CN112410071A - Method and system for separating refinery unsaturated dry gas - Google Patents

Abstract

The invention belongs to the technical field of refinery dry gas recovery, and provides a method and a system for separating unsaturated dry gas in a refinery. The invention is used for recovering the carbon dioxide component by a multi-stage absorption-high-low pressure flash evaporation-desorption mode, simultaneously reduces the consumption of the lean solvent and the desorption load, and finally reduces the energy consumption of the whole device. And the ethylene-rich gas extracted from the tower top of the rough separation tower is sent to a demethanizer or an alkali washing tower of the ethylene device, and the ethane-rich gas extracted from the tower bottom of the rough separation tower is sent to a cracking furnace of the ethylene device. The invention has simple flow, high carbon recovery rate, low energy consumption, low investment, respective treatment of rich ethylene and rich ethane and basically no influence on the operation of an ethylene device.

Description

Method and system for separating refinery unsaturated dry gas

Technical Field

The invention belongs to the technical field of refinery dry gas recovery, and particularly relates to a method and a system for separating refinery unsaturated dry gas.

Background

The dry gas is mainly from primary and secondary processing devices of crude oil and also from partial chemical devices. At present, most of dry gas generated by refineries in China is used as fuel to be burnt, and some dry gas is even put into a torch to be burnt, so that the utilization value is low, and the serious waste of resources and the environmental pollution are caused. Because the dry gas contains a large amount of carbon dioxide components, wherein ethylene is a good chemical product, and ethane is a very ideal cracking raw material, if the dry gas is recycled, the economic benefit is greatly improved, and the environmental pollution is reduced.

The dry gas is generally divided into saturated dry gas and unsaturated dry gas according to the olefin content, wherein the ethylene content in the catalytic dry gas is higher, generally more than 5 mol%, and belongs to the unsaturated dry gas, the saturated dry gas is generally PSA desorption gas, light hydrocarbon dry gas, hydrogenation dry gas, coking dry gas, reforming dry gas and the like, the ethane content and the propane content in the saturated dry gas are higher, and the olefin content is very low.

At present, methods for recovering a carbon dioxide component from refinery dry gas mainly comprise a cryogenic separation method, a cold oil absorption method, a Pressure Swing Adsorption (PSA) method and the like, and various methods have advantages and disadvantages. The cryogenic separation method is developed and improved for decades, has mature technology, perfect process and high product purity and recovery rate, and is a common method for cracking ethylene. However, the method generally needs to carry out gas separation at a low temperature of about-100 ℃, has large cold load and complex refrigeration process, and also needs to carry out pretreatment on raw material gas, thus leading to large investment of the device. In addition, the cryogenic separation method is generally suitable for areas with centralized refineries and large dry gas byproduct quantity, and the method has poor economy under the condition that the refineries in China are relatively small in scale and relatively dispersed. The pressure swing adsorption method can realize normal temperature operation, has high automation degree, simple operation, low energy consumption and environmental protection, but the method has huge equipment, more complex control system, lower purity of the obtained ethylene and low recovery rate, and usually needs to adopt multi-stage pressure swing adsorption to obtain polymerization-grade ethylene, thereby increasing the occupied area and equipment investment.

The cold oil absorption method belongs to physical absorption, and realizes the separation of dry gas by utilizing the solubility difference of each component in the dry gas in an absorbent, wherein the absorbent is generally mixed carbon four, mixed carbon five, liquefied gas and the like, firstly, non-condensable gas components such as methane, hydrogen and the like are absorbed and removed, and then, components C2 and C3 are recovered by a desorption method. The cold oil absorption method comprises an intermediate cold oil absorption method (-40 to-20 ℃) and a shallow cold oil absorption method (more than 0 ℃), and the existing cold oil absorption method has the problems of large circulation quantity of a poor solvent, high energy consumption and the like because desorption is separated by thermal desorption.

The patent CN 109553504A provides a method and a device for recovering refinery saturated dry gas by adopting a shallow cold oil absorption technology, the process comprises the steps of compression, absorption, desorption, reabsorption desorption and the like, the absorption temperature is 5-15 ℃, the operation condition is mild, but the absorption temperature is high, so that the solvent circulation is high, the device energy consumption is high, and the contents of C3 and heavy components in the absorbed methane hydrogen are high, so that two sets of absorption-desorption systems are required, and the process is complicated.

