PROCESS FOR THE CONVERSION OF HYDROCARBONACEOUS FEEDSTOCK

Abstract

A hydrocarbonaceous feedstock is converted in a process comprising the following steps: (a) the feedstock is contacted with hydrogen under hydrotreating conditions to yield hydrotreated product; (b) the hydrotreated product is subjected to a separation treatment to separate at least hydrogen from the hydrotreated product to obtain a liquid hydrotreated product stream; (c) at least a portion of the liquid hydrotreated product stream is subjected to a stripping treatment to separate a light product from the liquid hydrotreated product stream, leaving a heavy hydrotreated product; (d) the heavy hydrotreated product is separated under reduced pressure into a least one gaseous stripped fraction and at least one liquid stripped fraction in a separation zone, wherein at least a portion of a liquid stripped fraction is reheated and recycled to the separation zone, in which process at least a portion of the liquid stripped fraction is reheated by heat exchange between at least a portion of the liquid hydrotreated product stream and/or at least a portion of the heavy hydrotreated product.

Description

The present invention relates to a process for the conversion of a hydrocarbonaceous feedstock. In particular, it relates to a process for the conversion of a hydrocarbonaceous feedstock that comprises sulphur compounds.

Such processes are well known. An example of such a process is known from U.S. Pat. No. 4,822,480. This document describes a process for the catalytic hydrodesulphurisation of a hydrocarbonaceous feedstock in which the feedstock is contacted with hydrogen under hydrodesulphurisation conditions. Hydrogen and other gaseous components are separated from the hydrodesulphurisation product, and one or more liquid products are subjected to a stripping treatment. In the stripping section a portion of the bottom fraction is reboiled and recycled to be used as stripping gas.

When the stripping is done in the way according to the process of U.S. Pat. No. 4,822,480 the separation is capital and energy intensive, since the portion of the bottom fraction that is being reboiled is reheated in an expensive furnace, requiring additional fuel. This is carried out at the detriment of the efficiency of the process. Another example were the bottom fraction is being reboiled is U.S. Pat. No. 3,481,859.

The present invention has as an objective to remedy these deficiencies.

Accordingly, the present invention provides a process for the conversion of a hydrocarbonaceous feedstock comprising the following steps:

(a) the feedstock is contacted with hydrogen under hydrotreating conditions to yield hydrotreated product;
(b) the hydrotreated product is subjected to a separation treatment to separate at least hydrogen from the hydrotreated product to obtain a liquid hydrotreated product stream;
(c) at least a portion of the liquid hydrotreated product stream is subjected to a stripping treatment to separate a light product from the liquid hydrotreated product stream, leaving a heavy hydrotreated product;
(d) the heavy hydrotreated product is separated under reduced pressure into a least one gaseous stripped fraction and a liquid stripped fraction in a separation zone, wherein at least a portion of the liquid stripped fraction is reheated and recycled to the separation zone, in which process at least a portion of the liquid stripped fraction is reheated by heat exchange between at least a portion of the liquid hydrotreated product stream and/or at least a portion of the heavy hydrotreated product.

The process according to the invention has the advantage that heat of process streams is being used to reboil the heavy stripped bottoms from the fractionator, thereby saving on fuel. The optimum temperatures for the stripping treatment and subsequent fractionation makes that it is very advantageous to first use the available heat of the heavy hydrotreated product for reboiling before it is fractionated. This reverse use of the available heat provides a clear energy benefit and is to a certain extend self-regulating with respect to temperature requirements of the stripping treatment and fractionation.

