Chemical and Petroleum Engineering, Vol. 45, Nos. 9–10, 2009
RESEARCH, DESIGN, CALCULATIONS, AND OPERATING EXPERIENCE PROCESSES AND EQUIPMENT FOR CHEMICAL AND OIL-GAS PRODUCTION INVESTIGATION AND ANALYSIS OF A NEW GENERATION OF STRAIGHT-THROUGH CENTRIFUGAL SEPARATION ELEMENTS
A. G. Zibert and G. K. Zibert
Results are presented for investigations and analyses of straight-through centrifugal separation elements simultaneously fulfilling the function of the coalescence (enlargement) of drops, which makes it possible to improve the effectiveness of the centrifugal-separation process, and the function of drop separation from a gaseous flow. A mathematical model is presented for determination of drop diameter as a function of radius of the stationary installed ejector (deflector) along the axis of a straight-through centrifugal element, the difference between the densities of the liquid and gas, the surface tension of the liquid, and the tangential velocity component of the flow. Coalescence of liquid drops is confirmed not only by computational relationships, but also by photographs obtained during experiments.
Separators for the isolation of free and drop liquid from natural- or petroleum-gas flows (Fig. 1a) and separation sections of absorbers for the drying of moisture from gas have, in recent years, been built primarily on the basis of straightthrough centrifugal elements, which ensure a higher output from the equipment, a lower tendency to mechanical clogging and hydrate precipitation, individual streams of separated gas and liquid flows, and the possibility of assembling the devices through manholes and traps doors. Vessels with straight-through centrifugal separation and contact elements have come into widespread use by the company Gazprom. To determine the basic trend in further refinement of separation equipment, we analyzed methods and devices employed for the separation, and exposed their basic deficiencies. Elevated carry-off of liquid with the gas – more than 10–15 mg/m3 [1, 2] – is a major disadvantage of separators with straight-through centrifugal elements; this is dictated by the following: 1) existence of the splitting of liquid drops in the ejector of gas-flow ejector of the straight-through centrifugal element; and 2) splitting of liquid drops in the tube regulating the gas flow in the separation element; and 3) existence of secondary carry-off of liquid with the gas as a result of splitting of liquid drops by the intersecting gas flow exiting the film remover downward in the direction opposite to the discharge of liquid.
UK RusGazInzhiniring Controlling Company, Podolsk, Moscow Oblast, Russia. Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 9, pp. 3–5, September, 2009. 0009-2355/09/0910-0521 ©2009 Springer Science+Business Media, Inc.
521
Fig. 1. Diagram showing separator (a) and straight-through centrifugal element (b): 1) vortex generator; 2) branch pipe; 3) ejector; 4) support ring; 5) film remover; 6) coalescing teeth; 7) gas-discharge channels; 8) liquid-discharge channel.
Fig. 2. Breakaway of liquid film along generatrix of cone.
According to results of analysis of the performance of existing straight-through centrifugal elements and their design solutions, we have developed, fabricated, and tested a new generation of straight-through centrifugal elements (Fig. 1b) for separators of natural and casing-head gases. In a convoluted gaseous flow, small-diameter liquid drops are accumulated in the axial section of the straightthrough centrifugal element, proceed onto the convex surface of the ejector, and wet it, transforming into a liquid film, which moves along an Archimedes spiral under the action of centrifugal forces; here, the current radius Rb of the spiral on which a particle of liquid is found, is increased, and the thickness of the wetting film decreased. To separate the liquid from the gaseous flow, it is expedient to increase the radius of motion of the liquid, since the centrifugal force acting on a drop is increased here. Thinning of the liquid film leads, however, to a decrease in the diameter of the liquid drops being stripped away, and, of course, to an increase in the carry-off of liquid with the gaseous flow.
522
Fig. 3. Cone formed from liquid beyond limits of ejector.
The cohesive force of the liquid drops is equal to the force of surface tension [2], and is determined from the relationship Fco = πdσcosϕ, where σ is the surface tension in N/m; d is the diameter of a drop in m; and ϕ is the angle of twist of the gas flow. The force of inertia (separating force) Fs = ma, where m is the mass of a drop in kg, and a is the centrifugal acceleration of the motion of a drop in m/sec2. If the force of inertia Fs is lower than the tangential force of cohesion Fco on the maximum diameter of the ejector, the liquid film on descending from the ejector is separated from the latter, and continues to maintain the shape of the ejector in the form of a liquid film basin, which disrupts the gaseous flow at radius Rb, and splits it into finely disperse liquid drops, resulting in their increased carry-off. This phenomenon is illustrated in Figs. 2 and 3. For the case in question, Fs < Fco, or ma < πdσcosϕ, where a = wτ2 /Rb, and wτ is the tangential velocity component of the flow in m/sec. A gradual increase in radius Rb on the expanding cone of liquid along the path of motion of the flow, and a reduction in its thickness will lead to a change in the sign in the inequality between the inertial and cohesive forces: Fs > Fco.
