Process for dehydration and/or desalination and for simultaneous fractionation of a petroleum deposit effluent

A process for the dehydration and/or desalination and simultaneous fractionation of a petroleum deposit effluent containing oil, associated gas and water which can be saline, which process comprises: PA1 (a) at least one step for separating the liquid and gaseous phases at the pressure P1 for removal of the gas, producing a gaseous fraction G1, on the one hand, which is removed and a liquid fraction L1, on the other hand, which is sent to step PA1 (b) at least one step for separating, at least partly, the two liquid phases mixed in the liquid fraction L1, the aqueous phase being partly removed and the oil phase containing a quantity of residual aqueous phase being sent to step (c); PA1 (c) at least one distillation step carried out at a pressure P2 which is less than, or at the most equal to, the pressure P1 in step (a), in a distillation zone C1, said distillation being carried out in the presence of the oil phase coming from step (b), said zone C1 comprising an internal heat exchange zone and a boiling zone, and enabling a gaseous fraction G2, on the one hand, and a liquid fraction L2, on the other hand, to be recovered, said fraction L2 being constituted by a liquid oil phase and an aqueous liquid phase which is not miscible with the liquid oil phase; and PA1 (d) at least one step for separating the two liquid phases mixed in the liquid fraction L2, the oil phase being sent into said internal exchange zone, and then removed.

BACKGROUND OF THE INVENTION 
The invention is concerned with a process for dehydration and/or 
desalination and simultaneous fractionation of a petroleum deposit 
effluent containing oil, associated gas and water which can be saline. 
Treating a petroleum deposit effluent usually comprises an oil-gas 
fractionation step which is intended to adjust the droplet point of the 
oil produced in order to make it thermodynamically stable under storage 
conditions. 
The treatment also generally comprises a step in which an aqueous liquid 
phase which is not miscible with the oil phase is separated and which may 
possibly both be saline. In fact, an aqueous phase is usually produced at 
the same time as the liquid and gaseous hydrocarbons, and must be removed 
in order to satisfy water content requirements and salt content 
requirements in the oil produced. Moreover, to reduce the content of salt 
in the oil produced, it is often necessary to remove salt from the oil by 
mixing the oil with fresh water or water whose salt content is less than 
that of the water produced at the same time as the oil, then separating 
the liquid phases of water and oil. 
The treatment can also comprise a de-acidification step of the oil 
produced, this step consisting mainly of extracting the major part of the 
sulfurated hydrogen (H.sub.2 S) for toxicity and corrosion related 
reasons. 
These steps are usually simultaneous, the production effluent undergoing a 
series of successive expansions, usually 3 or 4, from the pressure at 
which the effluent issues from the well to a pressure close to atmospheric 
pressure, with, on each expansion, removal or recompression of the gas 
produced and removal of the aqueous phase by decantation possibly with 
fresh water being injected between expansions to produce a desalination 
effect. The effluent is usually heated before the last expansion in order 
to satisfy vapor pressure requirements and H.sub.2 S requirements of the 
oil produced, and to facilitate separation of the aqueous phase. 
For the oil produced, the necessary requirements are as follows: 
For vapor pressure, the criterium usually held is the vapor pressure at 
37.8.degree. C. expressed in Pascals, or Reid Vapor Pressure (RVP). This 
requirement is usually between 8 and 12 p.s.i. 
For the H.sub.2 S content, the usual requirement is 60 ppm by weight. 
For the water content, the requirement is usually between 0.5 and 1%. 
The gas produced on each expansion contains heavy constituents 
(C.sub.4.sup.+) which it is not necessary to draw off from the oil to 
stabilize it. It is desirable to recover these heavy constituents to 
reinject them into the oil because this increases the amount of stabilized 
oil produced, whilst lowering its density which will increase its selling 
value. Moreover, the presence of these heavy constituents in the gas 
produced confers upon it a high hydrocarbon dew point which is 
unfavourable in terms of its marketing. 
Recompression of the gas produced on each expansion and also partial 
condensation thereof by cooling allows a part of the heavy constituents to 
be recovered, but also excessively increases investment costs as the 
amount of compression needed increases. 
