US3240833A - Process and apparatus for treating hydrocarbon stream to remove contaminant - Google Patents

Description

March 15, 1966 J. G. CASH 3,240,833

PROCESS AND APPARATUS FOR TREATING HYDROGARBON STREAM TO REMOVE CONTAMINANT Filed May 28, 1962 ETHANE PROPANE RIC INVENTOR.

ml Ek -\r P)- J.G. CASH BY f y A 7'TORNE VS NGL United States Patent PROCESS AND APPARATUS FOR TREATENG HYDROCARBON STREAM T0 REMQVE CONTANHNANT Jerrold G. Cash, Sweeny, Tex., assignor to Phillips Petroleum Company, a corporation of Delaware Filed May 28, 1962, Ser. No. 198,268 6 Claims. (Cl. 26(9-676) This invention relates to hydrocarbon treating. In one aspect the invention relates to a process for removing an unwanted contaminant from a hydrocarbon stream by contact with a treating agent. In another aspect the invention relates to apparatus for treating a hydrocarbon stream with a treating agent to remove a contaminant.

Frequently, it is desirable to remove a contaminant from a hydrocarbon stream. A very useful contaminant removal process comprises contacting the hydrocarbon stream with a treating agent having an afiinity for the contaminant. This contacting results in a treated hydrocarbon stream and a stream of treating agent containing the contaminant, the separate streams being removed from the contacting zone. To prevent foaming and entrainment of the treating solution in the treated hydrocarbon stream, it is preferable to maintain the hydrocarbon stream in the vapor state and to prevent any condensation of the hydrocarbon during treatment.

Although it is possible to prevent condensation by increasing the temperature of the contact zone, this is not desirable in most instances because of its adverse effect on the treating step. For example, when treating with an amine, such as diethanolamine, the effectiveness of the treating agent in removing the contaminant is reduced since the affinity of the treating agent for the contaminant is reduced at higher temperatures. On the other hand, it is possible to prevent condensation by reducing the pressure in the contact zone, but this, in many instances, is undesirable because of the relatively high pumping head required to pump the treated stream into the separation means. It is desirable to accommodate variations in feed during treatment of the hydrocarbon stream.

An object of my invention is to treat a hydrocarbon stream for the removal of a contaminant therefrom.

Another object of my invention is to prevent foaming and entrainment of a treating agent while treating a vaporous hydrocarbon with a treating agent,

Another object of my invention is to prevent condensation of the hydrocarbon while treating a vaporous hydrocarbon stream With a treating agent.

Another object of my invention is to permit treating in the vapor phase of a relatively large quantity of a relatively high-boiling component at a temperature below its boiling point.

Another object of my invention is to control a hydrocarbon treating process.

Another object of my invention is to provide hydrocarbon treating apparatus and control means therefor.

Other aspects, objects and the advantages of my invention are apparent in the Written description, the drawing and the claims.

With the temperature and pressure of treating determined, if only vaporous material is fed to the treating zone, and if a component is present which boils higher than the treating temperature, the amount of this component which can be treated is determined by the amount of a lower-boiling component present. The composition of a vapor in equilibrium with the liquid in a multicomponent system is determined by the temperature and pressure. Larger amounts of a relatively high-boiling component can be removed with larger amounts of a relatively low'boiling component since the composition of the vapor is constant, the lighter component thus carrying the heavier component in the overhead vapor.

