WO2013144171A1

WO2013144171A1 - Process for depressurizing a gas mixture comprising freezable species

Info

Publication number: WO2013144171A1
Application number: PCT/EP2013/056453
Authority: WO
Inventors: Marc FOURNERAUT; Eric DEKEYZER
Original assignee: Total SE
Current assignee: TotalEnergies SE
Priority date: 2012-03-26
Filing date: 2013-03-26
Publication date: 2013-10-03
Anticipated expiration: 2014-09-26
Also published as: MY168587A; BR112014023788B1; BR112014023788A2

Description

PROCESS FOR DEPRESSURIZING A GAS MIXTURE COMPRISING

FREEZABLE SPECIES

Field of the invention The invention relates to a process for preventing the blockage of a line during depressurization of a gas mixture comprising freezable species in emergency situation. Furthermore, the invention relates to a depressurization system for implementing such a process.

The invention is aimed in particular to address the problems due to crystallization of C02 during depressurization in emergency situation of a pressurized capacity containing a gas having a high percentage of C02. The invention is particularly useful where the depressurization conditions are favorable to the formation of C02 crystals, and where the formation of such crystals presents a risk of blocking or plugging the depressurization line due to deposition, adhesion and/or agglomeration of said crystals onto said line. The invention applies in particular to any gaseous mixtures having a certain content of C02 which are subjected to pressure and temperature conditions below the triple point of the gaseous C02. In such conditions, any temperature loss due to decompression is likely to cause crystallization of a part of the C02 contained in the gas mixture. The invention can also be extended to any gas containing compound(s) which may pass from the gaseous phase to the solid phase during depressurization of said gas.

Background

When a gas mixture containing a high percentage of C02 is depressurized, the temperature of the gas decreases. As a result, the gas may pass in the zone of C02 solid phase, and crystals of C02 may appear (see Figure 1).

Initially, the gas phase pressure and temperature is below the C02 triple point. Thus, during depressurization, as soon as the temperature and pressure gas conditions enter in the zone of C02 solid phase, uncontrolled risk of blockage exists, depending on the depressurization time, gas flow rate, gas composition, initial and transient temperature of the pipe wall, etc...

During depressurization, the pressure and temperature conditions can become favorable to the formation of a certain quantity of crystals for a certain period for a certain system. It has been shown during experiments that even when the pressure and temperature come back to gas phase position in the phase diagram, the crystals do not disappear instantaneously, and that disappearance can take longer when crystals are agglomerated. The risk of accumulation up to blockage of the depressurization line is therefore obvious as soon as a sufficient quantity of crystals is formed during the depressurization, according to the characteristics of said line (size, metallurgy, roughness, temperature, etc .). The evaluation and the prevention of the plugging risk are not presently possible, and so a solution to suppress said risk and to guarantee the safe depressurization in an emergency situation is needed.

According to some standard norms and rules of the industry, a pressurized vessel is to be provided with a depressurization device. This device is an integral part of the design and safety of the vessel and facility. For instance, the device should be capable of depressurizing down to 7 bar in 15 minutes or half the pressure in 15 minutes according to API 521 (Guide for Pressure-Relieving and Depressuring Systems). This required decompression is an example of cases where the pressure and temperature conditions will be in the zone where C02 crystallization and particles deposition will occur and crystals will have a high probability to accumulate in the line.

The accumulation of the deposits of C02 crystals, but also the detachment of adherent deposits afterwards from the walls of the line because of depressurization or vibrations of the line possibly due to the depressurization along with non-instantaneous phase transformation, can block the line and hinder the depressurization. The objective of this invention is to avoid the blockage of the line during depressurization by an innovative system.

The present invention meets all these needs by providing a safe static passive depressurization process and system thereof that does not require any chemical injection or heating device and that is thus intrinsically safe. The process and system of the present invention is particularly suitable in the case of emergency depressurization since it maintains the depressurization flow rate in any case.

Summary

One object of the present invention is a process for depressurizing a gas Gl from a pressure PI to a pressure P2, said gas Gl comprising freezable species and being contained in a capacity (6), said process comprising the following steps:

1) Providing said capacity (6) with a depressurization system which can be fed with said gas Gl, said depressurization system comprising in the direction of flow a first automatic valve (1) in closed position, a line (2) comprising a gas G2 free of freezable species at a pressure PI, a second automatic valve (3) in closed position, and a restriction orifice (4),

2) Opening the first automatic valve (1) so that gas Gl and G2 come into contact at pressure PI,

3) Opening the second automatic valve (2) so that gas Gl and G2 flow through the restriction orifice (4), thereby recovering the gas Gl whose pressure is lowered to a pressure P2.

