WO2011089635A1

WO2011089635A1 - Integrated process for treating refinery waste water, containing ammonia and hydrogen sulphide, and refinery exhaust acid gas containing hydrogen sulphide

Info

Publication number: WO2011089635A1
Application number: PCT/IT2011/000016
Authority: WO
Inventors: Adalberto Bestetti
Original assignee: Siirtec-Nigi S.P.A.
Priority date: 2010-01-22
Filing date: 2011-01-19
Publication date: 2011-07-28
Also published as: EA022555B1; CN102844275A; EA201290680A1; ITMI20100080A1; IT1397745B1; CN102844275B

Abstract

A process for treating refinery waste water mainly containing hydrogen sulphide and ammonia in addition to acid gases effluent mainly containing hydrogen sulphide, said process comprising the following steps: a) stripping in the stripping tower (100) said waste water (1) at a pressure ranging from 8 to 20 bar g in order to produce a gaseous overhead stream (2) essentially containing hydrogen sulphide and water and a liquid bottom stream (4) essentially containing aqueous ammonia; b) stripping in a second stripping tower (110) the stream (4) at 1 to 3 bar g in order to produce a gaseous overhead stream (5-11) of ammonia essentially pure on dry basis and a liquid bottom stream (6) containing less than 1 ppm of hydrogen sulphide and less than 5 ppm of ammonia, said water composition having the requirements to be discharged into the sewer; c) oxidation on oxidizer deficiency to nitrogen and water of the ammonia contained in the stream (11) in the thermal oxidation unit (170), to produce an outlet stream (12) containing 50-150 ppm in volume of ammonia and 80-200 ppm in volume of nitrogen oxides; d) thermal oxidation of the stream (12) in the thermal oxidiser unit (160).

Description

INTEGRATED PROCESS FOR TREATING REFINERY WASTE WATER, CONTAINING AMMONIA AND HYDROGEN SULPHIDE, AND REFINERY EXHAUST ACID GAS CONTAINING HYDROGEN SULPHIDE.

DESCRIPTION

The present invention discloses an integrated process for treating refinery waste water mainly containing ammonia and hydrogen sulphide and in the same time acid gas, always from refinery, mainly containing hydrogen sulphide, this process enabling a quantitative removal of ammonia and hydrogen sulphide contained in the above mentioned streams, therefore obtaining purified water having a content in residual ammonia equal or less than 5 ppm and a content of residual hydrogen sulphide equal or less than 1 ppm, and exhausted gas containing less than 1500 ppm volume of S0₂, less than 150 mg Nm³of NO_x and less than 1.3 mg/Nm³ of ammonia.

The hydro-desulphurization and the catalytic cracking processes, refinery usual processes, convert sulphur containing organic compounds into hydrogen sulphide that is subsequently separated from the fuel by scrubbing with solvents whose regeneration gives rise to off-gas (acid) having an high hydrogen sulphide content. Before discharging these effluents into the atmosphere, hydrogen sulphide is converted to elemental sulphur that is removed mainly as a liquid.

Moreover, the hydro-desulphurization process, apart from the hydrogen sulphide conversion, produces ammonia from nitrogen containing organic compounds comprised in the oil fractions. The so produced ammonia gathers into the hydrocarbons scrubbing water from which it is removed by low pressure steam. According to the current industrial practice, streams by the above stripping processes, usually made by steam, are sent to Claus plants in order to make at the same time a partial oxidation of both H₂S and NH₃.

However, the sulphur recovering plants can accept acid gas streams and ammonia containing streams only if the composition of the mixture NH₃/H₂S in the overall stream, resulting from blending the above two streams, is equal or less than 35/65 mol. Streams having a NH₃ content greater than 35% mol. entail the risk of an excessive increase of the thermal Claus reactor operating temperature, by consequence causing damage to the reactor covering or depositing ammonia salts upon the relatively cold plant parts; therefore the plant stops and no more production is allowed.

The growing diffusion of oil having a high content of nitrogen-based products causes an unavoidable increase of said NH₃/¾S ratio, beyond the limit acceptable for a correct running of the Claus plant.

