PURIFICATION PROCESS
The present invention relates to an improved process capable of reducing the sulphur content of middle distillates. Sulphur in middle distillates e.g. diesel and gas oil can be present in a number of different species. These species can be classified as one of the following sulphur types: mercaptans, sulphides, di-sulphides, thiophenes, benzothiophenes (BT) and di-benzothiophenes (DBTs) comprising both unhindered di- benzothiophenes (unhindered-DBTs) and hindered di-benzothiophenes (hindered-DBTs). It is desirable to remove such compounds from the diesel for environmental reasons. This may be carried out by hydrodesulphurisation.
In a typical hydrodesulphurisation process, the middle distillate is reacted with hydrogen in the presence of a suitable catalyst. During this reaction, the sulphur compounds are converted to hydrogen sulphide gas, which is easily removed. Generally, however, aromatic sulphur species such as BT are more difficult to hydrodesulphurise than aliphatic sulphur compounds (eg. mercaptans, sulphides, di-sulphides) and DBTs are more difficult to hydrodesulphurise than the mono- aromatic species. The degree of substitution within each sulphur type can also have a large impact on the ease of hydrodesulphurisation. Thus, dibenzothiophenes with at least one ethyl or methyl substituent in one or both aromatic rings (hindered-DBTs) tend to be more difficult to treat than unhindered-DBTs and in particular 4, 6 dimethyldibenzothiophene is particularly difficult to hydrodesulphurise. Middle distillate is often desulphurised in a low pressure hydrotreating reactor.
Low pressure hydrotreating of middle distillate feed that contains relatively reactive sulphur species is able to provide oil products with a low sulphur concentration
compared to that of the untreated feed. Typically, the sulphur content of such a middle distillate stream may be reduced to 500 ppm by weight (expressed as elemental sulphur) by low pressure hydrotreating.
However when middle distillate streams which contain relatively high levels of DBTs are treated by low pressure hydrotreating they provide oil products with higher sulphur concentrations relative to those described above because the DBTs and particularly the hindered-DBTs are relatively unreactive when treated with hydrogen. Consequently the latter stream typically requires a larger low pressure hydrotreating reactor than required when treating streams which contain lower levels of DBTs and particularly hindered-DBTs. Alternatively, the stream can be hydrotreated using a high pressure hydrotreating reactor. However these large volume low pressure reactors or high pressure hydrotreating reactors are expensive and the higher pressure hydrodesulphurisation process is energy inefficient.
Attempts to reduce the sulphur content to below 500 ppm in this way have proved relatively uneconomic. This is because at low sulphur concentrations e.g. below 500 ppm, a significant proportion e.g. 80-100%, usually 90-100% of the sulphur present in the hydrotreated middle distillate is in the form of DBTs.
We have now found a process of improved energy efficiency capable of reducing the sulphur content of a middle distillate. Accordingly the present invention provides a process to remove sulphur containing compounds from a middle distillate stream which comprises the steps (a) separating said middle distillate stream containing DBT into at least two streams to provide at least one first stream having a reduced DBT content and at least one second stream having an increased DBT content (b) passing said first stream through at least one lower pressure reactor in the presence of hydrogen to produce a third stream of lower sulphur content and (c) passing the second stream through at least one higher pressure reactor in the presence of hydrogen to produce a fourth stream of lower sulphur content.
Preferably the lower pressure reactor is a low pressure hydrotreating reactor and the high pressure reactor is a high pressure hydrotreating reactor. Usually said first stream also has a reduced hindered-DBT content and said second stream also has an increased hindered-DBT content.
Preferably hydrogen is separated from the third and fourth streams, exiting said higher pressure hydrotreating reactor and said lower pressure hydrotreating reactor respectively. The depleted third and fourth streams may then be combined or preferably maintained as separate streams for subsequent use in middle distillate blending. Alternatively the third and fourth streams may be combined prior to the removal of hydrogen.
The middle distillate stream may be one or more petroleum fractions with a boiling range of 150-450°C, preferably 190-390°C. Advantageously the middle distillate stream is a combination of said petroleum fractions. Examples of suitable petroleum fractions include light gas oils (LGO), heavy gas oils (HGO), light cycle oils (LCO), coker gas oils (CGO) and Visbroken gas oils (VBGO). These petroleum fractions contain sulphur compounds, such as BT, unhindered-DBT and hindered-DBT.
Preferably the middle distillate stream is a stream for blending with other components e.g. kerosene to produce an automotive diesel. Usually this middle distillate stream contains between 50-100%wt e.g. 70-90%wt of LGO, 0-40%wt e.g. 10-30%wt of HGO, 0-40%wt e.g. 10-30%wt of LCO and 0-20%wt e.g. 5-15wt% of CGO and/or VBGO.
