CA2568726C - Process for desulphurization of a hydrocarbon stream with a reduced consumption of hydrogen - Google Patents

Abstract

A two-stage hydrotreating process is disclosed wherein a hydrocarbon stream is first desulphurized followed by a de-hydrogenation step, which process comprises in combination contacting the feed and hydrogen over a hydrotreating catalyst at hydrotreating conditions, heating the hydrotreated effluent and hydrogen-rich gas from the hydrotreating reactor and contacting said effluent and hydrogen gas over a hydrotreating catalyst in a post-treatment reactor at a temperature sufficient to increase the polyaromatic hydro-carbon content and lower the hydrogen content of said effluent.

Description

Process for Desulphurization of a Hydroaarbon Stream with a Reducad Consumption of Hydrogen HACKGFiOVND OF THE INVENTION
1. Field of the Invention The present invention. relates to a catalytic conversion process known as hydr,otreati.ng involving hydrogen and hy-drocarbons containingiheteroatoms such as nitrogen and sul-phur, More particularly, the invention relates to an im-proved process for removing sulphur and nitrogen compounds from middle distilla.te hydrocarbon streams wherein the chemical hycirogen cdnsumption is signifiCantly reduced.
Still more particularly, the invention relates to a process where the hydrotreated hydrocarbon stream is heated and pas5ed through a high=temperature post-reactor thereby low-ering the r.Let amount of hydrogen co-nsumed in the hy-drotreating process.

2. Description of Priqr Art Hydrotreating of hydrocarbon streams is carried out to re-duce the amount of'sulphur and nitrogen compounds in a hy-drocaxbon St:ream: He4eby, the impact of these compounds that upon combustion in an engine may form SOx and NOx is significantly reduced., As many countries are tightening the specifications for a1.lowed sulphur content in transporta-tion fuels, it is increasingly important to cost-effectively perform desulphurization of petroleum frac-tions.

A very significant operating cost related to hydrotreating is the prodLiction of hydrogen gas. Typically, hydrogen is produced by steam reforming of natural gas or as a by-product from gasolin& reforming (platforming). In the hy-drotreating proces~ , hydrogen is consumed not only by hy-drodesulphuzizati.an, hydrodenitrogenation and hydrodeoxy-genation reactions but also by saturation of aromatic hy-drocarbons. Furthermore, hydrocracking reactions and satu-ration of ol.efinic hydrocarbons are also taking place in the hydrotreating reactor consume hydrogen. Typically, saturation of aromatic=s causes the greatest contribution to the total hydrogen consumption.

In order to comply with more stringent specifications for the content of sulphur in the refinery products, it is nec-essary to increase the conversion of sulphur in the feed.
Typically, this will mean that the amount of hydrogen used in the process will iricrease. As the supply of hydrogen is expensive and may be scarce, it is desirable to increase the conversion of sulphur with little or no extra hydrogen consumption.
Typical conditions for a hydrotreating reactor are hydrogen pressures of 15-100 b'ars, liquid hourly space velocity of 0.5-4 m' oi1/m' catalyst/h, and temperatures ranging from 310 C to 400 C. The exact conditions will depend on feed-stock type, the required degree of sulphur removal and the desired run length. Typically, the reactor temperature is initially (start of run) at the lower end of the above range, and as the catalyst deactivates the reactor tempera-ture is raised to compensate for loss of activity. When the design temperature fo'r the reactor is approached, 'the run is normally ended. The design temperature is decided by the metallurgy of the reactor.

The chemical consumption of hydrogen in the hydrotreating process is not constant from start of run to end of run.
Typically, at the lower start-of-run temperature, the hy-drogen consumption i.s: higher since the saturation of aro-matics is an exothermic process that is favoured at low temperatures. Thus, chemical equilibrium between aromatxcs and hydrogenated counterparts dictates a relatively high degree of saturation and therefore relatively high hydrogen consumption. As the run progresses and the reactor average temperature is xaised; the reaction slowly shifts towards lesser aromatics saturation, and therefore the hydrogen consumption typically decreases during the run with the product sulphur conterit being more or less constant.

sLTmd$1RY oF THE INVENTION

it is the general obj'ective of the invention to provide a simple hydrotreating Orocess for desulphurization of a hy-drocarbon stream, where the hydrogen consumption is sig-nificantly reduced.

Accordingly, the invention provides a process comprising the steps of contacting the feedstock with hydrogen over a hydrotreating catalyst at hydrotreating conditions under conditions being effective in hydrotreating, heating the effluent and contacting said effluent with a hydrotreating catalyst at condi'~ions being effective in dehydrogenation of naphthenic and aromatic hydrocarbons.