SUMMARY OF THE INVENTION 
It has been discovered that oil-gas fractionation, on its own or in 
association with de-acidification of the oil produced, can be carried out 
with a high oil yield and with considerable savings being made on the 
amount of heating, on the amount of compression and in the size of the 
installation, and this can be particularly important in a context such as 
marine petroleum production. 
It has also been discovered that the water which is usually produced with 
liquid and gaseous hydrocarbons makes a favourable contribution to the 
fractionation of the oil and gas. 
It has also been discovered that these treatment operations can be carried 
out in at least one expansion step unless it is normal practice to carry 
out the fractionation operations by way of successive expansions. 
Generally speaking, the process for dehydration and/or desalination and 
simultaneous fractionation of a petroleum deposit effluent containing oil, 
associated gas and water which can be saline is characterised in that it 
comprises: 
(a) at least one step where the liquid and gaseous phases are separated at 
the pressure P1 for removal of the gas, producing, on the one hand, a 
gaseous fraction G1 which is removed, and, on the other hand, a liquid 
fraction L1 which is sent to step (b); 
(b) at least one step where the two liquid phases mixed in the liquid 
fraction L1 are at least partly separated, the aqueous phase being partly 
removed and the oil phase which contains a residual quantity of aqueous 
phase being sent to step (c), 
(c) at least one distillation step which is carried out at a pressure P2 
which is less than or at the most equal to the pressure P1 in step (a), in 
a distillation zone C1, said distillation being carried out in the 
presence of the oil phase coming from step (b), said zone C1 comprising an 
internal heat exchange zone and a boiling zone, and enabling a gaseous 
fraction G2 to be collected, on the one hand, and a liquid fraction L2 to 
be collected, on the other hand, said fraction L2 being constituted of a 
liquid oil phase and of an aqueous liquid phase which is not miscible with 
the liquid oil phase, and 
(d) at least one step For separating the two liquid phases which are mixed 
in the liquid Fraction L2, the oil phase being sent to said internal 
exchange zone, and then removed. 
It has also been discovered that by using an internal heat exchange zone it 
is possible to make substantial savings on the heat which is to be 
supplied to the boiler for distillation zone C1. 
It has also been discovered that if the treated effluent contains an acid 
gas, and, in particular, H.sub.2 S, it is possible to satisfy the two 
requirements of thermodynamic stability and H.sub.2 S content by adjusting 
the temperature of the boiler and the number of stages in the distillation 
step. In particular, by virtue of the separating power of the distillation 
operation, the present invention makes it possible for H.sub.2 S to be 
removed to the specification for effluents containing up to several % by 
weight of H.sub.2 S, and this without any loss of high grade hydrocarbons 
(C.sub.4.sup.+) from the top of the distillation zone. 
It has also been discovered that the internal heat exchange zone enables 
the distillation residue to be cooled, thereby increasing the 
thermodynamic stability thereof and making savings on subsequent cooling. 
Another discovery, and one which forms one of the main objects of the 
present invention, is that the presence of water in the effluent treated 
during step (c) of the process according to the invention brings about 
heteroazeotropic distillation which helps increase efficiency of the oil 
and gas fractionation. 
The process according to the invention will be described in Further detail 
hereinafter in conjunction with FIG. 1. The description is directed, more 
particularly, to the treatment of a petroleum deposit effluent containing 
oil, associated gas and water which can be saline.

The effluent to be treated arrives via the conduit 1. It is expanded at the 
pressure P1 for removal of the gas (between 1 and 10 MPa, fop example) in 
the valve V1, whence it issues in a partly vaporized state through the 
conduit 2 and is mixed with a liquid fraction coming from the flask B2 
arriving through the conduit 3. 
The phases of the mixture thus obtained are separated in the flask B1. The 
gaseous phase is removed through the conduit 4, possibly mixed with a 
gaseous phase coming from the flask B2 arriving via the conduit 11, and 
removed from the process. The aqueous phase is partly removed from the 
process via the conduit 5. The liquid hydrocarbon phase containing a small 
fraction of the aqueous phase which has not been separated (fop example, 
between 1% and 5%) is removed via the conduit 6, and is then expanded in a 
valve V2 until a pressure P2 between the pressure P1 and atmospheric 
pressure is reached. The pressure F2 is between 0.15 and 1 MPa, for 
example. 