Besides, the growing sensibility to the environmental problems is leading local authorities to set even more rigid emission limits. Regarding in particular the waste water, ammonia content less than 5 ppm weight and hydrogen sulphide content less than 1 ppm weight are often requested. As the gas emissions, less than 1500 ppm volume S0₂, less than 150 mg/Nm³ NO_x and less than 1.3 mg/Nm³ ammonia are requested. It has to be pointed out that efficiency in the sulphur recovery greater than 99.8% is necessary for obtaining this S0₂ value.

Now it has been found a process for treating refinery waste water mainly containing hydrogen sulphide and ammonia, and acid gas mainly containing hydrogen sulphide but often also ammonia, able to overcome the above mentioned drawbacks.

The present invention consists in an integrated process allowing:

(i) a sulphur recovery efficiency greater than 99.8%,

(ii) a quantitative removal of ammonia and hydrogen sulphide from the refinery acid water so obtaining a purified water having an ammonia content not greater than 5 ppm and a hydrogen sulphide content not greater than 1 ppm,

(iii) the removal of excess ammonia gas, referring to the amount acceptable by Claus plant, by thermal oxidation in an incinerator able to ensure a NO_x content lower than 150 mg/Nm in the exhausted gas to be released into the atmosphere. For a better understanding of the present invention, reference is made to Fig. 1 relating to the prior art process and to Figs. 2-4 relating to the present invention process. A plain comparison between these two technologies allows a more direct and in-depth comprehension of the present invention. Description of Fig. 1 and related process according to the prior art

In Fig 1 :

200 is a stripping column;

210 is a Claus plant for converting hydrogen sulphide into sulphur;

220 is a TGCU unit, i.e. a unit for recovering the Claus plant tail gas by a catalytic reduction of sulphur containing products, mainly S and S0₂;

230 is a thermal oxidation unit;

240 is the chimney of the off-gas to be released into the atmosphere.

In the plant described in Fig. 1, waste water (1), mainly containing ammonia and hydrogen sulphide, is introduced in the stripping column (200) operating at a pressure from 0.7 to 1.2 bar g giving rise to a bottom aqueous stream of purified water (3) and to a gas stream (2). The acid gas stream (4) and the gas stream (2) are sent to the Claus plant (210) along with the tail gas recycle stream (5) from the unit (220) used for treating tail gas of Claus plant. Claus plant converts large part of hydrogen sulphide into liquid sulphur, stream (6).

Stream (8), from TGCU unit (220), mainly contains N₂, C0₂, H₂0, S0₂ and traces of unconverted H₂S. Said stream is sent to the thermal oxidation unit (230) that converts to S0₂ the residual hydrogen sulphide. Stream (9), from the thermal oxidation unit (230), has almost the same composition as stream (8), except for the fact that H₂S is not present. Then said stream is sent to unit (240) giving rise to gas stream (10) that is discharged into the atmosphere.

This plant is a valid technical solution only if the molar ratio NH₃/H₂S in the final stream deriving from mixing stream (2) and acid gas stream (3) and sent to Claus reactor (20) is equal or less than 35/65.

Description of Fig. 2 (configuration A) and operation of the related plant according to the invention. 100 is a first stripping column operating at high pressure not described by the prior art, 1 10 is a second stripping column operating at low pressure as described by the prior art,

120 is a guard tower not described in the prior art too,

140 is equivalent to 210 in Fig. 1.

150 is equivalent to 220 in Fig. 1.

160 is equivalent to 230 in Fig. 1.

170 is a unit, not disclosed by the prior art, consisting in a thermal oxidation unit of NH₃.

180 is equivalent to 240 in Fig. 1.

The present invention envisages other two configurations (Fig. 3, configuration B and Fig. 4, configuration C), said configurations being different embodiments of the main configuration of Fig. 2.

All these configurations will be discussed below.

