南华大学化学化工学院毕业设计
stream passed through line 13. The make-up sulfuric acid utilized should contain as little water as commercially feasible and in any event should be a concentrated sulfuric acid containing less than 5% by weight of water. Tower 11 utilized to obtain the intimate contact of the ascending gases with the descending liquid may be any of those of conventional design normally utilized for obtaining intimate gas-liquid contact, such as those used for gas absorption. The tower should however be constructed of a material resistant to the acid medium such as Hastelloy C. The tower may be of packed type or the tray or plate type, the tray or plate type being preferred. The packing or trays should be arranged, and be of an amount or number, so as to assure intimate gas-liquid contact so as to accomplish the required mass transfer of the ethylene into the descending liquid. Where trays or plates are utilized, there should generally be at least about 10 of these trays or plates, with from about 10 to 40 trays or plates generally being sufficient; and, when using packing, the amount of the packing should be that which is roughly equivalent to such number of trays or plates. The liquid to gas ratio should generally be such that the weight of the descending liquid in the tower, that is the combined weight of lines 12 and 13, to the weight of the light ends stream fed through line 10 is at least 5:1, generally within the range of 10:1 to 350:1, and more preferably within the range of 100:1 to 200:1. The contacting of the liquid with the gas, and thus the conditions within tower 11, should be at temperatures within the range of about 25.degree. C. to 150.degree. C., preferably within the range of about 65.degree. C. to 100.degree. C., and at superatmospheric pressures ranging from slightly above atmospheric, that is about 1.05 atmospheres absolute, to 20 atmospheres absolute, preferably about 1.2 to 10 atmospheres absolute. Since the light ends stream will be taken from a vacuum distillation system operating at subatmospheric pressure, it will
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南华大学化学化工学院毕业设计
be necessary to pass the light ends stream through a compressor 14 for increasing the pressure from subatmospheric to superatmospheric, and this increase in pressure will also cause an increase in the temperature of the gaseous light ends stream.
The foregoing range is of temperatures and pressures are general in nature, and it will be recognized that the temperatures and pressures will not be equal throughout tower 11. The pressure at the base of tower 11 will be greater than at the upper end of the tower by an amount sufficient to overcome the hydraulic pressure of the descending liquid pressure drop due to vapor velocity. Just as the pressure varies at the base and the upper end of the tower, the temperature will also vary, with the gas being cooled as it rises through the tower, and with the descending liquid being heated as it descends in countercurrent contact with the rising gas. Generally the temperature of the gases entering the base of tower 11 through line 10 should be within the range of about 25.degree. C. to 150.degree. C., preferably 65.degree. C. to 100.degree. C. The initial temperature of the descending liquid, that is the liquid resulting from a combination of lines 12 and 13, should be within the range of 40.degree. C. to 150.degree. C. The reaction is slightly exothermic, and the liquid may increase very slightly in temperature after contact with the gas, but the temperature of the liquid withdrawn from the base of tower 11 through line 16 should generally be about the same as that fed to the upper end of the tower. The temperature of the gas removed overhead of tower 11 through overhead line 15 will also be about the same temperature as the liquid fed to the upper end of the tower.
The temperature and pressure control is very important to the operation of the present invention, and the desired results will not be obtained unless temperature and pressure are properly controlled. The pressure is important in that too much pressure will cause various components of the
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南华大学化学化工学院毕业设计
light ends stream to dissolve in the sulfuric acid medium, with these components thus ultimately being returned to the reaction zone of the ethyl acrylate process where they will lower the efficiency of the process. On the other hand, if the pressure is too low then the amount of ethylene absorbed into the sulfuric acid medium will be insufficient, and the recovery of ethylene values will be minimal. Operating at too high a temperature will result in carbon dioxide formation from the ethylene since the sulfuric acid is a strong oxidizer. High temperatures will also cause polymerization of components present in the light ends stream. If temperatures are too low, then the desired reaction of the ethylene with the sulfuric acid to form the sulfates will be inhibited.
