Separation of mixed hydrocarbon stream comprising n-butenes using divided wall distillation column, preparation of n-butenes, and methods of separation thereof

Abstract

Disclosed is a process for separating a mixed hydrocarbon stream comprising 1-butene, 2-butene, heavier olefins, the process comprising introducing the mixed hydrocarbon stream into a feed section of a divided wall distillation column. The divided wall distillation column has a housing defining a middle vapor-liquid contact region, an upper vapor-liquid contact region, and a lower vapor-liquid contact region. The middle vapor-liquid contact region contains at least one vertically oriented partition that divides the middle vapor-liquid contact region into at least a feed section defined by the housing and the partition and a side draw section defined by the housing and the partition. Thermal energy is provided to the divided wall distillation column via a bottom reboiler. Additional thermal energy is provided to the divided wall distillation column by withdrawing a liquid fraction from the divided wall distillation column via side draw, heating and partially evaporating the liquid fraction in a side draw evaporator, and directing the heated, partially evaporated fraction back to the divided wall distillation column. The process maximizes the proportion of thermal energy provided at relatively low temperature levels while still minimizing the total energy demand of the process. The method further allows for a reduction in the number of trays in the feed section of the middle vapor-liquid contact zone with all other variables constant.

Description

[0001] The present invention relates to a method for separating a mixed hydrocarbon stream containing n-butene and a method for producing n-butene. Butene is a group of four isomeric substances: 1-butene, cis-2-butene, trans-2-butene, and isobutene. 1-butene and the two 2-butenes belong to the group of linear butenes, while isobutene is a branched olefin. Linear C4-olefins, namely, 1-butene, cis-2-butene, and trans-2-butene, are also collectively referred to as "n-butene". Butene is currently mainly obtained on an industrial scale by the fractionation of C4 streams. C4 streams are mixtures of different hydrocarbons having four carbon atoms, which are produced as by-products in the production of ethylene and propylene in mineral oil crackers.

[0002] In the future, changes in feedstock conditions and the reorganization of refining processes may lead to a shortage of mineral oil-derived C4 streams. An alternative source of butene is the dimerization of ethylene. Ethylene can be derived from the fermentation or pyrolytic conversion of renewable feedstocks.

[0003] 2-Butene can undergo metathesis with ethylene to produce propylene. This reaction opens up a production route for propylene that is independent of mineral oil.

[0004] The n-butene obtained during the dimerization of ethylene is a mixed stream containing 1-butene and 2-butene. 1-Butene and 2-butene may have significantly different reactivities in several downstream reactions. For example, essentially only 2-butene reacts in the metathesis reaction, while 1-butene is essentially inert. The dimerization of ethylene is accompanied by the formation of heavier olefins. Therefore, the effluent from the dimerization of ethylene is separated by distillation. The conventional distillation of the dimerization mixture can be carried out with two distillation columns. In the first column, the light components containing ethylene, 1-butene, and 2-butene are separated as the overhead distillate, and the heavy components containing pentene, hexene, and octene are recovered from the bottom of the first distillation column. The light components are further distilled in the second column to separate the 1-butene stream (which may contain unreacted ethylene) from the 2-butene stream.

[0005] US2012 / 095275 discloses a method for producing propylene and 1-butene. The method includes dimerizing ethylene in the presence of a dimerization catalyst to produce a dimerization mixture containing 1-butene and 2-butene. The dimerization mixture is distilled to produce a 1-butene stream containing 1-butene and ethylene, a 2-butene stream, and a heavy stream. The 2-butene stream is reacted with ethylene in the presence of a metathesis catalyst to produce a metathesis mixture containing propylene, ethylene, and 2-butene. US2012 / 095275 describes that the distillation of the dimerization mixture can be carried out in a single-column distillation or a dividing-wall column.

[0006] During the distillation process, the heat demand in the sump of the column generally has to meet three requirements, namely, heating the liquid to the sump temperature, supplying the energy for separating the feed components, and generating stripping vapor. The dividing-wall column offers the advantages of both lower investment costs and reduced energy demand during operation. However, since the dividing-wall column is a single column with only one common sump (different from a two-column separation sequence), the energy must be input at a temperature level corresponding to the higher of the two sump temperatures in a conventional two-column separation sequence. This is due to the fact that in the single-column design of the dividing-wall column, the high boilers are always present in the sump, resulting in a high boiling point temperature of the bottom liquid located in the sump.

[0007] The technical problem on which the present invention is based is to provide a method for separating 1-butene and 2-butene using a dividing-wall column, which maximizes the proportion of thermal energy provided at a relatively low temperature level while still minimizing the total energy demand of the method. In addition, a dividing-wall column with the lowest possible number of theoretical stages should be used.

[0008] This problem is solved by operating a dividing-wall column with a sidedraw evaporator.

