Patent Description:
An alpha-olefin is an important material which is used in comonomers, cleaning agents, lubricants, plasticizers, and the like and is commercially widely used, and in particular, <NUM>-hexene and <NUM>-octene are often used as a comonomer for adjusting the density of polyethylene in the production of linear low-density polyethylene (LLDPE).

The alpha-olefins such as <NUM>-hexene and <NUM>-octene are produced representatively by an oligomerization reaction of ethylene. The ethylene oligomerization reaction is performed by an oligomerization reaction (trimerization reaction or tetramerization reaction) of ethylene using ethylene as a reactant, and the product produced by the reaction includes unreacted ethylene as well as a multicomponent hydrocarbon mixture including the desired <NUM>-hexene and <NUM>-octene. The product is subjected to a separation process by a distillation column, in which the unreacted ethylene is recovered and reused in the oligomerization reaction of ethylene.

The unreacted ethylene may be recovered as a gas phase or a liquid phase. In order to recover the unreacted ethylene and reuse it in the ethylene oligomerization reaction, a pressure of a recovered unreacted ethylene stream should be increased to a pressure of a reactor. Conventionally, in order to increase the pressure of the unreacted ethylene stream to the pressure of the reactor using a pump, the unreacted ethylene stream should be a liquid phase, but to this end, the unreacted ethylene stream should be cooled to -<NUM> or lower using a refrigerant at a very low temperature due to a low boiling point of ethylene. Otherwise, when the unreacted ethylene is recovered as a gas phase, a compressor should be separately installed for increasing the pressure of the unreacted ethylene stream recovered for being reused in the reactor to the pressure of the reactor,.

As such, the conventional method of recovering unreacted ethylene to reuse it in the oligomerization reaction has problems of having high investment costs and being economically infeasible, for example, using a refrigerant at a very low temperature, separately installing a compressor, or the like. <CIT> discloses a reactor system for monomer oligomerization. A first discharge stream from the reactor is fed to a first separating device. A second discharge stream is fed to a second separating device. The top stream from the second separating device contains unreacted monomer. The solvent is recycled to a mixer, where it is mixed with the top stream from the second separating device before entering the reactor. <CIT> discloses a reactor system for ethylene oligomerization. Unreacted ethylene from a first separating device is recycled and combined with recycled solvent before it enters the reactor.

In order to solve the problems described in the Background Art, an object of the present disclosure is to provide a method for preparing an oligomer having reduced investment costs.

That is, an object of the present disclosure is to provide a method for preparing an oligomer in which in an oligomer production process, in recovering unreacted ethylene to reuse it in an oligomerization reaction, investment costs are reduced and economic feasibility is improved since there is no need to use a refrigerant at a very low temperature or install a separate compressor.

In one general aspect, a method for preparing an oligomer includes: supplying a feed stream including a monomer to a reactor and performing an oligomerization reaction; supplying a first discharge stream from the reactor to a first separation device and supplying a second discharge stream from the reactor to a second separation device; recovering the monomer in the second separation device as an upper discharge stream; supplying the upper discharge stream including the monomer from the second separation device to a monomer dissolution device to dissolve the upper discharge stream in a solvent supplied to the monomer dissolution device; and supplying a discharge stream from the monomer dissolution device to the reactor, wherein the discharge stream from the monomer dissolution device is a liquid stream including the monomer dissolved in the solvent, and
rge stream from the monomer dissolution device is a liquid stream including the monomer dissolved in the solvent, and wherein the monomer is ethylene and the oligomer is an alpha-olefin.

According to the method for preparing an oligomer of the present disclosure, the recovered unreacted monomer is dissolved in the solvent and supplied to the reactor, whereby there is no need for a refrigerant at a low temperature and installation of an additional compressor which were required in the conventional method of recovering unreacted monomer.