According to my invention, condensation of a vaporous hydrocarbon during treatment with a treating agent to remove a contaminant is prevented by recycling a light fraction separated from the treated hydrocarbon, and controlling the amount of the light fraction recycled responsive to the measurement of the amount of a component in a separation step preceding the treatingstep. A hydrocarbon feed is separated to produce a stream comprising a lower-boiling component, a higher-boiling component, and containing a contaminant, and a heavier stream. The stream containing the higher-boiling component, the lower-boiling component and the contaminant is contacted in an extraction zone with a treating agent having an afiinity for the contaminant to be removed. The contacting is carried out at a temperature intermediate the boiling point of the lower-boiling component and the higher-boiling component at the pressure of the treating zone. A stream containing the treating agent and the contaminants removed from the hydrocarbon are withdrawn from the treating zone, the treating agent being discarded or regenerated for further use, and a substantially contaminant-free hydrocarbon stream withdrawn from the treating zone and separated .into a first relatively light fraction and a second relatively heavy fraction. The light fraction contains the predominant amount of the lower-boiling component While the heavy fraction contains the predominant amount of the higherboiling component. That is, of the amount in the total stream, the predominant amount of the lower-boiling component is in the light fraction and the predominant amount of the higher-boiling component is in the heavy fraction.

My invention is not limited to the treatment of hydrocarbon streams having only two components. Where the stream contains appreciable quantities of three or more components, the relatively light fraction contains at least one of the lower-boiling components while the relatively heavy fraction contains at least one of the higher-boiling components. Condensation of a heavier component of the hydrocarbon stream in the contacting zone is prevented by recycling a sufiicient amount of a light fraction to maintain the feed in the vapor state at the temperature and pressure existing in the pressure zone. The amount of the light fraction recycled is determined by measuring the amount of a particular component in the pretreatment separation zone.

The treating agent utilized in the practice of my invention is a material having a marked afiinity for the contaminant to be removed as compared with the afiinity for the hydrocarbon stream being treated. For example, the hydrocarbon stream can be contacted with a caustic solution for the removal of an acid gas such as carbon dioxide or hydrogen sulfide. As another example, the hydrocarbon stream can be treated with an aliphatic amine for removal of the same acid gases. Of the aliphatic amines, alkanol amines have been particularly useful, especially monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA). Further, combinations of agents are included in the broad designation treating agent. For example, a combination of an agent having an afiinity for an acid gas and one having an afiinity for moisture, such as a solution of an amine and a liquid polybasic aliphatic alcohol such as triethanolamine and diethylene glycol, can be used. The use of an amine solution, which has added to it an alkali metal compound, such as for example sodium hydroxide, sodium sulfide or sodium carbonate, is useful in some situations.

My invention also is applicable to treating processes in which two or more treating steps are employed, such as a caustic treating step following an amine treating step.

In such an instance, the light fraction can be recycled to one or more of the contacting steps or can be recycled to the feed prior to the first or a subsequent step where necessary to prevent condensation. Any hydrocarbons in which the components are maintained in the vapor state during treatment are suitable for operation according to my invention. Since it is easier to maintain the relatively light hydrocarbons in the vapor state, my invention finds more applicability to these relatively light hydrocarbons. My invention finds particular utility in the treatment of a stream comprising relatively large amounts of ethane and propane from which hydrogen sulfide or carbon dioxide must be removed.

In the contacting step of my invention, vaporous hydrocarbon is intimately contacted with a liquid treating agent in which the hydrocarbon is insoluble but which agent has an aflinity for the contaminant to be removed. Either the vaporous hydrocarbon or the liquid treating agent can be the continuous phase in the contacting zone. Suitable means for effecting contact between the vaporous hydrocarbon and the liquid treating agent include packed towers, columns containing bubble cap trays or valve trays, vessels with stirring means, flow conduits or vessels with turbulence-inducing means such as baflles, or other means for efliecting intimate contact of the hydrocarbon and the treating agent.

Further, according to my invention there are provided hydrocarbon treating means comprising a treating vessel having means for contacting the vaporous hydrocarbon and the liquid treating agent, and having a hydrocarbon inlet for the contaminant containing hydrocarbon, a treating agent inlet, a treated hydrocarbon outlet and a treating agent outlet. The hydrocarbon inlet communicates with the overhead of a distillation column from which the hydrocarbon feed is produced. The hydrocarbon outlet communicates with the hydrocarbon separator for separating the treated stream into a relatively low-boiling fraction and a relatively high-boiling fraction and means for withdrawing the two fractions separately are provided. A recycle conduit is connected with a light fraction outlet and with the hydrocarbon inlet of the treating vessel and means are provided to control the flow through the recycle conduit to permit flow of the desired amount of light fraction to prevent condensation of a portion of the hydrocarbon during the contacting step. Flow control means are provided in the recycle conduit and the set point of the flow control means is adjusted responsive to means for measuring the amount of the particular component in the upper portion of the distillation column.