The present invention relates to a process for treating refinery waste water mainly containing hydrogen sulphide and ammonia in addition to acid gases refinery effluent mainly containing hydrogen sulphide (sometimes containing also ammonia), said process comprising the following steps:

a) stripping in the stripping tower (100) said waste water (1) at a pressure ranging from 8 to 20 bar g, preferably at 15 bar g, in order to produce a gaseous overhead stream (2) essentially containing hydrogen sulphide and water and a liquid bottom stream (4) essentially containing aqueous ammonia;

b) stripping in a second stripping tower (1 10) the stream (4) at 1 to 3 bar g, preferably at 1 bar g, in order to produce a gaseous overhead stream (5-1 1) of ammonia essentially pure on dry basis and a liquid bottom stream (6) containing less than 1 ppm weight of hydrogen sulphide and less than 5 ppm weight of ammonia, said water stream (6) having the requirements to be discharged into the sewer; c) oxidation on oxidizer deficiency to nitrogen and water of the ammonia contained in the stream (11) in the thermal oxidation unit (170), to produce an outlet stream (12) containing 50-150 ppm volume of ammonia and 80-200 ppm volume of nitrogen oxides;

d) thermal oxidation of the stream (12) in the thermal oxidiser unit (160).

In a preferred embodiment of the invention (Fig. 2, configuration A), the gas stream of pure on dry basis ammonia (5) is sent to a guard column (120) where a stream (7) of slightly alkaline water (pH 7.5-9) is continuously recycled. The purpose of this column (120) is to hold hydrogen sulphide possibly present in stream (5), in this way preventing its discharge into the atmosphere through exhausted gases (8). A small amount of demineralised water, stream (9), is fed overhead the guard tower in order to maintain the pH solution constant, while a part of the recycle, stream (10), is recycled to the high pressure column in order to ensure the complete recovery of hydrogen sulphide. At the exit from the guard tower (120) a gas stream of essentially pure on dry basis ammonia (11) that is subsequently sent to the ammonia thermal oxidation unit (170) is obtained.

Regarding the composition of the streams to be cleaned, the liquid aqueous stream (1) usually has an ammonia concentration from 2.1% to 4.22% molar and H₂S concentration from 1.1 to 3.3 % molar, the complement to 100 being mainly water.

Stream (3) is called "acid gas". This term is used for meaning a gas stream whose main component is H₂S possibly mixed with C0₂. However acid gases can contain also NH₃, particularly from 2% vol. to 15% vol. As an exception, acid gases can contain NH₃ in an amount up to 20% vol., as it is possible to note in the experimental example based on real data. Typically stream (3) contains H₂S in an amount from 80% to 95% molar on dry basis and C0₂ from 3% to 18% molar on dry basis. Stream (3) can contain also hydrocarbons in a quantity usually smaller than 2% molar.

It is to be underlined that both acid gas (3) and liquid stream (1) are coming from the same origin, i.e. the hydro-desulphurisation and/or the catalytic cracking. The composition of the above streams depends on different factors, in particular the kind of refinery and its operation conditions. However the important parameter is not the individual stream (1) and (3) composition, but the molar ratio NH₃/H₂S of the mixture of streams (2) and (3), said mixture being produced in the Claus burner stack. In fact the Claus burner carries out the stream (2) and (3) blending before inlet into the Claus reactor. As said above, the present invention solves in particular the problem when the molar ratio NH₃/H₂S in the streams (2) and (3) mixture is greater than 35/65. However the process of the present invention can operate also in the case the molar ratio NH₃/H₂S is lower than 35/65, due to the flexibility of the process of the present invention.

Referring now to Fig. 2, configuration A, the process of the present invention operates in the following way:

Refinery waste water mainly containing ammonia and hydrogen sulphide is sent, line (1), to a first stripping column (100) operating at a pressure from 8 to 20 bar g, preferably at about 15 bar g, where the separation of hydrogen sulphide from the aqueous solution is carried out.

From the head of this stripping column a gas stream (2) is extracted at a temperature from 120°C to 200°C, preferably at about 140°C, said stream essentially containing hydrogen sulphide and water. The above stream (2) is suitable to be treated in a Claus plant (140) able to receive acid gas (3) also from different refinery units, for example from amine scrubbing units. The aqueous solution (4), extracted from the bottom of column (100) and almost totally containing ammonia, is fed, upon de- pressurization by a proper lamination valve, to a second stripping column (110) placed in sequence to the first one but operating at a pressure from 1 to 3 bar g, preferably at about 1 bar g.

