Patent Application
20210324275
The present invention relates to the field of separation of hydrocarbons and similar compounds by fractionation according to boiling point.
Separation of hydrocarbons and similar compounds by boiling point is a common process in refineries and petrochemical processes. An increased specificity of the separation is often related to an increased yield or value of products.
Separation in a distillation column may be aided by reboiling of the heavy fraction, but this may be problematic for some feeds, which may react or decompose in the fractionation column, with yield loss and/or precipitation and fouling of the equipment as consequences.
It is common to aid the separation by the addition of a stripping medium, which enhances fractionation by lowering the partial pressure of the light components, which thus are more completely vaporized.
Typical stripping media are steam and hydrogen, but also fuel gas (methane, ethane and possibly propane) have been used. The stripping medium will be separated with the lightest fraction and will typically end up as a waste stream, which is either combusted or released otherwise, and thus the use of a stripping medium may be costly. Therefore, the amount of stripping medium is moderated in consideration of the balance between additional purity or yield and the cost of stripping medium. In addition, the stripping medium may be problematic in itself; steam may cause corrosion challenges or water condensation and some stripping media such as fuel gas may be dissolved in the product with the consequence of requiring a later clean-up of product.
The present invention seeks to improve the separation quality, without increasing the cost of using a stripping medium during separation of hydrocarbons by recycling of the stripping medium.
According to the present disclosure a non-condensing stripping medium such as elemental nitrogen, methane or fuel gas is directed to contact a hydrocarbon mixture.
A method for separation may either be a fractionation process having several outlets based on boiling point or it may be a simpler stripping process wherein only a single liquid outlet and a single gaseous outlet are used.
A stripping medium shall in the following be construed as a light component directed to support a separation process.
Fractionation shall in the following be construed as a separation process of molecules according to boiling point, by distillation.
In the following the abbreviation wt/wt % shall be used to signify weight percentage.
In the following the abbreviation vol/vol % shall be used to signify volume percentage for a gas.
Where pressures for means of separation such as distillation columns are discussed in the following, the pressure is in accordance with the terminology of the art determined at the top of the column, i.e. typically the lowest pressure of the column.
A broad aspect of the present disclosure relates to a process for separation of a hydrocarbonaceous liquid phase from a gas phase comprising the steps of
In a further aspect the process further comprises the steps of
In a further aspect the operating pressure of said means of separation is from atmospheric pressure to 2 barg and said pressurizing of said recycled stripping medium involves increasing the pressure by 0.1 bar to 2 bar, with the associated benefit of designing to process for such a moderate pressurization of the recycled stripping medium being a low capital and operation cost of the means of pressurization, and specifically allowing the option of using a blower technology.
In a further aspect the operating pressure of said means of separation is from 0 mbar absolute to 200 or 500 mbar absolute and said pressurizing of said recycled stripping medium involves increasing the pressure by between 5 mbar and 50 or 200 mbar, with the associated benefit of designing to process for such a moderate pressurization of the recycled stripping medium being a low capital and operation cost of the means of pressurization, and specifically allowing the option of using a blower technology.
In a further aspect less than 5 wt/wt % of said feed is non-condensable at the boiling point of said stripping medium, with the associated benefit of the stripping medium being simple to separate from the lightest part of feed prior to recycle.
In a further aspect at least 95 wt/wt % or 99 wt/wt % of said feed is withdrawn from said means of separation in liquid form, with the associated benefit of the combination of non-condensable stripping medium and liquid product fractions being that an energy efficient separation of stripping medium from products is possible, allowing recycle of the stripping medium without excessive cooling or heating.
In a further aspect at least 90 vol/vol % of said stripping medium is non-condensable at 20° C. and 1 atmosphere, such as gases from the group comprising N2, H2, He, Ar, Ne, CO2, CH4 and C2H6, with the associated benefit of these constituents being gaseous at relevant process conditions, and typically being compatible with product specifications and being compatible with material specifications, including corrosion stability. Specifically, H2, CO2, CH4 and C2H6 are available in the process, N2 is available at low cost and easily separated from other streams, noble gases such as He, Ar and Ne are highly inert, CH4 and C2H6 may be taken as side streams from relevant process steps and CO2—or a mixture of CO2 and N2 may be obtained from the dried flue gas of a fired heater.
