US20240219022A1

US20240219022A1 - Method of controlling flare gas recovery system

Info

Publication number: US20240219022A1
Application number: US18/394,302
Authority: US
Inventors: Andre Luiz Machado De Oliveira
Original assignee: Petroleo Brasileiro SA Petrobras
Current assignee: Petroleo Brasileiro SA Petrobras
Priority date: 2022-12-28
Filing date: 2023-12-22
Publication date: 2024-07-04
Also published as: BR102022026909A2; BR102022026909B1

Abstract

The present invention relates to a method of controlling a flare gas recovery system, which meets all possible operational scenarios in a primary oil processing plant in a completely continuous, stable and transparent manner for the operator. In particular, the steps performed in the method proposed here include Initialization and Checking the availability of the main compressor, and Monitoring the system inlet pressure and Adjusting the compressor request. More precisely, the method described by the present invention is capable of operating in four operating modes, which comprise: a) a volume accumulation scenario for startup, referring to a system switched on with no compressor yet operating; b) a low burn scenario, referring to a system connected with a single compressor in operation; c) a high burn scenario, referring to a system connected with two compressors in operation; and d) a scenario of high sequential burns, referring to a system connected in “switch mode” between compressors.

Description

FIELD OF THE INVENTION

The present invention falls within the field of technologies applicable to the primary petroleum production and processing unit. More precisely, it falls within the field of mechanisms for recovering flare gas, which have been used in primary oil production and processing units. The present invention, in this sense, makes it possible to control flare gas recovery systems efficiently, continuously and automatically.

BACKGROUND OF THE INVENTION

A flare is a structure that releases vapor or flame through its top portion. More precisely, the flare is a gas combustion structure, which performs controlled burning of waste and, in this way, is mainly found in refineries, chemical plants, petrochemical plants, natural gas processing plants, exploration platforms, among others. The volume of all gas burned in the flare is computed and reported daily to the National Petroleum Agency (ANP), which assigns a maximum monthly burning limit for each unit in operation.
The gas burned in the flare is a loss both commercially and for the environment, as, when it is no longer re-injected into the well or exported, it becomes a volume released into the atmosphere, aggravating the greenhouse effect (GHG). Furthermore, the volume of salable gas that is not flared for operational safety reasons is also subject to the payment of royalties and, if this volume exceeds the monthly flare limit agreed by the agency, it will be a production limiter.
According to the ANP, in 2013, Brazil produced 28.2 billion cubic meters (m³) of natural gas, of which 1.3 billion m³were burned (4.6% of the total). On the other hand, the volume of flare gas recovered is not only an important factor in reducing greenhouse gas (GHG) emissions, but also a volume that can be sold and transformed into Carbon Credit, generating revenue for the Company in agreement to a more sustainable energy future.
For example, own Primary Oil Production and Processing Units of Petrobras, designed since 2012, already have a flare gas recovery system by default. Some units such as the P-62 and P-77 were able to break this system, but conventional control methods were not able to make it operate continuously, automatically and transparently for the operator, and the system was abandoned due to malfunction, recurrently.
Before the disclosure of the present invention, the flare gas recovery systems present in some production units had an operational control method with the following configurations:

- a) Control system switched on with 1 compressor in operation, in a low burn scenario; or
- b) Control system connected with 2 compressors in operation, in a high burn scenario.

The problem with this method is that it is not adapted to the great diversity of possible scenarios of the typical primary processing plant, making stability and operational continuity impossible in the units in which such a method is applied. In view of this limitation, the main problems with this simplified control method available in the prior art are:

- the method does not consider the special starting scenario wherein the gas volume is very low and insufficient for the compressor to enter steady state before being switched off due to time out, so that starts are only possible through artificial disturbances in the plant with the aim of increasing the volume of gas that goes to the flare until the compressor comes into operation, which, in addition to being undesirable, dangerous and slow, can put the entire production at risk;
- the method involves the need to always have both compressors available to be able to start the system, so that when one compressor is unavailable, the entire system is also unavailable;
- the method is not adapted to the real operational scenarios of the production plant, where most of the scenarios are composed of burning that starts and ends quickly (purging, depressurization of a system or replacement of a main compressor), and this requires a refined tuning and system learning method to estimate the appropriate moment at which the second compressor should enter and exit, so that it is only used in strictly necessary cases, but also does not leave too quickly to the point of taking the system to a TRIP;
- the method does not allow sequential high burnout scenarios, so that whenever one of the compressors goes out after a high burnout, it ends up being unavailable for a while, due to the electrical protections of minimum time between starts, but the real depressurizations are mostly very fast and occur sequentially (purging of the incoming compressor, followed by depressurization of the outgoing compressor);
- the method does not include the control of auxiliary systems according to the number of compressors in operation, as auxiliary systems are generally shared, this means that any stopped compressor can affect the quality of the other compressor, and
- the method does not carry out predictive control of large depressurization scenarios, causing the two compressors to come into operation, reach TRIP levels and remain unavailable for an unnecessary time.

