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The main results of the equilibrium calculation for the three separator stages are presented in Tables 6. I46 Tahlc 6. First separator stage; p I :6 hars. The composition of gas discharged from the three separator stages is given in Columns 5 of said Tables, whereas the composition of the stock-tank oil is given in Column 4 of Table 6. The useful GOR can be calculated using Eq. We have used the value 8. We have neglected z, ,the compressibility factor in the standard state, or have put it equal to unity. For finding liquid density pL3 we have used F i g.
It furnishes density of a liquid. Cl 1 Apparent density of crude containing methane and ethane, at 7- The starting data are listed in Tuhlc 6. The density of thc liquid containing C,, is then,. Introduced into Eq. I5 Since equilibrium calculations are rather time-consuming, it is recommended to find out beforehand whether two separate phases d o exist at all at the temperature and pressure in question.
If the well fluid is composed purely of hydrocarbons, this may be found out as follows.
On reducing the temperature of the mix at a given pressure, the dew point E is reached first and the bubble point H is reached thereafter F i g. If therefore, e. If both answers are yes, then the mix is two-phase in both cases and it. A t the dew point, one mole say of vapour keeps equilibrium with a liquid drop of evanescent size. Hence, in a very good approximation, z,. If the value obtained is greater than unity, the mix is two-phase, because in order to make said value approach unity it is necessary to increase the K i s and, by Fig.
Now by Fig. At the bubble point, one mole say of liquid keeps equilibrium with a gas bubble ofevanescent size. If the value of the equation is greater than unity, this means once more that the mix is two-phase. The well fluid often contains some water, too.
I t is necessary to keep in mind when performing calculations concerning such a fluid that the common vapour pressure qf immiscible liquids is entirely independent of the proportion of the components in the liquid phase, being equal to the sum of the vapour pressures of the components,. Find the vapour pressure of a water-pentane mix at 50 "C. On heating this mix, the bubble point will occur at that temperature where the sum of the two vapour pressures equals the external pressure.
The bubble-point temperature of the mix is, then, lower than that of any individual component water- vapour distillation.
It is often necessary to determine the temperature at which water starts to condense out of the hydrocarbon-water system that is, to establish the dew point of the mixture. Since each component generates its partial vapour pressure independently of the other components, the task is essentially to find the temperature at which the sum of partial pressures equals the vapour pressure.
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I39 0. What is the dew point at 1. The calculation results are given in 7uhlc 6. On cooling f r o m a higher temperature, the system will first attain I t reveals that, e. The curves show that, on increasing separator pressure, all components will at first condense to an increasing extent. The increment in condensate due to unity increase in pressure is highest in the case of methane and.
I c, 0. After Campbell and Whinery and Campbell The curves of components heavier than butane exhibit, however, peaks at which the liquid recovery is maximum for a given temperature, being reduced on further pressure increase by retrograde vaporization. This phenomenon is of considerable interest as regards the production of a liquid stable at atmospheric pressure. Graph 1 in Fig. The volume of stock-tank oil, composed largely of pentane and heavier components, will increase up to a pressure about of41 bars to decrease again at higher pressures.
A volume of liquid equal to the ordinate difference between the. Other well fluids separated at other temperature will give curves displaced against those given in the Figure, but their essential features will be same. The lower the temperature, the less will be the increment in C, and C, yield due to further decrease of temperature.
If the primary aim is to recover a maximum amount of stable liquid. I f the primary aim is to increase the total liquid yield, e. Low-temperature separation.bloomsuplivestgluc.cf
D & D Standard Oil & Gas Abbreviator, 6th Edition
Separatoi temperatures higher than 30 C are not be recommended as a rule. Influence of temperature upon liquid recovery. All three curves are seen to flatten out at temperatures below about - 7 "C, and to become practically independent of temperature and pressure.
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Recovery of stable stock-tank oil is governed below temperatures of - 7 "Clargely by wellstream composition. The factor having the most profound influence on stable-liquid yield is the molar ratio of C,,. Component A B C. The relative stable-liquid yields of the three wellstreams are plotted in F i g. The values at - 7 "C,marked in the Figure by dashed lines, have also been listed in the Table. Stable-liquid yield to be expected on separation below - 7 "C can be estimated by means of the formula. Relationships characterizing recovery above - 7 "C are more complicated. In Sections a , b and c we have analyzed the effects of temperature, pressure and wellstream composition, tacitly assuming two-stage separation with one separator and one stock-tank.
If the number of separation stages is more than two,. Stable stock-tank oil recovery 4. Hence, evaporation loss in open tanks will be less after stage separation. Increasing the number of stages from two to three brings a considerable improvement. According to an investigation on 13 wells, the recovery of stable stock-tank oil was improved by 8 percent on an average with a range from 3 to 22 percent.
On increasing stage number from three to four, the improvement in recovery tends to be much less, and four-stage separation is not usually economical. Liquids A and B passing from the first-stage into the second derive from one and the same wellstream, but first-stage temperature was - Z, Component C 0. The Figure reveals further that stable stock-tank oil yield is maximal in all three cases at the same, second-stage pressure of about 3. Pressure in the first separator stage is limited by the maximum possible wellhead pressure or, more precisely, by the maximum incoming flowline pressure.
Third- stage pressure is either atmospheric or is determined by the design features of the gas-gathering system. The most favourable second-stage pressure can be determined either by in-plant experiment, time-consuming equilibrium calculations, or approximative cal- culations.
Equilibrium calculations are to be carried out for numerous values of pressure, in the manner outlined in the foregoing section, which is a tedious procedure. The methods of approximation are more rapid but less accurate. Several such methods are known.
In a rough approximation, second-stage pressure can be calculated from the relationship. A more accurate result for a low-molecular-weight gas-liquid mix can be obtained by a procedure which expresses second-stage pressure by one of two equations Whinery and Campbell The first one is used when the relative molecular. Determining the A factor. M , in Fig. The procedure may be used if the third stage is atmospheric, in which case the result is accurate to within k5 percent. I0 Let the well fluid be characterized by Table 6.
Let first-stage pressure be What is the most favourable second-stage pressure'?
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Since M , is greater than I , we take Eq. Basic separator types. The compositions of the liquid and gas discharged from a separator differ somewhat from those predicted by calculation. In addition to the fact that calculation procedures based on auxiliary diagrams d o not model real-life conditions quite accurately, this is due to the circumstance that the liquid will contain gas bubbles and the gas will contain mist. Of these, it is the liquid content of the gas that may cause trouble in the transportation system.
Modern separators are designed so that the mist content of the discharged gas be less than 0. In most gas-liquid separators, the elements ensuring separation may be divided in three groups. These serve to effect a first separation of liquid and gas, mainly by centrifugal force and the force of gravity. They include the inlet separating element, bames, liquid passages, and that part of the separator between liquid surface and inlet in vertical separators or above the liquid level in horizontal and spherical separators.
Related D&D Standard Oil & Gas Abbreviator, 6th Edition
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