In a further aspect the feed has an initial boiling point of at least 100° C., 200° C. or 300° C., with the associated benefit of a process for separation of heavy feed being especially suitable for being operated with recycle of stripping medium as the separation of stripping medium will be simpler, the positive effect on separation higher and the benefits from avoiding thermal cracking in the absence of reboiling of a fraction of the feed will also be higher.
In a further aspect the 95% boiling point of said feed is 400° C., 500° C. or 600° C. with the associated benefit of a process being able to separate a heavy feed while avoiding a very heavy bottoms product.
In a further aspect the ratio between the amount of stripping medium and the amount of feed directed to the means of separation is from 10 NL/kg, 40 NL/kg or 100 NL/kg to 200 NL/kg, 400 NL/kg or 1000 NL/kg, with the associated benefit of providing a balance between a low cost at low ratios and a high separation efficiency at higher ratios.
In a further aspect wherein the stripping medium comprises at least an amount of a gas originating from the gas source supplying blanketing gas for product tanks, with the associated benefit of provision of blanketing gas already being enabled on the premises of a refinery, and the requirements to e.g. inertness of blanketing gas being similar to the requirements for a stripper gas.
In a further aspect from 1%, 2% or 5% to 10% of said recycled stripping medium is withdrawn as a purge with the associated benefit of such a purge removing oxygen and other undesired impurities from the process, such that the level of impurities is kept below critical limits.
In a further aspect said recycled stripping medium is heated by heat exchange, with the associated benefit of such heating being an increased efficiency of separation of dissolved light components.
A further aspect of the present disclosure relates to a process for production of a product boiling in the diesel range comprising the steps of
A further aspect of the present disclosure relates to a fractionation section comprising a means of separation having a feed inlet, a means of separation stripping medium inlet, one or more product outlets and a vapor outlet, and a means of pressurization having an inlet and an outlet characterized in said vapor outlet being in fluid communication with the inlet of said means of pressurization, and said outlet of said means of pressurization being in fluid communication with said means of separation stripping medium inlet.
with the associated benefit of such a fractionation section being able to operate with recycle of the stripping medium, and thus highly efficient in separation at a moderate operational cost, in comparison with a fractionation section not recycling the stripping medium.
In a further aspect the fractionation section further comprises a side column, having a side column feed inlet, a side column stripping medium inlet, a side column vapor outlet and a side column liquid outlet, wherein said side column stripping medium inlet is in fluid communication with the outlet of said means of pressurization, with the associated benefit of such a fractionation section being well suited for the side columns operating with a high amount of stripping medium, resulting in an increased separation efficiency.
In a further aspect the fractionation section, further comprises a bottoms stripper having a bottoms stripper stripping medium inlet, a stripper vapor outlet, a bottoms stream inlet and a stripped bottoms outlet, and where said means of separation further has a bottoms outlet, and wherein said bottoms stream inlet is configured for being in fluid communication with said bottoms outlet, optionally via a means of heating, wherein said bottoms stripper stripping medium inlet is configured for being in fluid communication with said outlet of said means of pressurization optionally via a means of heating, and wherein said stripper vapor outlet is in fluid communication with said means of separation stripping medium inlet, with the associated benefit of such a fractionation section being suited for minimizing the amount of purge necessary from a process producing HPNA by increasing the separation efficiency of the bottoms stripper.
A further aspect of the present disclosure relates to a hydrocracker section comprising a hydrocracking reactor having an inlet and an outlet and a fractionation section comprising a means of separation having a feed inlet, a make up stripping medium inlet, one or more product outlets and a vapor outlet, and a means of pressurization having an inlet and an outlet characterized in said hydrocracker section being configured for directing an amount of product from the hydrocracking reactor outlet to the means of separation feed inlet vapor outlet being in fluid communication with the inlet of said means of pressurization, and said outlet of said means of pressurization being in fluid communication with said stripping medium inlet, with the associated benefit of such a process providing a highly effective separation of diesel from heavy distillate, thus allowing an increased yield of valuable product boiling in the diesel range.
