Chemistry and Technology of Fuels and Oils, Vol. 37, No. 3, 2001

EQUIPMENT RECONSTRUCTION OF AN ABSORBER FOR REMOVAL OF HYDROGEN SULFIDE AND CARBON DIOXIDE FROM NATURAL GAS V. M. Berdnikov, Yu. N. Lebedev, K. V. Baklashov, B. I. Belinskii, E. M. Prokhorov, and T. M. Zaitseva

UDC 66.021.3

The absorber for removing hydrogen sulfide and carbon monoxide from natural gas with aqueous solutions of amine was rebuilt by specialists from Kedr-89 Scientific and Industrial Company together with workers from Astrakhan GPP. The essence of the reconstruction was to replace the bubble sieve trays with centrifugal trays and VAKUPAK structured packing. Industrial tests showed that the absorber with the new contact devices ensures output with respect to the gas in normal conditions of: desulfurized: 130,000 m 3 /h; separated, 196 – 200,000 m3 /h. These indexes are 8 – 10% higher than the projected indexes. The hydrogen sulfide content in the treated gas is no greater than 0.0073 g/m 3 (admissible: 0.012 g/m 3). The maximum carbon dioxide content in the treated gas is 2.68 ppm (standard: 150 ppm). At Astrakhan GPP, natural gas is treated to remove hydrogen sulfide and carbon dioxide in a 72SO1 absorber with an aqueous solution of amine. This absorber, designed by the French company Technip, is equipped with 33 sieve (perforated) trays with overflows. The upper part of the absorber 2.6 m in diameter contains 10 double-flow trays and the lower part, 4 m in diameter, contains 23 quadruple-flow plates. The pressure in the absorber is 6.9 MPa, the average gas density in the operating conditions is 49 kg/m 3 , and the liquid:gas mass ratio = 6.5. A drawback of this absorber is that TABLE 1 Theoretical plate number

Pressure, MPa

Temperature, °C

Flow of streams in trays, kg/h liquid

vapor

Section I 1

0.610

47.3

363532.8

124329.0

2 3

0.611 0.611

47.9 50.8

363585.7 0.0

124626.3 124679.2

Section II 4 5

0.612 0.613

62.1 63.3

1031310.8 1033070.0

117004.6 117910.7

6

0.614

Section III 71.1

1434508.5

119669.9

7

0.614

86.4

1503221.9

157291.2

8

0.615

88.1

1500000.0

226004.7

____________________________________________________________________________________________________ Astrakhan’gazprom Limited Liability Company. GAZPROM OJSC. Kedr-89 Scientific and Industrial Company. Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 3, pp. 20 – 23, May – June, 2001. 0009-3092/01/3703–0167$25.00 ©2001Plenum Publishing Corporation

167

TABLE 2 Tray No.

Molecular weight

Density (in operating conditions), kg/m3 Vapor phase

kg/h

m3 /h (real)

Flow rate

1

18.40

46.221

124328.91

2689.871

2

18.41

46.126

124626.29

2701.891

3

18.41

45.605

124679.18

2733.912

4

18.42

43.529

117004.63

2687.948

5

18.43

43.344

117910.71

2720.321

6

18.56

42.356

119669.89

2825.337

7 8

21.25 24.60

46.384 54.660

157291.20 226004.68

3391.064 4134.727

1

27.12

Liquid phase 1030.75

363532.78

352.687

2

27.12

1030.36

363585.67

352.872

3

27.12

1028.74

0.00

0.000

4 5

23.17 23.19

1012.22 1013.14

1.031·10 1.033·10

6

24.34

1036.40

1.434·10

7 8

24.76 24.92

1077.34 1091.56

1.503·10 1.533·10

6 6 6 6 6

1018.856 1019.672 1384.126 1395.310 1404.607

surfactants enter the aqueous solution of amine — corrosion inhibitor, products of corrosion, C 7 – C 8 liquid hydrocarbons, etc., form stable foam and cause premature flooding of the unit. Kedr-89 Scientific and Production Company (SPC) together with Astrakhan GPP proposed redesigning the absorber to improve its operation: replacing the bubble sieve trays with new contact devices — centrifugal trays [1] and VAKUPAK structured packing [2]. The gas reacts with the liquid in these devices in drops and films so that foam is actively broken down. Their output in comparison to bubble trays is 30 – 50% higher. Comparative tests of contact devices of the old and new designs were performed on an experimental setup using a model system before reconstruction began [3]. Data were obtained for hydrodynamic calculation of the absorber. The mass-transfer (kinetic) characteristics of the absorption process were determined by computer modeling using Hysim software from Hyprotech. Aqueous solutions of diethanolamine (DEA) and diethanolamine mixed with methyldiethanolamine (MDEA) were used as the absorbent. The operating parameters of the absorber and its gas-liquid loads in different sections are reported in Tables 1 and 2. An engineering design for reconstruction of the 72SO1 absorber in the ZU-272 plant was proposed using experimental data and the results of computer modeling. An overall view of the absorber is shown in Fig. 1. Upper section 1 contains 8 centrifugal trays with 16 centrifugal elements in each tray. A separator with string-type drip pans is located above the centrifugal trays. Middle section 2 contains 8 centrifugal trays with 52 centrifugal elements in each tray. A specially designed distributing tray is used for feeding the coarsely regenerated absorbent solution. Two beds of VAKUPAK structured packing 3.5 and 2.6 m high are placed in lower section 3 of the absorber; rectangular-cap liquid distributors are installed over the upper and lower beds. Collecting-distributing trays 4 for uniformly distributing the wet gas entering the absorber over the packing, collecting the saturated absorbent solution, and transferring it to the tower still are installed under the lower packing bed. The absorber operates as follows: wet gas enters underneath the unit under the distributing tray, passes 168