[0009] This problem is solved by a method for separating a mixed hydrocarbon stream containing 1-butene, 2-butene, heavier olefins, and unreacted ethylene if present, the method comprising:

[0010] introducing the mixed hydrocarbon stream into the feed section of a dividing-wall distillation column having a shell defining a middle vapor-liquid contact zone, an upper vapor-liquid contact zone above and in communication with the middle vapor-liquid contact zone, and a lower vapor-liquid contact zone below and in communication with the middle vapor-liquid contact zone,

[0011] the middle vapor-liquid contact zone containing at least one vertically oriented dividing wall that divides the middle vapor-liquid contact zone into at least a feed section defined by the shell and the dividing wall and a sidedraw section defined by the shell and the dividing wall,

[0012] supplying thermal energy to the dividing-wall distillation column via a bottom reboiler in communication with the lower vapor-liquid contact zone, at least partially condensing the vapor escaping from the upper vapor-liquid contact zone via a condenser in communication with the upper vapor-liquid contact zone, and recycling at least a portion of the condensate as reflux liquid to the upper vapor-liquid contact zone,

[0013] withdrawing 2-butene as a sidedraw from the sidedraw section, withdrawing 1-butene and unreacted ethylene if present from the upper vapor-liquid contact zone, and withdrawing heavier olefins from the lower vapor-liquid contact zone; and

[0014] Additional thermal energy is provided to the dividing wall distillation column by withdrawing a liquid fraction from the dividing wall distillation column via a side draw, heating and partially evaporating the liquid fraction in a side draw evaporator, and directing the heated, partially evaporated fraction back to the dividing wall distillation column.

[0015] The present invention maximizes the proportion of thermal energy provided at a relatively low temperature level while still minimizing the total energy requirement of the process. With all other variables held constant, the present invention further allows for a reduction in the number of trays in the feed section of the middle vapor-liquid contact zone.

[0016] As used herein, the term "heavier olefins" means olefins having more than four carbon atoms. For example, the mixed hydrocarbon stream contains 4% to 10% by weight, preferably 5% to 8% by weight of 1-butene; 60% to 95% by weight, preferably 75% to 90% by weight of 2-butene; 1% to 27% by weight, preferably 2% to 15% by weight of heavier olefins and 0% to 3% by weight, preferably 0% to 2% by weight of ethylene. The heavier olefins mainly include olefins having an even number of carbon atoms, especially hexene. The mixed hydrocarbon stream may contain smaller concentrations of olefins having an odd number of carbon atoms, such as propylene and pentene.

[0017] The mixed hydrocarbon stream can be the effluent stream of an ethylene dimerization process.

[0018] Accordingly, the present invention also relates to a process for preparing n-butene, the process comprising:

[0019] introducing an ethylene stream into a dimerization zone containing a dimerization catalyst and contacting the ethylene stream with the dimerization catalyst, operating the dimerization zone under conditions effective to produce a mixed hydrocarbon stream comprising 1-butene, 2-butene, heavier olefins and, if present, unreacted ethylene, and separating the mixed hydrocarbon stream by the above-described method.

[0020] The dividing wall distillation column has at least one inlet port. The inlet port is for introducing the mixed hydrocarbon stream into the feed section of the dividing wall distillation column.

[0021] When a dividing wall distillation column is used, a 2-butene stream is withdrawn from the dividing wall distillation column as a side draw from the side draw section. The dividing wall distillation column also produces an overhead distillate stream comprising 1-butene and any unreacted ethylene. The dividing wall distillation column also produces a bottoms stream comprising heavier olefins.

[0022] A dividing wall distillation column is generally a vertically oriented cylindrical column having a shell with a cylindrical inner surface. As used herein, the phrase "vertically oriented" means forming an angle with the horizontal plane generally between about 85° and about 95°, and preferably between about 87.5° and 92.5°. The dividing wall distillation column has three distillation vapor-liquid contacting zones - an upper zone, a middle zone, and a lower zone. The middle vapor-liquid contacting zone contains at least one dividing wall or partition which is a generally vertically oriented plane. Generally, similar to the longitudinal axes of the upper and lower vapor-liquid contacting zones, the longitudinal axis of the middle vapor-liquid contacting zone is also vertically oriented. The dividing wall divides the middle vapor-liquid contacting zone into two segments, i.e., a feed side segment and a side draw side segment. In the case of a single dividing wall and neglecting the area occupied by the thickness of the dividing wall and the thickness of the column wall, the area of any horizontal cross-section of the middle region of the column is divided between the feed side segment and the side draw side segment. The division of the horizontal cross-section of the column between the two segments need not be equal. The division depends in part on the composition of the feed stream entering the middle vapor-liquid contacting zone and the proportion of the feed in the vapor phase. The area of the feed side segment can be about 30% to about 60% of the area of any horizontal cross-section. The area of the side draw side segment is generally about 40% to about 70% of the area of any horizontal cross-section.