The patent CN 104557387B adds a rough separation tower on the basis of the patent CN 109553504A, realizes the separation of rich ethylene gas and rich ethane gas, and makes the separation utilized respectively, but the problems of large solvent circulation amount and high energy consumption of the device are not solved fundamentally.

Patent CN 104892340 a discloses a three-tower apparatus and method for recovering ethylene and ethane from oil absorption dry gas, which increases the recovery rate of methane and reduces the methane content in the ethylene product by increasing the cooling and flashing modes and reducing the absorption operation temperature in front of the desorption tower, but the cooling and flashing technology is essentially equivalent to adding a dephlegmator system at the top of the desorption tower, the flash tank is equivalent to a gas-liquid buffer tank behind the dephlegmator, and is finally equivalent to performing secondary methane and ethylene separation in the dephlegmator mode, thereby achieving the purpose of improving the recovery rate of methane, but not solving the problem of high process energy consumption.

The invention discloses a method for separating refinery dry gas by an intercooling oil absorption method, which comprises the steps of compression, dry gas pretreatment, absorption, desorption, cold energy recovery, rough separation and the like, wherein liquefied gas in a refinery is used as an absorbent, a cold box-expander system is arranged for recovering lost absorbent and carbon dioxide, and the method has the advantages of low absorbent cost, low loss, high carbon dioxide recovery rate, no need of an ethylene refrigeration compressor and the like.

In conclusion, the existing process for recovering carbon from refinery dry gas generally has the problems of large investment and high energy consumption. Therefore, the invention develops a process with low energy consumption, small investment and high recovery rate around the recycling and separate treatment of ethylene and ethane.

Disclosure of Invention

In order to realize reliable recycling of refinery dry gas and solve the problems of large investment, high energy consumption and the like, the invention provides a method and a system for separating refinery unsaturated dry gas. The invention adopts the intercooling oil absorption technology, recovers the carbon two components by a multi-stage absorption-high and low pressure flash evaporation-desorption mode, can obtain solvents with different purities by a high and low pressure flash evaporation mode, simultaneously flashes part of carbon two gases, can greatly reduce the use amount and desorption load of lean solvents, and finally reduces the comprehensive energy consumption of the whole device. The method has the advantages of simple flow, small solvent consumption, low comprehensive energy consumption of the device, and no influence on the operation of the ethylene device due to the respective treatment of the ethylene-rich gas and the ethane-rich gas.

The invention provides a system for separating unsaturated dry gas in a refinery plant, which comprises:

the system comprises a dry gas pretreatment system, a dry gas precooling heat exchanger, a multi-stage absorption tower, a high-pressure zone flash tank, a low-pressure zone flash tank, a carbon dioxide concentration gas compressor, a desorption tower and a rough separation tower.

The dry gas pretreatment system is connected with a dry gas cooler and then connected with a multi-stage absorption tower, the bottom of the absorption tower is sequentially connected with a high-pressure zone flash tank and a low-pressure zone flash tank, the top of the high-pressure zone flash tank is connected with a dry gas compressor, the bottom of the high-pressure zone flash tank is respectively connected with the multi-stage absorption tower and the low-pressure zone flash tank, the top of the low-pressure zone flash tank is connected with a carbon dioxide concentration gas compressor, the bottom of the low-pressure zone flash tank is respectively connected with the multi-stage absorption tower and a desorption tower, the top of the carbon dioxide concentration gas compressor and the top.

The invention provides a method for separating refinery unsaturated dry gas, which comprises the following steps:

(1) the unsaturated dry gas of the refinery after deacidification, drying and compression treatment of a dry gas pretreatment system, wherein the ethylene content in the dry gas is more than 5 mol%, particularly catalytic cracking dry gas, is cooled and then sent into a multi-stage absorption tower for treatment, the gas phase at the top of the multi-stage absorption tower is recycled and sent into a fuel gas pipe network or a PSA device, and the liquid phase at the bottom of the multi-stage absorption tower is sent into a high-pressure flash evaporation zone for treatment;

(2) the high-pressure flash evaporation area is provided with a plurality of stages of high-pressure flash evaporation areas for flash evaporation, the gas phase obtained by flash evaporation is returned to the compression section of the dry gas pretreatment system, part of the obtained liquid phase is used as a semi-lean solvent to circularly return to the multi-stage absorption tower, and the other part of the obtained liquid phase is sent to the low-pressure flash evaporation area for treatment;