The middle distillate stream usually has a total sulphur content (expressed as elemental S) of 1000-50000ppm S, preferably 5000-20000ppm S e.g. 15000ppm S, a DBT content of 100-20000ppm S, preferably 1000-5000ppm S e.g. 3000ppm S, and a hindered-DBT content of 50-5000ppm S, preferably 100-l OOOppm S e.g. 500ppm S. Usually the liquid hourly space velocity LHSV of the middle distillate stream is between 0.05-20 h"1, preferably between 0.1-10 h"1, and most preferably between 0.3-3 e.g. 1.5 h"1. The middle distillate stream is usually separated in step (a) by distillation to produce at least one first stream having a reduced DBT content with a boiling range of 200-330°C, preferably between 240-290°C and at least one second stream having an increased DBT content with a boiling range of usually between 260-400°C, preferably between 290-350°C. The middle distillate stream may be separated in a purpose built distillation column e.g. a fractional distillation column, or preferably in an existing crude distillation unit (CDU) or fluid cracking catalyst (FCC) main fractionator.
Typically the distillation column is a fractional distillation column comprising a column body containing plates or packing, a reboiler to continuously heat the middle distillate stream to between 200-500°C, preferably between 250-380°C e g 340°C, and a overheads condenser wherein part of the vapour produced is condensed and the resulting liquid contacted with more vapour in the column
Usually the first stream comprises a total sulphur content of 2000-40000ppm S, preferably 5000-20000ppm S e g lOOOOppm S, a DBT content of 50-5000ppm S, preferably 250-lOOOppm S e.g 750ppm S, and a hindered-DBT content of 50-500ppm S, preferably 50-200ppm S e g lOOppm S Usually the second stream comprises a total sulphur content of 5000-40000ppm
S, preferably 10000-30000ppm S e g 20000ppm S, a DBT content of 200-40000ppm S, preferably 2000- lOOOOppm S e g 5000ppm S, and a hindered-DBT content of 100- lOOOOppm S, preferably 200-2000ppm S e g lOOOppm S
The volume ratio of the first stream to the second stream is usually in the range 1 10 to 10 1, preferably 20 80 to 70 30 e g 50:50
The middle distillate stream can be divided into more than two streams e g 2-6, each of different DBT (and usually hindered-DBT) level Step (b) may also involve more than one lower pressure reactor e g 2-6 and step (c) more than one higher pressure reactor e g 2-6 The lower pressure hydrotreating reactor(s) is/are typically at a hydrogen pressure of 10-40bar preferably 15-35bar and especially 20-30bar e g 25bar whilst the higher pressure hydrotreating reactor is typically at a hydrogen pressure of 40-100bar, preferably 45-75bar and especially 50-60bar e g 55bar The hydrogen pressure difference between at least one hydrotreating reactor in step (b) and one in step (c) is usually at least 5bar, preferably between 10-60bar, and most preferably between 15- 55bar e g 30bar Preferably the hydrotreating reactors are operated at a temperature between 270 to 430°C preferably between 290 to 400°C, and most preferably between 320 to 390°C
The hydrotreating reactors usually comprise a fixed bed hydrodesulphurisation catalyst In the presence of the hydrodesulphurisation catalyst, the hydrogen reacts with the sulphur compounds in the middle distillate to release sulphur in the form of hydrogen sulphide and, optionally, also to saturate at least some of the unsaturated components
present in the middle distillate. The products of steps (b) and (c) comprise middle distillate/hydrogen mixtures of reduced S content which are usually each introduced into a separator where any unreacted hydrogen gas is removed. The unreacted hydrogen from at least one separator and usually all separators is preferably recycled for re-use to step (b) and/or (c). In one preferred embodiment of the invention unreacted hydrogen from step (b) is combined with unreacted hydrogen from step (c) and reused in step (c)
The hydrogen: middle distillate volume ratio in the hydrotreating reactors is usually between 20-2000 Nm3 hydrogen : lm3 middle distillate, preferably 50-1500: 1, and most preferably between 100-700: 1 e.g 500: 1. Any suitable hydrodesulphurisation catalyst may be used in the hydrotreating reactors. Such a catalyst may comprise an active component which is dispersed on a catalyst support. Examples of active components include molybdenum and tungsten compounds. Molybdenum sulphide is preferred. Optionally, a catalyst promoter, may be used in combination with the catalyst. Examples of catalyst promoters include cobalt and nickel. The active component, and optional promoter, may be supported on any suitable catalyst support, such as silica, and/or gamma alumina Where a gamma alumina support is employed, it may also comprise amounts of silica and/or phosphorus
Typically the weight of catalyst used in the hydrotreating reactors is 5- 1000 tonnes, preferably 50-500 tonnes and most preferably 100-200 tonnes e.g 150 tonnes In a preferred embodiment of the invention hydrogen is passed as stripping gas into the base of a distillation column e.g. a fractional distillation column and usually the middle distillate enters the fractional distillation column towards the top of the column e g 75-100%) of the column distance towards the top. The middle distillate feed may be heated to between 200-500°C, preferably between 250-380°C e.g 340°C before entering the column Typically the column has a pressure of between 10-40bar and most preferably between 25-35bar e.g. 28bar The middle distillate stream is separated into a first stream having a reduced DBT content, which is usually a overhead vapour stream and a second stream having an increased DBT content, which is usually a bottoms liquid stream In one embodiment of the invention the distillation column is combined with the lower pressure hydrotreating reactor in the same integral unit such that the first stream is passed directly into the lower pressure hydrotreating reactor part of the unit together