Therefore, in summary, it is clear that the state of the art lacks a methodology capable of allowing an easier startup and a stable operating regime that takes into account the real operational scenarios found in flare gas recovery systems.

STATE OF THE ART

Below, some technologies available in the field of invention are discussed, with the aim of making the limitations of the given state of the art even more crystal clear.
The document CN 107327852 A describes a control method for an FPSO (Floating Production Storage and Offloading Unit) flare gas recovery system. Said control method comprises the following steps: the gas pressure in a flare scrubber is detected through a pressure sensor mounted on the flare scrubber, and the detection value of the pressure sensor is received by a controller; whether the detection value is less than or equal to a first defined value is judged by a comparison; if so, a first switching valve is controlled by the controller to be opened, the gas recovery system starts to work, and a second switching valve is maintained in a closed state; otherwise, the second switching valve is controlled by the controller to be opened, the first switching valve is maintained in a closed state, and the gas recovery system stops working. According to the proposed method, different switching valves are controlled to be opened or closed according to the pressure in the flare scrubber and the gas in the flare scrubber is treated through two different pipelines.
The document US 2018149357 AA describes a method for recycling flue gas back to a processing facility that selectively employs different numbers of ejector legs depending on the flue gas flow rate. The ejector legs include parallel-channeled ejectors, wherein each ejector has a combustion gas inlet and a motive fluid inlet. Valves are arranged in the piping upstream of the combustion gas and motive fluid inlets at the ejectors and are selectively opened or closed to allow flow through the ejectors. The flare gas flow rate is monitored and distributed to a controller, which is programmed to calculate the required number of ejector legs to accommodate the amount of flare gas. The controller is also programmed to direct signals to actuators connected to the valves, which open or close the valves, to change the capacity of the ejector legs so they can handle varying combustion gas flow rates.
In view of this scenario, it is clear that none of the documents available in the prior art describe a method capable of controlling flare gas recovery systems, wherein the systems present four operating conditions, such as (i) an initial condition of operation, wherein no compressor is operating; (ii) a condition with a compressor in operation (low burn); (iii) a condition with two compressors in operation (high burn), and (iv) a condition with “switch mode” between compressors (high sequential burn).

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a method of controlling a flare gas recovery system, comprising the steps of: Step A) Initialization and Checking the availability of the main compressor, and Step B) Monitoring the system inlet pressure and Compressor request adjustment.
In the preferred embodiment of the invention, the described method controls flare gas recovery systems in: a volume accumulation scenario for startup, referring to a system turned on with no compressor yet operating; a low burn scenario, referring to a system connected with a single compressor in operation; a high burn scenario, referring to a system connected with two compressors in operation, and a high sequential burn scenario, referring to a system connected in “switch mode” between compressors.
Preferably, the method now disclosed acts to control gas compression systems composed of liquid ring type compressors and their sealing accessory and three-phase separation systems, optionally composed of one or more compressors depending on the volume of gas to be recovered and target recovery scenarios; wherein the systems optionally further comprise electric motors, automation panel to implement interlocking and control logic, knock-out vessels at the base of the flare to accumulate gas from primary processing and support the separation of liquid and vapor phases, quick opening valves, rupture discs, and PV valve for load control and gas recirculation from compressor discharge to suction.
In particular, in Step A) of the proposed method, once the flare gas recovery system is connected, it is checked whether there is a compressor determined as the main compressor, so that if there is no compressor determined as the main one, the method invites the user to define a main compressor, and if a main compressor has already been determined, the method leads to filling the system referring to the two compressors, by forming the liquid ring of the compressors; wherein the method sequentially leads to the closure of the flare valves, with the flare gas recovery system subjected to control already in operation.
Regarding Step B), after closing the flare valve in step A), the method leads to monitoring the pressure at the inlet of the system subjected to control, so that if it is identified that the system inlet pressure is greater than Ps (min), the method leads to the activation of the main compressor, and if it is identified that the system inlet pressure is not greater than Ps (min), monitoring of the system inlet pressure continues to be carried out. Furthermore, when it is identified that the system inlet pressure is greater than Ps (min) and the main compressor is turned on, the system inlet pressure is also monitored, so that if the inlet pressure is greater than Ps (max) and the PV recycle valve is closed for more than T seconds, the backup compressor is turned on, and if not, monitoring of the system inlet pressure remains in operation. Furthermore, when the backup compressor is activated, the method checks whether the “switch mode” mode is activated, where if it is not activated, the method continues to monitor the system inlet pressure, while the switch mode is activated, the backup and main compressors are led to alternate this primary and secondary positioning. Finally, following the alternation between compressors, monitoring of the system inlet pressure continues to be carried out, so that it is checked whether the PV recycle valve has an opening greater than PVopen % for more than Ta seconds, where if it is concluded that yes, the backup compressor is turned off and the first analysis is returned to check the need to activate the backup compressor, while if not, the method leads to remaining monitoring the system inlet pressure.
Additionally, and in parallel with the steps discussed A) and B), the method now described also performs a stop control of the flare gas recovery system in a predictive manner, wherein if it is identified that depressurization of the system is not supported by the recovery, the system is forcibly stopped.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a flowchart illustrating the steps of the method of the present invention.