In addition, when the recovered unreacted monomer is dissolved in the solvent, the unreacted monomer may be dissolved at a relatively high temperature, and the solvent in which the unreacted monomer is dissolved is a liquid phase, and thus, the pressure of the unreacted monomer is increased to the pressure of the reactor only by using a pump and the unreacted monomer is reintroduced to the reactor, thereby reusing the unreacted monomer.

The terms and words used in the description and claims of the present disclosure are not to be construed limitedly as having general or dictionary meanings but are to be construed as having meanings and concepts meeting the technical ideas of the present disclosure, based on a principle that the inventors are able to appropriately define the concepts of terms in order to describe their own inventions in the best mode.

In the present invention, the term "stream" may refer to a fluid flow in a process, or may refer to a fluid itself flowing in a pipe. Specifically, the "stream" may refer to both a fluid itself flowing in a pipe connecting each apparatus and a fluid flow. In addition, the fluid may refer to a gas or a liquid.

Hereinafter, the present invention will be described in more detail for better understanding the present invention.

According to the present invention, a method for preparing an oligomer is provided. As the method for preparing an oligomer, a method for preparing an oligomer including: supplying a feed stream including a monomer to a reactor to perform an oligomerization reaction; supplying a first discharge stream from the reactor to a first separation device and supplying a second discharge stream from the reactor to a second separation device; recovering the monomer in the second separation device as an upper discharge stream; supplying the upper discharge stream including the monomer from the second separation device to a monomer dissolution device to dissolve the upper discharge stream in a solvent supplied to the monomer dissolution device; and supplying a discharge stream from the monomer dissolution device to the reactor, wherein the discharge stream from the monomer dissolution device is a liquid stream including the monomer dissolved in the solvent, and
wherein the monomer is ethylene and the oligomer is an alpha-olefin, may be provided.

According to the present invention, in a step of supplying a feed stream including a monomer to a reactor to perform an oligomerization reaction, the feed stream including the monomer may be supplied to the reactor, and the oligomerization reaction of the monomer may be performed in a liquid phase in a lower portion of the reactor. The oligomerization reaction may refer to a reaction in which a monomer is oligomerized. The oligomerization may be referred to as trimerization or tetramerization depending on the number of monomers to be polymerized, and these are collectively called multimerization.

According to the present invention, the monomer is ethylene and the oligomer is an alpha-olefin. The alpha-olefin is an important material which is used in comonomers, cleaning agents, lubricants, plasticizers, and the like and is commercially widely used, and in particular, <NUM>-hexene and <NUM>-octene are often used as the comonomer for adjusting the density of polyethylene in the production of linear low-density polyethylene (LLDPE). The alpha-olefin such as <NUM>-hexene and <NUM>-octene may be produced by for example, a trimerization reaction or a tetramerization reaction of ethylene.

According to an exemplary embodiment of the present invention, a step of oligomerizing a monomer may be performed in a reactor appropriate for a continuous process, and preferably performed under a reaction system including one or more reactors selected from the group consisting of a continuous stirred tank reactor (CSTR) and a plug flow reactor (PFR).

According to an exemplary embodiment of the present invention, the oligomerization reaction of the monomer may be performed by a homogeneous liquid phase reaction, a slurry reaction having a catalyst system in a partially or entirely not dissolved form, a two-phase liquid/liquid reaction, or a bulk phase reaction or gas phase reaction in which the product acts as a main medium, in the presence or absence of a solvent, by applying the reaction system and a common contact technology. Preferably, the step of oligomerizing a monomer may be performed in the homogeneous liquid phase reaction.

According to an exemplary embodiment of the present invention, the step of performing an oligomerization reaction may be performed at a temperature of <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. In addition, the step of performing oligomerization may be performed under a pressure of <NUM> bar to <NUM> bar, <NUM> bar to <NUM> bar, or <NUM> bar to <NUM> bar. When ethylene is oligomerized within the temperature range and the pressure range, a selectivity to a desired alpha-olefin may be excellent, a by-product amount may be decreased, operational efficiency of a continuous process may be increased, and the costs may be reduced.