with the hydrogen. The hydrotreated first stream i.e. the third stream is then separated from the hydrogen by a first separator. Hydrogen at pressure of between 40-100bar, preferably between 50-90bar e.g. 70bar is combined with the second stream and passed into the higher pressure hydrotreating reactor. The hydrotreated second stream i.e. the fourth stream is then separated from the hydrogen by a second separator.
In a further embodiment of the invention the hydrogen separated from the hydrotreated first stream is compressed to a pressure of between 40-100bar, preferably between 50-90bar e.g. 70bar and combined with the hydrogen separated from the hydrotreated second stream. The combined hydrogen is mixed with the second stream exiting the distillation column prior to entering the higher pressure hydrotreating reactor. The process is capable of being used to reduce the sulphur level of the middle distillate to less than 500 ppm, preferably less than 200 ppm, most preferably, less than 100 ppm, and especially less than 50 ppm e.g. down to at most 10 or 5 ppm. In another aspect the invention provides apparatus for removal of sulphur containing compounds from a middle distillate which comprises
(a) at least one distillation column comprising at least one inlet port, at least two exit ports, at least one inlet line and at least two exit lines,
(b) at least one lower pressure hydrotreating reactor comprising at least one inlet port, at least one exit port, at least one inlet line and at least one exit line and (c) at least one higher pressure hydrotreating reactor comprising at least one inlet port, at least one exit port, at least one inlet line and at least one exit line and wherein at least one exit port of said distillation column is capable of being in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor and at least one exit port of said fractional distillation column is capable of being in fluid communication with at least one inlet line of the lower pressure hydrotreating reactor.
In a preferred embodiment of the invention the apparatus comprises (d) at least one hydrogen separator comprising at least one inlet port, at least two exit ports, at least one inlet line and at least two exit lines and wherein at least one exit port of each of the hydrotreating reactors is capable of being in fluid communication with at least one inlet line of at least one hydrogen separator.
The distillation column is usually a fractional distillation column wherein fluid is recirculated to and from the body of the column via a reboiler and a overheads
condenser Any inlet, outlet or conduit that facilitates the fluid recirculation does not represent any of the inlet or outlet ports or lines as herein described above
Preferably the apparatus comprises at least two hydrogen separators each comprising at least one inlet port, at least two exit ports, at least one inlet line and at least two exit lines Usually at least one exit port of the lower pressure hydrotreating reactor is in fluid communication with at least one inlet line of at least one first hydrogen separator and at least one exit port of the higher pressure hydrotreating reactor is in fluid communication with at least one inlet line of at least one second hydrogen separator
In preferred embodiment of the invention at least one exit port of the first hydrogen separator is in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor and most preferably at least one exit port of the second hydrogen separator is also in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor.
Preferably the distillation column comprises at least two inlet ports and at least two inlet lines, at least one for gas e.g hydrogen and at least one for liquid e.g. for middle distillate Advantageously the apparatus also comprises at least one pump in fluid communication with at least one inlet line of the distillation column, in particular the liquid line for the middle distillate stream
Usually the apparatus also comprises at least one pump and at least one heater connected in series and in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor and at least one pump and at least one heater connected in series and in fluid communication with at least one inlet line of the lower pressure hydrotreating reactor
The apparatus also usually comprises at least one gas compressor in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor Usually the apparatus comprises at least two gas compressors wherein at least one first gas compressor is in fluid communication with at least one inlet line of the lower pressure hydrotreating reactor and at least one second gas compressor is in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor In an alternative embodiment of the invention the apparatus comprises
(a) an integral unit comprising a distillation column and a lower pressure hydrotreating reactor comprising at least one inlet port, at least two exit ports, at least one inlet line and at least two exit lines
(b) at least one higher pressure hydrotreating reactor comprising at least one inlet port, at least one exit port, at least one inlet line and at least one exit line and wherein at least one exit port of said integral unit is capable of being in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor.