In one embodiment, the first automatic valve is a full port valve, such as a Blow-Down Valve.

In one embodiment, the second automatic valve is full port valve, such as a Blow-Down Valve.

In one embodiment, the restriction orifice is a perforated plate. In one embodiment, the freezable species include carbon dioxide. In one embodiment, the gas Gl originates from a production unit. In one embodiment, the gas G2 is an inert gas, such as nitrogen.

In one embodiment, the length of the line is at least of 10 m, particularly from 35 m to 300 m, more particularly from 100 m to 250 m, even more particularly from 150 m to 200 m. In one embodiment, the depressurization system is connected to a dispersion device, in particular a vent or a flare.

In one embodiment, P2 is from atmospheric pressure to 10 bar.

A second object of the present invention is a depressurization system for implementing the process of the invention comprising a first automatic valve connected to a line, in particular with a length of at least 10 m, said line being connected to a second automatic valve, said second automatic valve being connected to a restriction orifice.

In one embodiment, the first automatic valve is a full port valve, such as a Blow-Down Valve. In one embodiment, the second automatic valve is full port valve, such as a Blow-Down Valve.

In one embodiment, the restriction orifice is a perforated plate.

In one embodiment, the length of the line is from 35 m to 300 m, in particular from 100 m to 250 m, more particularly from 150 m to 200 m. Brief description of the figures

Figure 1 represents the phase diagram of carbon dioxide.

Figure 2 schematically represents the process and depressurization system of the invention.

Detailed description One object of the present invention is a process for depressurizing a gas Gl from a pressure PI to a pressure P2, said gas Gl comprising freezable species and being contained in a capacity, said process comprising the following steps:

(1) Providing said capacity with a depressurization system which can be fed with said gas Gl, said depressurization system comprising in the direction of flow a first automatic valve in closed position, a line comprising a gas G2 free of freezable species at a pressure PI, a second automatic valve in closed position and a restriction orifice, (2) Opening the first automatic valve so that gas Gl and G2 come into contact at pressure PI,

(3) Opening the second automatic valve so that gas Gl and G2 flow through the line and the restriction orifice, thereby recovering the gas Gl whose pressure is lowered to a pressure P2.

The gas Gl may be any gas stream obtained from natural gas or a petroleum reservoir. Such gas contains hydrocarbons, primarily methane, and often acid contaminants, such as carbon dioxide or hydrogen sulphide. Carbon dioxide can be present in large amounts up to 70% molar or over. For most of the applications of these gas streams, the contaminants need to be removed, either partially or almost completely, depending on the application and the contaminant. The process of the invention applies to a natural gas before treatment, during treatment or after treatment, in particular during treatment, more particularly during a cryogenic separation treatment.

The gas Gl can comprise from 15 to 99 molar % of carbon dioxide, typically from 20 to 80 molar % of carbon dioxide.

According to the invention, the gas Gl comprising freezable species is contained in a capacity. For instance, the gas Gl may originate from a production unit, such as a cryogenic separation column. The temperature of the gas Gl in the capacity may be typically comprised between - 60 °C and -40 °C. The pressure PI of the gas Gl in the capacity may be typically comprised between 30 bar and 40 bar.

According to the invention, « freezable species » means any compound which can pass from a gaseous state to a solid state when the gas Gl is subjected to depressurization. Typically, the freezable species include carbon dioxide. Depending on the composition of gas Gl, freezable species may also include hydrates. In the following description, it will be referred to carbon dioxide, but the invention can apply to other freezable species which may pass from the gaseous phase to the solid phase during depressurization of the gas Gl .

Now referring to figure 2, the depressurization system of the present invention comprises a first automatic valve 1, a line 2, a second automatic valve 3 and a restriction orifice 4. The gas Gl is contained in a capacity 6 at pressure PI . The capacity 6 is connected to the first automatic valve 1 of the depressurization system.

At the initial time of the process of the invention, the first automatic valve 1 and the second automatic valve 3 are both in closed position. A gas G2 free of freezable species is injected in line 2 at pressure PI . Said gas G2 may be a non-flammable inert gas or a hydrocarbon gas. For safety reasons, the gas G2 is preferably a non-flammable gas inert gas, such as nitrogen.