According to an exemplary embodiment of the present invention, the feed stream including the monomer may include a gaseous monomer, a liquid monomer, and a solvent.

The gaseous monomer included in the feed stream may be supplied as, for example, a feed stream including an ethylene monomer separated from a naphtha cracking center (NCC) process as it is or after being subjected to a storage step. In addition, the gaseous monomer may include a stream recovered in the oligomer production process.

The liquid monomer supplied to the reactor as the feed stream may include a stream recovered in the oligomer production process. In addition, the liquid monomer may include a stream in which a gaseous monomer recovered in the oligomer production process is dissolved in a solvent.

The solvent included in the feed stream may refer to a solvent for dissolving the recovered gaseous monomer. In some cases, the solvent may be further supplied to the reactor as a separate feed stream in addition to the solvent for dissolving the monomer.

According to an exemplary embodiment, in a step of supplying a first discharge stream from the reactor to a first separation device and supplying a second discharge stream from the reactor to a second separation device, a common separation column may be used as the separation device.

According to the exemplary embodiment of the present invention, the first discharge stream may be a stream including the gaseous monomer. The first discharge stream is supplied to the first separation device, and the upper discharge stream including the gaseous monomer and the lower discharge stream including the liquid monomer may be supplied to the reactor. Here, the upper discharge stream from the first separation device may be mixed with the gaseous monomer stream supplied to the reactor in a mixer and supplied to the reactor, or may be separately supplied to the reactor.

According to an exemplary embodiment of the present invention, the second discharge stream may be a stream including the liquid monomer. The second stream may be supplied to the second separation device and separated into the gaseous upper discharge stream including the monomer and the liquid lower discharge stream including the solvent and the oligomer. Here, the monomer may be recovered in the oligomer production process and reused by a step of recovering the monomer in the second separation device as the upper discharge stream. The monomer is recovered as the upper discharge stream from the second separation device and the upper discharge stream from the second separation device may be supplied to the reactor after passing through the monomer dissolution device. In addition, in the lower stream from the second separation device, the solvent and the oligomer may be separated by an additional separation process, and the separated solvent may be supplied to the reactor. In addition, the separated oligomer may be separated again into a trimer and a tetramer of the monomer by an additional separation process.

According to an exemplary embodiment of the present invention, in a step of dissolving the monomer recovered in the second separation device, a refrigerant may or may not be used. As an example, in the step of dissolving the monomer recovered in the second separation device, when the refrigerant is used, one or more selected from the group consisting of brine (an aqueous ethylene glycol solution), an aqueous propyl glycol solution, and a refrigerated coolant may be used as the refrigerant. Here, the refrigerant may include <NUM> parts by weight to <NUM> parts by weight, <NUM> parts by weight to <NUM> parts by weight, or <NUM> parts by weight to <NUM> parts by weight of ethylene glycol or propyl glycol, based on a total of <NUM> parts by weight of the refrigerant. As another example, in the step of dissolving the monomer recovered from the second separation device, when the refrigerant is not used, a coolant may be used. This may be selected depending on the temperature required for dissolving the recovered monomer in the solvent, and depending on the content of the recovered monomer to be dissolved in the solvent, the refrigerant or the coolant which may cool the monomer to the temperature or lower at which the monomer may be dissolved, may be selected. For example, the refrigerant or the coolant which may dissolve <NUM> wt% to <NUM> wt%, <NUM> wt% to <NUM> wt%, or <NUM> wt% to <NUM> wt% of the monomer recovered from the second separation device may be selected.