FIG. 2 is an illustrative diagram of the initial and final states of pressure, volume and temperature upstream downstream of the flare gas recovery compressors, given their compression ratio and a time limit after startup.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of controlling a flare gas recovery system, which meets all possible operational scenarios in a primary petroleum processing plant in a completely continuous, stable and transparent manner for the operator. In FIG. 1 a representative flowchart of the present invention is proposed. More precisely, the method described by the present invention is capable of operating in four modes of operation, which include:

- a) a volume accumulation scenario for startup, referring to a system switched on with no compressor still in operation;
- b) a low burn scenario, referring to a system connected with a single compressor in operation;
- c) a high burn scenario, referring to a system connected with two compressors in operation;
- d) a scenario of high sequential burns, referring to a system connected in “switch mode” between compressors.

Initially, for the purposes of clarifying the implementation of the method of the invention, it is worth noting that this proposed control method can be applied to all primary oil production and processing units, owned and chartered, which are based on a system of gas recovery from the flare consisting, for example, of two liquid ring compressors (main and backup), their accessories (sealing, refrigeration and three-phase separator) and opening valves to allow the reopening of the flare if necessary (depressurization). An example of this composition is illustrated in FIG. 2 . More specifically, a conventional flare gas recovery system, wherein the method object of the present invention is perfectly capable of acting, comprises:

- a) Gas compression system composed of liquid ring type compressors and their sealing accessory and three-phase separation systems. The compression system can be composed of 1 or more compressors according to the volume of gas to be recovered and the desired recovery scenarios. In oil and gas primary processing plants, arrangements with 2 compressors should be used to obtain better energy efficiency, keeping one compressor off most of the time, and turning it on only in some operational scenarios, such as during main compressor depressurization or the purging of turbogenerators and turbochargers, at which time the main compressor and secondary compressor must be operating, as detailed in the rest of the document;
- b) Electric motors to drive the compression units and their control centers;
- c) Automation panel for implementing interlocking and control logic;
- d) Knock-out vessels at the base of the flare to accumulate gas from primary processing and help separate the liquid and vapor phases;
- e) Quick opening valves (QOVs) in the quantity and redundancy required according to the required level of integrity. These valves enable the relief of gas from the plant to the flare (burner) and the alignment of gas from the plant to the Compression Units;
- f) Rupture discs or buckling pins in parallel with the quick opening valves (QOVs) to guarantee an adequate level of safety in the event of any electromechanical failure of the flare opening system, and
- g) PV valve for load control and recirculation of gas from the compressor discharge to the suction, with the purpose of maintaining a stable operating condition of the compressors even in low burning scenarios: flow less than 500 Nm³/h.

It should be understood that this system configuration mentioned above is a configuration most commonly found in petroleum production units. The method of the present invention is described, preferably, to suit a system of this order. However, it must be understood that, through variations in the composition of the system of the production unit under control, the method may be adjusted to perfectly meet the given new specific composition, and the adjustments will certainly still be covered by the definitions provided by the present invention.
Additionally, in relation to its implementation, the method of the present invention comprises a set of steps in the form of a set of instructions stored in an available memory associated with the automation panel that is already installed in the unit, the steps being executed by the electronics of the flare gas recovery control system.
Observing the considerations set out above, in an even more particular way, the present invention comprises the following steps, which are represented in the flowchart proposed in FIG. 1 :