(3) the low-pressure flash evaporation zone is provided with a plurality of stages of low-pressure zone flash evaporation, the gas phase obtained by flash evaporation is sent to the inlet of a carbon dioxide concentration gas compressor, part of the obtained liquid phase is used as a secondary lean solvent to circularly return to a multi-stage absorption tower, and the other part of the obtained liquid phase is sent to a desorption tower for treatment;

(4) mixing the gas phase obtained by the treatment of the desorption tower with the gas phase in the low-pressure flash evaporation area to obtain carbon dioxide concentrated gas, returning most of the liquid phase obtained by the carbon dioxide desorption tower to the multistage absorption tower as a lean solvent, taking the small part of the liquid phase as extracted carbon four, sending the extracted carbon four out of a boundary area, and externally supplementing a fresh carbon four absorbent;

(5) and feeding the carbon dioxide concentrated gas into a rough separation tower, obtaining ethylene-rich gas at the top of the rough separation tower, and obtaining ethane-rich gas at the bottom of the rough separation tower.

The specific working principle is as follows:

the front-end flow of the invention is consistent with the deep cooling process and the middle cooling oil absorption process, and dry gas enters the separation unit after pretreatment, drying and pressurization. The process is used for treating refinery dry gas (3-5 MpaG) after pretreatment, drying and pressurization, wherein the dry gas is unsaturated dry gas with ethylene content higher than 5 mol%, and particularly is catalytic dry gas.

The invention selects mixed carbon four as an absorbent, the circulating poor solvent comprises 80-95 mol% of carbon four, the balance is a small amount of carbon three and carbon five, the circulating sub-poor solvent comprises 50-80 mol% of carbon four, and the circulating semi-poor solvent comprises 30-70 mol% of carbon four.

The absorbent used in the present invention is not limited to the carbon four-cut fraction, and may be any of various absorbents commonly used in the art that satisfy the above-mentioned absorption requirements. Specifically, it may be a carbon three cut, a carbon four cut or a carbon five cut commonly used in the art, preferably alkane components in the carbon three, carbon four and carbon five cuts, and more preferably a carbon four cut containing n-butane and isobutane or a liquefied gas containing a saturated carbon three cut and carbon four cut. The carbon-three absorbent and the carbon-four absorbent can be refinery liquefied gas or mixed carbon-four components, the dosage of the absorbent is not particularly limited in the invention, and can be determined by those skilled in the art according to actual conditions, which are well known by those skilled in the art and will not be described herein again.

In the step (1), the compressed dry gas is cooled to-15 to-40 ℃, propylene refrigeration is adopted in the cooling treatment, and primary refrigeration to tertiary refrigeration is adopted in combination with the operation temperature of other equipment in the process.

In the step (1), the theoretical plate number of the multistage absorption tower is preferably 30-60, the operation pressure is 3.5-5 MPaG, the tower top temperature is preferably-15 ℃ to-35 ℃, and the tower kettle temperature is preferably-20 ℃ to-30 ℃. The multi-stage absorption tower is not provided with a reboiler and a condenser, a plurality of intercoolers, preferably two intercoolers, are arranged at the upper section and the lower section of the tower, the intercoolers at the upper section are respectively used for recovering liquid phase cold energy from a low-pressure zone flash tank to a desorption tower, and the intercoolers at the lower section adopt propylene at the temperature of minus 40 ℃ for refrigeration.

The technology is understood that the refinery dry gas after pretreatment and pressurization is precooled, then enters the bottom of a multi-stage absorption tower to be in countercurrent contact with a semi-lean solvent, a sub-lean solvent and a lean solvent in sequence to absorb carbon dioxide and heavier components in materials, hydrogen and methane gas which are not absorbed are mainly obtained at the top of the multi-stage absorption tower, and the obtained gas is sent to a fuel gas pipe network or a PSA device for hydrogen recovery after cold recovery.

In the step (2), one or more flash tanks can be arranged in the high-pressure zone flash tank, the flash tanks are sequentially connected in series under reduced pressure, and the pressure of the last flash tank is 0.1-0.3 MpaG. And (2) feeding the gas phase obtained by treatment of each flash tank in the high-pressure flash zone into each stage of a corresponding compressor of the dry gas pretreatment system according to the pressure, wherein one part of the obtained liquid phase is used as a semi-lean solvent to circulate to the multistage absorption tower, and the other part of the obtained liquid phase is fed into the low-pressure flash zone, wherein the liquid phase entering the low-pressure flash zone is the liquid phase of the last flash tank, the semi-lean solvent can be the liquid phase of each flash tank, the semi-lean solvent enters the lower section of the multistage absorption tower, the feeding plate is preferably 20-45, and the temperature of.