[0023] Each dividing or partition wall is typically a baffle, which is preferably imperforate. Each partition wall can be a single piece or can be composed of multiple segmented pieces fixed together (such as by welding or bolting). The baffle is typically rectangular with two faces and four edges. One face of the baffle faces the feed side section of the middle vapor - liquid contact area, and the other face faces the side draw side section. The pair of opposite side edges of the partition wall are typically fixed to the inner surface of the column wall of the middle vapor - liquid contact area, and preferably, each of the pair of side edges is sealingly joined to the inner surface wall, such as by seal welding, so that if there is passage between the attached edge and the column wall, the fluid in one section of the middle vapor - liquid contact area does not communicate with the fluid in any other section of the middle vapor - liquid contact area. The top edge of the partition wall defines the top of the middle vapor - liquid contact area and the bottom of the upper vapor - liquid contact area. The bottom edge of the partition wall defines the bottom of the middle vapor - liquid contact area and the top of the lower vapor - liquid contact area. All four edges do not have to be straight. For example, the side edges can be shaped or rounded according to the column wall profile to facilitate attaching the partition wall to the column wall. Also, the top edge can be shaped or segmented in a way that helps with the attachment or assembly between the partition wall and the trays or other column internals in the top of the middle vapor - liquid contact area and / or the bottom of the upper vapor - liquid contact area. Similarly, the bottom edge can be shaped to enhance the fit between the partition wall and the trays or internals at the bottom of the middle vapor - liquid contact area and / or the top of the lower vapor - liquid contact area.

[0024] The thickness of the partition wall can be any suitable thickness subject to mechanical requirements for the structural strength of the partition wall, attachment to the column wall, or attachment to other column internals (including other partition walls in the column). The partition wall can include two walls with a gas space therebetween. The partition wall can be constructed of any suitable material, and it is believed that preferably, the partition wall and the column wall shell are of the same material. The partition wall material is typically carbon steel. The surfaces of these faces of the partition wall are typically smooth.

[0025] The vapor-liquid contacting device is in the upper, middle, and lower vapor-liquid contacting zones of the dividing wall distillation column. Any suitable vapor-liquid contacting device can be used. Suitable vapor-liquid contacting devices include trays and packings. As used herein, the term "tray" includes sieve trays. Sieve trays typically consist of a solid tray or deck with a plurality of openings and are mounted on support rings within the column. Suitable sieve trays include crossflow trays such as sieve trays, fixed valve trays, or bubble cap trays. Packings are used in the vapor-liquid contacting zone as a supplement to or in place of trays. The vapor-liquid contacting zone is typically designed based on hydraulic performance (e.g., pressure drop, flooding, and load) and mass transfer performance (e.g., height equivalent to a theoretical plate (or HETP)).

[0026] In the feed side section of the middle vapor-liquid contacting zone, one or more trays are typically located above the height of the inlet port of the feed side section and below the top edge of the dividing wall, and one or more trays are typically located between the inlet port and the bottom of the dividing wall. One or more trays are typically in the side draw section, and one or more trays are typically in the lower vapor-liquid contacting zone. The tray spacing in any of these zones or sections can be the same as or different from the tray spacing in other zones or sections, and can also be the same as or different from the spacing within the same zone or section. Typically, the tray spacing at the height where the feed stream is introduced into the column is greater than the tray spacing of other trays. Trays typically have a tray efficiency of 80%, but trays with higher or lower efficiencies can be used. As used herein, tray efficiency is the degree of approach to equilibrium, which is defined as the ratio of the actual change in vapor composition as the vapor passes through the tray to the change that would occur if the vapor had reached equilibrium with the liquid leaving the tray.

[0027] The trays below the inlet of the mixed hydrocarbon stream in the feed side section act as a stripping section to reduce the concentration of 1-butene without significantly reducing the concentration of 2-butene and heavier olefins in the descending liquid. The V / L below the inlet of the mixed hydrocarbon stream in the feed side section and the temperature at the bottom of the dividing wall in the feed side section are important parameters for controlling the concentration of 1-butene in the side draw stream. The trays in the lower vapor-liquid contacting zone act as a stripping zone to not only further reduce the concentration of 1-butene but also reduce the concentration of 2-butene in the descending liquid in order to obtain a highly concentrated bottoms stream containing heavier olefins and the least amount (if any) of 2-butene. The trays above the side draw outlet in the side draw side section act as a stripping section to reduce the concentration of 1-butene in the descending liquid. The trays below the side draw outlet in the side draw side section act as a rectifying section to reduce the concentration of heavier olefins in the ascending vapor.

