Chemical and Petroleum Engineering, Vol. 46, Nos. 5–6, 2010

DESIGN OF A HYDRODYNAMIC ABSORBER WITH A Π-SHAPED GAS FLOW INLET AND OUTLET

V. P. Aleksandrov1 and N. I. Salikhov2

A design is provided for uniform gas flow distribution over the length of a layer of activated coal in a radial absorber with a stationary absorbent layer. Calculated results confirmed by experimental data indicate a high degree of absorbent layer utilization.

In order to clean ventilation discharges from sewerage pumping stations (SPS) from hydrogen sulfide (H2S) at the Runovskaya SPS (Moscow) [1] two ATS-1200 absorbers operating in parallel were installed with a nominal throughput of 1200 nm3/h with a vertical layer of “coal” absorbent 30 cm thick. The equipment is located in the suction line of the fan, and the linear velocity of cleaned air in the absorbent bunker cross section is on average 0.35 m/sec. With an initial H2S concentration on average of 1.425 mg/m3 and an RSH of 0.232 mg/m3 the device provides the required level of cleaning from foul smelling substances over 650 days of continuous operation. Since the results achieved for absorber reliability and operating life with a stationary layer are determined to a considerable extent by the distribution of air being cleaned over the catalyst layer cross section, i.e., the aerodynamics of the equipment, aerodynamic calculations are considered in this article that are proposed as a basis in developing its construction, and also their confirmation by experimental data. Taking account of the recommended classical work for gas cleaning equipment dynamics of Idelchik [2, 3] a Π-shaped layout was used for the gas flow input and output in the absorber (see Fig. 1). Calculation of equilibrium distribution over the length of the “coal” layer may be provided by the equation [2]

Δv =

(ξcol ′ − 1) ρ

wi2 w2 + (ξcol ′′ + 1) ρ i 2 2 , 2δpav

(1)

where Δv is the relative degree of gas flow nonuniformity; ξ′col and ξ″col are hydraulic resistance coefficients respectively for the distributing and gathering collectors; wi is gas flow rate in the initial collector cross section, m/sec; ρ is gas flow density, kg/m3; and δpav is pressure loss, calculated fort he average gas flow rate in the absorbent layer, Pa. The value of ξcol is found by the equation [2] ξcol ≈ 0.5λ c L / Deq ,

1 2

State Research Institute of Industrial and Sanitary Gas Cleaning (NIIOGAZ), Moscow, Russia. Moscow State University of Engineering Ecology (MGUIE), Moscow, Russia. Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 5, pp. 47–48, May, 2010.

312

0009-2355/10/0506-0312 ©2010 Springer Science+Business Media, Inc.

(2)

Fig. 1. Layout of absorber: 1) diffuser; 2) distributing collector; 3) reducer; 4) gathering collector.