According to an exemplary embodiment of the present invention, the monomer included in the upper discharge stream from the second separation device may be dissolved in the solvent supplied to the monomer dissolution device in the monomer dissolution device. Specifically, the upper discharge stream including the gaseous monomer from the second separation device may be supplied to the monomer dissolution device, and mixed with the solvent supplied to the monomer dissolution device to dissolve the gaseous monomer in the solvent. Here, the solvent may be one or more selected from the group consisting of n-pentane, n-hexane, n-heptane, cyclohexane, methylcyclohexane, octane, cyclooctane, decane, dodecane, benzene, xylene, <NUM>,<NUM>,<NUM>-trimethylbenzene, toluene, ethylbenzene, chlorobenzene, dichlorobenzene, and trichlorobenzene. As a specific example, the solvent may be used in combination of two or more. As a more specific example, the solvent may be a mixture of methylcyclohexane and decane. When two or more solvents are mixed and used as the solvent, in a step of mixing the solvent having a high boiling point and dissolving the monomer in the solvent, the monomer may be liquefied at a higher temperature and a dissolution rate of the monomer may be improved.

According to an exemplary embodiment of the present invention, a temperature of the solvent supplied to the monomer dissolution device may be in a range of <NUM> to <NUM>, and a pressure thereof may be in a range of <NUM> bar to <NUM> bar. For example, the supply temperature of the solvent may be in a range of <NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>, and the supply pressure thereof may be <NUM> bar to <NUM> bar, <NUM> bar to <NUM> bar, or <NUM> bar to <NUM> bar. By supplying the solvent to the monomer dissolution device in the range of temperature and pressure, the monomer may be liquefied at a relatively high temperature in the monomer dissolution device, and the pressure of the discharge stream from the monomer dissolution device may be increased to the pressure of the reactor only by use of a pump.

According to an exemplary embodiment of the present invention, the content of the solvent supplied to the monomer dissolution device may be in a range of <NUM> to <NUM> times, <NUM> to <NUM> times, or <NUM> to <NUM> times the sum of the oligomer product and the produced by-product in the second discharge stream discharged from the reactor. By supplying the solvent having the content in the range described above is supplied, the oligomer product may be efficiently produced with a similar amount of the solvent to the amount of the solvent which has been used in the conventional method for preparing an oligomer.

According to the present invention, a step of supplying a discharge stream from the monomer dissolution device to the reactor is included, and the discharge stream from the monomer dissolution device is a liquid stream including the monomer dissolved in the solvent. Specifically, the upper discharge stream from the second separation device includes the gaseous monomer, and a separate compressor for increasing the pressure up to the pressure of the reactor should be installed. Otherwise, there is a method of cooling the gaseous monomer to a liquid phase to increase the pressure of the monomer to the pressure of the reactor by the pump, but in this case, a refrigerant at a very low temperature is needed for cooling the gaseous monomer to a liquid phase.

According to an exemplary embodiment of the present invention, an operating temperature of the monomer dissolution device and a temperature of the discharge stream from the monomer dissolution device may be in a range of -<NUM> to <NUM>. For example, the operating temperature of the monomer dissolution device and the temperature of the discharge stream from the monomer dissolution device may be in a range of -<NUM> to <NUM>, <NUM> to <NUM>, or <NUM> to <NUM>. In the conventional method for preparing an oligomer, in order to cool the recovered gaseous monomer to a liquid phase, a refrigerant at a very low temperature is used to cool the monomer to a temperature of about -<NUM> or less, and thus, there was a problem of increased costs due to the use of an expensive refrigerant. However, in the method for preparing an oligomer according to the present invention, the recovered gaseous monomer is dissolved in the solvent and then supplied to the reactor, thereby solving the above problem.

According to an exemplary embodiment of the present invention, an operating pressure of the monomer dissolution device and a pressure of the discharge stream from the monomer dissolution device may be in a range of <NUM> bar to <NUM> bar. For example, the operating pressure of the monomer dissolution device and the pressure of the discharge stream from the monomer dissolution device may be in a range of <NUM> bar to <NUM> bar, <NUM> bar to <NUM> bar, or <NUM> bar to <NUM> bar. Since the discharge stream from the monomer dissolution device is a liquid phase, the pressure may be increased to the pressure of the reactor only by using the pump, and no additional device or process is required, and thus, the process may be simplified and the process costs may be reduced.