[0028] The skilled person realizes that, with all other variables held constant, the number of trays in the vapor-liquid contact zone generally varies directly with the V / L ratio. As used herein, the V / L ratio, or simply V / L, is the ratio of the number of moles of the upflowing vapor (V) to the number of moles of the downflowing liquid (L). The designer of the vapor-liquid contact zone for carrying out distillation arrives at the optimum number of trays and the optimum V / L ratio by making a trade-off or balance between, on the one hand, the capital cost of the distillation column and, on the other hand, the operating cost. In the design of a dividing-wall distillation column, this trade-off is considered in respect of each of the upper, middle, and lower vapor-liquid contact zones; within the middle vapor-liquid contact zone, it is considered for both the feed-side section and the side-draw section; and within the feed-side section, it is considered for the trays above and below the feed inlet. Thus, the skilled person realizes that the number of trays in the vapor-liquid contact zone of a dividing-wall distillation column can vary according to the V / L in that zone.

[0029] Suitably, a dividing-wall distillation column (including an upper vapor-liquid contact zone, a middle vapor-liquid contact zone, and a lower vapor-liquid contact zone) comprises from 80 to 200, preferably from 130 to 170, theoretical stages, where the 1st theoretical stage is the lowermost theoretical stage in the dividing-wall distillation column. A theoretical stage in a distillation process is a hypothetical zone or stage in which the liquid phase and the vapor phase of the substance to be distilled are brought into thermodynamic equilibrium with each other. The greater the number of theoretical stages, the higher the efficiency of the separation process. The concept of theoretical stages and the calculations thereof are well known to the skilled person. Since actual physical trays, plates, or trays, or similar devices (such as, for example, a packed bed containing Raschig rings or other structured inserts) rarely represent 100% efficient equilibrium stages, the actual number of trays is generally more than the required number of theoretical stages. For a fixed heat flow supplied to the column, if the number of theoretical stages in the column is below the range given above, 1-butene and 2-butene may not be separated from each other sufficiently. At the same time, if the number of theoretical stages in the column exceeds the range given above, the investment cost increases undesirably, while the energy consumption does not decrease significantly further.

[0030] Suitably, the dividing wall is configured such that the bottom edge of the dividing wall is at a height such that the 2nd to 50th, preferably the 4th to 30th, more preferably the 5th to 15th theoretical stages, where the 1st theoretical stage is the lowermost theoretical stage in the dividing-wall distillation column. The vertical extent of the dividing wall can be from 10 to 150, preferably from 80 to 120, theoretical stages.

[0031] Suitably, the inlet port is located at a height from 1 theoretical stage above the bottom edge of the dividing wall to 1 theoretical stage below the top edge of the dividing wall, preferably from 50 theoretical stages above the bottom edge of the dividing wall to 1 theoretical stage below the top edge of the dividing wall.

[0032] Suitably, the position of the 2-butene withdrawal port is at a height from 1 theoretical stage above the bottom edge of the dividing wall to 1 theoretical stage below the top edge of the dividing wall, preferably from 1 theoretical stage above the bottom edge of the dividing wall to 50 theoretical stages below the top edge of the dividing wall.

[0033] The liquid fraction to be directed to the sidestream evaporator is withdrawn from the dividing-wall distillation column via a sidestream draw. Preferably, there are liquid-holding trays and downcomers through which the descending liquid can be withdrawn from the column. The liquid collected on the liquid-holding trays preferably flows to the downcomer and is withdrawn from the dividing-wall distillation column by the downcomer.

[0034] Preferably, the liquid fraction is withdrawn from the lower vapor-liquid contact zone, for example at a height of from the 2nd to the 40th, more preferably from the 3rd to the 6th theoretical stage, where the 1st theoretical stage is the lowest theoretical stage in the dividing-wall distillation column. Even more preferably, the heated, partially vaporized fraction is directed back to the lower vapor-liquid contact zone, in particular within 20, in particular 2, in particular 1 theoretical stage below or above the sidestream draw from which the liquid fraction is withdrawn.

[0035] The condenser is in communication with the upper vapor-liquid contact zone. The condenser can condense the vapor into a liquid as a total condenser or a partial condenser for the incoming vapor, and such condensers are known to the person skilled in the art. The condenser can be outside the column or located within the column. An internal condenser can be located within the column, directly above the upper vapor-liquid contact zone, and thus there is communication between the condenser and the upper vapor-liquid contact zone. For an external condenser, the communication with the upper vapor-liquid contact zone can be via one or more ports, conduits, and / or accumulators. Thus, nozzles attached to the shell of the dividing-wall distillation column vessel can be connected to the condenser to allow the vapor to flow from the upper vapor-liquid contact zone to the condenser. Nozzles attached to the shell of the column vessel can be connected to the condenser to allow the liquid to flow from the condenser to the upper vapor-liquid contact zone. An accumulator can be located between the condenser and the upper vapor-liquid contact zone to collect the condensed liquid and separate these liquids from the uncondensed vapor, and the accumulator can be in communication with the condenser and the upper vapor-liquid contact zone via ports or conduits. A pump can be used to pump the liquid from the accumulator to the upper vapor-liquid contact zone, and the liquid flow can be regulated by a control valve. Although the vapor outlet port from the dividing-wall distillation column and the liquid inlet port into the dividing-wall distillation column are preferably separate ports, they can be the same port through which the vapor of relatively lower density rises and the liquid of relatively higher density falls.