The hydrocarbonaceous feedstock can be selected from a variety of hydrocarbon-containing streams and fractions. The hydrocarbonaceous feedstock can be crude oil, a straight run fraction of a crude oil, a fraction of after vacuum fractionation, a deasphalted oil, an oil originating from tar sands or shale oil. The conversion process can be a hydrofinishing process in which the oil is marginally changed, it may be a hydrocracking process in which the average chain length of the oil molecules is reduced, it may be a hydrodemetallisation process in which metal components are removed from the hydrocarbonaceous feedstock, it may be a hydrogenation process in which unsaturated hydrocarbons are hydrogenated saturated, it may be a hydrodewaxing process in which straight chain molecules are isomerised, or it may be a hydrodesulphurisation process in which sulphur compounds are removed from the feedstock. It has been found that the present process is particularly useful when the hydrocarbonaceous feedstock comprises sulphur compounds and the hydrotreating conditions comprise hydrodesulphurisation conditions. The process is very advantageous in the treatment of sulphur-containing feedstocks that contain so-called refractory sulphur compounds, i.e., dibenzothiophene compounds.

The hydrotreating conditions that can be applied in the process of the present invention are not critical and can be adjusted to the type of conversion to which the hydrocarbonaceous feedstock is being subjected. Generally, the hydrotreating conditions comprise a temperature ranging from 250 to 480° C., preferably from 320 to 400° C., a pressure from 10 to 150 bar, preferably 20 to 90 bar, and a weight hourly space velocity of from 0.1 to 10 hr⁻¹, preferably from 0.4 to 4 hr⁻¹. The skilled person will be able to adapt the conditions in accordance with the type of feedstock and the desired conversion.

The catalyst used in the present process can also be selected in accordance with the desired conversion. Suitable catalysts comprise at least one Group VB, VIB and/or VIII metal of the Periodic Table of the Elements on a suitable carrier. Examples of suitable metals include cobalt, nickel, molybdenum and tungsten, but also noble metals may be used such as palladium or platinum. Especially when the hydrocarbonaceous feedstock comprises sulphur, the catalyst suitably contains a carrier and at least one Group VIB and a Group VIII metal. Whereas these metals can be present in the form of their oxides, it is preferred to use the metals in the form of their sulphides. Since the catalyst may normally be produced in their oxidic form the catalysts may subsequently be subjected to a pre-sulphiding treatment which can be carried out ex situ, but is conducted preferably in-situ, in particular under circumstances that resemble the actual conversion.

The metals are suitably combined on a carrier. The carrier may be an amorphous refractory oxide, such as silica, alumina or silica alumina. Also other oxides, such as zirconia, titania or germania can be used. For hydrodewaxing processes, crystalline aluminosilicates, such as zeolite beta, ZSM-5, mordenite, ferrierite, ZSM-11, ZSM-12, ZSM-23 and other medium pore zeolites, can be used. When the hydrotreating conditions entail hydrocracking, the catalyst may advantageously comprise a different zeolite. Suitable zeolites are of the faujasite type, such as zeolite X or Y, in particular ultra-stable zeolite Y. Other, preferably large pore, zeolites are also possible. The zeolites are generally combined with an amorphous binder, such as alumina. The metals are suitably combined with the catalyst by impregnation, soaking, co-mulling, kneading or, additionally in the case of zeolites, by ion exchange. It is evident that the skilled person will know what catalysts are suitable and how such catalysts can be prepared.

After the contact of the feedstock with the catalyst and hydrogen a hydrotreated product is obtained. This product contains unconverted hydrogen, and may, depending on the hydrotreating conditions, contain hydrocarbons with the same or fewer carbon atoms than in the hydrocarbons that were present in the feedstock, and/or hydrogen sulphide, ammonia and other gaseous compounds. To remove these gaseous compounds, the hydrotreated product is subjected to a separation treatment. Hydrogen and optionally other gaseous products are separated from the hydrotreated product to yield a liquid hydrotreated product stream. Typically, the hydrotreated product will have a temperature ranging from 250 to 480° C., preferably from 320 to 400° C. and a pressure from 10 to 150 bar, preferably from 20 to 90 bar. The hydrotreated product is subjected to separation suitably by adjusting the pressure and/or temperature. One might reduce the temperature in a so-called hot high pressure separator, in which the pressure is substantially the same as the pressure at the hydrotreating conditions. Due to a lowering of temperature, suitably at least 75% wt of the hydrotreated product is liquid, and this liquid fraction is recovered as the liquid hydrotreated product stream that becomes available at a temperature ranging from 100 to 350° C., preferably from 150 to 280° C. Suitably, the temperature is lowered by 20 to 380° C. Alternatively, one may reduce both temperature and pressure. In the latter case, one has preferably first separated excess hydrogen from the hydrotreated product before the pressure is reduced to yield the liquid hydrotreated product stream.