The hydrocarbon phase issuing from the valve V2 via the conduit 7 can be 
mixed with a quantity of fresh water which circulates in the conduit 15 
(this quantity of water can be between 10 and 100% of the quantity of oil 
circulating in the conduit 6) in order to reduce its salt content by the 
effects of dilution; the mixture obtained is introduced (step (c)) into 
the distillation zone C1. This distillation zone comprises a boiling zone 
B101 and an internal heat exchange zone Z101 into which the liquid oil 
phase coming from step (d) and issuing from said boiling zone B101 by 
rising along said internal heat exchange zone Z101 reheats the liquid L1 
and the vapor circulating counter-current in the distillation zone C1. The 
aqueous phase injected though the conduit 15 can be constituted preferably 
of fresh water, but also of water with a salt content of less than that of 
the residual aqueous phase contained in the oil phase circulating in the 
conduit 6 in such a way as to have a desalination effect. 
The boiling zone B101 is constituted of two parts: in the part disposed 
under the distillation zone C1, the mixture of the two liquid oil and 
water phases is brought to boiling point by the external application of 
heat, whilst in a lateral part separated by a vertical baffle and supplied 
by an overflow of liquid over said baffle, the two liquid oil and water 
phases are separated by decantation, which enables the aqueous phase to be 
removed via the conduit 14, and the oil phase to be removed via the 
conduit 12 by means of the pump P101. 
The temperature in the boiling zone B101 is usually between 100.degree. C. 
and 250.degree. C., and preferably between 100.degree. C. and 150.degree. 
C. 
The residue from the distillation operation is composed of an oil phase and 
an aqueous phase which are separated by decantation; the oil phase 
satisfies Reid Vapor Pressure requirements (RVP requirements), H.sub.2 S 
and water content. After rising through the internal heat exchange zone 
Z101, the oil phase is removed via the conduit 8. 
The vapor distillate is removed via the conduit 9; it can be at least 
partly recompressed in the compressor K1, from the pressure P2 to the 
pressure P1, then usually cooled in the heat exchanger E1 by an external 
fluid which can, for example, be water, or air, or any other cooling fluid 
available locally. This sequence of recompression and cooling enables a 
liquid phase which has a large content of high grade hydrocarbons to be 
condensed. Since the quantity of liquid fraction arriving in the 
distillation zone C1 via the conduit 7 is much greater than the quantity 
of gas issuing through the conduit 9, the temperature of the gas issuing 
from the distillation zone C1 via the conduit 9 is close to the 
temperature of the liquid fraction circulating in the conduit 7. It is 
usually between 40.degree. and 80.degree. C. 
The liquid-vapor mixture thus formed is removed from the heat exchanger E1 
to the flask B2 via the conduit 10. The liquid fraction constituted of a 
mixture of a liquid oil phase and a liquid aqueous phase is removed from 
the flask B2 via the conduit 3 and is mixed with the effluent coming from 
the valve V1 and arriving via the conduit 2, so as to be sent to the flask 
B1 (step (a), as already described, hereinabove. Since the liquid aqueous 
phase obtained after condensation at the exit from the exchanger E1 is 
constituted of water with a salt content of virtually zero, since it 
originates from condensation, it can be separated and reinjected through 
the conduit 15 in such a way that it contributes to the desalination 
process. 
The vapor phase is removed from the flask B2 via the conduit 11 and is 
mixed with a gaseous fraction coming from the flask B1 and arriving via 
the conduit 4, as already described hereinabove. 
Recompression of the gaseous phase issuing from the top of the column C1 
can be effected in one or more compression steps; it is, however, 
advantageous if the recompression is effected in one single step in order 
to limit the number of compressors; to this end, the value of the pressure 
P2 can be selected which is intermediate between the pressure P1 and 
atmospheric pressure P0 and in a ratio such that P2/P0 is at least equal 
to half of P1/P2, for example. 
Of course, the gaseous phase G2 which is recompressed at the pressure P1 
could be mixed with the gaseous phase G1 and removed from the process 
directly without cooling, but it is much more advantageous to cool the 
gaseous phase G2 after recompression in order to condense a part of the 
phase G2, and, after separation in the flask B2, to recycle the liquid 
phase thus obtained to the flask B1, as already described hereinabove. 
