Process for separation of hydrocarbon vapors and apparatus therefor

Separation of hydrocarbon vapors from an admixture of hydrocarbon gases and hydrocarbon liquid is achieved by passing the admixture into the upper portion of a vapor rfecovery zone comprising a vertical, elongated separation zone, withdrawing hydrocarbon gases from an upper portion of the separation zone, the admixture being introduced into the vapor separation zone below the point at which the hydrocarbon gases are withdrawn from said separation zone. The liquid introduced into the separation zone flows in a generally downward, vertical direction to a point in the lower portion of the vapor recovery zone, changing direction and flowing in an upward, substantially vertical direction forming a column of liquid. The is passed from the vertical liquid column to an oil retaining zone having an upper, gaseous zone and a lower liquid zone. The vertical liquid column maintains a seal between the gaseous zone of the oil retaining zone and the gas-containing upper portion of the separation zone.

BACKGROUND OF THE INVENTION 
This invention relates to a process for the recovery of hydrocarbon gas or 
vapors from hydrocarbon liquids and to apparatus for effecting such 
recovery. More particularly, this invention relates to separation of 
entrained hydrocarbon gas from crude oil prior to passing the oil to 
storage. 
In the past, crude oil withdrawn from an oil well has been passed to a 
heater treater or separator for separation of water and hydrocarbon vapors 
from the oil, and the oil is thereafter passed to oil storage tanks. 
Normally, there is a considerable amount of hydrocarbon vapors entrained 
in the oil passed from the heater treater to the storage tanks. When a 
certain preset or predetermined pressure is reached inside the storage 
tanks, the hydrocarbon vapors are released into the atmosphere resulting 
in waste as well as atmospheric pollution. 
SUMMARY OF THE INVENTION 
Surprisingly, a process and apparatus have been discovered which will 
enable collection of the hydrocarbon gases without their release to the 
atmosphere. The process of the present invention involves passing the oil 
containing entrained hydrocarbon gases from the heater treater to a vapor 
recovery unit in which the hydrocarbon gases can be separated from the oil 
and recovered without contamination of the oil or the gases, or pollution 
of the atmosphere. 
The process of the present invention comprises passing a hydrocarbonaceous 
gas-liquid admixture into the upper portion of a vertical, elongated 
separation zone, and withdrawing hydrocarbon vapor from an upper portion 
of the vapor recovery zone. The admixture is introduced into the upper 
portion of the vapor recovery zone below the point at which said 
hydrocarbon vapors are withdrawn from said vapor recovery zone. The liquid 
fraction flows in a generally downward, vertical direction to a point in 
the lower portion of said vapor recovery zone, at which the liquid changes 
direction and then flows in an upwardly, substantially vertical direction 
in a manner such as to form a liquid seal as said liquid is passed from 
said vapor recovery zone. The liquid is then passed to a storage zone. 
The vapor recovery unit of the present invention comprises an elongated, 
vertical separator having an inlet means for introducing the oil and 
entrained gas from the heater treater and an outlet means for passing the 
oil to the oil storage tanks, as well as a second outlet means for 
withdrawing the separated gas for passage to a gas treatment plant for 
compression and expansion to form liquefied petroleum gas (LPG). The vapor 
recovery unit of the present invention is provided with a "dip tube", 
which may be an internal dip tube or external dip tube. The liquid-vapor 
admixture must enter the vapor recovery unit at a height determined by the 
absolute pressure on the vapor line leaving the top of the vapor recovery 
unit and the specific gravity of the liquid. Likewise the height of the 
vapor recovery unit is determined by the absolute pressure on the vapor 
line leaving the top of the unit along with the specific gravity of the 
liquid. The liquid falls by gravity to the lower portion of the unit and 
then up the dip tube out of the vapor recovery unit into the flow line 
leading to the oil storage tanks. Alternatively, the vapor recovery unit 
may be designed having the dip tube on the outside of the unit. In this 
alternative, the liquid and vapor admixture flows from the heater treater 
into the vapor recovery unit and the resulting separated liquid flows out 
of the bottom of the unit into the dip tube located outside the unit and 
thereafter into the oil storage tanks. The purpose of the dip tube is to 
provide a means of transferring the liquid from the vapor recovery unit to 
the oil storage tanks while maintaining a liquid seal. The liquid seal 
prevents air from being pulled into the vapor line leaving the top of the 
vapor recovery unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, an admixture of crude oil, hydrocarbon gas and 
water are withdrawn from crude oil producing well 10 and passed by means 
of flow line 12 to heater treater or separator 14 for removal of water. 