According to an exemplary embodiment of the present invention, the method for preparing an oligomer may satisfy the following General Formula <NUM>: <MAT> wherein M1 is a content of the monomer in the upper discharge stream from the second separation device, and M2 is a content of the monomer in the discharge stream from the monomer dissolution device. Specifically, General Formula <NUM> may mean a dissolution rate of the monomer dissolved in the solvent, in the discharge stream from the monomer dissolution device, when the monomer recovered as the upper discharge stream from the second separation device is dissolved in the solvent in the monomer dissolution device and discharged as the discharge stream from the monomer dissolution device. Here, the dissolution rate of the monomer dissolved in the solvent may be <NUM> wt% to <NUM> wt%, <NUM> wt% to <NUM> wt%, or <NUM> wt% to <NUM> wt%. As such, <NUM> wt% or more, or the entire amount of the recovered monomer is dissolved in the solvent and recovered to the reactor to have an excellent reuse rate of the monomer, thereby improving oligomer production efficiency and reducing process costs.

According to an exemplary embodiment of the present invention, the second discharge stream from the reactor is supplied to a third separation device, an upper discharge stream from the third separation device may be supplied to the reactor, and a lower discharge stream from the third separation device may be supplied to the second separation device. Specifically, the second discharge stream of a liquid phase discharged from the reactor is supplied to the third separation device before being supplied to the second separation device to perform gas-liquid separation, and then the upper discharge stream including the gaseous monomer is supplied to the reactor and the lower discharge stream including the liquid monomer may be supplied to the second separation device. As such, a separation step is added in a manner of supplying the second discharge stream from the reactor to the third separation device before supplying it to the second separation device, thereby decreasing an amount of the monomer recovered as the upper discharge stream in the second separation device. Thus, it is possible to liquefy the monomer at a higher temperature than the temperature in the case of dissolving the monomer recovered as the upper discharge stream from the second separation device in the solvent in the monomer dissolution device.

An apparatus for preparing oligomer is provided. As the apparatus for preparing oligomer, an apparatus for preparing oligomer including: a reactor for oligomerizing a feed stream including a supplied monomer, supplying a first discharge stream to a first separation device, and supplying a second discharge stream to a second separation device; a first separation device for being supplied with the first discharge stream from the reactor; a second separation device for being supplied with the second discharge stream from the reactor and supplying an upper discharge stream to a monomer dissolution device; and a monomer dissolution device for dissolving the supplied upper discharge stream from the second separation device in a supplied solvent and supplying a discharge stream to the reactor, may be provided.

The apparatus for preparing oligomer may be an apparatus for performing the process according to the method for preparing an oligomer described above.

The apparatus for preparing oligomer may be described with reference to the following <FIG>. For example, the apparatus for preparing oligomer includes a reactor <NUM> for oligomerizing a feed stream including a supplied monomer, and in the reactor <NUM>, a first discharge stream including a gaseous monomer may be supplied to a first separation device <NUM> and a second discharge stream including a liquid monomer may be supplied to a second separation device <NUM>.

The feed stream supplied to the reactor <NUM> may include the monomer and the solvent. Specifically, the feed stream may include the gaseous monomer, the liquid monomer, and the solvent. The feed stream including the gaseous monomer may include a gaseous monomer stream directly supplied to the reactor <NUM> and the gaseous monomer recovered as an upper discharge stream from the second separation device <NUM>. The gaseous monomer stream directly supplied to the reactor <NUM> and the gaseous upper discharge stream recovered in the first separation device <NUM> may be separately supplied to the reactor <NUM>, or supplied to the reactor <NUM> as a mixed discharge stream mixed in a mixer (not shown).

In addition, among the feed stream supplied to the reactor, a stream including the liquid monomer may include a lower discharge stream including the liquid monomer separated in the first separation device <NUM> and a discharge stream from the monomer dissolution device <NUM> in which the gaseous monomer separated in the second separation device <NUM> is dissolved in the solvent. Here, the upper discharge stream from the second separation device <NUM> may be supplied to the reactor <NUM> by a pump <NUM>, after the upper discharge stream is dissolved in the solvent supplied to the monomer dissolution device <NUM> in the monomer dissolution device <NUM>.