In one embodiment, at least a portion of the liquid hydrotreated product stream is directly passed to a stripping zone to separate volatile hydrocarbons and impurities such as hydrogen sulphide and ammonia from the desired liquid hydrocarbons. The stripping treatment may be conducted with nitrogen, carbon dioxide or low-molecular weight hydrocarbons as stripping gas. However, it is preferred to carry out the stripping with steam. Typically, steam is readily available in refineries at different pressures and different temperatures. Moreover, stripping with steam has proven to be very efficient. It is convenient to conduct the stripping treatment at conditions that are most advantageous relative to the conditions at which the liquid hydrotreated stream becomes available or at which light hydrocarbons can be optimally stripped off. Suitably the stripping is conducted at a temperature ranging from 100 to 350° C., preferably from 130 to 240° C. and a pressure ranging from 1 to 50, preferably from 1.5 to 10 bar.

The heavy hydrotreated product that is obtained after the stripping treatment may contain some stripping gas, e.g., water, and, possibly, an undesirable amount of relatively light, e.g. naphtha, hydrocarbon components. This stripping gas and optionally these light hydrocarbon components are advantageously removed from the heavy hydrotreated product since the stripping gas may interfere with subsequent treatments of the hydrocarbons contained in the heavy hydrotreated product and the light hydrocarbon components may dilute the desired product. Conveniently, the heavy hydrotreated product is separated under reduced pressure into at least one gaseous stripped fraction and at least one liquid stripped fraction. The gaseous stripped fraction will typically comprise a small portion of the stripping gas and some of the light hydrocarbon products, the gaseous stripped fraction will comprise the majority of the stripping gas, e.g., steam, and the undesired light hydrocarbon compounds, whereas the liquid stripped fraction will have a reduced proportion of stripping gas and light hydrocarbon components. The separation may be carried out in a flash vessel wherein the heavy hydrotreated product is exposed to a reduced pressure. Although this separation method may be satisfactory for some applications, a more strict separation between various hydrocarbon fractions may be desired. Therefore, the separation is, preferably, effected in a fractionation column, in which the heavy hydrotreated product is exposed to a reduced pressure, whereby the more volatile components, including the stripping gas and optionally relatively light hydrocarbon components, will evaporate and can be withdrawn from the column as a gaseous stripped fraction. The heavier compounds will remain liquid and can be withdrawn as a liquid stripped fraction. The conditions in such a separation step, such as flashing vessel or fractionation column, include suitably a temperature of 80 to 300° C., preferably from 120 to 250° C., more preferably from 120 to 200° C. and even more preferably from 150° C. to 200° C. and a pressure ranging from 0.005 to 1 bar, preferably from 0.05 to 0.5 bar, more preferably from 0.15 to 0.5 bar.

One advantage of the process according to the invention resides in that a portion of the liquid stripped fraction is initially used for reboiling of at least a portion of the liquid stripped fraction before being fed to the fractionation flash zone. In that fractionator flash zone the reheated fraction acts as stripping medium, which promotes the vaporisation of light products in the heavy hydrotreated product. For further removal of the light products, the fractionator suitably comprises a stripping section below the flash zone.

Although the use of a fractionation column or flashing vessel has been illustrated, it is evident that also other separation methods, operating at reduced pressure, can be applied.

In the above-described first embodiment of the invention the heating of at least a portion of the liquid stripped fraction is being reboiled via heat exchange with at least a portion of the liquid stripped fraction before it is fed to the fractionator.