The first automatic valve 1 and second automatic valve 3 are preferably full flow valves, i.e. valves whose opening is as large as the section of the line 2. Such full flow valves include for instance full port valves, also named full bore ball valves. A full port valve has an over- sized ball so that the hole in the ball is the same size as the line, which results in lower friction loss. In one particular embodiment, the first automatic valve 1 and second automatic valve 3 are Blow-Down Valves (BDV).

According to step (2) of the process of the invention, the first automatic valve 1 is opened so that gas Gl and G2 come into contact at pressure PI inside line 2. In order to avoid any pressure loss in line 2, the first automatic valve 1 is fully opened. Since they are at the same pressure PI, at this point gas Gl and gas G2 substantially do not form any gaseous mixture.

Then, according to step (3) of the process of the invention, the second automatic valve 3 is opened so that gas Gl and G2 flow through the line 2and the restriction orifice 4. By "restriction orifice", it is meant any perforated device which allows the control of the flow rate of gas Gl and the depressurization of gas Gl . Typically, the restriction orifice 4 is a perforated plate, i.e. a plate perforated with multiple holes. In one embodiment, the restriction orifice 4 is sized to allow a depressurization time of around 15 minutes from P 1 to P2, as required by the API 521 (Guide for Pressure-Relieving and Depressuring Systems).

As well known by the person skilled in the art, the depressurization of a gas generally leads to a decrease of the gas temperature. Prior to reaching the restriction orifice 4, the gas Gl flows through line 2 and is heated by heat exchange with the line 2. The dimensions and materials of the line 2 are adjusted so that the temperature of the gas Gl at the restriction orifice 4 is increased to a point where the gas Gl is not under the pressure and temperature conditions which would lead to crystallization of carbon dioxide during depressurization. The calculation of the required dimensions of the line 2 can be made by using a simulation software, such as the BlowDown program from the Imperial College (M.A. Haque, S.M. Richardson, G. Saville: Process Safety and Environmental Protection, Vol 70, 3-17, 1992)

The dimension calculation of line 2 is made from various parameters, including the volume of the gas Gl contained in the capacity 6, the temperature of the air surrounding the line 2 (typically > 15 °C), the temperature and pressure of gas Gl at the inlet of the line 2 (typically from - 60 °C to - 40°C and from 30 to 40 bars) and the material of the line 2. The line 2 is a conventional line used in gas production units. Typically, the line 2 is made in stainless steel, but any other material able to accumulate heat by contact with the surrounding air may be used.

In one embodiment, the gas Gl is contained in a cryogenic separation column whose height is from 40 m to 50 m and diameter is 4 m to 6 m including a reflux drum. It contains from 10 to 30 % molar of carbon dioxide, and is at a pressure PI from 30 to 40 bar and at a temperature from -60 °C to -40 °C. According to this embodiment:

- the length of the line 2 between the first automatic valve 1 and the second automatic valve 3 can be from 10 m to 300 m, in particular from 35 m 100 m to 250 m and, more particularly from 150 m to 200 m, - the diameter of the line 2 can be from 0.05 m to 1 m, in particular from 0.1 m to 0.9 m, more particularly from 0.2 m to 0.8 m,

- the line 2 can be made of stainless steel. As an example: - Gas Gl is contained in a cryogenic separation column of 45m height and 4.6m

diameter with a reflux drum of 9 m height and 2.5m diameter,

PI is 36 bar,

Temperature of the head of the column: -54°C

Diameter of the line 2: 51 cm, If the temperature of the air surrounding the line 2 is 20 °C, the required length of the line 2 to avoid crystallization of C02 during depressurization of gas Gl to atmospheric pressure is 44 m.

If the temperature of the air surrounding the line 2 is 0 °C, the required length of the line 2 to avoid crystallization of C02 during depressurization of gas Gl to atmospheric pressure is 62 m.

Just as a precautionary measure, the depressurization system of the invention may further comprise a gas/liquid/solid separator 5 for collecting any crystal or liquid which may form during the first minutes of the depressurization of the gas despite line 2. To this end, the gas/liquid/solid separator 5 is preferably connected downstream the restriction orifice 4 in its vicinity. The gas/liquid/solid separator 5 can be any vessel able to separate gas from solids and liquids. If crystallization of C02 happens downstream of the depressurization device 4 despite line 2, the crystals will be accumulating in the gas/liquid/solid separator 5. Minimum distance will be maintained between restriction orifice 4 and the gas/ liquid/solid separator 5 or the restriction orifice 4 can be placed directly at the entrance or inside the gas/ liquid/solid separator 5. The gas/ liquid/solid separator 5 can be directly connected to the second automatic valve 3. The gas/ liquid/solid separator 5 is designed to avoid any C02 crystal carry-over through the downstream line.