The partial technology is understood as that light components such as methane and hydrogen absorbed are separated through reduced pressure flash evaporation, sent into a dry gas compression system and mixed with feed materials and then returned to a multi-stage absorption tower, so that the recovery rate of the methane and the hydrogen can be improved, the content of the methane and the hydrogen in the final carbon dioxide concentrated gas can be reduced, and the recovery rate of the methane and the hydrogen can be regulated and controlled through flash evaporation pressure, so that the content of the methane and the hydrogen is low and controllable. By pumping the semi-lean solvent out of the high-pressure flash evaporation zone, the dosage of the lean solvent can be greatly reduced, the treatment capacity of a subsequent carbon dioxide desorption tower is reduced, and the energy consumption is finally reduced.

In the step (3), one or more flash tanks can be arranged in the flash tank in the low-pressure region, the flash tanks are sequentially connected in series under reduced pressure, the pressure of the last flash tank is 0-0.1 MpaG, the gas phase obtained by the treatment of the low-pressure flash zone is used as a part of carbon dioxide concentration gas, the gas phase enters a carbon dioxide concentration gas compressor to be pressurized and sent out of a boundary region, a part of the obtained liquid phase is used as a secondary lean solvent to be circulated back to the multistage absorption tower, and a part of the obtained liquid phase is sent into the desorption tower, wherein the liquid phase sent into the desorption tower is the liquid phase of the last flash tank, the secondary lean solvent can be each liquid phase, the secondary lean solvent enters the middle part of the multistage absorption tower, the feeding plate is preferably 5.

The partial technology is understood as that partial carbon dioxide components are recovered by pressure reduction flash evaporation instead of thermal desorption, and the content of the carbon quadruple components in the recovered carbon dioxide product gas and the carbon dioxide flash evaporation amount are controlled by regulating the flash evaporation pressure, so that the desorption amount of a subsequent desorption tower is reduced, and the energy consumption is reduced. By setting the secondary lean solvent circulation, the consumption of the lean solvent can be continuously reduced, the treatment capacity of a subsequent desorption tower is reduced, and the energy consumption is continuously reduced.

In the step (4), the residual three components of carbon dioxide are recovered by a thermal desorption mode of a desorption tower, the theoretical plate number of the desorption tower is 20-60, the operation pressure is 0.5 MpaG-4 MpaG, the tower top temperature is-35-40 ℃, and the tower kettle temperature is 60-130 ℃.

The partial technology is understood that after a part of carbon dioxide products are recovered by reduced pressure flash evaporation, the residual carbon dioxide three components are recovered by a thermal desorption mode of a desorption tower, and the carbon dioxide three-enriched product gas obtained at the tower top is mixed with the carbon dioxide enriched gas recovered by reduced pressure flash evaporation and then sent to a rough separation tower. Most of the liquid phase obtained from the tower bottom is used as a poor solvent, is cooled to-15 to-40 ℃ through heat recovery, returns to the top of the multi-stage absorption tower, and the rest part is sent out of a battery limit. In order to ensure the flow rate of the lean solvent in the system, it is further preferred that the fresh carbon tetra-absorbent is replenished during the cooling of the lean solvent, and the present invention has no particular requirement on the temperature of the replenished fresh carbon tetra-absorbent itself.

The partial technology is understood that in the invention, part of liquid phase extracted from the tower kettle of the desorption tower is sent out of a boundary region, and the fresh carbon four absorbent is supplemented, so that the aim of preventing the accumulation of heavy components in the solvent is fulfilled, the tower kettle temperature of the desorption tower is overhigh, and the absorption effect and the stable operation of the device are influenced. The extraction amount of the tower kettle of the desorption tower and the supplement amount of the fresh carbon four absorbent are different according to different compositions of dry gas raw materials, more heavy components above C4 in a plurality of gas raw materials need to be extracted and supplemented in large amount, and if the heavy components are less, the extraction amount is less or only intermittent extraction is needed.

In the steps (3) and (4), the obtained carbon dioxide concentrated gas mainly contains 30-90 mol% of carbon dioxide, 10-50 mol% of carbon III, and the content of methane is lower than 5 mol% and is controllable.