Preferably the apparatus also comprises (c) at least one hydrogen separator comprising at least one inlet port, at least two exit ports, at least one inlet line and at least two exit lines and wherein at least one exit port of said integral unit and at least one exit port of the higher pressure hydrotreating reactor are capable of being in fluid communication with at least one inlet line of at least one hydrogen separator.
Preferably the apparatus comprises at least a first and second hydrogen separator each comprising at least one inlet port, at least two exit ports, at least one inlet line and at least two exit lines.
In a further embodiment of the invention at least one exit port of the first hydrogen separator is in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor and most preferably at least one exit port of the second hydrogen separator is also in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor.
Preferably the integral unit comprises at least two inlet ports and at least two inlet lines, at least one for gas e.g. hydrogen and at least one for liquid e.g. for middle distillate. Preferably the hydrogen enters the distillation column (typically a fractional distillation column as herein described) of the integral unit via a bottom inlet port and the middle distillate enters the fractional distillation column of the integral unit a via a top inlet port. Advantageously the apparatus also comprises at least one pump and at least one heater connected in series and in fluid communication with at least one inlet line of the integral unit, in particular the liquid line.
The apparatus also usually comprises at least one gas compressor wherein at least one exit port of the first hydrogen separator is in fluid communication with at least one inlet line of the higher pressure hydrotreating reactor via at least one gas compressor. Preferably at least one exit port of the second hydrogen separator is also in fluid
communication with at least one inlet line of the higher pressure hydrotreating reactor via at least one gas compressor.
The present invention provides a more economical process for the production of a middle distillate streams with a sulphur content below 500 ppm by minimising the volume of the high pressure hydrotreating reactor and avoiding the use of larger volume low pressure hydrotreating reactors.
The invention will now be described and illustrated with reference to the accompanying drawings in which figures 1 and 2 are flow diagrams for the hydrodesulphurisation process. The valves are omitted for reasons of clarity. In figure 1 the middle distillate stream is passed via feed line (1) into a first pump
(2) and into the fractional distillation column (4) via inlet port (3). The fractional distillation column comprises a reboiler at the bottom of the column (wherein the middle distillate stream is heated to preferably between 350-400°C e.g. 340°C) and a overheads condenser at the top of the column. The reboiler and condenser are not shown for reasons of clarity. The middle distillate stream is separated by fractionation into a first stream having a reduced DBT content and a second stream having an increased DBT content. Usually the first stream is a vapour whilst the second stream is a liquid.
The first stream exits the distillation column (4) via exit port (5) and is then passed into line (7) to a second pump (8). Hydrogen via line (1 1) is passed through a gas compressor (12) and mixed with the first stream. The mixture is then passed into a lower pressure hydrotreating reactor (13) via a first heater (9) and via inlet port (10).
The second stream exits the distillation column (4) via exit port (6) and is then passed into line (15) to a third pump (16). Hydrogen via line (21) is passed through a gas compressor (22) and mixed with the second stream. The mixture is then passed into a higher pressure hydrotreating reactor (19) via a second heater (17) and via inlet port (18).
The hydrotreated first stream mixed with hydrogen exits the lower pressure hydrotreating reactor (13) via the exit port (14) and enters a separator (25) via line (23) and inlet port (24). Hydrogen exits the separator (25) via a first exit port (27) and passes into line (26) whilst middle distillate of reduced sulphur content exits the separator (25) via a second exit port (28) and passes into line (29).
The hydrotreated second stream mixed with hydrogen exits the higher pressure hydrotreating reactor (19) via the exit port (20) and enters a separator (32) via line (30) and inlet port (31). Hydrogen exits the separator (32) via a first exit port (33) and passes into line (36) whilst middle distillate of reduced sulphur content exits the separator (32) via a second exit port (34) and passes into line (35).
In figure 2 hydrogen is passed into gas compressor (37) via line (38) and then into an integral unit (40) via a first inlet port (39). The integral unit (40) comprises a fractional distillation column (40a) and a lower pressure hydrotreating reactor (40b). Preferably the hydrogen is at a pressure of 28-30bar. The middle distillate stream is passed via feed line (la) into a first pump (2a) through a heater (45) (wherein the middle distillate stream is heated to preferably between 350-400°C e.g. 340°C) and then into the integral unit (40) via a second inlet port (43). The middle distillate stream is separated by fractionation with column (40a) into a first stream having a reduced DBT content and a second stream having an increased DBT content. Usually the first stream is a vapour whilst the second stream is a liquid. The first stream with hydrogen is then passed into a lower pressure hydrotreating reactor part (40b) of the integral unit (40).