The gas Gl exiting the depressurization system is at pressure P2. Pressure P2 is lower than PI . In one embodiment, P2 is from atmospheric pressure to 10 bar, more particularly from atmospheric pressure to 5 bar.

In one embodiment, the depressurization system is connected to a dispersion device 7 via to the restriction orifice 4 or the gas/liquid/solid separator 5 if applicable.

In one embodiment, the dispersion device 7 is a vent or a flare. Of course, the process of the invention can be combined with other means such as the heating of the depressurization line 2, for instance if the temperature of air surrounding line 2 is not high enough to ensure enough heating of the gas Gl to avoid crystallization of carbon dioxide, or by a coating of the line to further reduce the risk of crystals adhesion in the line. A second object of the present invention is a depressurization system for implementing the process of depressurization of the invention as described above, comprising (by referring to Figure 2) a first automatic valve 1 connected to a line 2, in particular with a length of at least 10 m, said line 2 being connected to a second automatic valve 3, said second automatic valve 3 being connected to a restriction orifice 4.

The first automatic valve 1, the line 2, the second automatic valve 3 and the restriction orifice 4 are as described previously.

In one embodiment, the first automatic valve 1 is a full port valve, such as a Blow-Down Valve. In one embodiment, the second automatic valve 3 is full port valve, such as a Blow-Down Valve.

In one embodiment, the restriction orifice 4 is a perforated plate.

As explained previously, the dimensions and materials of the line 2 are adjusted so that the temperature of the gas Gl at the restriction orifice 4 is increased to a point where the gas Gl is not under the pressure and temperature conditions which would lead to crystallization of carbon dioxide during depressurization.

In one embodiment, the length of the line 2 is from 35 m to 300 m, in particular from 100 m to 250 m, more particularly from 150 m to 200 m, in particular when the line 2 is made of stainless steel. In one embodiment, the depressurization system may further comprise a gas/liquid/solid separator 5 connected downstream the restriction orifice 4 in its vicinity.

Claims

1. A process for depressurizing a gas Gl from a pressure PI to pressure P2, said gas Gl comprising freezable species and being contained in a capacity (6), said process comprising the following steps:

3) Opening the second automatic valve (3) so that gas Gl and G2 flow through the restriction orifice (4), thereby recovering the gas Gl whose pressure is lowered to a pressure P2.

2. The process according to claim 1, wherein the first automatic valve (1) is a full port valve, such as a Blow-Down Valve.

3. The process according to claim 1 or 2, wherein the second automatic valve (3) is full port valve, such as a Blow-Down Valve.

4. The process according to any one of claims 1 to 3, wherein the restriction orifice (4) is a perforated plate.

5. The process according to any one of claims 1 to 4, wherein the freezable species of gas Gl include carbon dioxide.

6. The process according to any one of claims 1 to 5, wherein the gas Gl originates from a production unit.

7. The process according to any one of claims 1 to 6, wherein the gas G2 is an inert gas, such as nitrogen.

8. The process according to any one of claims 1 to 7, wherein the length of the line (2) is at least of 10 m, in particular from 35 m to 300 m, more particularly from 100 m to 250 m, even more particularly from 150 m to 200 m.

9. The process according to any one of claims 1 to 8, wherein the depressurization system is connected to a dispersion device (7), in particular a vent or a flare.

10. The process according to any one of claims 1 to 9, wherein P2 is from atmospheric pressure to 10 bars.

11. A depressurization system for implementing the process according to any one of claims 1 to 10, comprising a first automatic valve (1) connected to a line (2), said line (2) being connected to a second automatic valve (3), said second automatic valve (3) being connected to a restriction orifice (4).

12. The depressurization system according to claim 1 1, wherein the first automatic valve (1) is a full port valve, such as a Blow-Down Valve.

13. The depressurization system according to claim 11 or 12, wherein the second automatic valve (3) is full port valve, such as a Blow-Down Valve.

14. The depressurization system according to any one of claims 11 to 13, wherein the restriction orifice (4) is a perforated plate.

15. The depressurization system according to any one of claims 11 to 14, wherein the length of the line (2) is at least 10 m, in particular from 35 m to 300 m, more particularly from 100m to 250m, even more particularly from 150 to 200m.