And (5) feeding the obtained carbon dioxide concentrated gas into a coarse separation tower, sending ethylene-rich gas obtained at the tower top to an alkaline washing tower or a demethanizer of an ethylene device according to the content of the acid gas, sending the ethane-rich gas obtained at the tower bottom to a cracking furnace of the ethylene device, wherein the theoretical plate number of the coarse separation tower is 20-80, the operation pressure is 0.5-4.0 MPaG, the ethylene-rich gas mainly contains 20-80 mol% of ethylene and 10-40 mol% of ethane, and the ethane-rich gas mainly contains 50-90 mol% of ethane and less than 5 mol% of ethylene.

In the invention, the desorption tower and the rough separation tower are provided with reboilers to ensure that the recovery rate of the carbon dioxide concentrated product obtained from the tower top and the purity of the ethane-rich gas and the ethylene-rich gas meet the process requirements, and the heating medium of the reboilers can adopt low-pressure steam, and can also adopt low-temperature hot oil or low-temperature hot water of a refinery.

The method for utilizing the refinery mixed dry gas has the following advantages:

(1) in the invention, solvents with different purities can be obtained by adopting a high-pressure flash evaporation mode and a low-pressure flash evaporation mode, the consumption of the poor solvent is greatly reduced, meanwhile, part of carbon dioxide gas can be flashed out by the low-pressure flash evaporation mode, the desorption load can be reduced, and the comprehensive energy consumption of the whole device is finally reduced;

(2) in the invention, two products are obtained by rough separation: the ethylene-rich gas and the ethane-rich gas can be sent to a cracking furnace and an alkaline washing tower or a demethanizer of an ethylene device according to respective composition characteristics, so that respective composition utilization is fully exerted, the comprehensive utilization and respective treatment of ethylene and ethane are realized, and the adverse effect on the ethylene device is avoided;

(3) in the invention, because different treatment modes are adopted for the saturated hydrocarbon gas and the unsaturated hydrocarbon gas, no requirement is made on refinery dry gas, and the process adaptability is strong;

(4) in the invention, the amount of the absorbed agent carried in the light component gas of methane and hydrogen at the top of the multi-stage absorption tower is less, and a cold box-expander system or a reabsorption system is not required to be arranged, thereby greatly reducing the equipment investment;

(5) in the invention, the lowest operation temperature of other systems except the secondary lean solvent circulating pipeline is not lower than-40 ℃, so that the equipment and pipelines of the systems can be made of common low-temperature carbon steel, and the equipment investment is saved.

Drawings

FIG. 1 is a schematic view of a system for separating unsaturated dry gas of a refinery plant according to the present invention

In the figure: 1, a dry gas pretreatment system; 2, a dry gas precooler; 3, a multi-stage absorption tower; 4, a high-pressure zone flash evaporation tank; 5 a low-pressure zone flash tank; 6 carbon two concentrated gas compressor system; 7 a desorption tower; 8 lean solvent cooling heat exchanger; 9, roughly dividing the tower; a, refinery dry gas; b, absorbing tail gas; c, adding carbon dioxide to extract concentrated gas; d, a lean solvent; e lean solvent; f, semi-lean solvent; g, extracting carbon four; h, fresh make-up solvent; i ethylene-rich gas; j ethane-rich gas.

Detailed Description

The technical solution of the present invention will be clearly and completely described below. It is to be understood that the described embodiments are merely a few embodiments of the invention, and not all embodiments. All other embodiments that can be modified or adapted by one of ordinary skill in the art based on the embodiments of the present invention are within the scope of the present invention.

Fig. 1 is an example of a method and a system for separating unsaturated dry gas in a refinery provided by the present invention, in which a plurality of dry gas pre-cooling heat exchangers, a plurality of absorption tower intercoolers, a plurality of carbon dioxide concentration gas compressor stages, a high-pressure zone flash tank, and a low-pressure zone flash tank may be provided as required, in this embodiment, two dry gas pre-cooling heat exchangers, two absorption tower intercoolers, two high-pressure zone flash tanks, and two low-pressure zone flash tanks are provided, and a carbon dioxide concentration gas compression system is provided with three stages, and in this embodiment, only a second lean solvent cycle is provided according to an optimization result, and a semi-lean solvent cycle is:

the composition of the dry gas is shown in table 1, the refinery dry gas comprises catalytic dry gas, coking dry gas, PSA (pressure swing adsorption) analysis gas, hydrocracking dry gas and ethylbenzene alkylation tail gas, and the composition given in table 1 is the mixed composition of the dry gas.