[0036] At least a portion of the condensate is recycled as reflux liquid to the upper vapor-liquid contact zone, while the remainder is withdrawn as an overhead stream from the dividing wall distillation column. Generally, the reflux ratio is in the range of from 20 to 90, preferably from 30 to 70. The reflux ratio is defined as the ratio of the reflux liquid flow rate to the overhead stream withdrawal rate. The overhead stream contains predominantly 1-butene, with the remainder being ethylene, such as from 60% to 99% by weight, preferably from 80% to 99% by weight of 1-butene.

[0037] A bottom reboiler is in communication with the lower vapor-liquid contact zone. The reboiler is capable of vaporizing a liquid into a vapor, and such a reboiler is typically a partial reboiler for the incoming liquid and is known to the person skilled in the art. The reboiler can be an external reboiler or an internal reboiler located within the column. The internal reboiler can be located within the column, directly below the lower vapor-liquid contact zone, and thus there is communication between the reboiler and the lower vapor-liquid contact zone. For an external reboiler, the communication with the lower vapor-liquid contact zone can be via one or more ports and / or conduits. Thus, a nozzle attached to the shell of the dividing wall distillation column vessel can be connected to the reboiler to allow liquid to flow from the lower vapor-liquid contact zone to the reboiler. A pump can be used to pump the liquid from the column to the reboiler, and the liquid flow rate can be regulated by a control valve. Alternatively, the reboiler can be a so-called thermosyphon reboiler, where the reboiling changes the density of the material being reboiled, and the density change in turn causes flow through the reboiler. A nozzle attached to the shell of the column vessel can be connected to the reboiler to allow vapor, or possibly a vapor-liquid two-phase mixture, to flow from the reboiler to the lower vapor-liquid contact zone. Although the outlet liquid and vapor of the liquid from the dividing wall distillation column to the inlet ports of the dividing wall distillation column are preferably separate ports, they can be the same port, through which the liquid of relatively higher density falls and the vapor of relatively lower density rises. Preferably, the evaporation rate in the bottom reboiler is in the range of from 2% to 15% by weight, more preferably from 5% to 12% by weight.

[0038] The distillation is typically carried out at a pressure (at the top of the dividing wall distillation column) of from 3 to 8 bar absolute, preferably from 3.6 to 6.7 bar absolute. The column bottom temperature is generally in the range of from 100 °C to 150 °C, preferably from 110 °C to 130 °C, where the temperature at the top of the dividing wall distillation column is in the range of from 30 °C to 70 °C, preferably from 30 °C to 50 °C. The heating in the bottom reboiler is supplied by a heat transfer medium (such as heating steam at 3 to 40 bar absolute) having a temperature of from 130 °C to 250 °C, preferably from 135 °C to 160 °C. The temperature difference between the heat transfer medium and the bottom liquid is suitably within 5 K to 25 K.

[0039] According to the present invention, additional thermal energy is provided to the dividing wall distillation column by using a sidestream evaporator. For this purpose, a liquid fraction is withdrawn from the dividing wall distillation column via a sidestream, and the liquid fraction is heated and partially evaporated in the sidestream evaporator. The heated and partially evaporated fraction is led back to the dividing wall distillation column. Preferably, a liquid storage tray and a downcomer are present, and the descending liquid can be withdrawn from the column by the downcomer. A pump can be used to pump the liquid fraction from the column to the sidestream evaporator, and the flow rate of the liquid fraction to the sidestream evaporator can be adjusted by a control valve.

[0040] The heating in the sidestream evaporator is preferably achieved with a heat transfer medium (such as hot water) having a temperature of 65°C to 125°C, preferably 65°C to 99°C, more preferably 70°C to 90°C. The temperature difference between the heat transfer medium and the liquid fraction is suitably within 5K to 25K. Thus, a significant proportion of the total thermal energy provided to the dividing wall distillation column can be provided via the sidestream evaporator at a relatively low temperature level (i.e., a temperature lower than the temperature required for the bottom reboiler). This can allow heat to be recovered from low-grade waste heat generated by a variety of industrial and commercial processes and operations. When the waste heat is low-grade, such as waste heat having a heat temperature below 100°C, conventional heat recovery systems cannot operate at an efficiency sufficient to make energy recovery cost-effective.

[0041] By using a sidestream evaporator, 2-butene in the sidestream section of the dividing wall column can be driven upward even if the energy input via the bottom reboiler is reduced. This is especially true if a high evaporation rate of the liquid fraction is achieved in the sidestream evaporator. Preferably, 10% to 90% by weight, more preferably 40% to 80% by weight of the liquid fraction is evaporated in the sidestream evaporator.