(1)
Breakaway of the liquid film will then occur with the formation of finely disperse drops; this ensures a developed specific interphase surface within the volume of the straight-through centrifugal element; this surface is 7052–18070 m2/m3 and 1726–3359 m2/m3 for an element 0.06 and 0.1 m in diameter, respectively [2]. The greatest specific interphase surface is provided by a large amount of finely disperse drops; this is a positive factor for straight-through centrifugal mass-exchange elements, but a negative factor for the separation elements. Insignificant centrifugal forces act on finely disperse drops due to their small mass, and the drops are easily carried-off by the gas flow.
523
It is established experimentally that for condition (1), breakaway of the liquid film takes place along the generatrix of the cone, where the rupture stresses due to inertial forces uniformly applied to the conical lining are maximum, as for any material shell, ring, or torus. Breakaway phenomena of the liquid shell are recorded in Figs. 2 and 3. The equilibrium condition of the forces acting on a liquid ring with a radius Rb Fb = Fco is observed at the moment of initiation of breakaway of the liquid film, where Fb = ma is the centrifugal force of the breakaway of a ring at radius Rb in N. The total breakaway force of the ring at the point of force application
∑
Fb =
maRb πd 2 = P, d 4
(2)
where d is the diameter of a drop, which is approximately equal to the thickness of the liquid film of the cone at the moment of breakaway, and P is the strength of the ideally pure liquid (allowable stress), which can be calculated from the formula [3, 4]
P = pG −
2.2 ⋅ 10 8 σ 3 / T log Aτ
,
(3)
where PG is the saturated-vapor pressure of the liquid in Pa; A = 1014–1036 is a factor before the exponent in (sec·cm3)–1; τ is the average time of expectancy of liquid breakaway (in seconds in 1 cm3 of liquid) in sec·cm3; and T is the absolute temperature in K. The strengths of certain liquids, which are calculated from formula (3) in Pa are: water – 1600, benzene – 400, and ethyl alcohol – 270. The values of Rb and d can be determined from Eq. (2) and from the equation for the diameter of a drop [2] d=
1 wτ
6Rbσ cos ϕ , ρ1 − ρ2
where ρ1 and ρ2 are the densities of the liquid and gas in kg/m3. We propose an ejector design that provides for automated breakaway of the liquid film at the maximum radius of the ejector with simultaneous enlargement of the diameter of the drops stripped from the ejector and provision for the required section for passage of gas between liquid drops. The ejector is a part of the shell in the form of a solid of revolution, the convex portion of which is directed toward the flow of gas. Inclined (along the path of rotation of the gas flow) teeth are positioned along the periphery of the ejector. The radius Rb is set equal to half the radius of the branch pipe of the centrifugal element. This value is determined experimentally based on minimization of the hydraulic resistance of the straight-through centrifugal element at the effective gas velocities. Investigations conducted on straight-through centrifugal elements with a flat edge indicated that the average diameter of drops split-off from its edge is equal to 10–4 m [2] when the spacing between drops is 2d. The average diameter of drops on the ejector equipped with 24 teeth is 3.2·10–4 m, i.e., the drop diameter on a tooth of the ejector is increased by more than three times, while the mass of the drop is increased by an order here. The increase in the diameter of a drop and its mass ensures increased effectiveness of the separation of liquid from the gas. Figure 4 shows liquid drops that have been enlarged on the inclined teeth of the ejector, and the distance between them increased for the passage of gas, i.e., deformation of the liquid film is recorded on the inclined teeth with the formation of coarse drops at their tips. 524
Fig. 4. Enlargement of liquid drops on tooth of ejector.
The splitting of liquid drops by an intersecting gas flow discharged from beneath film remover 5 (see Fig. 1b) in the direction opposite to the discharge of liquid was eliminated by implementing an additional subassembly for centrifugal separation in the space between the film remover and outside wall of branch pipe 2. For this purpose, channels 7 for the discharge of gas, which are situated on the inside radius of the ring, and channels 8 for the discharge of liquid, which are located on the outside radius of the ring, are introduced to support ring 4 (see Fig. 1b, section A–A). Other indicated deficiencies are also similarly eliminated in the new generation of separation elements. The new generation of separators with straight-through centrifugal multifunctional elements, which are intended to provide the following, are developed on the basis of analysis of the performance of existing separation equipment: • multistage separation; • enlargement of liquid drops; • forced breakaway of a liquid drop along the maximum diameter of the ejector to exclude its splitting by the gaseous flow; • recirculation of the gas-liquid flow; and • preliminary selection of the liquid.
REFERENCES 1. 2. 3 4.
G. K. Zibert and I. E. Ibragimov, “Mass-exchange straight-through centrifugal elements,” Khim. Neft. Mashinostr., No. 6, 2–5 (1996). G. K. Zibert and I. E. Ibragimov, “Determination of the surface area of mass transfer in a straight-through contactseparation element with an internal central surface,” Khim. Neft. Mashinostr., No. 6, 5–7 (1996). A. B. Zel’dovich, “Kinetic theory of liquid separation,” Zh. Eksprim. Teor. Fiz., 12, No. 11–12, 525 (1942). G. K. Zibert, Promising Technologies and Equipment for Preparation and Refining of Hydrocarbon Gases [in Russian], Nedra, Moscow (2005).
525