TABLE 1

Name (R)	Refinery dry gas
		Temperature, C	40
Pressure, MpaG	3.77
		Mass flow rate, kg/hr	30542
Molar flow, kmol/hr	2010
		Composition in mol%
Hydrogen gas	41.73
		Carbon monoxide	0.58
Oxygen gas	0.59
		Nitrogen is present in	7.18
Methane	28.29
		Ethane (III)	14.82
Ethylene	2.05
		Propane	2.65
Propylene (PA)	0.30
		N-butane	1.13
Isobutane	0.06
		1-butene	0.02
Isobutene	0.01
		Cis-butenediol	0.02
N-pentane	0.49
		Isopentane	0.06
N-hexane	0.01

The method for separating the unsaturated dry gas in the refinery plant comprises the following specific steps:

(1) the dry gas of each device in the refinery enters the process of the invention after pretreatment, compression, drying and mixing, and the pressure of the dry gas entering the process is 3.75 MpaG. Cooling the dry gas to-23 ℃ through a dry gas precooling heat exchanger, and sending the cooled dry gas to the bottom of the multistage absorption tower;

(2) in the multistage absorption tower 3, the absorbent is a mixed carbon four absorbent, the total absorbent amount is 58t/h, wherein the lean solvent amount is 23t/h, the sub-lean solvent amount is 35t/h, the lean solvent is sprayed from the top of the multistage absorption tower, and the sub-lean solvent is sprayed from the middle of the multistage absorption tower and is in countercurrent contact with the dry gas for absorption. The theoretical plate number of the multistage absorption tower is 30, the operation pressure is 3.6MpaG, the tower top temperature is-28.4 ℃, and the tower kettle temperature is-17.2 ℃.

(3) Liquid phase materials from the tower bottom of the multistage absorption tower are sent to a high-pressure zone flash tank for processing, cold energy recovery is carried out on gas phase materials from the tower top of the multistage absorption tower by heat exchange with dry gas to 33 ℃, the gas phase materials mainly comprise unabsorbed methane, hydrogen and the like and are sent to a fuel gas pipe network or a PSA device for processing, and if the gas phase materials are sent to the fuel gas pipe network, throttling depressurization is preferably carried out to 0.9MpaG, and then cold energy recovery is carried out on the gas phase materials by heat exchange with the dry gas to 33 ℃.

(4) Preferably, the liquid phase material from the tower bottom of the multistage absorption tower is depressurized to 1MpaG and then sent into a high-pressure zone first-level flash tank, the liquid phase at the bottom of the high-pressure zone first-level flash tank is depressurized to 0.3MpaG and then sent into a high-pressure zone second-level flash tank, the gas phase (rich in hydrogen and methane) at the top of the high-pressure zone flash tank is sent into a dry gas compression system according to different pressures, and the liquid phase at the bottom of the high-pressure zone second-level flash tank is sent into a low.

(5) Preferably, the liquid phase from the high-pressure zone secondary flash tank is depressurized to 0.13MpaG and then is sent to the low-pressure zone primary flash tank, the liquid phase at the bottom of the low-pressure zone primary flash tank is depressurized to 0MpaG and then is sent to the low-pressure zone secondary flash tank, the gas phase at the top of the low-pressure zone primary flash tank is sent to a carbon dioxide concentration gas primary compressor, the gas phase at the top of the low-pressure zone secondary flash tank is sent to a carbon dioxide concentration gas secondary compressor, and the carbon dioxide concentration gas (1.9MpaG) pressurized by a carbon dioxide concentration gas compression system is mixed with the gas at the top of the desorption tower to be used. The liquid phase at the bottom of the secondary flash tank in the low-pressure area is divided into two branches, one branch is used as a secondary lean solvent (35t/h and 59 ℃) for circular absorption, and the other branch sequentially passes through a intercooler of a multi-stage absorption tower and a dry gas precooling heat exchanger for cold recovery to 33 ℃ and enters a desorption tower.

(6) The theoretical plate number of the desorption tower is 30, the operation pressure is preferably 3.5MpaG, the tower top temperature is 40.8 ℃, and the tower bottom temperature is 152 ℃. Most of the liquid phase at the bottom of the desorption tower is circularly absorbed as a lean solvent (23t/h) after being supplemented with fresh solvent. The lean solvent is cooled to 0 ℃ by a-6 ℃ propylene refrigerant through a lean solvent cooling heat exchanger, and is cooled to-35 ℃ by a-40 ℃ propylene refrigerant through a lean solvent cooling heat exchanger, and then is sent to the top of the multistage absorption tower.

