A M E T H O D OF S A V I N G C A U S T I C EXTRACTION

SODA IN A L K A L I

OF H Y D R O R E F I N I N G G A S O L I N E

F. Kh. U r a z a e v , D. F. V a r f o l o m e e v , F. G. A k h m e t o v , M. N. S t e k o l ' s h c h i k o v , a n d E. G. B u t a k o v

UDC 004.18 : 681.322.1 : 62-631.2

In a plant for hydrorefining diesel fuel from high-sulfur petroleums, 1.0-2.0% of gasoline distillate is evolved in stabilizing the product. In it there is about 1.5% dissolved hydrogen sulfide. The alkali-washing unit provided by the design did not ensure the required degree of hydrogen sulfide removal. This has hindered the preparation of high-quality fuels because of the formation of pyrophoric iron in the reservoirs and has increased the fire hazard in the trade depot. The scheme of additional gasoline refining in stabilization blocks of AVT units and thermal cracking units which has been introduced in a number of plants is associated with additional operational costs and with difficulties in removing the elemental sulfur which is formed in the oxidation of hydrogen sulfide by atmospheric oxygen dissolved in the gasoline. Moreover, of the methods indicated above, in the Ukrainian Petroleum Refining Plant, we have checked out the variants of gasoline refining with a solution of monoethanolamine (MEA) and caustic soda with vigorous contacting of the gasoline and reagents in a centrifugal pump.

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Fig. 1. Scheme of a pilot plant for blowing out hydrogen sulfide from gasoline: 1) column; 2) packing; 3) spray trap; 4) gas meter; 5) measuring cylinder; 6) sampler; 7) thermometer well; 8) valve; 9) manometer. Streams: I) original gasoline; II) hydrogen gas; III) gasoline after blowing; IV) used gas. Fig. 2. Degree of removal of hydrogen sulfide (a) and of mercaptan sulfur (b) as a function of gas factor. Pressure, arm: 1) 0.05; 2) 2.0; 3) 3.5. Ukrainian Petroleum Refining Plant. Translated from t
9 Consultants Bureau, a division of Plenum P u b l i s h i n g Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for arty purpose w h a t s o e v e r without permission o f tl~e publisher. A copy o f this article is available from the p u b l i s h e r for $15.00.

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Fig. 3. Effect of temperature on degree of hydrogen sulfide removal at values of gas factor shown (m3/ma): 1) 6; 2) 5; 3) 4, Fig. 4. Effect of gas factor on saturated gasoline vapor pressure. For designations of curves, see Fig. 2. It was established that, upon thorough mixing, the loss of MEA solution with the gasoline is increased, and the refining process is m a d e more expensive thereupon. In alkali-washing the gasoline with Caustic soda, its consumption is about 15 kg per metric ton of gasoline, which is 45 times as much as the a l k a l i coasumption in treating gasoline from thermal cracking of Aral petroleum tar. To ascertain the possibility of blowing out hydrogen sulfide from gasoline with hydrocarbon gas, a pilot plant was set up by the e x p e r i m e n t a l - i n v e s t i g a t i v e works of the plant, which was connected up with the streams of gasoline and gas of an L-24-7 plant, The scheme of the unit is shown in Fig. 1. Hydrocarbon gas freed from hydrogen sulfide passed into the lower part of the column and through a layer of porcelain packing. The hydrocarbon gas, saturated with hydrogen sulfide, was discharged into the atmosphere through a spray trap and a gas meter. The starting gasoline entered the upper part of the column, was purged with the countercurrently ascending gas, and was discharged from the lower part of the column. The amount of gasoline was measured with a graduate. A sampler was connected to the stripped gasoline stream. A gas meter showed the amount of gas fed to the blowing operation. The characteristics of the original gasoline which was subjected to blowing are given below: Density, Ol0 ..... 0,735 Fractional composition, "C i n i t i a l bp . . . . . . . . . . 40 10% . . . . . . . . . . ~ . 64 50% . . . . . . . . . . . . 116 90% . . . . . . . . . . . . 155 end of boiling range . . . . . 186/98,5% Octane number (by motor method) 61 Co~ntent of sulfur compounds,* 0 bv wt.: -Hydrogen sulfide . . . . . . 1,25--1,67 Mercaptans ........ 0,0032--0,005 Elemental sulfur .... 0,0010--0,0020 Sulfides .... 0,0250--0.015,0 Disulfides .... Absent Residual sulfur .... 0,045--0,166 The hydrocarbon gas fed into hydrogen sulfide blowing had the following composition (wt, %): Hydrogen . . . . . Methane . . . . . . Ethane . . . . . . Propane . . . . . . Butane . . . . . . Pentane . . . . . . Hydrogen sulfide Density of gas feed

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16,5 33,6 23, 7 9.4 Up to 0,012 0,953 k g / m a

