In atmospheric refineries, oil is usually separated into four distillate fractions and the remainder is fuel oil. The by-product is a mixture of hydrocarbon gases, often containing hydrogen sulfide, which is formed from unstable sulfur compounds when oil is heated. The most common in our country is the installation of AT according to the scheme of double evaporation and double rectification (Fig. 1.2).

Oil dehydrated and desalted at CDU is additionally heated in heat exchangers and fed to the separation in the partial topping column (K-1). The hydrocarbon gas and light gasoline leaving the top of this column are condensed and cooled in air and water coolers and enter the irrigation tank. Part of the condensate is returned to the top of column K-1 as a phlegm. The topped oil from the bottom of column K-1 is fed into a tubular furnace, where it is heated to the required temperature and enters the atmospheric column (K-2). Heavy gasoline is taken from the top of K-2, and fuel fractions are removed from the side, through the stripping columns: kerosene, light and heavy diesel.

Rice. 2.3. Schematic diagram of the atmospheric unit:

1 – topping column; 2 – atmospheric column; 3 - stripping clone; 4 - tubular furnace; 5 - condenser-refrigerator; 6 - heat exchanger; 7 - reflux capacity;

I - oil with ELOU; II - gasoline vapors; III - phlegm; IV - light gasoline; V – stripped oil; VI - pairs of heavy gasoline; VII - heavy gasoline; VIII - circulating irrigation; IX, X, XI - side straps; XII - kerosene fraction; XIII - light diesel fraction; XIV - heavy diesel fraction; XV - fuel oil; XVI - water vapor; XVII - gases.

The atmospheric column, in addition to acute irrigation (phlegmy), has three circulation irrigations (or two), which remove heat below the plates for the selection of side straps. Superheated steam is supplied to the lower parts of the atmospheric and stripping columns (under the lower trays) to strip lightly boiling fractions. Fuel oil is removed from the bottom of K-2, which is sent to the vacuum distillation unit.

Description of the atmospheric column

Atmospheric column K-2 is a complex column consisting of three simple columns (Fig. 2.2). Excess heat in the column is removed from the top of the column by means of an acute evaporative reflux and along the height of the column by two intermediate circulating refluxes.

The number of circulating irrigations will be taken equal to the number of side fractions.

Based on the literature data, we will take the following number of plates in the concentration part of the column: in sections of gasoline, kerosene and diesel fuel - 8 plates each. For each circulation irrigation, we will take 2 plates. In the stripping part of the column and in the stripping sections, we will take 6 plates each. Thus, with two circulating refluxes in the column, the total number of trays in the atmospheric column will be 34.

Schematic diagram of the atmospheric column

Fig.2.

Pressure

Let's take the pressure at the top of the column (above the top, 34th plate) 140 kPa. This is slightly above atmospheric and is necessary to overcome hydraulic resistance during the passage of distillate vapor through the condenser-cooler.

We accept valve trays for installation in the column. According to reference data, the hydraulic resistance of one valve disc is = 0.6 kPa. We calculate the absolute pressure under each plate along the height of the column, starting from the top (Table 2.5).

Table 2.5

Physical characteristics by column height

	plate number	Pressure under the plate	Density liquids on	Molecular weight of the liquid on the plate	The temperature on the plate
Gasoline section
Kerosene section
Diesel section
Diesel section
Distillation part

Atmospheric unit is designed for separation of desalted oil by distillation into dry gas, head fraction, and NK fractions - 140 0С, 140 - 180 0С, 180 - 240 0С, 240 - 290 0С, 290 - 360 0С, fuel oil (residue of atmospheric distillation) - fraction > 3600 С.

From the bottom of the column K-1, the stripped oil is taken by pumps N-3 / 1.2, and is pumped through the P 1/2 furnaces in 4 parallel streams, where it is heated to a temperature of 360 ° C and fed into the column K-2 for 46 trays.

At the bottom of K-2, superheated water vapor is supplied through the “NC” valve of the flow regulator position 956.

From the top of the K-2 column, gas, gasoline vapors and water enter through air condensers T 17 / 1-4, where they are cooled to a temperature of 33-400C and then into the E-3 tank. The gas from the top of the tank E-3 is discharged to the flare.