(7) And (3) feeding the carbon dioxide concentrated gas into a rough separation tower, wherein the number of theoretical plates of the rough separation tower is 40, the operating pressure is 1.8MPaG, the operating reflux ratio is 13, an ethylene-rich product is arranged at the top of the rough separation tower, and an ethane-rich product is arranged at the bottom of the rough separation tower.

The ethylene-rich product obtained was 1200kg/h with the composition shown in Table 2.

TABLE 2

Composition of	mol％
		Hydrogen gas	0.05
Methane	31.16
		Ethane (III)	28.26
Ethylene	40.18

The ethane-rich product obtained was 10905kg/h with the composition shown in Table 3.

TABLE 3

Composition of	mol％
		Methane	0.07
Ethane (III)	79.56
		Ethylene	1.86
Propane	14.06
		Propylene (PA)	1.66
N-butane	1.41
		Isobutane	1.22

The composition of the lean solvent, the second lean solvent is shown in Table 4.

TABLE 4

Name (R)	Lean solvent	Sub-lean solvent
			Mass flow rate, t/hr	23	35
Composition in mol%
			Methane	0.00	0.04
Ethane (III)	0.66	21.06
			Ethylene	0.01	0.93
Propane	4.22	9.99
			Propylene (PA)	0.32	1.03
N-butane	45.96	33.25
			Isobutane	27.02	16.70
1-butene	0.52	0.40
			Isobutene	0.31	0.25
Butene of trans-butene	0.05	0.04
			Cis-butenediol	0.49	0.38
N-pentane	18.19	14.16
			Isopentane	1.98	1.54

In this example, the carbon recovery was 91.4% and the ethane recovery was 94.9%.

Claims (14)

1. A method for separating refinery unsaturated dry gas is characterized by comprising the following steps:

(1) carrying out deacidification, drying and compression treatment on the catalytic cracking dry gas by a dry gas pretreatment system, wherein the ethylene content in the catalytic cracking dry gas is more than 5 mol%, cooling the catalytic cracking dry gas, sending the catalytic cracking dry gas into a multistage absorption tower for treatment, recycling cold energy from a gas phase at the top of the multistage absorption tower, sending the gas phase into a fuel gas pipe network or a PSA (pressure swing adsorption) device, and sending a liquid phase at the bottom of the multistage absorption tower into a high-pressure flash evaporation area for treatment;

(2) the high-pressure flash evaporation area is provided with a plurality of stages of high-pressure flash evaporation areas for flash evaporation, the gas phase obtained by flash evaporation is returned to the compression section of the dry gas pretreatment system, part of the obtained liquid phase is used as a semi-lean solvent to circularly return to the multi-stage absorption tower, and the other part of the obtained liquid phase is sent to the low-pressure flash evaporation area for treatment;

(3) the low-pressure flash evaporation zone is provided with a plurality of stages of low-pressure zone flash evaporation, the gas phase obtained by flash evaporation is sent to the inlet of a carbon dioxide concentration gas compressor system, part of the obtained liquid phase is used as a secondary lean solvent to circularly return to a multi-stage absorption tower, and the other part of the obtained liquid phase is sent to a desorption tower for treatment;

(4) mixing the gas phase obtained by the treatment of the desorption tower with the gas phase in the low-pressure flash evaporation area to obtain carbon dioxide concentrated gas, returning most of the liquid phase obtained by the carbon dioxide desorption tower to the multistage absorption tower as a lean solvent, taking the small part of the liquid phase as extracted carbon four, sending the extracted carbon four out of a boundary area, and externally supplementing a fresh carbon four absorbent;

(5) feeding the carbon dioxide concentrated gas into a rough separation tower, obtaining ethylene-rich gas at the top of the rough separation tower, and obtaining ethane-rich gas at the bottom of the rough separation tower; the treatment method of the rough separation tower comprises the following steps: and (3) sending the carbon dioxide concentrated gas to a rough separation tower, wherein the theoretical plate book of the rough separation tower is 20-80, and the operating pressure is 0.5-4 MPaG.

2. The method for separating refinery unsaturated dry gas according to claim 1, wherein the multistage absorption tower treatment method comprises: and (3) sending the cooled dry gas to a multistage absorption tower to contact with a carbon four absorbent, wherein the number of theoretical plates of the multistage absorption tower is 30-60, the operating pressure is 3.5-5 MpaG, the temperature of the top of the tower is-15 ℃ to-35 ℃, and the temperature of the bottom of the tower is-20 ℃ to-30 ℃.