I

II 1

À

À —À

À

III II

IV

B B —B

B

2

C

C 3

4

C —C

V

VI

Fig. 1. Diagram of the Astrakhan GPP absorber: 1, 2, 3) upper, middle, and lower sections; 4) cooling-distributing tray; I) scrubbed gas; II, IV) finely and coarsely regenerated DEA solution; III) regeneration gas; V) gas to treatment; VI) saturated DEA solution. 169

Fig. 2. VAKUPAK structured packing unit. through the two packing beds, 16 centrifugal trays, and the string separator. Finely regenerated amine solution — 60% DEA with 40% MDEA — is fed into the upper centrifugal tray, goes out of the 8th tray (counted from the top), and is transferred to the lower section for spraying the packing. Coarsely regenerated amine solution enters the middle absorber section in a centrifugal tray, passes through eight centrifugal trays, and goes to the liquid distributor in the upper packing bed, where two liquid streams are mixed: overflow from the upper section of finely regenerated amine solution and the stream of coarsely regenerated amine solution from the middle section. The highest reflux density — up to 120 m 3 /(m 2 ×h) is thus created on the structured VAKUPAK packing, but its operation is still efficient. An overall view of the VAKUPAK packing unit, which is a pack of vertical plates parallel to the axis of the column, is shown in Fig. 2. The plates consist of a punched sheet with horizontal corrugations having curved prongs with openings directed downward. The prongs maintain the defined distance between plates. The 350×440×1000 mm packing units are installed through a hatch on a support grate by layers with 90° rotation, closely filling the cross section of the unit. The lower layer with a total height of 3.5 m consists of 8 rows (each 440 mm high), and the upper layer consists of 6 rows with a total height of 2.64 m. The liquid, in the form of a film, spreads over the corrugations, and film movement is preserved with any liquid and gas loads. For this reason, stable efficiency is guaranteed in a wide range of loads. The gas phase zigzags from the bottom up, and the distance between sheets ensures low pressure loss in the packing. Intensive internal phase mixing is ensured by the prongs on the tilted sides of the corrugations. A contact element 380 mm in diameter is the fundamental unit of the centrifugal tray (Fig. 3). This element consists of a swirler, reflector, and separating shell. Liquid overflows from tray to tray in pipes installed in the tray region between elements. The trays operate as follows. The gas goes down into the contact element, is swirled in the swirler, the liquid coming out of the overflow pipe is taken up and discharged into the separating shell. High gas output of the tray is ensured because the centrifugal forces that arise in swirling of the phases cause efficient separation of the liquid after contact and reduce entrainment of drops to the stage above. Foam is simultaneously actively broken down in the separating shell so that the throughput of the tray for the liquid phase is 1.5 – 2 times higher than for bubbling trays. In addition to high output, the wide range of stable efficient operation is also an advantage of centrifugal trays: when the output varies by 2 – 3 times, high separating efficiency is preserved. 170

Fig. 3. Diagram of centrifugal tray contact element.

According to the technical plan agreed upon with Astrakhan GPP, Kedr-89 SPC has manufactured all of the contact internals for the ZU-272 absorber unit. After assembly of these internals in March 2000, operation of the absorber began. The industrial tests of the redesigned absorber conducted from April 20 – 27, 2000, according to the program agreed upon with Astrakhan GPP, showed: • the maximum output for desulfurized gas was 130,000 m 3/h (or 196 – 200,000 m 3 /h for separated gas) in normal conditions, which is 8-10% higher than projected; • the hydrogen sulfide content in the treated gas did not exceed 0.0073 g/m 3 with an admissible value of 0.012 g/m 3; • the maximum carbon dioxide content was 2.68 ppm versus the standard of 150 ppm. After one year of operation, the 72SO1 absorber is operating stably and providing the required separating efficiency and maximum pressure differential in the tower of 0.025 MPa.

REFERENCES 1. RF Patent No. 2122881. 2. RF Patent No. 2118201. 3. Yu. N. Lebedev, A. I. Vladimirov, and V. D. Kos’min, Khim. Tekhnol. Topl. Masel, No. 10, 20 – 21 (1997). 171