Chemical and Petroleum Engineering, Vol. 47, Nos. 7–8, November, 2011 (Russian Original Nos. 7–8, July–August, 2011)
DEVELOPMENT AND STUDY OF VORTEX APPARATUS FOR PREPARING ASSOCIATED PETROLEUM GAS FOR TRANSPORT
T. N. Bokovikova and S. Yu. Savitskii
In this article, a triple-stream vortex tube design is analyzed and methods of optimization of its operation are described. Keywords: associated petroleum gas, vortex apparatus, gas purification, 3-stream vortex apparatus, HYSYS.
Presently, a huge number of oil, oil and gas, and oil condensate fields are being developed and operated in the world. The composition of the formation fluid varies widely with the type of the field, but the well products contain associated gas (casing-head gas) in different quantities. The volumes of associated gas yield depend directly on the oil yield. Associated petroleum gas is a mixture of high-volatile and low-boiling natural hydrocarbons, the main method of utilization of which is flaring at the sites of recovery of natural hydrocarbons. In Russia, the approximate annual loss of this valuable raw material and energy carrier comprises 15 bln m3. Gas flaring causes not only irreplaceable losses of hydrocarbons but also ecological damage to the environment. The most common way of utilization of associated petroleum gas is to feed it into trunk gas line or use it as a fuel in gas turbine and gas piston power plants. But for these kinds of utilization of associated gas one has to use complicated technological schemes for prior preparation of the gas, its purification and drying to the requirements of operative normative documents with employment of a large number of process equipment, which helps extract steam (water vapor) and high-boiling hydrocarbon components. The technological schemes for drying and purification of associated gases, which are based on absorption, adsorption, and low-temperature condensation processes with employment of refrigerating machines, entail substantial energy (power) and material consumption and operational costs, which renders them unprofitable in field conditions, especially if the fields are small. Economically more effective are low-temperature processes based on gas-dynamic expanding machines. For instance, in implementation of low-temperature separation processes, vortex tubes (VT) occupy a special place. In terms of thermal efficiency, VTs occupy an intermediate position between a throttle and an expander. At a relatively small pressure differential, the temperature may drop markedly. Alongside cold generation, they, in their three-stream version, ensure condensation and separation of the liquid from the whirling stream simultaneously [1]. The range of developed and used VTs is extremely wide [2]. For instance, in some VT designs intended for generating cold (refrigeration), the temperature on the apparatus axis attains –200°C. The goal of this work was to develop a VT that possesses both thermal efficiency significantly exceeding the existing relatives and maximally possible separation properties. The vortex apparatus developed by Zhidkov [3] was taken as the base for developing a high-efficiency three-stream VT (TVT–HE). The original apparatus belongs to vortex apparatuses and can be used in gas, chemical, and oil industries for cold or heat generation and for purification of gas mixtures from condensed fractions, particularly for low-temperature purification of natural gas from higher hydrocarbons and sulfur compounds. Kuban State Technological University, Krasnodar, Russia. Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 8, pp. 27–29, August, 2011. 0009-2355/11/0708-0545 ©2011 Springer Science+Business Media, Inc.
545
Fig. 1. Original vortex apparatus: 1) cylindrical body; 2) diaphragm; 3, 4) outlet tubes; 5) diffuser; 6) drive.
The VT contains a rectangular nozzle inlet (Venturi-type tube) and a device for controlling the flow rate of compressed gas that includes an operating unit fixed in the nozzle section and made in the form of a body having unevenly narrowing cross section with provision for reciprocating motion. The longitudinal section of the original vortex apparatus is shown in Fig. 1. The diffuser is fixed relative to the body by rods. The operating unit mounted with the provision for reciprocating motion is placed in the nozzle inlet. The drive of the pneumatic actuating mechanism is fixed on the body. The vortex apparatus operates as follows. Compressed natural gas flows in through the connecting pipe, passes through the annular gap, and then flows tangentially at the velocity of sound into the VT through the diffuser containing the rectangular nozzle inlet, whirls and separates into two streams: cold stream passing through the diaphragm and hot stream flowing in the opposite direction. For varying the gas load, the control pulse Pc is varied, whereupon the operating unit moves, reducing or increasing the area of the flow section of the nozzle. The compressed natural gas increases the control pulse Pc. In this case, the operating unit made in the form of a body having a variable cross section descends vertically and thereby reduces the area of the flow section. Because of the fact that the operating unit is made in the form of a body with a variable cross section, it does not deform during the operation in a wide range of variation of load and differential pressure. This VT design is reliable but has some flaws, which affect both the separation properties of the apparatus and its thermal efficiency. The shape of the operating body, i.e., of the flow rate regulator and inlet pipe, because of their aerodynamic inefficiency, causes additional turbulization (whirling) of the inflowing stream, which adversely affects the degree of thermal separation inside the apparatus. From the aerodynamic standpoint, for reducing turbulization of the stream it is essential that the extreme generatrix of the flow rate regulator in the inlet pipe repeated the geometry of the apparatus wall. It is also essential to vary the geometry of the inlet pipe by placing in it a guide element (deflector). This deflector will help reduce turbulization of the stream if the control element is in the “incompletely open” state. In order to universalize the design, the apparatus body is provided with a spot for fixing an electrical and a pneumatic drive for controlling the reciprocating motion of the operating unit. The possibility of selection of the type of drive for controlling the operating body makes it easy to maximally adapt the VT to operation in any automation method adopted in the customer’s facilities. Variations of the geometry of the operating body and inlet pipe help use the TVT-HE in a wide flow rate range without disturbing the structure of the fluid 546
Fig. 2. Circular impeller (vane): 1) apparatus housing; 2) impeller (vane); 3) place for installing drive; 4) control rod.