To carry out step (c) of the process according to the invention, it is 
possible to carry out the step in a device such as that described 
hereinafter in conjunction with FIG. 2. 
The device mainly comprises: 
A boiling zone B101 constituted of a flask which has, on the one hand, a 
capacity which enables a boiling device known in the art, such as an 
electric heating device, a pin for the circulation of a heat conductive 
fluid, or a heating tube, for example, to be immersed in the residue from 
the distillation operation, and the flask comprising, on the other hand, a 
decantation zone which is separated by a vertical baffle of the 
above-mentioned size and in which zone the two liquid oil and water phases 
are separated by decantation which enables the aqueous phase to be removed 
via the conduit 14 and the oil phase to be removed via the conduit 12 by 
the pump P101; and 
an internal heat exchange zone Z101 which is placed above the boiling zone 
and which is constituted by two circulation spaces; the mixture of oil to 
be degassed and of water arriving at the top of the distillation device C1 
via the conduit 7 flows by the force of gravity, counter-current to the 
vapor rising in the boiling zone B101 through said internal heat exchange 
zone Z101, said vapor then being removed from the distillation device C1 
via the conduit 9. In the other space, the liquid oil phase which issues 
from the boiling zone B101 flows from the bottom to the top via the 
conduit 12 by means of the pump P101 and re-enters the heat exchange zone 
via the conduit 13, and then issues from said heat exchange zone Z101 via 
the conduit 8. 
The internal heat exchange zone Z101 can be designed in various ways, some 
of which will be described hereinafter. 
By way of example, the internal heat exchange zone Z101 can be constituted 
of vertical tubes in which the mixture of oil to be degassed and water 
flows in the form of a film which falls onto the inner walls of the tubes, 
whilst the liquid phase of oil which rises in the boiling zone B101 
circulates outside the tubes in the calendria. The inner wall of said 
vertical tubes can be smooth, but they can also have regions of 
unevenness, or they can be subjected to a surface treatment to promote 
transfer of material and heat between the phases circulating inside the 
tubes, and also promoting transfer of heat between the phases circulating 
on either side of the tube walls. By way of example, the geometry of the 
inner surface of the tubes can be such that it promotes the appearance of 
waves within the falling liquid film, or it can be such that it has 
channels in the axis of the tubes in such a way as to increase the inner 
surface area of them, or it can be provided with a deposit of solid 
agglomerated particles which promote nucleation of vapor droplets within 
the falling liquid film. The inner wall of the tubes is preferably 
moistened by the oil phase. 
Said vertical tubes can also be filled with a bulk filling, ie., packing 
material such as with balls, rings, or saddle-shaped members. Preferably, 
the largest dimension of a filling element is less than one eighth the 
diameter of said tubes. 
Said vertical tubes can also be filled with a structured filling 
constituted, for example, of metal gauze, metal wool, plates or 
crosspieces such as used, for example, in static mixers. 
Another possible configuration consists in contacting the outer surface of 
the tubes with the oil mixture to be degassed and with the water and the 
vapor rising from the boiling zone B101 through the calendria, the liquid 
phase of oil rising by the pump P101 From the boiling zone B101 inside the 
tubes. In this case, the calendria can be empty or filled with a bulk or 
structured filling. In this case, the outer surface of the tubes can be 
smooth or it can have regions of unevenness, or it can be subjected to a 
surface treatment which will promote transfer of material and heat 
transfer between the phases circulating outside the tubes, and also 
transfer of heat between the phases circulating on either side of the tube 
walls. 
The outer wall of the tubes is preferably moistened by the oil phase. 
The internal heat exchange zone Z101 can also be structured in such a way 
that the two circulation spaces are delimited by an assembly of vertical 
plates. 
The distillation device C1 can also comprise a distribution device for the 
mixture of oil to be degassed and of water in the corresponding 
circulation space of the internal heat exchange zone Z101 when this space 
is composed of many parts (for example, constituted by tubes). These 
devices are known to those skilled in the art. 
The following example illustrates the invention. 
EXAMPLE 
In this example, the procedure followed is the same as illustrated in FIG. 