where λc is channel hydraulic flow coefficient; L is channel length, m; Deq = 4Fi/Π is collector initial cross section equivalent diameter, m; Fi is initial rectangular cross sectional area, m2; and Π is initial section perimeter (2.4 m). Equation (1) holds with ξcol < 1 and ratio L/Deq. In the equipment in question, L = 1000 mm, Deq = 330 mm. According to [4], the hydraulic resistance coefficient for a rectangular channel depends on Re number and channel relative roughness Δ. Channel wall roughness is taken as 0.1 mm, and the roughness of the w alls and grid bounding the absorbent, is 10–15 mm. Then the average roughness Δav is 6.06 mm, and the relative roughness Δ = Δav/Deq = 6.06/330 = 0.018 (mm). With an initial gas flow rate in the channel wi = 1.66 m/sec and air kinematic viscosity coefficient at 20°C ν = 15·10–6 m2/sec, Re = wiDeq /ν = 3.6·104, that according to the diagram provided in [4], gives a value of friction coefficient λ = 0.048. For a rectangular channel λc = knλ, where kn is a coefficient specifying the ratio of channel width (0.2 m) to height (1 m). With 0.2/1.0 = 0.2 [4] kn = 1.2. Whence λc = 1.2·0.048 = 0.057. Thus, according to Eq. (2), ξcol ≈ 0.086. It should be noted that the channel hydraulic resistance coefficient (both separating and collecting) should include apart from friction loss, local losses (in turning, separation and merging of flows). Considering them an insignificant quantity, the channel cross section in calculations was taken as constant, and loss was calculated without considering the reduction in gas velocity over the channel length. Calculation of hydraulic losses δpav was made on the basis of experimental data obtained in the Runov SPS installation: gas volumetric flow rate at the absorber inlet 1480 m3/h; total gas pressure at the absorber inlet 685 Pa, at the outlet 230 Pa, area of “coal” layer 1 m2, coal layer thickness 0.3 m. This the absorber hydraulic resistance is 685 – 230 = 455 Pa. A change in total pressures pt.in and pt.out directly in the air and water ventilation network is due to the following local resistances (see Fig. 1): • in the diffuser with six separation walls; • in the separating collector; • in the activated coal layer; • in the gathering collector; • in the reducer. The diffuser hydraulic resistance coefficient with separation walls, according to [4], is ξd = 0.54. Correspondingly, the loss in the diffuser with gas flow rate w1 = 8.3 m/sec are air density ρa = 1.2 kg/m3 is 313

Δpd = ξd ρa

w12 1.2 ⋅ 10.32 = 0.54 ≅ 34 Pa . 2 2

The reducer hydraulic resistance coefficient, according to [5], ξr = 0.23. whence the hydraulic loss in the reducer is Δpr = ξ r ρa

w12 1.2 ⋅ 10.32 = 0.23 ≅ 15 Pa . 2 2

In view of the insignificant values of hydraulic resistance in friction in the separating and gathering collectors, they may be ignored. Thus, the hydraulic resistance of the “coal” layer with a gas flow rate passing through the absorber of 1480 m3/h, Δpla = Δpab – (Δpd + Δpr) = 406 Pa. The hydraulic resistance of the absorbent layer Δpla (Pa) is calculated by the equation Δpla = δpla = ξla ρ

2 wav , 2

(3)

where ξla is layer hydraulic resistance coefficient, and wav is average gas velocity in the coal layer, m/sec. With a planned throughput of 1200 m3/h, the hydraulic loss is δpla = 4025 ⋅ 1.2 ⋅

0.332 = 263 Pa . 2

Substituting the values obtained in Eq. (1), we obtain Δv = 0.00054, i.e., velocity deviation in the absorbent layer to a greater or lesser extent does not exceed 0.054%. If in Eq. (1) an additional pressure loss in collectors is introduced, considering turning of the gas stream, the relative degree of nonuniformity should increase somewhat, but in this case the loss should not exceed 1%. Use in an actual absorber of a separating collector of wedge shape (variable cross section) there should be an additional capacity for uniform gas flow distribution over the absorbent layer cross section. Thus, according to the calculation and experimental data, within an absorber there is quite uniform distribution of gas flow rate over the absorbent layer cross section, that makes it possible to achieve high efficiency during a prolonged operating life.

REFERENCES 1. 2. 3. 4. 5.

314

V. I. Lazarev, R. B. Baranova, L. E. Zharkova, et al., “Deodorization of ventilation discharges of sewerage pumping stations,” Ekol. Proizv., No. 4, 64–68 (2010). I. E. Idelchik, Aerodynamics of Engineering Equipment [in Russian], Mashiniostroenie, Moscow (1983). I. E. Idelchik, Aerodynamics of Industrial Equipment [in Russian], Energiya, Moscow (1964). I. E. Idelchik, Hydraulic Resistance Handbook [in Russian], Mashiniostroenie, Moscow (1975). I. E. Idelchik, Hydraulic Resistance Handbook [in Russian], Gosenergoizdat, Moscow (1960).