 0,0001

S8 0,0000 0,0050 0,0004

CH30H 0,0000

H20 7,2810 34,0290 9,3133 17,0863

HCN 0,0000

02 0,0000 13,7230 1,0427 20,9500 4,0000

Total 99,981 100,001 99,983 100,000 99,999

Flow available

(m3/hr) 10.702 1.123

Flow taken

(Nm3/hr) 8.632 710 9.341 6.716 15.169

P (Kpa(abs)) 94 95 100

T (°C) 41,0 132,0 47,9 500,0 933,0

LHV (MJ/m3) 2,24 0,01 2,07

Claims

Process to treat a sulphur compounds comprising gaseous feed having a sulphur compounds content of below 5 vol.% to obtain a gas poor in sulphur compounds comprising the following steps,

(b) continuously discharging the sulphur compounds comprising gaseous feed into the pre-heated combustion space and separately continuously discharging an oxygen comprising gas having a temperature of between 150 and 550 °C into the pre-heated combustion space resulting in a continuous flameless combustion at a temperature of between 800 and 1200 °C wherein the sulphur compounds are oxidized to sulphur dioxide and in that any hydrocarbons are combusted thereby obtaining a flue gas comprising sulphur dioxide, carbon dioxide, water and between 1 and 10 vol.% non- reacted oxygen, which flue gas is continuously discharged from the combustion space, and

(c) separating sulphur dioxide from the flue gas as discharged from the combustion space to obtain the gas poor in sulphur compounds, and

(d) increasing the temperature of an oxygen comprising gas having a temperature of below 100 °C to the oxygen comprising gas used in step (b) by indirect heat exchange against the flue gas as discharged from the combustion space.

Process according to claim 1, wherein the lower heating value of the sulphur comprising gaseous feed or the lower heating value of the sulphur comprising gaseous feed is between 1 and 30 MJ/Nm³.

Process according to claim 2, wherein the lower heating value is between 1.5 and 15 MJ/Nm³.

Process according to any one of claims 1-3, wherein in step (b) a sulphur compounds comprising gaseous feed having a lower heating value of less than 1 MJ/Nm³ is used in admixture with a hydrocarbon comprising gaseous co-feed having a lower heating value of more than 20 MJ/Nm^ .

5. Process according to any one of claims 1-4, wherein the content of sulphur

compounds is below 2 vol.%.

6. Process according to any one of claims 1-5, wherein the sulphur compounds

comprising gaseous feed further comprises the off gas as obtained in a Claus Process, the off-gas obtained in a SCOT process and/or the off gas as obtained in a Superclaus

Process.

7. Process according to any one of claims 1-6, wherein the oxygen comprising gas in step (b) has a temperature of between 300 and 550 C.

8. Process according to any one of claims 1-7, wherein the flue gas comprising sulphur dioxide, carbon dioxide, water contains between 1 and 6 vol% non-reacted oxygen.

9. Process according to any one of claims 1-8, wherein the oxygen comprising gas is air.

10. Process according to any one of claims 1-9, wherein in step (b) a hydrocarbon

comprising gaseous co-feed is discharged into the combustion space.

11. Process according to claim 10, wherein the hydrocarbon comprising gaseous co-feed is a natural gas comprising co-feed.

Process according to any one of claims 10-11, wherein the hydrocarbon comprising gaseous co-feed comprises an off-gas as obtained in a refinery process.

13. Process according to any one of claims 1-12, wherein the flue gas is indirectly cooled in the combustion space or after it is discharged from the combustion space against evaporating water. Process according to any one of claims 1-13, wherein sulphur dioxide is separated from the flue gas in step (c) by contacting the flue gas with a calcium hydroxide comprising solid compound.