Ammonia is separated and extracted from the second column head, stream (5), at a temperature from 100°C to 150°C, preferably at about 120°C, and sent to the above described guard tower (120). From the bottom of the column (110) an aqueous stream (6) containing hydrogen sulphide in a quantity lower than 1 ppm weight and ammonia in a quantity lower than 5 ppm weight is extracted. Therefore said aqueous stream is suitable to be discharged into surface water.

Said very low level of polluting products can be obtained by column (100) operating in the following way:

heat to the reboiler equal to 50-100 kcal for each kg of fed solution, preferably 72 kcal/kg; reflux in the column from 5 to 20 kg per m³ feed, preferably 11 kg/m³;

at least 30 separation steps.

As the second column (1 10) the preferred operating conditions are the following:

. - heat to the reboiler equal to 60-120 kcal for each kg of the solution fed to the unit, preferably 90 kg/m³;

reflux in the column from 140 to 250 kg per m feed, preferably 225 kg/m ;

at least 35 separation steps.

It has to be noted that the multiple stripping unit, i.e. (100) + (1 10), can operate also as a single stripping unit in the case the quantity of ammonia to be removed from waste water is low, conveying stream (1) downstream the lamination valve - stream (19) of Fig. 3 configuration B - to the column (1 10) excluding columns (100) and (120) to operate. In this case a single gas stream is obtained from the stripping unit, said stream consisting of a wet mixture of hydrogen sulphide and ammonia, which can be fed directly to the Claus unit.

Coming back to Fig. 2 configuration A, stream (5) from the head of the low pressure column (110) contains about 84% volume of ammonia and 16% volume of water at a temperature preferably of 120°C. In the preferred embodiment of the invention, said stream (5) is sent to the guard column (120) where a stream (7) of slightly alkaline water (pH 7.5-9) is continuously recycled. The purpose of this column (120) is to hold hydrogen sulphide possibly present in stream (5), in this way preventing its discharge into the atmosphere through exhausted gases (8). A small amount of demineralised water, stream (9), is fed overhead the guard tower (120) in order to maintain the pH solution constant, while a part of the recycle, stream (10), is recycled to the high pressure column (100) in order to ensure the complete recovery of hydrogen sulphide.

In another embodiment (see Fig. 4, configuration C), particularly for reasons relating to operating flexibility, stream (5) can be divided in two streams, i.e. stream (19) feeding the guard column (120) and stream (20) that, by joining the ammonia stream (2), gives rise to the ammonia stream (21) to be sent to the Claus unit (140). Stream (20) is controlled in order to have a molar ratio NH₃/H₂S in the final stream (deriving from all feeding streams) sent to the Claus unit (140) lower than 35/65, while the ammonia in excess, stream (19), is carried to the thermal oxidation unit (170).

Later, Fig. 2 and Fig. 4 are equivalent. From the guard column (120) ammonia stream (1 1) essentially pure on dry basis is extracted, said stream being disposed by converting ammonia into nitrogen and water according to the following reaction (I):

2NH₃ + 3/2 0₂ — -> N₂ + 3H₂0 (I)

The above partial oxidation reaction (I) takes place in the relevant thermal oxidation unit (170), air being the oxidizer. If necessary also pure oxygen or enriched air can be used.

The oxidation reaction (I) is carried out in a small oxidizer deficiency (for example using a molar ratio 0₂/NH₃ of 0.75/1 or slightly lower than this ratio), then in reducing conditions, in order to get, in the stream (12) coming out from the thermal unit (170), an ammonia residue from 50 ppm vol. to 150 ppm vol and a nitrogen oxides content from 80 to 200 ppm vol. The reaction (I) is carried out at a temperature from 1350°C to 1500°C, preferably at 1500°C, the value 1350°C being the lowest temperature at which ammonia is converted at an acceptable rate.

Then stream (12) is conveyed into the thermal oxidation unit of the tail gas coming from the sulphur recovery plant (160) operating at a temperature from 850°C to 950°C, preferably at about 900°C, in a slight oxygen excess, let say in oxidizing conditions, in order to ensure the greatest conversion of the de-nitrification reaction (II), illustrated by nitrogen monoxide, but valid for every nitrogen oxide.