In another embodiment at least a portion of the liquid hydrotreated product stream, obtained after separation of e.g. hydrogen from the hydrotreated product, is first passed to a heat exchanger before being fed to the stripping treatment flash zone so that the heat exchange of said at least a portion of the liquid stripped fraction is conducted with said at least a portion of the liquid hydrotreated product stream, that has been recovered from the fractionator column. The advantage of such operation resides in that the heat exchange is conducted without the temperature drop over the stripping zone that takes place. When the liquid hydrotreated product stream has arrived at the stripping treatment flash zone it may be freed from gaseous light components. The heavy hydrotreated product thus obtained is passed to a fractionator as described above to yield a gaseous stripped fraction and a liquid stripped fraction. At least a portion of the liquid stripped fraction is reheated in the heat exchanger through which the liquid hydrotreated product stream is passed, and the thus heated vapour portion of the liquid stripped fraction is recycled to the fractionator stripping section. Hence, in this embodiment the heat exchange of said at least a portion of the liquid stripped fraction is conducted with said at least portion of the liquid hydrotreated product stream.

Whereas both embodiments above describe the heat exchange with either a portion of the liquid hydrotreated product stream or at least a portion of the heavy hydrotreated product, it is also within the scope of the present process when at least portions of the liquid hydrotreated product stream and of the heavy hydrotreated product are combined and used in the heat exchange of at least a portion of the liquid fraction, obtained from the fractionator column. Alternatively, the liquid stripped fraction may be subjected to heat exchange with at least a portion of the liquid hydrotreated product stream and at least a portion of the heavy hydrotreated product sequentially, in any order.

It is advantageous to reheat at least 90% wt of the liquid stripped fraction. It is even more preferred to reheat the entire liquid stripped fraction. In that way the reheated liquid stripped fraction is separated, so that light hydrocarbons that may have vaporised be recycled to the fractionation stripping section, whereas the heavier components in the liquid stripped fraction remain liquid and be separated from these more volatile compounds, and be recovered as products.

The temperature to which at least said portion of the liquid stripped fraction is reheated can be established by the skilled person, based on the nature and boiling range of the liquid fraction. Suitably, the at least portion of the liquid stripped fraction is reheated to a temperature of from 125 to 350° C., preferably from 150 to 220° C. The liquid stripped fraction is suitably recovered as middle distillate product. Suitable products include kerosene and diesel. The recovery of such products can be achieved by conventional means, such as fractionation.

The gaseous stripped fraction may contain stripping gas. It is therefore advantageous to remove such gas from the gaseous fraction. This may be accomplished by conventional means, such as distillation, membrane separation or any other conventional means. When the stripping gas is steam it is advantageous to cool the gaseous stripped fraction to condense the steam. This is an easy way to remove the steam from the gaseous stripped fraction. An additional advantage of this method resides in the option to condense also the heavier hydrocarbon fraction that is being entrained with the stripping gas. This is typically a naphtha fraction. The naphtha fraction may be at least partly recovered as product and/or partly recycled to the fractionator as reflux.

FIG. 1 shows a simplified flow scheme of an embodiment of the present invention.

FIG. 2 shows an alternative embodiment of the present process.

FIG. 1 shows a line 1 via which a hydrocarbonaceous feedstock is passed through a heat exchanger 2 and to which a hydrogen containing gas is added via a line 3. The combined feedstock and hydrogen-containing gas is passed through a furnace 4 and the heated feedstock is passed via a line 5 to a hydrotreating reactor 6. The reactor can be provided with one or more catalyst beds. When the reactor contains two or more catalyst beds, quench gas or a quench liquid may be introduced between the catalyst beds. The hydrotreated product is withdrawn from the reactor 6 via a line 7 and passed through the heat exchanger 2 to pre-heat the hydrocarbonaceous feedstock in line 1. The hydrotreated product is forwarded to a separator 8 in which gaseous products such as hydrogen, hydrogen sulphide and ammonia, are removed and withdrawn via a line 9, and wherein a liquid hydrotreated product stream is obtained via a line 10. The gaseous products may be separated in a gas treating section to remove hydrogen sulphide, ammonia and any gaseous hydrocarbons that may have been formed. The thus purified hydrogen can suitably be recycled to the hydrotreating reactor, e.g., via combining it with the hydrogen-containing gas in the line 3.