- Step A) Initialization and Main Compressor Availability Check;
- Step B) Monitoring system inlet pressure and adjusting compressor demand.
- Step A) is the initialization step of the method, wherein the scenario of this step comprises the system connected with 0 (no) compressors in operation, which is a scenario of volume accumulation to enter steady state within the expected time of “startup”. This scenario is necessary for the automatic startup of the flare system, allowing the initial suction pressure to be high enough for the final discharge pressure to reach an adequate value, ensuring a steady state, within X seconds after starting the compressor. To do this, the following are considered: the compression ratio of each liquid ring compressor, the volumes and temperatures at the inlet and outlet of the compression system, the initial and final pressure values at the inlet and outlet of the compression system, the maximum suction pressure before the TRIP system, the system suction pressure control set and the minimum discharge pressure value in steady state. The calculation must be carried out according to the following parameters, and with the logic shown in the diagram in FIG. 2 :
- total volume of the knock-out vessels and vessel outlet pipes according to their dimensions until arrival at the compressor in the initial and final startup scenarios (Vsi and Vsf). Note that Vsi=Vsf;
- temperature of the process gas at the compressor inlet in the initial and final startup scenarios (Tsi and Tsf);
- total volume of the pipes according to their dimensions from the compressor exit to the arrival at the vapor recovery system, where the recovered gas is sent, also in the initial and final startup scenarios (Vdi and Vdf). Note that Vdi=Vdf;
- temperature of the process gas at the compressor outlet, initial and final startup scenarios (Tdi and Tdf);
- compression ratio of the main compressor of the system (Rc). Note that for liquid ring compressors, this compression ratio is fixed;
- the discharge pressure in the initial and final startup scenarios (Pdi and Pdf), and
- the suction pressure in the initial and final startup scenarios (Psi and Psf). Note that Pdi=Psi because the pressures are equalized in suction and discharge while compression is not operating.

In knowledge of these parameters, using the real gas equation, PV=n ZRT, the minimum initial suction pressure value necessary for the system to enter steady state after a maximum time limit allowable by the design of the compression system can be found: discharge pressure must be at a value above the low pressure TRIP value and the suction pressure must be at the control set value. Like this:
$P . V \dot{=} n . Z . R . T;$
(Psi).(Vsi)=(ni).Z.R.(Tsi) (a)

- (ni)=(Psi). (Vsi)/Z.R. (Tsi), which is the amount of matter initially available in the compressor suction;

(Pdf).(Vdf)=(nf).Z.R.(Tdf) (b)

- (nf)=(Pdf). (Vdf)/Z.R. (Tdf), which is the amount of material available at the end of the compressor discharge starting time.

Thus, there is the rate ns, which is the amount of matter supplied to leave the initial conditions and reach the final conditions of the system, defined as:
ns=nf−ni. (c)
Through the previous definitions, it is clear that nf is known and calculable, and that ni depends on Psi (initial suction pressure), which is the value of interest to achieve stability at system startup. Dividing equation (a) in time:
(Psi).(Vsi/t)=(ns/t).Z.R.(Tsi) (d)
For a defined compression ratio Rc, it is known that the value of Vsi/t is equivalent to the compressor flow rate, constant in the case of liquid ring compressors. Thus, the value of Psi for a steady state is found by defining the maximum allowed time “t” in equation (d) and solving:
(Psi).Rc=(nf−(Psi).(Vsi)/Z.R.(Tsi))/t.Z.R.(Tsi),
with Psi being the only unknown, for a given time t.
The time t depends on the construction conditions of the compressors. In the case studied, it was defined as 30 seconds.
In this step A), once the flare gas recovery system is connected, it is checked whether there is a compressor determined as the main compressor. If there is no compressor determined as main, the method invites the user to define a main compressor. Meanwhile, if a main compressor has already been determined, the method leads to the filling of the system relating to both compressors, by the formation of the liquid ring of the compressors. Subsequently, the method leads to the closure of the flare valves, with the flare gas recovery system subjected to control already in operation.
Step B) is a step performed after closing the flare valve in step A). In step B), the proposed method leads to the monitoring of the pressure at the inlet of the system subjected to control through a pressure sensor. If the sensor identifies that the system inlet pressure is greater than Ps (min), the method leads to the activation of the main compressor. On the other hand, if it is identified that the system inlet pressure is not greater than Ps (min), monitoring of the system inlet pressure continues to be carried out.
In the first case, wherein it is identified that the system inlet pressure is greater than Ps (min) and the main compressor is turned on, the system inlet pressure also remains monitored. If the inlet pressure is greater than Ps (max) and the PV recycle valve is closed for more than T seconds, the backup compressor is turned on. If not, monitoring of the system inlet pressure continues to be carried out.
Next, once the backup compressor is activated, the control system checks whether the “switch mode” is enabled. This mode increases the availability of the system as a whole, however it can be disabled by the operator through the Graphical Interface of the flare gas recovery system. If it is not activated, the method continues to monitor the system inlet pressure. If the switch mode is activated, the backup and main compressors alternate their functions, with the compressor that was on for the longest time, the main one, being turned off first, so that, if the plant needs it, it can be turned on again at a shorter time, as this will have a lower thermal image of its electrical assembly.
Following alternation between compressors, the method continues to monitor the system inlet pressure. At this point, it is checked whether the PV recycle valve is opening greater than the value defined as minimum opening percentage (PVopen %) for a time longer than a minimum time also defined (Ta seconds). If yes, the backup compressor is turned off and the first analysis is returned to check whether the backup compressor needs to be activated. If not, the method leads to continuing to monitor the system inlet pressure.
In addition to the steps discussed above, the method of the present invention performs stop control of the flare gas recovery system in a predictive manner. This is a parallel function of the method. With each cycle of the Programmable Logic Controller of the unit, the method monitors the production very high depressurization valves and compressors of the unit. If it is identified that system depressurization is not supported by recovery, the system is forcibly stopped, as the volume of gas released is inevitably greater than the recovery capacity system of the system.
The present invention is defined here in terms of its preferred embodiment. However, a person skilled in the art is perfectly capable of observing what modifications can be made to the information described here, these modifications still being covered by the same scope of the matter described and claimed.