Gasoline from the tank E-3 is fed to the intake of the pump H-4/1.2 and then in two streams it goes to the top of K-2 in the form of acute irrigation and the second stream - the balance excess of gasoline is pumped out through the cooler T-15a to E-6.

Excessive heat of the K-2 column is removed by three circulating irrigations: 1st circulation irrigation from the fifteenth tray K-2 is fed to the pump H 1.2, pumped through the air coolers T-30 and returned to the 14th tray K-2; 2nd irrigation from the twenty-fifth tray K-2 is taken by the pump H-23 / 1.2, air cooler T-32 and returned to the twenty-fourth tray of the column K-2; The third circulation irrigation is taken from the thirty-fifth tray K-2 by the pump H-15 / 1.2, pumped through the heat exchangers T-5 / 1.2, T-31, T-46 and returned to the thirty-fourth tray K-2.

4 side pumps are removed from the K-2 column - a fraction of 120 - 180 ° C is removed in the eleventh and thirteenth plates on the upper plate K-6. Superheated water vapor is fed down K-6. The stripped fractions are returned to the eleventh plate K-2, - the fraction 180 - 240 ° C is removed from the twenty-first and twenty-third plates to the upper plate K-7. Superheated water vapor is fed down the K-7 column. The stripped fractions are returned to the twentieth plate of K-2, - the fraction 240 -290 0С is removed from the thirty-first and thirty-second plates of K-2 to the upper plate of the K-9 column. Superheated water vapor is fed down the K-9 column. The stripped light fractions are returned to the thirty-first plate K-2, - the fraction 290 - 350 0С from the thirty-ninth plate K-2 is fed to the pumps N-20, N-15/2 and is pumped through the T-20 reboiler, T-6 heat exchangers and T-12, air cooler T-46 and are being withdrawn from the installation.

Table 13 Material balance of column K-2

	% mass oil	% mass for semi-fueled fuel. oil	thousand tons/year
Semi-leaned oil
Fraction 85-120 0 С
Fraction 120-240 0 C
Fraction 240-350 0 C

Temperature and pressure regime.

Column K-2 is supplied with stripped oil, irrigation and water vapor. From the column is displayed head strap - gasoline fraction 85-120 o C, side straps - fraction 120-240 o C, fraction 240-350 o C, irrigation, water vapor and the remainder - fuel oil. The overhead vapor and water vapor are discharged through the head pipe of the column, the liquid shoulder from the side of the column, and the remainder from the bottom of the column.

The temperature of the stripped oil entering the column K-2 is determined by the point of the RI curve of the stripped oil, which corresponds to the total selection of light oil products (it is assumed that light oil products completely evaporate at the point of input of raw materials into the column).

Previously, it was shown that the temperature value in the feed section of column K-2 is t p.s.2 = 305 o C, pressure P p.s.2 = 2 atm = 1520 mm Hg. Art.

The K-2 column, unlike the K-1 column, works with steam. Based on the factory data, the amount of water vapor introduced into the K-2 column (G 1) is 1.01.5% (we take 1%) in terms of semi-leaned oil, and supplied to the stripping column (G 2) - 26% ( we accept 2%) in terms of each side shoulder strap. Assuming respectively 1.5 and 2.0% of water vapor per flow, we get:

G 1 \u003d 0.01961765 \u003d 9617.65 kg / h 9618 kg / h

G 2 \u003d 0.02 179177 \u003d 3583.54 kg / h 3584 kg / h

G 3 = 0.02 191584= 3831.68 kg/h 3832 kg/h

The temperature of the vapors leaving the top of the K-2 column is set at the end of the OI curve of the overhead at a pressure corresponding to the partial pressure of its vapors in a mixture with water vapor.