Fig. 3. Flow diagram of the experimental setup: PI – pressure gage; TI – thermometer; MI – moisture gage; 1, 2, 3) flow rate regulators.
stream at the apparatus inlet. In order to augment the thermal separation efficiency and to improve the separation properties of the TVT–HE, it is necessary to analyze how the thermal separation occurs in the VT. According to [4], the center of the VT contains those portions of the stream that initially had very little store of kinetic energy (energy content), and separation of the stream elements having different tangential velocities in a centrifugal force field is the mechanism that ensures entry of these portions in particular into the vortex center. Let us assume that on the same radius there are two microvolumes in the spinning (whirling) gas, the circular velocity fluctuation of one of which is positive and of the other, negative. Occurrence of different tangential velocities at the same centripetal acceleration will cause separation of these elements: the “faster” gas will move away from the stream center and the “slower” gas will move toward the center. The stagnated (retarded) gas, which undergoes adiabatic cooling due to expansion under conditions of pressure drop, will accumulate in the central region with reduced static pressure. On the periphery the “fast” gas will experience partial deceleration on the wall, due to which the gas will heat up, i.e., the reason for thermal separation of the gas in the vortex tubes is centrifugal separation of the turbulent elements depending on the velocity. This means that the thermal efficiency of the apparatus developed will be influenced not only by the turbulence of the inflowing stream but also by the centrifugal acceleration occurring in the thermal separation chamber itself. For increasing thermal separation in the TVT, it is necessary to stabilize the vortex formed at the center of the thermal separation chamber. Modeling of the apparatus operation by the finite-elements method showed that for stabilizing the vortex at the center of the thermal separation chamber, it is necessary to use an impeller (a vane) placed around the circle (circumference) at an equal distant (Fig. 2). 547
TABLE 1 CO2
N2
C1
C2
C3
i-C4
n-C4
C5
C6+
Liquid, mg/m3
0.16
1.03
83.29
5.53
6.65
1.49
2.53
0.67
1.22
2270
TABLE 2 Condition number
Excess pressure Pin
Pout
Temperature, °C Tin
Tc
Th
Tc+h
ΔT
Liquid, mg/m3
TVT–HE 1
2
1.5
18
–11
10
4
29
1240
2
2.4
1.5
18
–17
18
3
35
1100
3
2.7
1.5
18
–20
22
0
38
870
Throttle 1thr
2
1.5
18
–
–
14
–
–
2thr
2.4
1.5
18
–
–
11
–
–
3thr
2.7
1.5
18
–
–
9
–
–
Note: Pin, Pout – inlet and outlet pressures; Tin, inlet temperature; Tc, Th, Tc+h – temperature of the cold, hot, and mixed cold and hot streams, respectively.
The design of the thermal separation chamber shown in Fig. 2 makes it possible to achieve the following. 1) Stabilize the thermal separation vortex right at the center of the separation chamber and restrain the vortex from moving toward the apparatus walls in order to prevent deflection of the vortex, which produces a favorable effect on thermal separation. 2) On account of local reduction of the flow section at the point where the impeller is installed, the gas stream acquires additional acceleration momentum, which facilitates thermal separation. 3) The impeller, because of having a large working surface area, helps condense the moisture formed in the thermal separation process. Since the impeller occupies the entire length of the apparatus, it effectively removes the condensed moisture from the thermal separation zone, which prevents its repeated evaporation in the hot stream zone. For quantitative determination of the changes made in the apparatus, a series of experiments were performed on the experimental setup (Fig. 3), where the designed apparatus was considered as a cold generator. The composition of the associated petroleum gas used in the experiment is given in Table 1. The tests of the TVT–HE operation were performed at 2.0–2.7 MPa inlet pressures, and the temperature of the incoming stream was taken as 18°C. These technological parameters are the worst conditions for TVT–HE operation. In order to make it possible to determine the level of operational efficiency of the experimental apparatus, data on the operation of the throttling device under the same conditions are cited in Table 2. The data obtained allow us to conclude that the proposed design of the vortex impeller (vane) and the shape of the operating body at the apparatus inlet do produce a positive effect on thermal separation in the TVT–HE. The cited TVT–HE performance data indicate increased thermal efficiency of the apparatus. Also, going by the data on the volumes of liquid found at the TVT–HE outlet, it can be concluded that the proposed design of the separating equipment package indeed effectively condenses moisture from the gas stream and removes it from the thermal separation region before its secondary evaporation in the hot stream zone. The design features enable us to divide the apparatus into two basically independent parts and thereby prevent secondary entrainment of the condensed liquid. 548
The proposed apparatus design helps not only dry natural gas to the Russian Industry Standard (OST) requirements for gas transport, retaining in this process the possibility of its transport by trunk pipeline, but also obtain additional cold stream for further use in associated petroleum gas preparation technology.
REFERENCES 1. 2. 3. 4. 5.
F. G. Tempel, Development of Gas Deposits and Gas Transportation [in Russian], Nedra, Leningrad (1970). A. P. Merkulov, Vortex Effect and Its Application in Engineering [in Russian], Mashinostroenie, Moscow (1969). M. A. Zhidkov, RF Patent No. 2035990, “Vortex apparatus.” A. F. Gutsol, “Rank effect,” Usp. Fiz. Nauk, June, 665–685 (1997). A. P. Gusev, R. M. Iskhakov, M. A. Zhidkov, and G. A. Kamarova, “System for associated petroleum gas preparation for transport employing controllable three-stream vortex tube,” Khim. Neftegaz. Mashinostr., No. 7, 16–18 (2000).
549