The first separation device <NUM> may be supplied with the first discharge stream from the reactor <NUM> and separate the stream into the upper discharge stream including the gaseous monomer and the lower discharge stream including the liquid monomer. Here, the upper discharge stream and the lower discharge stream from the first separation device <NUM> may be supplied to the reactor <NUM>.

The lower discharge stream from the second separation device <NUM> may include an oligomer product, a by-product, and the solvent. Here, the oligomer product, the by-product, and the solvent may be separated by an additional separation apparatus (not shown), and the separated solvent may be reused in the oligomer production process. In addition, when the oligomerization reaction is performed using an ethylene monomer as the monomer, the oligomer product may include <NUM>-hexene and <NUM>-octene. In this case, <NUM>-hexene and <NUM>-octene may be separated by an additional separation device (not shown) or separated by a separate process and used.

A third separation device <NUM> which is supplied with the second discharge stream from the reactor <NUM> and supplies the lower discharge stream to the second separation device <NUM> may be further included. Specifically, before the second discharge stream from the reactor <NUM> is supplied to the second separation device <NUM>, the second discharge stream is subjected to gas-liquid separation once by the third separation device <NUM>, and then an upper discharge stream from the third separation device <NUM> including the gaseous monomer may be supplied to the reactor <NUM> and a lower discharge stream from the third separation device <NUM> including the liquid monomer may be supplied to the second separation device <NUM>.

Hereinabove, the method for preparing an oligomer according to the present invention and the apparatus for preparing oligomer have been described and illustrated in the drawings, but the description and the illustration in the drawings are the description and the illustration of only core constitutions for understanding of the present invention, and in addition to the process and apparatus described above and illustrated in the drawings, the process and the apparatus which are not described and illustrated separately may be appropriately applied and used for carrying out the method for preparing an oligomer according to the present invention and the apparatus for preparing oligomer.

Hereinafter, the present invention will be described in more detail by the Examples.

For the process flowchart illustrated in <FIG>, the process was simulated using an Aspen Plus simulator from Aspen Technology, Inc. Here, ethylene (C2) as the monomer was supplied to the reactor <NUM> at a reaction amount of <NUM>,<NUM>/hr or more, reaction conditions of the reactor <NUM> were set to a temperature of <NUM> and a pressure of <NUM> bar, methylcyclohexane was used as the solvent, and an amount of the solvent supplied was an equivalent amount to the sum of the product and the by-product in the second discharge stream supplied to the second separation device <NUM> in the reactor <NUM>. The results are shown in the following Table <NUM>.

Referring to Table <NUM>, it was confirmed in Stream <NUM> that ethylene recovered as Stream <NUM> was dissolved at a dissolution rate of <NUM>%, and at this time, the temperature at which recovered ethylene is dissolved in the solvent in the monomer dissolution device <NUM> and discharged was <NUM>, and it was confirmed therefrom that the cooling temperature of the recovered ethylene was significantly higher than the conventional temperature of -<NUM>. In addition, the pressure of Stream <NUM> was <NUM> bar, and the pressure of Stream <NUM> after passing through the pump <NUM> was <NUM> bar, from which it was found that the pressure was increased to the same pressure as the reactor <NUM>.

The process was performed in the same manner as in Example <NUM>, except that methylcyclohexane was used as a first solvent and decane was used as a second solvent to simulate the process. Here, the first solvent was introduced at an equivalent amount to the sum of the product and the by-product, and the second solvent was introduced <NUM> times the sum of the product and the by-product. The results are shown in the following Table <NUM>.