At the top of the column, where there is a two-component mixture of gasoline and water vapors, based on Dalton's law:

where R b - partial pressure of gasoline vapors;

Total pressure at the top of the column;

Molar concentration of gasoline vapors:

First we find the density of fractions 85-120°С, 120-240°С, 240-350°С:

s 20 4 (85-120) = 0.7260 (85-120) = 0.7304

s 20 4 (120-240) = 0.8080 (120-240) = 0.8118

s 20 4 (240-350) = 0.8750 (240-350) = 0.8784

s 20 4 (350-c.c.) = 0.9810 (350-c.c.) = 0.9836

The molecular weight of gasoline 85-120 ° C is determined by the Cragg formula:

Then N b = = 274.05 kmol/h;

N water vapor \u003d \u003d 946.33 kmol / h

Since acute top reflux is used in the column, which is removed in the form of vapors together with balance gasoline vapor and water vapor through the top tube of the column, this reflux, which changes the molar concentration and partial pressure of gasoline vapors, must be taken into account when determining the temperature of the top of the column.

For normal operation of the atmospheric column, 1 - 2-fold irrigation is sufficient. In accordance with this recommendation, let's set the irrigation multiplicity to 2. Then the amount of sharp top irrigation will be:

G op \u003d 2G b \u003d 229622 kg / h \u003d 59244 kg / h; N op \u003d \u003d 548.1 kmol / h

Molar vapor concentration of gasoline: == 0.4649

The total pressure at the top of column K-2 is assumed to be equal to or slightly higher than atmospheric pressure. Let's take = 1.5 atm = 1 140 mm Hg. Art. Then the partial pressure of gasoline vapor will be:

P b \u003d 1.5 0.4649 \u003d 0.697 atm \u003d 530 mm Hg. Art.

Therefore, the temperature of the vapors leaving the top of the K-2 column will be t v2 \u003d 88 ° C.

According to factory practice, the temperature of the bottom of the atmospheric column should be 20-30 o C lower than the temperature in the feed section. Let's take t n2 \u003d 305 - 20 \u003d 285 o C.

The temperature of acute upper irrigation is 35 o C, let's take t op = 35 o C.

The temperature of the superheated water vapor supplied to the column is taken equal to = 350 ° C. This steam is usually obtained by overheating the exhaust (crumpled) steam from pumps with a pressure of 0.2-0.3 MPa in a coil located in a raw or special furnace.

The number of trays in the concentration section of the K-2 column will be set by the temperature difference between the feedstock input section (t p.s. -10 o C (take 6 o C):

36.2, accept = 37 plates.

Let for the selection of the kerosene fraction 120-240 o C we will divert from the 13th plate, then the diesel fraction 240-350 o C will be diverted from the 25th plate. The number of plates in the stripping section of the atmospheric column is from 5 to 7 pieces, let's take = 7 plates. Remote columns have from 4 to 8 plates depending on the required clearness of the head separation. Let's take the number of plates in the stripping section N strip. = 7.

The temperature of the side stream is set at the beginning of the corresponding RI curve, since the liquid side stream withdrawn from the column is on the tray at the boiling point. In the cross section for the withdrawal of side streams, there are also lighter-boiling components that reduce the partial vapor pressure, and therefore the true temperatures for the withdrawal of side streams are usually 10-20 ° C lower than the temperatures of the initial points of their RI curves built at atmospheric pressure.

Let's determine the temperature of the output of the kerosene fraction 120-240 o C in the same way as described above:

N CF \u003d \u003d 1087.24 kmol / h

N water vapor \u003d \u003d 946.33 kmol / h

The pressure on the 13th fraction removal plate, based on the fact that the pressure at the top of the column is 1.5 atm (1140 mm Hg), and in the feed section 2 atm (1520 mm Hg) and the difference between the plates should be 5-10 mm Hg . Let's check this assumption:

Therefore, the number of plates was chosen correctly. The pressure on the 13th plate will be:

mmHg. = 1.671atm

Partial pressure KF:

R CF \u003d 15 \u003d 1.6710.535 0.894 atm

The temperature of the output of the kerosene fraction from the column K-2 corresponds to the temperature of 0% distillation according to the RI curve, constructed at P KF = 0.894 atm = 679 mm Hg. Art. and is = 141 o C.