Also according to my invention, the overhead from the distillation column passes through an accumulator from which the feed to the treater is removed overhead as a vapor. The liquid level of the accumulator controls the heat supplied to a reboiler for the distillation column, a liquid stream from the accumulator being used as a recycle for the column, the amount being controlled by a flow controller. The temperature in the accumulator is controlled by passing a portion of the stream through a cooler into the accumulator and another portion through a bypass, through a sparger into the liquid portion of the accumulator, controlling the flow through the cooler responsive to the pressure in the column and the flow through the by-pass responsive to the temperature in the accumulater.

In the drawing, a fractional distillation vessel 19 is provided with an inlet 11, an overhead outlet 12 and a bottom outlet 13. The stream fed through inlet 11 comprises natural gas liquids and this stream is separated in such a manner that the overhead stream comprises C and C hydrocarbons, the heavier material being removed through outlet 13. A reboiler 14 is provided as shown.

The stream comprising the C and C hydrocarbons, predominantly ethane and propane, but also containing varying amounts of other hydrocarbons, and hydrogen sulfide and carbon dioxide, is partially condensed in condenser 15 and passed into accumulator 16, from which a stream of liquid is returned by pump 17 as reflux for column 10, while a vapor stream is transferred through heater 18 to treating vessel 19, and through hydrocarbon inlet 19 to treating vessel 21. Means 22 to feed a chemical treating agent to vessel 21 are provided as shown. A hydrocarbon outlet to withdraw treated hydrocarbon from vessel 21 is provided in the upper portion of the vessel and is designated by the numeral 23. Means to withdraw used treating agent are provided in the lower portion of the vessel and designated by the numeral 24. The treated hydrocarbon stream is passed through a cooler 26 and an accumulator 27 to hydrocarbon separator 28. Light fraction outlet 29 and heavy fraction outlet 31 are provided from separator 28. Separator 28 is further provided with a reboiler 31. The light fraction outlet is passed through heat exchanger 32 and cooler 33 into accumulator 34. A portion of the condensed stream is returned by pump 36 as reflux to separator 28 while the vaporous portion is removed through a light fraction conduit 37. The desired amount of the light fraction is returned through pipe 38 and heater 39 to outlet 12, and thus recycled to vessel 21. A motor valve 41, regulated by flow recorder-controller 42 controls the quantity of the light fraction recycled.

An analyzer recorder-controller 43 is provided and the output used to adjust the set point of controller 42. This analyzer can be a chromatographic analyzer and peak reader. Such analyzers are disclosed, for example, in US. 3,018,310 (1962) and the references there named, including US. 2,994,646 (1961), US. 2,875,606 (1959) and copending application Serial No. 727,606 filed April 10, 1958, now US. Patent No. 3,069,895, December 25, 1962. Accumulator 16 is provided with a level controller 44 which adjusts the set point of a flow controller 45 which controls the supply of heating fluid, for example steam, to reboiler 14. The overhead from column 10 enters accumulator 16 through a pipe 46 which is provided with a motor valve 47 controlled by a pressure controller 48, responsive to the pressure in the upper portion of column 10, and through a pipe 49 controlled by a motor valve 50 regulated by a controller 51 responsive to the temperature in the accumulator. That portion of the flow which passes through pipe 49 enters accumulator 16 through a sparger 52 in the liquid section thereof.