The hydrotreated first stream mixed with hydrogen exits the integral unit (40) via the first exit port (41) and enters a separator (25a) via line (23a) and inlet port (24a). The hydrotreated first stream exits the separator (25a) and passes into line (29a) via a first exit port (28a). Hydrogen exits the separator (25a) via a second exit port (27a). The hydrogen is usually at a pressure of 20-25bar. The hydrogen is passed via line (26a) through a compressor (47) (wherein the hydrogen is compressed to a pressure of usually 50bar). The hydrogen is then passed into line (36a) where it is mixed with hydrogen exiting a separator (32a). The combined hydrogen stream is then passed through a second compressor (48) (wherein the hydrogen is compressed to a pressure of usually 70bar) and subsequently mixed with the second stream prior to entering a high pressure hydrotreating reactor (19a).
The second stream exits the integral unit (40) via a second exit port (44) and is fed via line (15a) to a second pump (16a). Hydrogen via line (36a) (usually at a pressure of 70bar) is mixed with the second stream. The mixture is then passed into the high pressure hydrotreating reactor (19a) via inlet port (18a).
The hydrotreated second stream mixed with hydrogen exits the high pressure hydrotreating reactor (19a) via the exit port (20a) and enters the separator (32a) via line (30a) and inlet port (3 la) The hydrotreated second stream exits the separator (32a) and passes into line (35a) via a first exit port (34a) Hydrogen exits the separator (32a) via a second exit port (33a) and passes into line (36a) The hydrogen is usually at a pressure of 60bar The hydrogen is combined with hydrogen from line (26a) and recycled into the high pressure hydrotreating reactor (19a) as herein described above The following examples were simulated using a mathematical model of a hydrodesulphurisation pilot plant process Examples
A middle distillate stream having a boiling range of 200-360°C and a total sulphur content of 1 lOOOppm S with a unhindered-DBT content of 2500ppm S and a hindered- DBT content of 300ppm S (expressed as elemental sulphur) was hydrodesulphurised as described below Comparative Example (a)
The above middle distillate stream was passed through a low pressure hydrodesulphurising reactor with a LHSV of 2 Oh"1 The hydrogen partial pressure in the reactor was 30bar The reactor contained a fixed bed of a supported molybdenum sulphide catalyst The hydrodesulphurising reactor was heated to a temperature of 320°C
Once treated the middle distillate/hydrogen outlet stream was passed though a separator wherein the unreacted hydrogen was removed The product middle distillate stream contained lOOOppm of total sulphur with a unhindered-DBT content of 600ppm and a hindered-DBT content of 250ppm Comparative Example (b)
The above middle distillate stream was divided into two equal streams having the same boiling range One stream was passed through a low pressure hydrodesulphurising reactor at a LHSV of 2 0 h" and the other stream was passed through a high pressure hydrodesulphurising reactor at a LHSV of 2 0 h"1 The hydrogen partial pressure in the low pressure reactor was 30bar and in the high pressure reactor was 70bar Each reactor contained a fixed bed of the same catalyst as in comparative example (a) and both reactors were heated to a temperature of 320°C
Once treated the hydrogen and middle distillate outlet streams from both the reactors were passed separately though a separator wherein any unreacted hydrogen was removed and recycled. The outlet streams from the separators were then combined to produce a product middle distillate stream which contained 550ppm of total sulphur with a unhindered-DBT content of 300ppm and a hindered DBT content of 200ppm. Example according to the invention
The process as shown in Fig. 1 was performed. This was a repeat of comparative example (b) apart from the division of the above middle distillate stream into two streams by fractional distillation to provide a first stream with a boiling range of 240-290°C and sulphur content of όOOOppm and a second stream with a boiling range of 300-350°C and sulphur content of 15000ppm. The first stream was passed through the low pressure hydrodesulphurising reactor (13) and the second stream was passed through the high pressure hydrodesulphurising reactor (19), as shown in Fig 1.
The outlet streams (23) and (30) from the reactors was then separated in the respective separators (25) and (32) into hydrogen for recycling and middle distillate streams which were combined to produce a product middle distillate stream which contained 250ppm of total sulphur with a unhindered-DBT content of 1 lOppm and a hindered-DBT content of 120ppm.