4NH₃ + 2 NO + 2 0₂ -— » 3 N₂ + 6H₂0 (II)

The above de-nitrification reaction (II) is widely used in the SNCR processes (Selective Non

Catalytic Reduction). This reaction allows to abate both residual ammonia and NO_x until to have an exhausted gas discharged into the atmosphere (stream 8) comprising less than 150 mg/Nm³ NO_x, less than 1.3 mg/Nm residual ammonia and less than 1500 ppm S0₂. The exhausted gas from unit

(160) is sent, stream (18), to the chimney (180) and then discharged into the atmosphere as stream The Claus unit (140) is fed by refinery acid gas (3), by stream (2) and by stream (15) coming out from the unit concerning the recovery of the Claus plant tail gas (150). The above stream (15) is usually made up of N₂, H₂0 and H₂S, the last one being at most 3% volume. In another embodiment, (Fig. 4, configuration C), in the case of stream (5) partition, the Claus unit is fed by refinery acid gas (3), by stream (21) and by the recycle (15). The Claus plant operating conditions are well known to people skilled in the art, said process enabling the conversion of hydrogen sulphide into liquid sulphur. The Claus unit sulphur recovery efficiency is about 95% in the case it consists of two reactors in series, or about 97,5 in the case it consists of three reactors in series. Then the tail gas from Claus unit (140), let say stream (16), comprises noticeable amounts of hydrogen sulphide, apart from N₂, H₂0, H₂S, S0₂, COS, CS₂ and H₂. In order to increase sulphur recovery efficiency until to overcome 99.5%, the sulphur products having a high oxidation number (S0₂ and S) contained in the tail gas of the unit (140) are reduced to hydrogen sulphide by a catalytic reduction process carried out in the unit (150). The so produced hydrogen sulphide is recycled to the Claus plant (140) for a further conversion as stream (15). At the exit from unit (150), stream (17) is obtained, said stream essentially consisting of N₂, C0₂, H₂0, H₂S (usually from 200 to 350 ppm) and S0₂, usually from 20 to 50 ppm. Then the above stream (17) is sent to the thermal oxidation unit (160).

In the process of the present invention the exit streams are:

* * liquid sulphur (14)

** purified water (6)

** scrubbed gas (8)

As the ammonia stream (1 1), said ammonia could be separated from water and kept in the refinery or used in said refinery. However this is not always allowed due to several reasons, for example the refinery cannot stockpile ammonia nor cannot inside use such ammonia amounts.

The integrated process of the present invention enables to join the more and more strict requirement in relation to the removal of ammonia and hydrogen sulphide from liquid and/or gaseous streams and the need of refinery operating flexibility, deriving on the one hand from the always greater difference in the crude oil to be treated quality, on the other hand from the increasing demand of diesel having a very low content of sulphur.

As more details about operations already known in the prior art, we report below some references: Claus plants + TGCU: I. Pasquon, G, Guerreri, Principi della Chimica Industriale, Vol. 3, Metodi di separazione e di purificazione e loro applicazione all'industria chimica e petrolifera, pag. 353 e seg.: Clup, 1895 A. Kohl, R. Nielsen, Gas purification, Gulf Publishing Company, 1997;

H.G. Paskall, J.A. Sames, Sulphur Recovery, Sulphur Experts (Western Research). This reference contains also a chapter relating to Claus plant incinerator.

Incinerators: C.E. Baukal, The John Zink Combustion Handbook, CRC Press, 2001.

The following example is reported for a better understanding of the present invention.

EXAMPLE

A refinery has to treat 60m³/h of water polluted by ammonia (1.20% weight) and hydrogen sulphide (1.70% weight). For the reason that said water is in excess, in relation to the total balance of the refinery, it is necessary to purify it before its conveying into surface water, in order to have an ammonia residue lower than 5 ppm weight and a hydrogen sulphide residue lower than 1 ppm weight.