In refinery operation fractional distillation or fractionation, i.e. processes separation according to boiling point, is a key unit operation. The crude oil comprises many chemical components, having a wide boiling range, e.g. from 40° C. to more than 600° C. Fractional distillation processes are carried out on the crude oil, to provide the typical fuel fractions e.g. naphtha, kerosene, diesel, lubricant and bunker fuel. Fractional distillation is also carried out on a wide range of intermediate products, e.g. where an intermediate fraction with a homogeneous boiling range has been treated in a chemical process, such that products with different boiling point are produced. Typically, the chemical processes involve hydroprocessing, in which crude oil reacts with hydrogen, in the presence of catalysts.
Hydroprocessing may be in the form of hydrotreatment, which maintain the structure of the crude oil hydrocarbons, but release light components, such as NH3, H2O and H2S. Hydroprocessing may also be in the form of hydrocracking, in which the structure of hydrocarbons is broken down to form smaller compounds.
The separation of products is typically carried out in a fractional distillation process, where a means of separation, typically a distillation column with multiple trays and multiple outlets is used. The temperature decreases from the inlet towards the top of the distillation column, and from each outlet a fraction can be withdrawn, which comprises the condensed product. In a side column, this stream is divided in a liquid stream and a gaseous stream. The gaseous stream is returned to the main column. From such a column each outlet will provide a stream boiling in a defined range, but the separation will be imperfect with overlapping fractions.
One reason for the imperfect separation is that an amount of light products is dissolved in the liquid. Therefore, the main column and the side columns are often equipped with stripping medium streams, which can aid the separation, by the principle of decreasing the vapor pressure of the light products above the liquid.
Separation, especially in the case of simple gas/liquid separations, may be carried out at the process pressure, which typically is elevated. This is convenient especially where the products are to be further treated at elevated pressure. However, since the boiling point of a component is dependent on the pressure, separation may be more efficient at low pressure. The separation is often carried out slightly above atmospheric pressure, e.g. at 0.1 barg to 3 barg (where barg means bar gauge, e.g. pressure relative to atmospheric pressure), which involves the simplest equipment. Such separation is called atmospheric separation. Even more efficient separation may be carried out at reduced pressure, e.g. 0 bar to 0.2 or 0.5 bar (absolute pressure), which may also have the benefit of avoiding excessive heating to temperatures where the components are unstable. Such separation is called vacuum separation.
Stripping medium may beneficially be used to aid separation at all pressures, but the physical equipment to be used will differ, depending on the pressure of operation.
To increase the separation efficiency, especially for the highest boiling constituents, the heaviest bottom fraction may also be reboiled, i.e. directed to be heated, such that at least an amount of the bottom fraction is evaporated, and returned to the column. This improves separation, but during reboiling the high temperatures may lead to thermal cracking, and thus reduce the total recuperation of hydrocarbons and cause fouling of equipment leading to more frequent interruption of operation to clean the affected items.
The economy of a separation process depends on the balance between operational expense for the process and the value of the products. The products must adhere to standards, and thus a poor separation must be compensated by a restrictive limitation of boiling point. However, an increased yield of more valuable product may be obtained by improved separation, and therefore it may be beneficial to accept increased operational expense to gain product value. The operational expenses related to separation may also be balanced against the operational expense of downstream processes.
Use of stripping medium is one example of added operational expense which provides a net economic benefit. The typical stripping medium is steam, and the production, and thus production and use of steam is related to a consumption of energy, which has a cost. However, when a separation is aided by steam as stripping medium, the separation efficiency may be increased, such that each fraction comprises a high amount of components well suited for the fraction. After stripping, the temperature will be low and steam is collected as liquid water, which must be heated and vaporized to be used as stripping medium again.
If the amount of stripping steam is increased, the separation efficiency may be further increased, but the balance between cost and gain will reach a maximum at some level, beyond which the cost of steam and equipment size exceeds the value of improved separation. In addition, the use of steam increases the requirements to corrosion resistance of materials and causes a need for removal of water from the products.