1. The effluent to be treated which is a crude petroleum issuing from the 
production well arrives via the conduit 1 at a flow rate of 169.3 ton/h, 
of which 31.3 ton/h is free water; its temperature is 60.degree. C., its 
pressure is 30 MPa. It is expanded until the pressure P1 of 3 MPa is 
reached in the valve V1, whence it issues via the conduit 2 at the 
temperature of 37.degree. C. It is then mixed with a liquid L3 which comes 
From the flask via the conduit 3 at a flow rate of 2.15 ton/h, of which 
155 kg/h is free water; this liquid is at a temperature of 35.degree. C. 
and at a pressure of 3 MPa. The mixture thus obtained is sent to the flask 
B1 where the gaseous phase is completely separated from the liquid phases 
which are partially separated. The gaseous phase G1 is removed via the 
conduit 4 at a flow rate of 72.1 ton/h; the liquid L1 which is formed of a 
majority hydrocarbon phase and a minority aqueous phase is removed through 
the conduit 6 to the valve V2 at a flow rate of 68.5 ton/h, of which 321 
kg/h is aqueous phase; the aqueous liquid phase is removed from the 
process via the conduit 5 at a flow rate of 30.88 ton/h. In the valve V2, 
the liquid phase L1 is expanded at a pressure P2 of 1 MPa, and partly 
vaporized, and then mixed with a liquid aqueous phase L4, the salt content 
of which is very much less than saturation, which arrives through the 
conduit 15 at a flow rate of 2583 kg/h. The liquid-phases and the vapor 
phase thus obtained are at a temperature of 32.degree. C. and they enter 
the distillation zone C1 which comprises an internal heat exchange zone 
Z101 and a boiling zone B101 via the conduit 7. The temperature in the 
boiling apparatus B101 is 132.degree. C. The internal heat exchange zone 
Z101 is constituted of vertical tubes in which the oil to be degassed 
flows in the form of a film which falls on the inner walls of the tubes. 
The anhydrous hydrocarbon liquid phase L2 which issues from the boiling 
zone B101 by the pump P101 at 132.degree. C. is returned to the internal 
heat exchange zone Z101 in which it circulates outside the tubes, and it 
is then removed from the top of the distillation zone C1 via the conduit 8 
at 62.degree. C. at a flow rate of 50.9 ton/h. The aqueous phase is 
completely decanted in the boiling apparatus B101 and is removed from the 
process via the conduit 14 at a flow rate of 2736 kg/h. The vapor phase G2 
is removed from the top of the distillation zone Ci via the conduit 9 at a 
flow rate of 17.45 ton/h at a temperature of 53.degree. C. 
Issuing from the distillation zone C1 via the conduit 9, the gaseous phase 
G2 enters the compressor K1 which has an output of 520 kW, whence it 
re-emerges at the pressure of 3 MPa and at a temperature of 110.degree. C. 
and enters the heat exchanger El. In the heat exchanger El, the gaseous 
phase G2 is cooled to 35.degree. C. by cooling water external to the 
process, and this cooling operation causes condensation of a fraction of 
the gas in the form of a liquid L3. The mixture is removed from the heat 
exchanger E1 via the conduit 10 and is sent to the flask B2 in which the 
gaseous phase is separated from the liquid phases. The liquid LB formed 
from the hydrocarbon and aqueous liquid phases is mixed with the fluid 
coming from the valve V1 via the conduit 2, the gaseous phase is removed 
via the conduit 11 and is mixed with the gaseous phase G1 which circulates 
in the conduit 4, and the gaseous phase resulting from this mixture is 
removed at a temperature of 36.degree. C. and at a flow rate of 87.4 
ton/h. 
It has been stated hereinabove that the presence of water in the effluent 
treated during step (c) of the process according to the invention results 
in a heteroazeotropic distillation which helps increase the efficiency of 
the oil and gas fractionation. This is confirmed by the following 
comparison: if, in the example given hereinabove, the hydrocarbon and 
aqueous liquid phases ape completely separated in the flask B1, and if the 
liquid aqueous phase L4 arriving via the conduit 15 is suppressed, the 
liquid L1 entering the distillation zone C1 is anhydrous. With the same 
specification for the oil produced and with the same temperature in the 
boiling apparatus B101, the pressure in the distillation zone C1 must then 
be decreased to 716 kPa, and the output of the compressor K1 is increased 
to 690 kW.