In operation, when the amount of a low-boiling fraction entering column 10 is reduced, the amount of the higher-boiling component which passes overhead from accumulator 16 is also reduced. This results in a tendency for the liquid level in accumulator 16 to rise. The rising liquid level adjusts the set point of controller 45 to reduce the flow of heating fluid through reboiler 14, thus reducing the boil up in column 10. Controller 43 senses the presence of a selected heavier component in column 10 and, due to the reduced boil up, the amount of this com ponent decreases. As this component decreases, a signal is transmitted to controller 42 to increase the flow through recycle line 38 resulting in a higher ratio of lower boiling to higher boiling components in the accumulator. This causes increased amounts of the high-boiling component to be carried from accumulator 16 and a system to readjust to the changing feed. Similarly, when it is the higher-boiling component which decreases, the liquid level in accumulator 16 tends to fall, thus causing controller .4 to adjust controller 45 to increase the flow of heating fluid to reboiler 14 and increase the boil up in column 10. This causes an increase in the amount of the sampled component to appear in the upper portion of the column and be sensed by controller 43, thus sending a signal to controller 42 to reduce the flow of low-boiling recycle to the portion desirable for the reduced amount of higher-boiling component.

The maximum temperature which can be maintained in accumulator 16 is determined by the maximum temperature which can be permitted in treating vessel 21 and still obtain effective treatment. The composition of the vapor stream removed from accumulator 16 for a given group of components is determined by the temperature maintained in the accumulator. Therefore, when insuflicient quantities of the lighter components are present, a relatively small quantity of the heavier components is carried to treater 21. For example, when the heated stream is substantially all ethane and propane, and the temperature which must be maintained in accumulator 16 is less than the boiling point of propane at the treating pressure, the amount of propane which can be carried to the treater is determined by the amount of ethane which is present in accumulator 16. Flow controller 42 is adjusted to maintain the desired quantity of the lighter fraction. When the composition of the natural gas liquid being fed through inlet 11 changes appreciably, the set point of controller 42 must be adjusted to compensate for the change. That is, when the proportion of the light fraction decreases, the flow through flow controller 42 is increased and vice versa.

Table I below shows the eifect of temperature on the propane content of the vapor stream from accumulator 16 with variations in temperature. The liquid volume percent of propane in the vapor is based on a C /C ratio of 5/95. The values are determined for an accumulator pressure of 335 p.s.i.g.

In an example according to my invention a stream of natural gas liquids containing relatively large amounts of C and C and heavier and a relatively small amount of C and lighter is fed to inlet 11. Treating vessel 21 is a tower, 8 feet in diameter, containing 20 bubble-cap trays at a spacing of 24 inches. Distillation column 10 operates at a pressure of 340 p.s.i.g. and with temperatures at the top and bottom respectively of 127 F. and 247 F. Separator 28 also is a distillation column and operates at a pressure of 47 p.s.i.g. with top and bottom temperatures respectively of 62 and 172 F. Accumulator 16 operates at a pressure of 335 psig. and a temperature of 125 F. Since the vapor stream from accumulator 16 always is saturated vapor, to prevent condensation due to the slight amount of cooling occurring between the outlet of accumulator 16 and the inlet of treater 21, heater 1'8 15 used to raise the temperature about F, thus feeding the stream to treater 21 at 130 F. The stream of lean drethanolamine which is fed through pipe 22 is also temperature controlled and enters at a temperature of about 135 F. The pressure in treater 21 is 330 p.s.-i.g. The composition of the various streams is given in Table II below. It is seen that sufiicient C is recycled to permit continuous removal of all of the C in the overhead plus the C in the recycle and charge it to treater 21.

Controller 43 is adjusted to sense the amount of isobutane on an upper tray in column 10. When the amount of propane in the stream to column 10 decreases, since the vapor removed from accumulator is a function of the amount of ethane present, the liquid level in accumulator 16 starts to decrease, thus causing controller 44 to adjust controller 45 to increase the supply of steam to reboiler 14. This increases the boil up in column 10, causing an increase in isobutane at the point analyzed by controller 43. When this occurs, the set point of controller 42 is adjusted to reduce the flow of ethane through recycle 38. Reducing the ethane content reduces the vapor causing the gas yield from the accumulator to be reduced, thus tending to increase the accumulator liquid level. The accumulator level controller then reduces the reboiler steam in an effort to stop the level from rising and this in turn reduces the boil up in the tower causing the isobutane content on the sample tray to fall back to the desired amount. A reduction in the amount of ethane in the feed causes the level to rise, causing controller 44 to reduce the flow of steam by adjusting the set point of controller 45, thus reducing the boil up in column 10, decreasing the isobutane content where analyzed by an alyzer-controller 43 of causing an adjustment of the set point of controller 42 to increase the flow of ethane to recycle line 38.