Heater treater 14 is provided with a heating element (not shown) for 
heating the admixture under a pressure of about 12 psig to about 30 psig 
to a temperature of from about 120.degree. F. to about 140.degree. F. so 
as to cause separation of water from the admixture, said water being 
removed by line 24. These conditions also result in vaporization of a 
portion of the C.sub.1 -C.sub.5 hydrocarbons present in the crude oil. The 
C.sub.1 -C.sub.5 gases are removed from heater treater 14 by means of line 
16 through gas measuring meter 18, check valve 20 and line 22 by means of 
a vacuum in line 30. The gases withdrawn from line 22 are passed to a gas 
plant where the gas is liquefied and fractionated into various products, 
such as ethane-propane and butane-pentane fractions. 
Next, oil containing entrained C.sub.1 -C.sub.5 hydrocarbon gas is 
withdrawn from heater treater 14 by means of line 26 and is passed to 
vapor recovery unit 28 for separation of entrained gas from the oil. In 
vapor recovery unit 28, the entrained gas vapors are separated from the 
liquid and the gas is withdrawn from unit 28 by means of line 30 through 
compressor 32 and line 34 and is then passed through control valve 36 
which may be used to maintain pressure in unit 28. Thereafter, the gas is 
passed by means of line 38 through gas meter 40, line 42, check valve 44 
and line 46. The gas in line 46 is subjected to treatment identical to 
that of the gas withdrawn through line 22. Meanwhile, the separated oil in 
vapor recovery unit 28 is withdrawn by means of line 48 and passed through 
a "dip tube" configuration 50 and then by means of lines 52 and 54 into 
one or more oil storage tanks illustrated as oil storage tanks 56 and 58, 
respectively. Oil from storage tanks 56 and 58 is withdrawn by means of 
line 60 for disposition of the oil as desired. 
A preferred form the vapor recovery unit of the present invention is shown 
in FIG. 2 which substantially corresponds to vapor recovery unit 28 of 
FIG. 1. As shown in FIG. 2, hydrocarbon liquid and entrained vapor are 
passed by means of line 126 from the heater treater (not shown) to the 
vapor recovery unit 128. The liquid and vapor mixture enter the vapor 
recovery unit 128 above the liquid level in the unit at a height 
determined by the absolute pressure on the vapor line leaving the top of 
the vapor recovery unit and the specific gravity of the liquid. The liquid 
and vapor mixture can enter unit 128 below the liquid level in the unit, 
if desired. Likewise, the height of the vapor recovery unit is determined 
by the absolute pressure on the vapor line 130 leaving vapor recovery unit 
128 and the specific gravity of the liquid 132. The liquid falls by 
gravity to the lower portion of the unit, passes from unit 128 and then 
passes up dip tube 134 into flow line 136 passing to the oil storage tanks 
as shown in FIG. 1. 
An alternative vapor recovery unit of the present invention is shown in 
FIG. 3 in which vapor recovery 228 has dip tube 234 located inside the 
vapor recovery unit. The liquid and entrained vapors from the heater 
treater are introduced by means of line 226 in the same manner as the 
vapor recovery unit of FIG. 2 and the vapors are withdrawn by means of 
line 230 in the same manner as that shown in FIG. 2. The liquid passes 
downward by gravity as shown by the arrows, but must pass upwardly through 
the internal dip tube 234 to leave the vapor recovery unit and pass to the 
oil storage tanks. By providing the dip tube outside the vapor recovery 
unit as shown in FIG. 2, the diameter of the vapor recovery unit may be 
made smaller. 
The dip tube is a hollow elongated tube whose purpose is to provide a means 
of transferring the hydrocarbon oil fraction from the vapor recovery unit 
to the oil storage tanks while maintaining a liquid seal. The dip tube has 
an inside diameter equal to or greater than that of line 126 of FIG. 2. 
This liquid seal prevents air from being pulled from the oil storage tanks 
back into the vapor recovery unit and into the vapor line to 130 of FIG. 2 
resulting in a potentially explosive gas-oxygen mixture. The dip tube is 
shown as a generally L-shaped (FIG. 3) or Z-shaped (FIG. 2) conduit and is 
substantially the vertical portion of conduit 134 (FIG. 2) or conduit 234 
(FIG. 3) through which liquid passes upwardly after having flowed 
downwardly through the vapor recovery unit by gravity. Thus, the dip tube 
provides a seal in the form of a vertical column of liquid. 
As seen in FIG. 3, a vapor fraction is present in the space 232 above the 
liquid level 233 of the liquid fraction 236. Upon entering the vapor 
recovery unit the gas-liquid admixture entering unit 228 from line 226 
separates into a gaseous fraction which passes upwardly through the upper 
section of the unit 232 which contains baffles 238 and 240, which help 
prevent entrained liquids from passing into line 230 along with the 
gaseous hydrocarbon fraction. 
Depending upon the particular system, either a vacuum may exist in line 230 
or a positive pressure may exist in line 230. When a positive pressure 
exists in line 230, a compressor or pump 242 may be provided to assist in 
recovering more vapors from unit 228 and boosting the vapors to desired 
line pressure. Additionally, pressure control valve 244 may be utilized in 
vapor line 246 to maintain the desired pressure on line 230 and in unit 
228 which aids in controlling level 233 in unit 228. 