DETAILED DFTSCRIPTION OF THE INVMT=VN

The invention is explained in more detail, in the following description with reference to the drawings, in which Pig. 1 schematically shows process flows according to an embodi-ment of the invention.' Fresh feedstock is mixed with hydrogen and heat exchanged with process effluent;and passed through the hydrotreating reactor 1. The effluLant from hydrotreating reactor 1 is heated in a furnace 2 before passing to the posttreatment reactor 3. 'rhe tempetature employed in the postt'reatment reactor will typically be in the range of 350 C to 450 C
and will typically be; at least 10 C ha.gb,ez than the out].et temperature of the hydrotreater. The licquid hourly space velocity (LHSV) in the'posttreatment reactor will typically be in the range 2-20m' oil/m' catalyst/h, and the tot&l pzessure will be at the same level as that employed in the hydrotreating reactor: The hydrotreating reactor section may consist of one or:more reactors. Each reactor may have one or more catalyst b-eds.

The catalyst used',i.n the hydrotreatment reactor may be any catalyst used for hy0rotreating petroleum fractions known in the art. Likewise, the catalyst used in the posttreat-ment xeactor may be any catalyst used for hydrotreating pe-troleum fractions knowin in the art. Particular useful cata-lysts for use in the invention comprise at least one metal on a porous inorganic oxide support. Preferred catalysts are Ni-Mo, Co-Mo and 14i-W on alumina support.

The function of the posttreatment reactor is primarily to reduce the amount. of:hydrogen in the liquid product, but also to lower the sulphur and nitrvgen content. The reduc-tion of hydrogen in the liquid product will be balanced by an increase in the hydrogen purity of the effluent gas.

The effluent from the'posttreatment reactor containing liq-uids and gases can be passsed through a cooler and intro-duced into a gas-liquid separator where the hydrogen gas along with ammonia and hydrogen sulphide by-products from the hydrotreating reactions may be separated from the liq-ui.d product. The separated gases are usually recycled via a compressor back for reuse in the hydrogen stream. The recy-cled gas can. be passed through a scrubber to remove hydro-gen sulphide and amrnonia because of their inhibiting ef-fects on the kinetics; of hydrotreating and also to reduce corrosion in. the recycle circuit. As the present invention results in a higher content of purity in this recycle gas the amount of make-up; gas added to compensate for the by-drogen consumed in the hydrotreating process can be low-ered. Alternatively, the effluent gas can be utilized in other downstream proce~sses.

The present -inventionis illustrated in the following exam-5 ples of specific embodiments.

Facample 1 Feedstocks A and B(Table 1) were hydrotreated in a pilot plant consisting of two isothermal reactors in series.
Feedstock A is a strai;ght-run light gas oil (LGO) and Feed-stock B is a mixture df 70 wt% Feedstock A and 30 wt$ light cycle oil (L(M). The two reactors were loaded with the same volume of Ni-Mo/al-4imirta catalyst and the LHSV in each reac-tor was 1.0 h'', so that the overall LHSV was 0.5 h"1. 100%
hydrogen at a pressure of 50 bar was co-fed with the liquid stream. The tempetatuze of the first reactor was maintained at 3600C; the temperature of the second reactor at 400 C.
Product samples were ~aken of the liquid effluent from both the first and second reactor.
Table 1 ProDerties of feedstocks used__in the folloW~na exeunxpZes PrOperty Feedstock A Feedstock B
SG 0.8448 0.8738 S (wt %) 1.33 1.83 N (wt ppm) 108 237 H (wt %) 13.3 12.3 Aromatics (wt 16) Mono- 15.3 15.1 Di- 9.3 16.0 Tri- and higher 1.4 4,7 pistillation D2887 ( 0) 5 wt ~ 195 198 10 wt% 218 219 wtRs 267 262 50 wt$ 298 295 70 wt% 330 326 90 wt% 370 366 95 wt% 386 381 The properties of thetotal liquid products from reactor 1 are given in Table 2.

Table 2 Properties of nrbdudts_in Examnle .Z
Property Vroduct Al Product D1 S (wt %) 0.0013 0.0031 N (wt ppm) < 1 < 1 H (wt %) 14.1 13.4 Aromatics (wt Mono- 13.7 26.8 Di- 1.0 3.1 Tri- and higher 0.1 0.6 Products Al and B1 contain few polyaromatics as is typical at 360 C, which is considered a typical start-of-run (SOR) temperature. The saturation of polyaromatics to monoaromat-ics and of monoaromatxcs to naphthenes consumes a signifi-cant amount of hydrogen, which is reflected in the hydrogen content increase fromifaed to product.