The pressure in column 10 is controlled by pressure recorder-controller 48 through adjustment of motor valve 47. The temperature in accumulator 16 is maintained constant by controller 51 which senses the temperature in the accumulator and adjusts motor valve 50 to increase the flow through line 49 and sparger 52 when the temperature decreases, thus by-passing hot vapor directly to accumulator 16 to increase the temperature thereof.

Additional steps can be included in the over-all process. For example, in some instances it is desirable to have caustic wash, water wash and dehydration steps following the amine treating step. Means for separating the removed contaminant from the treating agent can be provided, as for example, means for heating and flashing the rich treating agent stream, thus separating the removed contaminant and providing lean treating agent to be re turned through pipe 22. It will be understood by those skilled in the art that many details, which have been eliminated from the drawing to simplify the description of the invention, can be added. For example, additional meters, controls, valves, pumps, duplication of the items of equipment described, etc. can be, and in most instances will be, provided. Other types of analyzer controllers than chromatographic analyzer and peak reader combination can be used where applicable. For example, under certain conditions infrared analyzers or other types of analyzers can be utilized.

Reasonable variation and modification are possible within the scope of my invention which sets forth process and apparatus for treating a contaminant-containing hydrocarbon stream, including recycling a portion of the light fraction of the treated hydrocarbon to prevent condensation during treatment, and control of the amount of recycle responsive to a measurement of a selected component in a distillation column preceding the treating step.

I claim:

1. A hydrocarbon treating process, comprising:

preparing a hydrocarbon feed comprising a lower-boil ing component and a higher-boiling component by separation of a hydrocarbon stream in a fractional distillation zone, said hydrocarbon feed comprising overhead products from said distillation zone; contacting said hydrocarbon feed containing a contaminant, in an extraction zone, with a treating agent having a chemical afiinity for said contaminant, at a temperature intermediate the boiling points of said lower-boiling component and said higher-boiling component at the pressure of said extraction zone; withdrawing a stream containing substantially all of said treating agent and said contaminant from said extraction zone;

withdrawing a substantially contaminant-free hydrocarbon stream from said extraction zone;

separating said relatively contaminant-free hydrocarbon stream into a first fraction containing the predominant amount of said lower-boiling component and a second fraction containing the predominant amount of said higher-boiling component;

recycling a sufiicient amount of said first fraction to said hydrocarbon feed to maintain the desired flow of said higher-boiling component in the vapor state at said temperature and said pressure in said extraction zone; and

controlling the amount of said first fraction recycled responsive to a measurement of the quantity of a selected component in said distillation zone at a point above the point at which said hydrocarbon stream enters said distillation zone.

2. The process of claim 1 wherein said contaminant is an acid gas and said treating agent is an aliphatic amine.

3. The process of claim 2 wherein said aliphatic amine is selected from the group consisting of monoethanolamine, diethanolamine and triethanolamine, and wherein said lower-boiling component is ethane and said higherboiling component is propane.

4. Hydrocarbon treating means, comprising:

a fractional distillation column;

means to feed a hydrocarbon stream to said column;

means to remove a relatively heavy stream as a bottom product from said column;

means to remove a relatively light product as an overhead product from said column;

a treating vessel for said overhead product and means to feed said overhead product to said vessel;

fluid separation means;

means to remove a treated stream overhead from said vessel and feed said treated stream to said fluid separation means;

means to remove a relatively light fraction from said fluid separation means;

means to remove a relatively heavy fraction from said fluid separation means;

means to recycle a portion of said light fraction to said overhead from said column;

means to control the flow of said recycle;

means to determine the quantity of a selected component in said column above said feed; and

means to adjust said means to control the flow responsive to the measurement of the quantity of a selected component in said column above the feed point.