Into the stripping column 11 a stripping gas such as steam is fed via a line 12. The stripping gas will entrain gaseous contaminants such as hydrogen sulphide and ammonia, as well as light hydrocarbons, such as hydrocarbons with 1 to 6 carbon atoms. These gaseous components are withdrawn via a line 13, cooled and passed to a low-pressure separator 14 in which the steam is condensed so that water, optionally with dissolved contaminants such as hydrogen sulphide and ammonia, is discharged via a line 16. Remaining gaseous components are withdrawn via a line 17. To the extent that these components include hydrocarbons these may be used as fuel. From the separator 14 also hydrocarbons are liquefied. These are withdrawn via a line 15. The hydrocarbons typically comprise naphtha. According to the embodiment of the figure they are recycled to the stripping column 11. It is also possible to at least partly recover the liquefied hydrocarbons as product.

From the stripping column 11 a heavy hydrotreated product is obtained and is removed therefrom via a line 18. The heavy hydrotreated product is passed through a heat exchanger 19 and subsequently passed to a fractionation column 20. In the fractionation column 20, which operates at a lower pressure than the one prevailing in line 18, the heavy hydrotreated product is separated in a gaseous stripped fraction that leaves the fractionation column 20 via a line 21 and a liquid stripped fraction that leaves the fractionation column 20 via a line 22. The gaseous stripped fraction in the line 21 can be recovered as product. It may also be further treated and/or purified. A suitable treatment includes a cooling of the gaseous stripped fraction to condense part of the gaseous stripped fraction. The condensate may comprise water, when steam is used as stripping gas, and relatively light hydrocarbon components. These can be separated, and water may be removed from the system. The relatively light hydrocarbon components, e.g. naphtha, can be at least partly recovered as product or partly recycled to the fractionation column as reflux.

The liquid stripped fraction in the line 22 is split into a product that is recovered via a line 23 and a portion in a line 24 that is subjected to heat exchange with the heavy hydrotreated product in the heat exchanger 19. The heated portion is recycled through the line 24 to the fractionation column 20 wherein it acts as stripping medium. Therefore, the fractionation column has been provided with a stripping section in its lower part to facilitate separation of light compounds from the heavy stripped liquid. Alternatively, the stripped fraction in line 22 can be subjected to heat exchange in its entirety, after which the heated stripped fraction is split into a fraction that is recovered as product and another fraction that is recycled to the fractionation column.

FIG. 2 shows a process wherein a hydrocarbonaceous feedstock in a line 101 is pre-heated in a heat exchanger 102 and, after addition of a hydrogen-containing gas via a line 103, is further heated in a furnace 104. The heated feedstock is subjected to hydrotreating conditions in a reactor 106. Hydrotreated product is withdrawn from the reactor 106 via a line 107 and passed via the heat exchanger 102 to a high-pressure separator 108a in which gaseous products, especially hydrogen, hydrogen sulphide and ammonia, are discharged via a line 109 and an intermediate liquid is passed via a line 110a to a low-pressure separator 108b. From separator 108b a gaseous fraction, comprising primarily light hydrocarbons, is discharged via a line 109b. Liquid hydrotreated product stream leaves the separator 108b via a line 110, and is passed to a heat exchanger 119, and subsequently to a stripping column 111. The light hydrocarbons in the line 109b are combined with the liquid hydrotreated product in line 110 before the liquid hydrotreated product enters the stripping column 111. Alternatively, separator 108b is omitted and the recovered hydrotreated liquid fraction from separator 108a is passed to heat exchanger 119, and subsequently to stripping column 111. Stripping gas is fed into the column 111 via a line 112. Gaseous compounds are discharged via a line 113, cooled and passed to a gas-liquid separator 114. From this separator 114 hydrocarbons are withdrawn via a line 115 and recycled to the stripping column 111, water with dissolved contaminants is withdrawn via a line 116, and remaining gaseous components are withdrawn via a line 117.