The temperature of the CF at the outlet of the stripping column is assumed to be 20 ° C lower than the temperature of the liquid at the inlet to the stripping section, i.e.:

141 - 20 = 121 o C

Let us determine the output temperature of the diesel fraction 240-350 o C in the same way as described above:

N DF = = 746.77 kmol/h

N water vapor \u003d \u003d 747.22 kmol / h

Pressure on the 25th fraction outlet plate:

25 = 1 140 + 25 10 = 1 390 mm Hg. Art. = 1.83 atm

Partial pressure DF:

R DF \u003d 25 \u003d 1.83 0.50.915 atm

The temperature of the diesel fraction withdrawal from the K-2 column corresponds to the temperature of 0% distillation according to the RI curve constructed at P DF = 0.915 atm = 695.4 mm Hg. Art. and is = 259 o C.

The temperature of the DF at the outlet of the stripping column is assumed to be 20 ° C lower than the temperature of the liquid at the inlet to the stripping section, i.e.:

259 - 20 = 239 o C

Thermal balance of column K-2.

Heat is supplied to the K-2 column with semi-leaned oil heated in the furnace, as well as with water vapor supplied to the bottom of the column.

Heat is removed with the top product - gasoline fraction, side strips - KF and DF and the remainder, and is also removed by sharp (evaporating) irrigation.

Calculation of the heat balance of the main distillation column K-2 is carried out similarly to the calculation of the heat balance of the pre-evaporation column K-1.

Heat input:

The amount of heat introduced by the raw material (semi-leaned oil) -Q mon, is determined taking into account the proportion of the vapor and liquid phases. The fraction of distillation e is determined from the RI curve of semi-leaned oil at the temperature of the feedstock inlet to the K-2 column, or, which is the same, heating in the furnace (305 ° C) and a pressure equal to the pressure in the feed section of the column (2 atm = 1,520 mm Hg st.). Graphically we get e = 0.415

Q pon \u003d G pon,

where Gpon - the amount of semi-leaned oil entering the column, kg/h;

e - fraction of distillation of semi-leaned oil at the heating temperature in the furnace;

907.86 kJ/kg - heat content of vapors of semi-leaned oil at the temperature of the exit from the furnace (calculated earlier in the heat balance K-1)

682.57 kJ/kg - heat content of the liquid phase of semi-leaned oil at the temperature of the exit from the furnace.

Q pon \u003d 961765 * (0.415 * 907.86 + (1-0.415) * 682.57) \u003d 746392491 kJ / h

The amount of heat introduced by water vapor:

Q water vapor = G water vapor q = G water vapor (-),

where G water vapor - the amount of water vapor, kg/h;

3176.59 kJ/kg - heat content of water vapor at the inlet to column K-2, kJ/kg;

2657.81 kJ/kg - heat content of water vapor at the outlet of column K-2, kJ/kg; (from Sardanashvili)

Q water vapor \u003d 9618 * (3176.59-2657.81) \u003d 4989626 kJ / h

Heat consumption:

with the top product: Q b = G b,

where G b - the amount of vapor of gasoline, kg / h;

255.07*(4-0.7304)-308.99 = 525 kJ/kg

Q b \u003d 29622 * 525 \u003d 15551550 kJ / h

with a side product: Q KF = G KF,

301.57 kJ/kg

Q CF \u003d 179177 * 301.57 \u003d 54034408 kJ / h

with side product:

Q DF = G DF,

where G mon - the amount of diesel fraction, kg/h;

588.09 kJ/kg

Q DF \u003d 191584 * 588.09 \u003d 112668635 kJ / h

with the remainder: Q rest = G rest,

where Q ost - the amount of residue (fuel oil), kg/h;

624.21 kJ/kg

Q rest \u003d 561382 * 624.21 \u003d 350420258 kJ / h

with acute (evaporating) irrigation: Q op = G op q op = G op (-),

where G op is the amount of acute irrigation (the composition of acute irrigation is identical to the top product), kg/h; with an irrigation multiplicity of 2, we get G op = 2G b;

525kJ/kg - heat content of irrigation vapor at the temperature of the top of the column t in2 = 88 o C;

71.57 kJ/kg

Q op \u003d 2 * 29622 * (525-71.57) \u003d 26863007 kJ / h

Find the amount of heat that needs to be removed by circulating irrigation:

Q c.o. \u003d Q incoming - Q flow \u003d (Q pon + Q water vapor) - (Q b + Q KF + Q DF + Q rest + Q op) \u003d (746392491 + 4989626) - (15551550 + 54034408 + 112668635 + 35040258 + 26863007 ) = 191844259kJ/h

Heat balance of column K-2 Table 14

Name	wt % oil	% wt. per half
semi-leaned oil
water vapor
fraction 85-120 0 С
fraction 120-240 0 C
fraction 240-350 0 C
acute irrigation
circulating irrigation

Calculate the amount of circulation irrigation G c.o. required to ensure normal operation of the column (kg/h):

where is the heat content of the liquid (phlegm) flowing down from the circulating irrigation outlet plate (at temperature t 1 on the 14th plate);

The temperature t 1 is taken on the basis of a uniform temperature difference between adjacent plates of 5-10 ° C (previously taken 6 ° C). Since the temperature of the output of the kerosene fraction from the 13th plate is 141 ° C, we get t 1 \u003d 141 + 1 6 = 147 ° C. The temperature of the inlet to the K-2 column of circulation irrigation is taken equal to t 2 = 80 ° C. We accept the density of the circulating liquid, based on the assumption of a uniform drop in this indicator for each plate. Then, taking into account the density of the kerosene fraction, we get:

0,8080+ 1 0,005 = 0,8130 = 0,8168

314.8 kJ/kg

161.47 kJ/kg

The flow rate of the circulating fluid will be:

G c.o. == 1251185.41251185 kg/h

Determination of the main dimensions of the K-2 column

The main dimensions of the main distillation column K-2 are determined in the same way as the dimensions of the pre-evaporator K-1.

When determining the diameter of the K-2 column, in order to establish the section that is most loaded with vapors, the volumes of vapors in the evaporation space (feeding section) of the column and under the plates from which irrigation is removed are checked.

1. Section under the 1st plate, on which cold spray flows (gasoline vapors, cold spray and water vapor supplied to the bottom of K-2 and stripping sections):

G steam \u003d G b + G cold. + G water vapor = 29622 + 59244 + (9618 + 3584 + 3832) = 105900 kg/h

2. Section under the 13th plate (circulating reflux, vapors coming from the stripping column, and the same total amount of water vapor):

G steam = G strip. + G water steam + G c.o. \u003d 1251185 + 0.19179177 + (9618 + 3584 + 3832) \u003d 1302263 kg / h,

where G is a strip. \u003d e CT G CF - the amount of vapor stripped in the stripping section (the fraction of distillation e CF = 0.19 is determined graphically in accordance with the temperature of the entry of the CF fraction into the stripping column, equal to 141 ° C)

3. Section under the 25th plate (circulating reflux, vapors coming from the stripping column, and water vapor):

G steam = G strip. + G water steam + G c.o. \u003d 0.25 * 191584 + 1251185 + (9618 + 3832) \u003d 1312531 kg / h,

where G is a strip. = e DF G DF - the amount of vapors stripped in the stripping section (the fraction of distillation e DF = 0.25 is determined graphically in accordance with the temperature of the entry of the DF fraction into the stripping column, equal to 259 ° C).

4. Section under the 37th plate (vapors of stripped oil and water vapor supplied to the bottom of K-2):

G steam = G oil steam + G water vapor \u003d e k-2 * G pon + G 1 \u003d 0.415 * 961765 + 9618 \u003d 408750 kg / h

As can be seen from the proposed calculations, the section under the 25th plate is the most loaded, where the steam load is: G steam = 1312531 kg/h.

Based on this, we calculate the volume of vapors according to the Mendeleev-Clapeyron equation:

Based on practical data, the linear velocity of vapors in the free section for column K-2 is w = 0.6 1.15 m/s. Let's take w = 1.0 m/s, then the cross-sectional area of ​​the column will be:

The column diameter is calculated by the equation:

In accordance with the standard, we accept the value of the diameter of the atmospheric column K-2 as D K-2 = 7 m.

The distance between the upper plate and the upper bottom of the column is taken equal to half the diameter of the column, that is, h 1 \u003d 7/2 \u003d 3.5 m.