Referring to Table <NUM>, when two solvents were used, ethylene recovered as Stream <NUM> was dissolved at a dissolution rate of <NUM>% in Stream <NUM>, as compared with Example <NUM>. In addition, the temperature at which the recovered ethylene is dissolved in the solvent in the monomer dissolution device <NUM> and discharged was <NUM>, and it was found that the recovered ethylene monomer may be liquefied at a higher temperature than the temperature in the case of using one solvent at <NUM>.

For the process flowchart illustrated in <FIG>, the process was simulated using an Aspen Plus simulator from Aspen Technology, Inc. Here, ethylene (C2) as the monomer was supplied to the reactor <NUM> at a reaction amount of <NUM>,<NUM>/hr or more, the reaction conditions of the reactor <NUM> were set to a temperature of <NUM> and a pressure of <NUM> bar, methylcyclohexane was used as the solvent, and an amount of the solvent supplied was an equivalent amount to the sum of the product and the by-product in the second discharge stream supplied to the second separation device <NUM> in the reactor <NUM>. The results are shown in the following Table <NUM>.

It was confirmed from Table <NUM> that in Stream <NUM>, ethylene recovered as Stream <NUM> was dissolved at a dissolution rate of <NUM>%. In addition, as compared with Example <NUM>, the third separation device <NUM> was further installed between the reactor <NUM> and the second separation device <NUM> to partially recover the ethylene monomer primarily in the reactor <NUM>, thereby decreasing the amount of the recovered ethylene monomer supplied to the monomer dissolution device <NUM> and liquefying the ethylene monomer at a higher temperature of <NUM>.

For the process flowchart illustrated in <FIG>, the process was simulated using an Aspen Plus simulator from Aspen Technology, Inc. Here, ethylene (C2) as the monomer was supplied to the reactor <NUM> at a reaction amount of <NUM>,<NUM>/hr or more, the reaction conditions of the reactor <NUM> were set to a temperature of <NUM> and a pressure of <NUM> bar, methylcyclohexane was used as the solvent supplied to the reactor <NUM>, and an amount of the solvent supplied was an equivalent amount to the sum of the product and the by-product in the second discharge stream supplied to the second separation device <NUM> in the reactor <NUM>. The results are shown in the following Table <NUM>.

Referring to Table <NUM>, in order to increase the pressure of Stream <NUM>-<NUM> including the recovered gaseous ethylene monomer to the pressure of the reactor <NUM> using the pump <NUM>, the stream should be a liquid phase. To this end, the condenser <NUM> is needed, and it was found from Stream <NUM>-<NUM> which is a stream including a condensed ethylene monomer in the condenser <NUM> that Stream <NUM>-<NUM> was at a temperature of -<NUM>, which is a significantly low temperature as compared with Examples <NUM> to <NUM>.

For the process flowchart illustrated in <FIG>, the process was simulated using an Aspen Plus simulator from Aspen Technology, Inc. Here, ethylene (C2) as the monomer was supplied to the reactor <NUM> at a reaction amount of <NUM>,<NUM>/hr or more, the reaction conditions of the reactor <NUM> were set to a temperature of <NUM> and a pressure of <NUM> bar, methylcyclohexane was used as the solvent supplied to the reactor, and the amount of the solvent supplied was an equivalent amount to the sum of the product and the by-product in the second discharge stream supplied to the second separation device <NUM> in the reactor <NUM>. The results are shown in the following Table <NUM>.

Claim 1:
A method for preparing an oligomer, the method comprising:
supplying a feed stream including a monomer to a reactor and performing an oligomerization reaction;
supplying a first discharge stream from the reactor to a first separation device and supplying a second discharge stream from the reactor to a second separation device;
recovering the monomer in the second separation device as an upper discharge stream;
supplying the upper discharge stream including the monomer from the second separation device to a monomer dissolution device to dissolve the upper discharge stream in a solvent supplied to the monomer dissolution device;
supplying a discharge stream from the monomer dissolution device to the reactor,
wherein the discharge stream from the monomer dissolution device is a liquid stream including the monomer dissolved in the solvent, and
wherein the monomer is ethylene and the oligomer is an alpha-olefin.