Heavy hydrotreated product is obtained in stripping column 111, is removed via a line 118, and passed to a fractionation column 120. Therein a gaseous stripped fraction is recovered via a line 121. A liquid stripped fraction is removed via a line 122. The liquid stripped fraction is split into a product that is being recovered via a line 123 and a portion that is subjected to heat exchange in the heat exchanger 119 and recycled to the fractionation column 120. Alternatively, the entire liquid stripped fraction may be fed into the heat exchanger 119, so that all volatile components in the fraction can be vaporised. These vaporised components are recycled to the fractionation column 120 via a line 124. The remaining liquid stripped fraction may then be withdrawn from the heat exchanger 119 via a line 123 and recovered as product.

It will be appreciated that the Figures do not show auxiliary equipment that is usually present, such as valves, pumps, compressors, expanders, control equipment etc. The skilled person will understand where this auxiliary equipment is desired.

Claims

1. A process for the conversion of a hydrocarbonaceous feedstock comprising the following steps:

(a) the feedstock is contacted with hydrogen under hydrotreating conditions to yield hydrotreated product;

(b) the hydrotreated product is subjected to a separation treatment to separate at least hydrogen from the hydrotreated product to obtain a liquid hydrotreated product stream;

(c) at least a portion of the liquid hydrotreated product stream is subjected to a stripping treatment to separate a light product from the liquid hydrotreated product stream, leaving a heavy hydrotreated product;

(d) the heavy hydrotreated product is separated under reduced pressure into a least one gaseous stripped fraction and at least one liquid stripped fraction in a separation zone, wherein at least a portion of a liquid stripped fraction is reheated and recycled to the separation zone,

in which process at least a portion of the liquid stripped fraction is reheated by heat exchange between at least a portion of the liquid hydrotreated product stream and/or at least a portion of the heavy hydrotreated product.

2. A process according to claim 1, in which the hydrocarbonaceous feedstock comprises sulphur compounds and the hydrotreating conditions comprise hydrodesulphurisation conditions.

3. A process according to claim 1, in which the hydrotreating conditions comprise a temperature ranging from 250 to 480° C., a pressure from 10 to 150 bar, and a weight hourly space velocity of from 0.1 to 10 hr−1.

4. A process according to claim 1, in which the catalyst comprises a carrier and at least one Group VIB and a Group VIII metal.

5. A process according to claim 1, in which the stripping is carried out with steam.

6. A process according to claim 1, in which the stripping is conducted at a temperature ranging from 100 to 350° C. and a pressure ranging from 1 to 50 bar.

7. A process according to claim 1, in which the separation zone in step (d) comprises a stripping section.

8. A process according to claim 7, in which the separation zone of step (d) comprises a rectifying section, in addition to a stripping section.

9. A process according to claim 1, in which the heat exchange with said at least portion of the liquid stripped fraction is conducted with said at least portion of the liquid hydrotreated product stream.

10. A process according to claim 1, in which the heat exchange with said at least portion of the liquid stripped fraction is conducted with said at least portion of the heavy hydrotreated product.

11. A process according to claim 1, in which at least 90% wt of the liquid stripped fraction is reheated by heat exchange.

12. A process according to claim 1, in which part of the liquid stripped fraction is recovered as middle distillate product.

13. A process according to claim 1, in which the gaseous stripped fraction is separated into at least a naphtha fraction and a water fraction.

14. A process according to claim 1, in which the separation zone is a fractionation column wherein the conditions are a temperature of 80 to 300° C. and a pressure ranging from 0.005 to 1 bar.

15. A process according to claim 1, in which said at least portion of the liquid stripped fraction is reheated to a temperature of from 125 to 350° C.