Height of the concentration part of column K-2 (n = 37):

h 2 \u003d (n - 1) H t \u003d (37 - 1) 0.600 \u003d 21.6 m

Height of the feed section of the column:

h 3 \u003d (2 3) H t \u003d 2 0.600 \u003d 1.2 m

The height of the stripping part of the K-2 column (n = 7):

h 4 \u003d (n - 1) H t \u003d (7 - 1) 0.600 \u003d 3.6 m

The distance from the liquid level at the bottom of the column to the bottom plate is taken equal to h 5 \u003d 1 2 m, so that the vapor is evenly distributed over the section of the column.

The height occupied by the liquid residue in the column is calculated based on a 5-10-minute supply of liquid at the temperature of the bottom of the column (329.4 ° C):

V ost \u003d 55.5 m 3 / h,

where is the absolute density of the residue at the temperature of the bottom of the column (285 ° C), kg / m 3:

981 - 0,522 (285 - 20) = 842,67 843

5 min \u003d 0.083 h - time reserve, h.

Hence the height occupied by the liquid residue:

We accept the height of the pedestal h 7 \u003d 4.0 m.

When calculating the height of the concentration section of the column, we take into account that 10 hatches are installed through 4 trays along the height of the column to ensure the installation and repair of the trays. In these sections, we take the distance between the plates H t \u003d 800 mm. Then:

h 2 \u003d 21.6 + 10 0.8 \u003d 29.6 m

Useful height of the column H floor (excluding the height of the support shell h 7):

H floor \u003d \u003d 3.5 + 29.6 + 1.2 + 3.6 + 2.0 + 1.44 \u003d 41.34 m.

Total column height:

H K-2 \u003d H floor + h 7 \u003d 41.34 + 4.0 \u003d 45.34 m? 46 m

The technology of primary oil refining is based on the separation of oil by the method of rectification into narrow oil fractions and is determined by the directions for using the fractions separated at the AVT units.

According to the type of operation of these installations, they distinguish:

1. Fuel (isolated fractions are intended mainly for the production of motor fuels)
2. Oily (it is envisaged to isolate narrow oily fractions)
3. Fuel - oil

Therefore, domestic oil refining units (AT and AVT) are characterized by a wide variety of distillation schemes used, depending on the range of produced fractions. However, in all cases, several basic principles are observed:

1. The process of primary distillation of oil is carried out in complex columns, characterized by the presence of several zones of feed input and selection of target products.
2. In the process of rectification, in order to provide heat supply to the system and reduce the partial pressure of oil vapors, sharp steam irrigation is widely used (superheated water vapor is introduced into the system).
3. For intermediate condensation of the vapor phase along the height of the column, remote cold circulating irrigations are used.
4. In rectification schemes, remote stripping columns (stripping - sections) are used, which leads to the appearance of additional recycle links in the system.
5. The raw material supply of production is often characterized by the presence of several oil suppliers, and hence fluctuations in time of the fractional composition of the feedstock.
6. The requirements for the quality of the separated fractions, primarily in terms of reducing the effect of superposition of neighboring fractions on each other, are constantly increasing.

These circumstances significantly complicate both the process implementation scheme and its constructive design. Separation technology (diagram) and design have a significant influence on each other and should be considered together. Therefore, the computational study of the process, and especially the procedure for its optimization, becomes an extremely complex task that cannot be solved without the use of the CMF.

Scheme of the atmospheric unit of the AVT installation

Scheme of the atmospheric unit of the AVT installation

The principle of operation of the atmospheric unit (AT)

The most common process implementation scheme for the AVT atmospheric unit is the scheme of double evaporation and double rectification of oil (Fig. 2.1). According to this scheme, the widely used one, which is included in the typical LK-6U blocks of many oil refineries in the Russian Federation, works.

Installation of CDU-AVT-6

Dehydrated and salt-free oil from the ELOU block (oil treatment unit - electric dehydration and desalting of oil) after heating to a temperature 195-205 about C due to the heat recovery of the material flows leaving the plant, it enters the separation in the column of partial topping of raw materials K-1.