*Determination of the group composition of the sulfur compounds was carried out under the direction of Yu. I. Khakhileva. 677

Experiments were conducted under the following conditions: pressure in column, 0.05; 2.0, and 3.5 atm; gas factor (ratio of volume of hydrocarbon gas per unit volume of gasoline), 1-12 mS/mS; temperature, 20, 40, and 60~ As a result of the experiments conducted, it was established that the gas factor is the basic parameter in the process of blowing out hydrogen sulfide (Fig. 2). At a value of 12 mS/m 3 for the gas factor, the curve which characterizes the removal of hydrogen sulfide becomes almost parallel to the abscissa and essentially approaches 100%. At a pressure of 0.05 atm and a gas factor over 10 mq/m ~, the gasoline treated by the indicated method passes the copper plate test without additional alkali leaching. With increase in pressure, the r e l a t i v e v o l a t i l i t y of the h e a v y gas components mixed with the hydrogen sulfide is increased, which retards the degree of hydrogen sulfide removal. Therefore, to obtain the same degree of gasoline refining at a higher pressure, a greater gas flow is required to blow out the hydrogen sulfide. Thus, if the gas factor is 4 mS/m 3 at a pressure of 0.05 arm for removing 95% of the hydrogen sulfide, then to ensure the same degree of hydrogen sulfide removai at a pressure of 2.0 arm,the gas factor must be increased to 9 mS/m 3. The dependence of the degree of hydrogen sulfide removal on gas factor at a pressure o f 2.0 atm is described by the equation: y = 0,054 ( x - - 12)3 + 100, where y is the degree of hydrogen sulfide removal, in % by wt.; x is the value of the gas factor, within the range from 1 to 12 mS/mS; 0.054 is an e m p i r i c a l constant which depends on the pressure; and 12 and 100 are e m p i r i c a l constants which c h a r a c t e r i z e the change in degree of hydrogen sulfide removal with gas factor. There is a small amount of mercaptans in the gasoline distillate; these apparently have a low boiling point, as is indicated by the drop in content of these on purging the gasoline distillate with gas. Thus, at a gas factor of 10-12 mS/m3, up to 70% of the mercaptans are removed. The effect of temperature on the extent of removal of hydrogen sulfide from gasoline was studied at t e m peratures of 20, 40, and 60~ and a gas factor of 4, 5, and 6 mS/m ~. It was established that as the temperature of the product was raised, the degree of hydrogen sulfide removal was increased (Fig. 3). The greater the gas factor, the greater the hydrogen sulfide removal. In checking the quality of the gasoline, it was estsblished that purging it with gas assists i n vaporizing volatile components, and the saturated vapor pressure is reduced (Fig. 4). Raising the pressure hinders evaporation. The data obtained were used in developing a scheme for blowing out hydrogen sulfide with hydrocarbon gas in an industrial L-24-7 unit. The selection of the construction and size of the apparatus thereupon was determined by the presence of idle equipment in the unit. The following scheme was set up. Refined gasoline is fed by a r e flux pump into the upper part of a K - 8 column. Since the column has a large diameter (2.2 m) and is filled with a packing of c e r a m i c rings, a channel breakthrough of gas and gasoline without the necessary contact is not e x cluded. Therefore the gasoline level in the column is raised to a height of 12 m. Gasoline from the bottom of the K-8 column is routed to an E-1 reservoir (an a l k a l i settler), where alkali extraction is carried out by the scheme provided in the design. Circulation of the a l k a l i solution is effected by a piston pump having an output of 1.5 mS/h. Then the a l k a l i - e x t r a c t e d gasoline passed to water washing in an E-2 reservoir and was run off to c o m m e r c i a l storage. The yield of gasoline from the unit fluctuates within a large range, since the gasoline from a nearby L-24-8 unit is also pumped to the blowing K-8 column for joint purging. Therefore, for r e l i a b i l i t y in contacting the gasol i n e and a l k a l i , a diaphragm mixer has been set up, with a controllable cross section which makes it possible, if necessary, to change the mixer section and increase the intensity of mixing. In the column of the industrial unit, where the linear velocity of the gases fed to blowing is considerably less than in the pilot unit, a reduction of vapor pressure is not noted. Introduction of the method of freeing gasoline from hydrogen sulfide by blowing it with a hydrocarbon gas in a diesel fuel hydrorefining unit has m a d e it possible to considerably improve the technical and economic f e a tures of gasoline refining because of the reduction in expenditure of e l e c t r i c a l energy and the decrease in caustic soda consumption from 18 to 0.35 kg per metric ton of gasoline. The stable production of h i g h - q u a l i t y gasoline in the unit has been ensured.

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When there is a hydrocarbon gas having a low hydrogen sulfide content in the plant, this method of removal of sulfur compounds can compete with a widespread method of removing hydrogen sulfide - thermal stabilization since the latter method requires a large expenditure of h e a t and is more complex in its technological features,

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