Now, according to the present disclosure, it is proposed to use a stripping medium being gaseous at standard temperature, such as N2, and furthermore to recirculate the stripping medium around the separation process, which is not possible when the stripping medium is steam, since the used steam is condensed as liquid water. The use of a gaseous stripping medium has the benefit that the separation of stripping medium from the light feed fraction being uncomplicated, especially if the boiling points of the stripping medium and the light feed fraction are remote from each other. The use of recirculation has the benefit that the cost of providing stripping medium is reduced, since only a minor amount released to the surroundings or dissolved in the separated feed fractions must be replenished. As the cost of stripping medium is reduced, the balance between the cost of stripping medium and the value of separation efficiency will be shifted towards a higher separation efficiency, and thus less requirement for distancing the fractionation cut point from the classification limits of products.
The requirements to the feed and the stripping medium for use in a process where the stripping medium is recycled include, as mentioned, that the boiling point is remote from that of the lightest feed, such that the product is at least 95 wt/wt % condensing, while the stripping medium is non-condensing in the fractionation section, which typically includes an overhead drum having a lower temperature than the column. The lowest temperature is typically in the overhead drum, which typically is 20-120° C. depending on environmental conditions. This means that for atmospheric fractionation it is preferred that the product does not include significant amounts of fuel gas, e.g. methane, ethane and propane. For a vacuum fractionation, even butane and pentane may be undesired in the product. The criterion that the stripping medium is non-condensing allows at least N2, H2, He, Ar, Ne, CO2, CH4 and C2H6 as stripping media. It is also beneficial that the stripping medium is inert and has a low solubility in the hydrocarbon products, to avoid a loss of stripping medium and a product in accordance with the specifications, which makes N2 a preferred stripping medium. Another benefit of N2 is that it is typically available in refineries in pure form, for the purpose of blanketing storage tanks.
While the concept is described above as it relates to an atmospheric fractionator, a similar concept may also be used for vacuum fractionators. For a vacuum fractionator, specific considerations must be made. It is in general important that the equipment is airtight when operating under vacuum, but when a gaseous stripping medium is recirculated, this becomes even more important, as leaks may introduce oxygen, which with recirculation may reach dangerous levels. Therefore, purge, scavenging with a solid or a liquid scavenger or selective conversion of oxygen may preferentially be included when stripping media is recirculated with a vacuum fractionator.
The present disclosure is also relevant for hydroprocessing, especially a hydrocracking process, in which a heavy feedstock is hydrocracked and directed to a fractionation process employing a recycled stripping medium. A hydrocracking process is especially well suited for being combined with such a fractionation process, as a cascade of vapor/liquid separators will ensure that the feed for separation contains little or no non-condensing product, such that purification of the stripping liquid is avoided or minimized. Hydrotreatment processes involving heavy products may also benefit from such a configuration, especially if the amount of light products is minimal.
In a further aspect a purge of stripping medium may be withdrawn with the objective of avoiding concentrating impurities. The impurities may gaseous product hydrocarbons or oxygen from equipment leaks. The purge stream may be directed to an absorbent or a reactor for removing the impurities. Removal of oxygen may be carried out by catalytic oxidation of hydrocarbons or hydrogen or liquid or solid scavenging and the gaseous products may be collected e.g. in an amine wash. Gaseous hydrocarbons may be directed to other means of separation.
The efficiency of separation according to the present disclosure and the prior art has been evaluated by studying the fractionation of the products of a hydrocracking process.
In Example 1 the product of a hydrocracking process is directed to a fractionation according to the present disclosure, as described in
In Table 1 it is seen that all products fulfill the specifications for the respective product types.
In Example 2 the product of a hydrocracking process is directed to a fractionation according to the prior art, as described in
The performance of the fractionation process is shown in Table 2, which also show the product fractions. The product qualities are similar to those of Example 1.
The product yields of Example 1 and Example 2 are compared in Table 3. It is seen that an extra yield of valuable products like kerosene and diesel is obtained and the yield of less valuable products like naphtha and unconverted oil (UCO) is reduced.
In addition to the increased yield, the use of recycled nitrogen as stripping medium in Example 1 will furthermore have the benefit of having a lower operational expense, compared to the required amount of steam in Example 2.
The examples thus document that recycled nitrogen provides an improved separation and thus a reduced yield loss while also being a cost effective alternative to steam.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2018 00763 | Oct 2018 | DK | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/078735 | 10/22/2019 | WO | 00 |