[0042] The sidestream evaporator is not particularly limited and can be selected from falling film evaporators, rising film evaporators, and forced or natural circulation evaporators or combinations thereof. A high evaporation rate can be achieved, for example, by using a circulation evaporator (such as a Robert evaporator) having an additional internal circulation. For a lower evaporation rate, a simple circulation evaporator may be sufficient.

[0043] The ratio of the thermal energy provided by the bottom reboiler to the thermal energy provided by the sidestream evaporator can be in the range of 10% to 90%.

[0044] A stream consisting essentially of 2-butene is withdrawn as a sidestream from the sidestream section. Typically, this 2-butene stream contains at least 98% by weight, preferably at least 99% by weight of 2-butene.

[0045] Processes for the dimerization of ethylene are known in the art. The dimerization catalyst can be a homogeneous catalyst or a heterogeneous catalyst. Examples of suitable homogeneous catalysts are taught in US 3,321,546, US 4,242,531, US 4,476,341, US 5,260,499, and US 5,414,178.

[0046] In a preferred embodiment, the dimerization catalyst comprises a nickel compound and an organoaluminum compound. Suitable nickel compounds include nickel salts of mono- or dicarboxylic acids, preferably acids having 5 to 20 carbon atoms, such as nickel oleate, nickel laurate, and nickel octanoate. Other nickel compounds include coordination complexes of organophosphines with nickel salts. Examples of such complexes are bis(triethylphosphine)nickel dichloride [Ni(Et3P)2Cl2], bis(triphenylphosphine)nickel octanoate, bis(triphenylphosphine)nickel dichloride, and bis(tricyclohexylphosphine)nickel dichloride. Suitable organoaluminum compounds include those having 1 to 2 alkyl groups and 1 to 2 halogen atoms per aluminum atom. These alkyl groups preferably have 1 to 5 carbon atoms. The halogen is preferably chlorine.

[0047] A particularly preferred dimerization catalyst comprises bis(triphenylphosphine)nickel octanoate and ethylaluminum dichloride. The molar ratio Ni:Al is typically from 0.9:1 to 1:0.9.

[0048] Another preferred dimerization catalyst comprises (1) an organoaluminum compound having the formula R n AlX 3-n wherein R is an alkyl group, X is a halogen, and n is 1 or 2; (2) a complex of a nickel salt of an organic or inorganic acid with a tertiary phosphine or a tertiary phosphite, the atomic ratio Al / Ni varying in the range from 1:1 to 100:1.

[0049] The dimerization reaction is typically carried out at a temperature in the range from 10 °C to 100 °C, preferably from 20 °C to 80 °C. Depending on the reaction temperature and pressure used, the dimerization reaction can be carried out in the liquid phase or the gas phase by contacting ethylene with the catalyst. The pressure of the dimerization reaction is usually from 1 to 40 bar absolute.

[0050] The dimerization reaction produces a dimerization mixture comprising ethylene, 1-butene, and 2-butene. Other olefins such as hexene and octene may be present in the dimerization mixture. It is preferred to minimize the amount of hexene, octene, and other higher olefins produced. Usually, this can be achieved by selecting an appropriate catalyst and controlling the ethylene conversion. Higher butene selectivity can be achieved by operating at a lower ethylene conversion.

[0051] Examples of reactors in which the reaction can be carried out are stirred tank reactors, stirred tank cascades, flow tube reactors, and loop reactors.

[0052] After the reaction is completed, the active catalyst is deactivated by treating the reaction effluent with an aqueous solution of a base (such as sodium hydroxide). The effluent from the dimerization zone is first subjected to alkali washing and drying, and then introduced into the feed section of the dividing wall distillation column.

[0053] The present invention is illustrated by the accompanying drawings and the following examples.

[0054] Figure 1 An apparatus for separating a mixed hydrocarbon containing n-butene using a dividing wall distillation column with a sidestream evaporator by a method according to the present invention is shown.

[0055] Figure 2 An apparatus for separating a mixed hydrocarbon containing n-butene using a dividing wall distillation column according to the prior art is shown.

[0056] Figure 3 An apparatus for separating a mixed hydrocarbon containing n-butene using a conventional two-column distillation sequence by a method according to the prior art is shown.