Exaaqple 2 Product Al from Exampie 1 and the effluent gas are further processed in the second reactor at a temperature of 400 C
and a total pressure;of 50 bar. The results are shown in Table 3.

Table 3 Pronerties of praduct! in Examale 2 Property Product A2 S (wt ~k) ;0.0001 N (wt ppm) < 1 H (wt 13.9 Aromatics (wt rtono- 19.0 Di- 13.0 Tri- and higher 0.4 The further hydrotreating has lowered the amount of sulphur as compared with prodilct Al, but due to the shifted aromat-ics equilibrium at th6 high temperature used in reactor 2, the amount of ai-omatics has increased and the hydrogen con-tent decreased. It is. observed that not only is the total amount of ar=omatics higher in product A2 as compared with product Al, but also the ratio of polyaromatics to monoaro-matics has increased. The savings in hydrogen consumption by passing gas and l.iquid through reactor 2 is approxi-mately 20 Nm' H,/m' oi~ ' Example 3 Product B1 from Example 1 and the effluent gas are further processed in the second reactor at a temperature of 400 C
and a total pressure of 50 bars. The results are shown in Table 4.

Table 4 Prop2x-ties of Ar duct , in Exainpl e 3 Property Product 52 S (wt 10.0002 N (wt ppm) < 1 H (wt 13.2 Aromatics (wt %) Mono- 125.8 Di- .1 Tri- and higher 1.2 Also in this case it i.s evident that the shifted equilib-rium at the high;<temperature has increased the amount of aromatics, urhil.st the' sulphur content has decreased. The effect of adding a.sedond high-temperature reactor is a re-duction in hydrogen consumption of approximately 20 Nm' H=/m' oil.
Examle 4 To test the deactivation of the catalyst in the posttreat-ment reactor at high temperatures a separate test was con-ducted using feedstock A from Example 1. Feedstvck A was hydrotreated with a NiMo catalyst in a pilot plant unit at 340 C, 30 barg H, pressure, LHSV = 2.0 h'3, H,/oil = 250 Nm'/m' using 100% HZ. The properties of the liquid product from this test are given in Table 5. This liquid product (product A3) was subs4quently used as a feed in a test that simulated the proposed posttreatment reactor reaction con-ditions. The test was conducted with a CoMo catalyst at 4000C, 30 barg H, pre;ssure, H,/oil = 200 Nm'/m' using 100!t H,. The LHSV was set 4t 21 h-' for the first 455 runhours and then changed to '42 h'1. The 1iquid products from the ].atter test are denominated "Product A4". The properties of Product Ad are listed in Table 5.

Table 5 Prope=ies gf;12roducts in Examole 4 Property Produ'ct ' Product Product Product Run hour Run hour Run hour = 48 = 455 = 605 LHSV= 21 LHSV= 21 LHSV= 42 h_' h 1 h-i S (wt ~) 0.0163 N (wt ppm) 18 Aromatics wt A

Mono- 23.0 20.3 20.5 21,0 Di- 2.5 4.3 4.2 3.8 Tri- and 0.4 0.7 0.6 0.6 higher Distillation D2887 ( C) 5 wt % 189 186 187 1.88 wt% 2- 1~' 211 212 212 30 wt% 260 260 259 259 50 wtt 292 292 290 291 70 wt96 322 322 320 321 90 wt% 366 366 364 365 95 wt$ 382 382 380 381 In this example the effect of the posttreatment reactor is 10 primarily to convert monoaromatics to diaromatics and triaromatics. At the;higher LHSV's used in this example, the distillation curve of the total liquid products is very similar to that of the feed, meaning that the yield loss due to cracking reac~'ions is very small. Furthermore, the example demon.strates !that the deactivation rate over the course of the testi5 is negligible.

Claims (4)

1. A process for reducing content of sulphur compounds in a hydrocarbon feedstock, which process comprises the steps of (a) contacting, the said feedstock with a hydrogen-rich gas over a hydrotreating catalyst at conditions be-ing effective in hydrogenation of hydrogenable com-pounds being present in the feedstock, (b) heating the effluent consisting of hydrotreated feedstock, hydrogen sulphide and hydrogen, and (c) contacting the said effluent with a hydrotreating catalyst at conditions being effective in dehydro-genation Of aromatic hydrocarbons.

2. A process of claim 1, wherein the temperature in step (c) is between 5°C and 100°C higher than the outlet tem-perature from step (a).

3. A process of claim 1, wherein LHSV in step (c) is be-tween 2 and 20 times rhe LHSV in step (a).

4. A process of claim 1, where the feedstock is charac-terised by having a 50% boiling point between 200°C and 400°C.