Besides it is necessary to remove sulphur contained in a gas stream (acid gas) of about 1040 Nm /h containing about 62% volume hydrogen sulphide and 20% volume ammonia from the fluid bed cracking unit and the hydro-cracking unit. The overall ammonia content (adding up the value from gas stream and that from acid gas) makes impossible to treat these two streams according to conventional procedures.

Finally, in order to comply with current regulations regarding the atmosphere emissions, the refinery must recover 99.8% of the sulphur contained in both the liquid effluents and the gas streams, and ensure a NO_x level in the chimney smokes not greater than 100 mg Nm . For this reason it is necessary to reach a high purity degree of treated water and to dispose of ammonia by means of a specially provided incinerator.

In this case the molar ratio ammonia/hydrogen sulphide in the global stream sent to the Claus unit is about 87% molar; then a prior art plant for recovering sulphur is not able to accept said stream. This problem is solved by a two step separation of ammonia from hydrogen sulphide and by the separation of both of them from refinery water according to the above described layout; the wet pure hydrogen sulphide stream by the high pressure step is conveyed to the Claus plant in order to convert hydrogen sulphide in elemental sulphur to be recovered by condensation. Instead ammonia is sent to a two step thermal oxidation unit (incinerator) in order to be converted to nitrogen and water, in this way reducing the NO_x formation to 100 mg/Nm³.

The 99.8%) sulphur recovery is provided by means of the downstream Claus plant installation of a unit for tail gas treating based on catalytic reduction of the sulphur based products therein contained.

Claims

1 A process for treating refinery waste water mainly containing hydrogen sulphide and ammonia in addition to acid gases effluent mainly containing hydrogen sulphide, said process comprising the following steps:

a) stripping in the stripping tower (100) said waste water (1) at a pressure ranging from 8 to 20 bar g in order to produce a gaseous overhead stream (2) essentially containing hydrogen sulphide and water and a liquid bottom stream (4) essentially containing aqueous ammonia;

b) stripping in a second stripping tower (110) the stream (4) at 1 to 3 bar g in order to produce a gaseous overhead stream (5-11) of ammonia essentially pure on dry basis and a liquid bottom stream (6) containing less than 1 ppm of hydrogen sulphide and less than 5 ppm of ammonia, said water composition having the requirements to be discharged into the sewer;

c) oxidation on oxidizer deficiency to nitrogen and water of the ammonia contained in the stream (1 1) in the thermal oxidation unit (170), to produce an outlet stream (12) containing 50-150 ppm volume of ammonia and 80-200 ppm volume of nitrogen oxides;

d) thermal oxidation of the stream (12) in the thermal oxidiser unit (160).

2. The process according to the claim 1 , in which the stripping of step (a) is operated at 15 bar g.

3. The process according to the claim 1, in which the stripping of step (b) is operated at 1 bar g.

4. The process according to the claim 1, wherein the temperature of the ammonia thermal oxidation unit (170) is kept in the range from 1350 tol500°C under a slight oxidizer deficiency, preferably with an ammonia/hydrogen sulphide ratio of 0.75/1, or slightly smaller than this ratio.

5. The process according to the claim 1 in which the stream (1) has the following composition: ammonia: from 2.1 to 4.22 % mol; hydrogen sulphide: from 1.1 to 3.3% mol, the balance being mainly water;

and the stream (3) has the following composition on dry basis: hydrogen sulphide: from 80 to 95 % mol.; carbon dioxide: from 3 to 18% mol.; hydrocarbons less than 2 % mol; ammonia from 0 to 20% volume.

6. The process according to the claim 1, wherein the overhead stream (5) of the stripping column (1 10) is being routed to the guard columns (120) before being sent to the ammonia thermal oxidiser unit (170) as stream (11).

7. The process according to the claim 6, wherein the stream (5) is being split in two streams, let say the stream (19) to be fed to the guard column (120) and the stream (20) which is being mixed with the stream (2) to originate the stream (21) to be fed to the Claus unit (140), the flow rate of said stream (20) being controlled in order that the ammonia to hydrogen sulphide molar ratio in the combined feed to the Claus unit (140) is lower than 35/65, the ammonia in excess, stream (19), being routed toward the thermal oxidiser unit (170).