[0057] Regarding Figure 1 ,, a dividing wall distillation column 101 with a sidestream evaporator 104 is shown. The dividing wall distillation column 101 includes a feed section, a middle vapor-liquid contact zone, an upper vapor-liquid contact zone above the middle vapor-liquid contact zone, and a lower vapor-liquid contact zone below the middle vapor-liquid contact zone. The middle vapor-liquid contact zone is divided into a feed section and a sidestream section by a vertically oriented dividing wall. A mixed hydrocarbon stream 1 containing 1-butene, 2-butene, and heavier olefins is introduced into the feed section of the dividing wall distillation column 101 via a side inlet. Thermal energy is supplied to the dividing wall distillation column 101 via a bottom reboiler 103. The vapor escaping from the upper vapor-liquid contact zone of the dividing wall distillation column 101 is condensed via a condenser 102. A part of the condensate is recycled as reflux liquid to the upper vapor-liquid contact zone, and another part of the condensate is withdrawn as a stream 2 containing 1-butene. A stream 4 containing heavier olefins is withdrawn from the lower vapor-liquid contact zone via the reboiler 103, and a stream 3 containing 2-butene is withdrawn as a sidestream from the sidestream section of the dividing wall distillation column 101. Additional thermal energy is supplied to the dividing wall distillation column 101 via the sidestream evaporator 104. For this purpose, a liquid fraction is withdrawn at a certain position in the lower vapor-liquid contact zone of the dividing wall distillation column 101 (i.e., below the bottom edge of the dividing wall). The liquid fraction is then heated and partially evaporated in the sidestream evaporator 104. The heated and partially evaporated fraction is led back to the lower vapor-liquid contact zone of the dividing wall distillation column 101.

[0058] Figure 2 Illustrates as Figure 1The dividing wall distillation column 201 shown in

[0059] Figure 3 A conventional two-column distillation setup with a first distillation column 301 and a second distillation column 304 is illustrated. A mixed hydrocarbon stream 1 containing 1-butene, 2-butene, and heavier olefins is introduced into the first distillation column 301. Thermal energy is provided to the first distillation column 301 via a bottom reboiler 303. The vapor escaping from the top of the first distillation column 301 is condensed via a condenser 302. A portion of the condensed liquid is recycled as reflux to the first distillation column 301, and another portion of the condensed liquid is withdrawn as a stream 1a containing 1-butene and 2-butene. A stream 4 containing heavier olefins is withdrawn from the first distillation column 301 via the reboiler 303. Then, the stream 1a is introduced into the second distillation column 304 for further fractionation. Thermal energy is provided to the second distillation column 304 via a bottom reboiler 306. The vapor escaping from the top of the second distillation column 304 is condensed via a condenser 305. A portion of the condensed liquid is recycled as reflux to the second distillation column 304, and another portion of the condensed liquid is withdrawn as a stream 2 containing 1-butene. A stream 3 containing 2-butene is withdrawn from the second distillation column 304 via the reboiler 306.

[0060] Using process simulation software to model the distillation system as Figures 1 to 3 illustrated. Table 2 shows the process conditions selected based on the following specifications and the composition of the mixed hydrocarbon stream used in the modeling process (as shown in Table 1):

[0061] 0.075% 1-butene in the 2-cis / trans-butene product stream,

[0062] 0.057% 2-trans-butene in the ethylene / 1-butene top distillate stream,

[0063] 2.5% 2-cis-butene in the light ends bottoms stream (pentene, hexene),

[0064] 3 mbar pressure drop per theoretical separation stage.

[0065] Table 1.

[0066] Component <![CDATA[Mass flow rate [kg h -1 > Ethylene 5.00 Propylene 1.50 1-Butene 30.24 2-cis-Butene 127.83 2-trans-Butene 278.87 1-Pentene 0.34 2-cis-Pentene 0.03 2-trans-Pentene 0.05 2-cis-Hexene 0.23 2-trans-Hexene 59.57 1-Heptene 5.00

[0067] Table 2.

[0068]

[0069] [1] Evaporation rate achieved in the sidestream evaporator = 70%

[0070] [2] Evaporation rate achieved in the sidestream evaporator = 10%

[0071] [3]Feed section of the dividing wall column

[0072] [4]Total number of stages in the lower vapor - liquid contact zone, the side draw section in the middle vapor - liquid contact zone, and the upper vapor - liquid contact zone of the dividing wall column

[0073] [5]First distillation column

[0074] [6]Second distillation column

[0075] [7]Supply to the reboiler (temperature level)

[0076] [8]Supply to the side draw evaporator (temperature level)

[0077] [9]Total supply

[0078]

[10] Supply to the reboiler of the first distillation column (temperature level)

[0079]

[11] Supply to the reboiler of the second distillation column (temperature level)

[0080] *Comparative example

[0081] For the conventional two - column operation according to Figure 3 , due to the presence of heavier olefins, the bottom temperature in the first distillation column 301 is high (120 °C). Due to the large boiling point difference, the heavier olefins can be easily separated, and a low reflux ratio is sufficient. Therefore, the required energy input (60 kW) is significantly lower compared to the downstream second distillation column 304. In the second distillation column 304, a high reflux ratio is required to achieve the desired purity because the boiling points of 1 - butene and 2 - butene are close to each other. For this purpose, a sufficient amount of thermal energy (180 kW) must be supplied. However, since the high - boilers have been separated, the bottom temperature is relatively low. This energy input can be carried out at a significantly lower temperature (56 °C).