5. The apparatus of claim 4 wherein said means to determine the quantity of a selected component comprises a chromatographic analyzer.

6. Hydrocarbon treating means, comprising:

a fractional distillation column;

means to feed a hydrocarbon stream to said column;

means to remove a relatively light product overhead from said column;

means to remove a relatively heavy stream from the bottom of said column;

an accumulator having a liquid zone and a sparger therein; acooler;

means to feed said relatively light fraction through said cooler to said accumulator;

means to by-pass said cooler and feed said relatively light fraction directly to said accumulator through said sparger;

means to control the flow through said cooler responsive to the pressure in the overhead from said column;

means to control the flow through said by-pass responsive to the temperature in said accumulator;

means to reflux said column with liquid from said accumulator;

means to control the flow of said reflux to said column;

a reboiler for said column;

means to control the flow of a heating fluid to said reboiler;

means to adjust said means to control said heating fluid responsive to the liquid level in said accumulator;

means to remove a vapor overhead from said accumulator;

a treating vessel;

means to feed said vapor to said vessel;

means to feed a treating fluid to said vessel;

means to remove said treating fluid from said vessel;

means to remove a treated stream from said vessel;

a fluid separator;

means to feed said treated stream to said separator;

a light fraction outlet for withdrawing a relatively lowboiling fraction from said separator;

means for withdrawing a relatively high-boiling fraction from said separator;

a recycle conduit connecting said light fraction outlet with said means to remove a relatively light product overhead from said column;

flow control means in said recycle conduit; and

means to measure the amount of a selected component in said column above the feed thereto and adjust said flow control means in said recycle responsive thereto.

References Cited by the Examiner UNITED STATES PATENTS 2,369,058 2/1945 Legatski 196-141 X 2,600,133 6/1952 Simms 196-132 X 2,710,278 6/1955 Gilmore 196-132 X 2,711,992 6/1955 Kerner 202- X 2,718,454 9/1955 Wylie 23-2 2,745,889 5/1956 Johnston et al. 260-676 2,813,920 11/1957 Cobb 55-51 X 2,860,030 11/1958 Goldtrap 23-3 2,868,701 1/1959 Berger 202-160 2,886,611 5/1959 King et al. 202-71 X 2,990,437 6/1961 Berger 202-160 X 3,002,818 10/1961 Berger 196-132 X 3,039,941 6/1962 Sweeney et al. 196-132 X ROBERT F. BURNETT, Primary Examiner.

GEORGE D. MITCHELL, Examiner.

Claims (1)

1. A HYDROCARBON TREATING PROCESS, COMPRISING: PREPARING A HYDROCARBON FEED COMPRISING A LOWER-BOILING COMPONENT AND A HIGHER-BOILING COMPONENT BY SEPARATION OF A HYDROCARBON STREAM IN A FRACTIONAL DISTILLATION ZONE, SAID HYDROCARBON FEED COMPRISING OVERHEAD PRODUCTS FROM SAID DISTILLATION ZONE; CONTACTING SAID HYDROCARBON FEED CONTAINING A CONTAMINANT, IN AN EXTRACTION ZONE, WITH A TREATING AGENT HAVING A CHEMICAL AFFINITY FOR SAID CONTAMINANT, AT A TEMPERATURE INTERMEDIATE THE BOILING POINTS OF SAID LOWER-BOILING COMPONENT AND SAID HIGHER-BOILING COMPONENT AT THE PRESSURE OF SAID EXTRACTION ZONE; WITHDRAWING A STREAM CONTAINING SUBSTANTIALLY ALL OF SAID TREATING AGENT AND SAID CONTAMINANT FROM SAID EXTRACTION ZONE; WITHDRAWING A SUBSTANTIALLY CONTAMINANT-FREE HYDROCARBON STREAM FROM SAID EXTRACTION ZONE;

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