Referring once again to FIG. 2 a height, h.sub.1 is shown, which is the 
distance from the liquid level to the top of the vapor recovery unit. The 
height, h.sub.1, is the minimum height required for the vapor recovery 
unit to extend above the liquid level inside the vapor recovery unit. Thus 
height, h.sub.3, must be sufficient to insure that the force of gravity on 
the liquid is greater than the difference between the absolute pressure in 
the gas space in the oil storage tanks and the absolute pressure which 
exists in line 130, along with pressure losses in the dip tube and the 
line leading from the dip tube to the oil storage tanks (lines 52 and 54 
in FIG. 1). This minimum height, h.sub.1, prevents liquid from being 
pulled from the vapor recovery unit 128 into the gas gathering line 130 in 
the event that the absolute pressure in line 130 is less than the pressure 
inside the oil storage tanks. The liquid level inside the vapor recovery 
unit varies according to the pressure of the liquid entering the vapor 
recovery unit through line 126 from the heater treater and according to 
the height of the dip tube 134, as well as the absolute pressure in line 
130 and according to the pressure loss due to friction in dip tube 134 and 
the line from the dip tube to the oil storage tanks, i.e., line 136 to the 
oil storage tanks. 
The height of the liquid inside vapor recovery unit 128 will be at least as 
high as the dip tube height h.sub.2. Because of frictional losses in dip 
tube 134, and in the line 136 leading from the dip tube to the oil storage 
tanks, this liquid level will be slightly higher than the highest point of 
the liquid in the dip tube. 
Surprisingly, the pressure losses due to friction in dip tube 134 and the 
line leading from the dip tube to the oil storage tanks is significant. In 
view of this frictional pressure loss, a control valve 144 may be utilized 
to ensure that the back pressure on the vapor recovery unit 128 from the 
process line 130 along with the force of gravity on the liquid is greater 
than the pressure inside the oil storage tanks. This prevents liquid from 
entering line 130 from the vapor recovery unit. 
Additionally, in accordance with a preferred embodiment of the present 
invention, a high liquid level shutdown device 62 shown in FIG. 1 may be 
utilized to prevent liquid from entering line 30 from vapor recovery unit 
28. In the event that the liquid level rises and reduces the height 
h.sub.1 of the space above the liquid level in vapor recovery unit 28 
below the minimum desired, liquid flows into shutdown tube 62 and trips a 
switch 64 which causes termination of the flow of liquid and gas to the 
vapor recovery unit 28 from heater treater 14 through line 26 (by means 
not shown). 
Referring again to FIG. 2, height h.sub.2, which is the length of the dip 
tube 134 is shown. This height, h.sub.2, is the minimum length of the dip 
tube, whether an internal or an external dip tube, which is positioned 
below the point where the liquid enters the oil storage tanks from the 
vapor recovery unit. The height h.sub.2 is sufficient to ensure that the 
pressure required to force the liquid out of the dip tube and into the oil 
storage tanks, along with the pressure losses due to friction, is greater 
than the difference between the absolute pressure inside the oil storage 
tanks and the absolute pressure in vapor removal line 130. In other words, 
the dip tube length must be of sufficient height to ensure that the 
pressure differential between the pressure in the oil storage tanks and 
the vapor take off line 130 will not permit air to be drawn into the gas 
gathering line 130. 
In order to further illustrate the present invention, the following 
non-limiting examples are presented for purposes of illustration. 
Example 1 
The following calculations were made in order to determine the minimum 
height h.sub.3 for the vapor recovery unit of FIG. 2 assuming a vacuum in 
line 130 of 2 inches of Mercury. 
In the following equations 
P.sub.t =pressure inside the storage tanks 
P.sub.a =atmospheric pressure=14.65 psia 
.rho.=density of the liquid=49.92 lb./ft.sup.3 
x=vacuum in inches of Mercury 
g=32.2 ft/s.sup.2 
g.sub.c =32.2 lb.sub.m .multidot.ft./lb.sub.f .multidot.S 
##EQU1## 
Thus, for a vacuum of 2 inches of Mercury in line 130 of FIG. 2, the height 
h.sub.3 must be at least about 3 feet. The aforesaid calculation assumes 
that pressure loss due to friction in the dip tube and connecting lines is 
negligible. 
Example 2 
This example illustrates the calculation for the height, h.sub.2, of the 
dip tube 134 in FIG. 2 assuming a positive pressure in gathering line 130 
of one psig. The constituents of the following equations have been defined 
in Example 1, except P.sub.g =the pressure in line 130 in psig: 
##EQU2## 
Thus, if for a positive presence of one psig, the dip tube should be 2.9 
feet in vertical height to provide a suitable liquid seal.