[0082] The energy input required for using the dividing wall distillation column 201 according to Figure 2 is 21% less compared to the conventional two - column operation according to Figure 3 . In the dividing wall distillation column 201, a high reflux ratio is required because the heavier olefins, 2 - butene, and 1 - butene are separated from each other simultaneously. Therefore, a high thermal energy input (191 kW) is required. Due to the high boiling point of the heavier olefins, the temperature in the bottom evaporator is correspondingly high (117 °C).

[0083] If the dividing wall distillation column is equipped with a side draw evaporator as shown in Figure 1 , the following advantages are obtained: For the same separation capacity, compared to that according to Figure 2The dividing wall distillation column without a sidestream evaporator requires fewer theoretical separation stages (see Table 2: 93 vs. 87 theoretical stages in the feed section). In contrast, the amount of thermal energy required increases slightly by 4 kW. Nevertheless, compared with the conventional two-column operation according to Figure 3 , a total of 47 kW can be saved, which corresponds to a 20% reduction in energy input. A significant advantage lies in the fact that a part of the required heat can be provided at a low temperature (58 °C or 56 °C) via the sidestream evaporator 104.

[0084] When operating at a high evaporation rate, Figure 1 the sidestream evaporator 104 in becomes particularly advantageous. This is for the following reasons: at an evaporation rate of 70%, 138 kW can be supplied at a temperature of 58 °C. Therefore, only 55 kW must be supplied at a high temperature of 123 °C. At an evaporation rate of 10%, most of the heat (173 kW) must be supplied at a temperature of 123 °C. Only a small part (20 kW) can be supplied at 56 °C. By achieving a high evaporation rate, the advantages of the conventional two-column operation (high energy input at low temperature) can be combined with the advantages of the dividing wall distillation column (energy saving and lower investment costs).

Claims

1. A method for separating a mixed hydrocarbon stream containing 1-butene, 2-butene, heavier olefins, and unreacted ethylene if present, the method comprising: introducing the mixed hydrocarbon stream into the feed section of a dividing wall distillation column having a shell defining a middle vapor-liquid contact zone, an upper vapor-liquid contact zone above and in communication with the middle vapor-liquid contact zone, and a lower vapor-liquid contact zone below and in communication with the middle vapor-liquid contact zone, the middle vapor-liquid contact zone containing at least one vertically oriented dividing wall that divides the middle vapor-liquid contact zone into at least the feed section defined by the shell and the dividing wall and a side draw section defined by the shell and the dividing wall, supplying heat energy to the dividing wall distillation column via a bottom reboiler in communication with the lower vapor-liquid contact zone, at least partially condensing the vapor escaping from the upper vapor-liquid contact zone via a condenser in communication with the upper vapor-liquid contact zone, and recycling at least a portion of the condensate as reflux liquid back to the upper vapor-liquid contact zone, withdrawing 2-butene as a side draw from the side draw section, withdrawing 1-butene and unreacted ethylene if present from the upper vapor-liquid contact zone, and withdrawing heavier olefins from the lower vapor-liquid contact zone; and providing additional heat energy to the dividing wall distillation column by: withdrawing a liquid fraction from the dividing wall distillation column via a side draw, heating and partially evaporating the liquid fraction in a side draw evaporator, and directing the heated, partially evaporated fraction back to the dividing wall distillation column.

2. The method according to claim 1, wherein The liquid fraction is withdrawn from the lower vapor-liquid contact zone.

3. The method according to claim 1 or 2, wherein The heated, partially evaporated fraction is directed back to the lower vapor-liquid contact zone.

4. The method according to any one of the preceding claims, wherein, 10% to 90% by weight of the liquid fraction is evaporated in the side draw evaporator.

5. The method according to any one of the preceding claims, wherein, The heating in the bottom reboiler is achieved with a heat transfer medium having a temperature of 130°C to 250°C, preferably 135°C to 160°C; and the heating in the side draw evaporator is achieved with a heat transfer medium having a temperature of 65°C to 125°C, preferably 65°C to 99°C, more preferably 70°C to 90°C.

6. The method according to any one of the preceding claims, wherein, The ratio of the heat energy provided by the bottom reboiler to the heat energy provided by the side draw evaporator is in the range of 10% to 90%.

7. The method according to any one of the preceding claims, wherein, The side draw evaporator is selected from a falling film evaporator, a rising film evaporator, and a circulation evaporator.

8. A method for preparing n-butene, the method comprising: introducing an ethylene stream into a dimerization zone containing a dimerization catalyst and contacting the ethylene stream with the dimerization catalyst, operating the dimerization zone under conditions effective to produce a mixed hydrocarbon stream containing 1-butene, 2-butene, heavier olefins, and unreacted ethylene if present, and separating the mixed hydrocarbon stream by the method according to any one of claims 1 to 7.

9. The method according to claim 8, wherein, The dimerization catalyst comprises a nickel compound and an organoaluminum compound.

10. The method according to claim 8 or 9, wherein The effluent from the dimerization zone is first subjected to alkali washing and drying and then introduced into the feed section of the dividing wall distillation column.

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