Of the liquid removed as a bottoms stream through line 16, a major portion thereof will be recycled through line 13 to the upper end of tower 11, while a minor portion will be removed through line 17 and passed to the reaction zone of the ethyl acrylate process as a source of make-up sulfuric acid. The volume of liquid removed through the line 17 will be roughly the same as the fresh make-up sulfuric acid fed through line 12, although the weight per unit time of these sulfate enriched liquids removed through line 17 will be slightly higher than the weight per unit volume of the fresh make-up sulfuric acid through line 12. The weight per unit time of gases removed overhead of tower 11 through line 15 will be less than the weight per unit time of the light ends stream fed through line 10 due to the removal of ethylene. Generally, about 50 to 98% of the liquid bottoms removed through line 16 will be recycled through line 13, with the remainder being withdrawn through line 17. The amount of line 16 will be about the amount of liquid passed to the upper end of the tower, plus the ethylene absorbed from line 10. The temperature of the liquid bottoms recycled through line 13 will be controlled by heat exchanger 18 prior to being recycled to the upper end of tower 11.
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南华大学化学化工学院毕业设计
The liquid recycle through line 13 is very important, and is critical to operation of the process, for several reasons. First, the make-up sulfuric acid through line 12 is too small a stream to provide good gas-liquid contact. Secondly the recycle stream has a relatively low concentration of sulfuric acid (as compared to the make-up stream) because of the presence of ethyl hydrogen sulfate and diethyl sulfate, and this lower concentration of sulfuric acid will diminish the danger of ethylene polymerization or decomposition. Thirdly, the presence of ethyl hydrogen sulfate in the recycle increases the ethylene absorption in the liquid. By operating according to the present invention, usually about 65% to 85% of the ethylene contained in the light ends stream can be recovered, more usually about 70% to 75%. The following example is given to illustrate the present invention, but is not to be taken as limiting the scope thereof. EXAMPLE
To an apparatus of the type illustrated in the FIGURE, there was passed to tower 11 through line 10 about 360 grams per hour of a synthetic feed similar to the gaseous light ends from the above described ethyl acrylate process. The gases flowing through line 10 were at a temperature of about 90.degree. C. and a pressure of about 2 atmospheres absolute after passing through compressor 14. The gaseous light ends stream contained, by weight, about 30% ethylene, 30% sulfur dioxide, and 40% nitrogen.
Tower 11 was approximately 1.5 meters in height and 5 centimeters in diameter and contained 15 trays of the sieve type. The pressure at the base of tower 11 was about 2.03 atmospheres absolute and pressure at the upper end of tower 11 was about 2.00 atmospheres absolute. The gaseous light ends stream passed through line 10 to the bottom of tower 11 entered at a point below the lower tray. Passed to the upper end of tower 11, at a point above the uppermost tray, through line 12 was about 480 grams per hour of fresh make-up sulfuric acid having a concentration of about 97%,
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南华大学化学化工学院毕业设计
the remaining 3% comprising water. Also fed onto the upper tray of tower 11 through line 13 was about 15 kilograms per hour of recycle liquid. The temperature of the fresh make-up sulfuric acid fed through line 12 was 30.degree. C., while the temperature of the liquid recycle fed to the tower through line 13 was 70.degree. C. after having passed through heat exchanger 18.
About 15.6 kilograms per hour of liquid bottoms was removed through line 16, such having a temperature of about 75.degree. C. upon removal from the lower end of tower 11, with about 15 kilograms per hour of the liquid bottoms being recycled through line 13 as mentioned above, and the remaining 0.6 kilograms per hour being withdrawn through line 17 as a source of make-up sulfuric acid for the ethyl acrylate process. The product withdrawn through line 17 consisted essentially of sulfuric acid having about 50% by weight of ethyl hydrogen sulfate and 5% by weight of diethyl sulfate dissolved therein, and this product was substantially free of sulfur dioxide.
The overhead gases removed through line 15 were at a temperature of 70.degree. C. and had a composition, by weight, of about 12% ethylene, 0.1% ethanol, 38% sulfur dioxide, and 50% nitrogen. The amount of sulfur dioxide in the gases removed through line 15 was substantially the same as that passed to the lower end of tower 11 through line 10, however, only about 30% of the ethylene in line 10 appeared in the overhead gases removed through line 15, the remaining 70% having been recovered in the form of ethyl hydrogen sulfate and diethyl sulfate through line 17.
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