3. The method for separating the unsaturated dry gas in the refinery according to claim 1 or 2, wherein the high-pressure flash tank is provided with at least one flash tank, when a plurality of flash tanks are arranged, the flash tanks are sequentially connected in series under reduced pressure, and the pressure of the last flash tank is 0.1-0.3 MpaG.

4. The method for separating unsaturated dry gas in a refinery according to claim 3, wherein the semi-lean solvent obtained in the high-pressure flash zone is the liquid phase in each flash tank, the semi-lean solvent enters the lower section of the multi-stage absorption tower, the feeding plate is 20-45 ℃, the temperature of the semi-lean solvent is-20 ℃ to-40 ℃, and the liquid phase fed into the low-pressure flash zone is the liquid phase in the last flash tank.

5. A method for separating unsaturated dry refinery gas according to claim 1, 2 or 4, wherein at least one flash tank is arranged in the low-pressure flash tank, when a plurality of flash tanks are arranged, each flash tank is connected in series under reduced pressure, and the pressure of the last flash tank is 0-0.1 MpaG.

6. The method of claim 5, wherein the sub-lean solvent obtained from the low-pressure flash zone is the liquid phase of each flash tank, the sub-lean solvent enters the middle of the multi-stage absorption tower, the temperature of the sub-lean solvent is 5-25 ℃, the temperature of the sub-lean solvent is-40 ℃ to-70 ℃, and the liquid phase fed into the desorption tower is the liquid phase of the last flash tank.

7. Method for the separation of refinery unsaturated dry gases according to claim 1, 2, 4 or 6, characterized in that said treatment of carbon dioxide concentrate gas compression: and increasing the pressure of the gas phase obtained in the low-pressure flash evaporation zone to 1-4 MPaG.

8. A method for the separation of refinery unsaturated dry gases according to claim 7, characterized in that the carbon concentration gas is a multi-stage compression.

9. A method for separating refinery unsaturated dry gas according to claim 1, 2, 4, 6 or 8, characterized in that said cooling treatment method comprises: cooling the treated refinery dry gas to-15 to-40 ℃, wherein propylene refrigeration is adopted for cooling treatment, and primary refrigeration to tertiary refrigeration is adopted for propylene refrigeration by combining with the operation temperature of other equipment of the process.

10. The method of separating refinery unsaturated dry gas according to claim 9, wherein the carbon four absorbent is a carbon three fraction, a carbon four fraction or a carbon five fraction containing alkane components.

11. A method for the separation of refinery unsaturated dry gases according to claim 1, 2, 4, 6, 8 or 10, characterized in that in the carbon four absorbent: the circulating poor solvent comprises 80-95 mol% of carbon four, the balance of a small amount of carbon three and carbon five, the circulating sub-poor solvent comprises 50-80 mol% of carbon four, and the circulating semi-poor solvent comprises 30-70 mol% of carbon four.

12. The method for separating unsaturated dry gas in a refinery according to claim 11, wherein the theoretical plate number of the desorption tower is 20-60, the operation pressure is 0.5 MpaG-4 MpaG, the tower top temperature is-35-40 ℃, and the tower kettle temperature is 60-130 ℃.

13. A method for separating unsaturated dry gas in refinery according to claim 1, 2, 4, 6, 8, 10 or 12, characterized in that the lean solvent returned to the multistage absorption tower for recycling is cooled to-15 to-40 ℃ and returned to the top of the multistage absorption tower as lean absorbent for recycling.

14. A system for separating unsaturated dry gas in a refinery is characterized by comprising a dry gas pretreatment system, a dry gas precooling heat exchanger, a multi-stage absorption tower, a high-pressure zone flash tank, a low-pressure zone flash tank, a carbon dioxide concentration gas compressor system, a desorption tower and a rough separation tower;

the dry gas pretreatment system is connected with a dry gas cooler and then connected with a multi-stage absorption tower, the bottom of the absorption tower is sequentially connected with a high-pressure zone flash tank and a low-pressure zone flash tank, the top of the high-pressure zone flash tank is connected with a dry gas compressor, the bottom of the high-pressure zone flash tank is respectively connected with the multi-stage absorption tower and the low-pressure zone flash tank, the top of the low-pressure zone flash tank is connected with a carbon dioxide concentration gas compressor, the bottom of the low-pressure zone flash tank is respectively connected with the multi-stage absorption tower and a desorption tower, the top of the carbon dioxide concentration gas compressor and the top.

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