Purpose K-1– selection of light gasoline and the main part of dissolved gases from oil to normalize the amount of gasoline hydrocarbons in the main column K-2 and stabilization of the mode of its operation with possible fluctuations in the composition of raw materials.

distillate vapors from K-1 are condensed in air and/or water coolers and separated in the C-1 separator into liquid (II) and gas (VIII) phases.

Part of the liquid phase is returned to K-1 as a phlegm, and the balance excess (fraction of light gasoline II) is removed from the plant.

The gas phase is diverted to a gas fractionation plant ( HFC). Partially stripped oil from the bottom K-1 enters the oven P-1, heated to a temperature 360-370 about C and served on the column feed plate K-2.

At the same time, part of the heated oil ( bottom product K-1) is returned to K-1 as a "hot jet" to create steam reflux in the exhaust section of the column.

Distillate vapors from the top K-2 condense in devices AVO and enter the separator C-2. Part of the liquid phase returns as phlegm to K-2, and the balance excess (heavy gasoline fraction III) is removed from the plant. From the intermediate plates of the strengthening section K-2 fuel fractions are removed in the form of side straps 180-220 about C, 220-280 o C and 280-350 o C, which are sent to stripping columns K-3, K-4 and K-5 respectively.

By the way, read this article too: Catalytic cracking unit

To the bottom of the K-2 column, as well as to the bottom of the stripping columns, superheated water vapor(stream IX) for stripping lighter fractions from product streams. Stripped fractions together with water vapor they return to the main column K-2 above the points of selection of side straps.

The use of stripping columns can significantly reduce the content of light fractions in the selected distillate products and thereby improve their quality.

Rice. 2.1. Schematic diagram of double distillation of oil in the atmospheric unit of the AVT installation: K - distillation columns;

P - oven; C - separators; T - heat exchangers. Flows: I - raw materials (oil with ELOU); II - light gasoline; III - heavy gasoline; IV - fraction 180-220 about C;

V - fraction 220-280 about C; VI - fraction 280-350 about C; VII - fuel oil; VIII - gas;

IX - water vapor

In the process of oil rectification, water vapor plays a special role, determined by the fact that water and hydrocarbons in the liquid phase are practically mutually insoluble and form separately boiling mixture.

Under these conditions, water vapor not only introduces into the system the heat necessary for stripping light hydrocarbons, but also reduces partial pressure oil vapors, which in turn leads to a decrease in the boiling point of the hydrocarbon (oil) phase and, at the same time, to increase in relative volatility all hydrocarbon pairs of components.

Therefore, the introduction of water vapor is to a certain extent equivalent to pressure drop in a distillation system, which is especially important for columns operating under vacuum.

On plates distillation columns AVT installations water vapor at the operating modes used does not condense, passes the entire column from the bottom up and condenses only in the external condensing units. The flow rate of water vapor in the atmospheric block is ( 1,2–3,5 ) % wt. Based on the raw materials of the plant.

By the way, read this article too: Sulfuric acid plant

The use of water vapor also leads to negative effects:

• the energy costs for the process increase;
• steam loads in distillation columns noticeably increase, since the molecular weight of water is significantly less than the molecular weight of the separated hydrocarbons;
• as a result, the diameters of distillation columns and their hydraulic resistance increase;
• oil products are flooded, which necessitates their subsequent drying;
• chemically polluted wastewater is generated.

Therefore, in world practice there is a tendency to use the hydrocarbon phase (gasoline and kerosene-gas oil fractions) as an evaporating agent instead of water.

However, in domestic practice, these solutions are not widely used. In the strengthening section of column K-2 (Fig. 2.1) there are 2 cold circulating sprays, which provide intermediate condensation of the steam flow in K-2.

At the same time, the flow rates of liquid irrigation (internal reflux) increase and a more complete selection of target fuel fractions is ensured. Cooling of circulating irrigation is carried out in remote refrigerators.

At different refineries, the operating modes of atmospheric unit columns, as well as instrumentation technological process can vary significantly, which confirms the need for optimization solutions in the analysis and improvement of the performance of each specific installation. Characteristic indicators of the operating modes of the atmospheric unit of the AVT-6 installation during the processing of West Siberian oil are given in Table. 2.1.

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