Claims

1. METHOD OF CONTROLLING FLARE GAS RECOVERY SYSTEM, characterized by comprising the steps of:

Step A) Initialization and Verification of main compressor availability, and

Step B) Monitoring the system inlet pressure and Adjustment of compressor request.

2. METHOD, according to claim 1, characterized by controlling flare gas recovery systems in:

a volume accumulation scenario for startup, referring to a system switched on with no compressors still in operation,

a low burn scenario, referring to a system connected with a single compressor in operation,

a high burn scenario, referring to a system connected with two compressors in operation, and

a scenario of high sequential burns, referring to a system connected in “switch mode” between compressors.

3. METHOD, according to claim 1, characterized by acting in the control of gas compression systems composed of liquid ring type compressors and their sealing accessory and three-phase separation systems, optionally composed of one or more compressors depending on the volume of gas to be recovered and the target recovery scenarios,

wherein the systems optionally further comprise electric motors, automation panel to implement interlocking and control logic, knock-out vessels at the base of the flare to accumulate gas from primary processing and support the separation of liquid and vapor phases, quick opening valves, rupture discs, and PV valve for load control and gas recirculation from compressor discharge to suction.

4. METHOD, according to claim 1, characterized in that Step A) initially considers an accumulation of volume to enter a steady state at the expected “startup” time, wherein the initial suction pressure is recognized at the point where it is high enough for the final discharge pressure to reach an adequate value, ensuring a steady state, within X seconds after starting the compressor, applying the real gas equation PV=n ZRT within the stipulated parameters.

5. METHOD, according to claim 4, characterized in that in Step A) once the flare gas recovery system is connected, it is checked whether there is a compressor determined as the main compressor, so that if there is no compressor determined as the main one, the method invites the user to define a main compressor, and if a main compressor has already been determined, the method leads to the filling of the system referring to the two compressors, by the formation of the liquid ring of the compressors,

wherein the method sequentially leads to the closure of the flare valves, with the flare gas recovery system subjected to control already in operation.

6. METHOD, according to claim 1, characterized in that in Step B)

after closing the flare valve in step A), the method leads to monitoring the pressure at the inlet of the system subjected to control through a pressure sensor, so that if it is identified that the system inlet pressure is greater than Ps (min), the method leads to the activation of the main compressor, and if it is identified that the system inlet pressure is not greater than Ps (min), monitoring of the system inlet pressure continues to be carried out,

wherein when it is identified that the system inlet pressure is greater than Ps (min) and the main compressor is turned on, the system inlet pressure is also monitored, so that if the inlet pressure is greater than Ps (max) and the PV recycle valve is closed for more than T seconds, the backup compressor is turned on, and if not, monitoring of the system inlet pressure remains in operation,

wherein, upon activation of the backup compressor, the method leads to checking whether the “switch mode” mode is activated, where if it is not activated, the method remains monitoring the system inlet pressure, while the switch mode is activated, the backup and main compressors are led to alternate this primary and secondary positioning, and

wherein, following the alternation between compressors, monitoring of the system inlet pressure continues to be carried out, so that it is checked whether the PV recycle valve has an opening greater than PVopen % for more than Ta seconds, where if it is concluded that yes, the backup compressor is turned off and the first analysis is returned to check the need to activate the backup compressor, while if not, the method leads to remaining monitoring the system inlet pressure.

7. METHOD, according to claim 1, characterized by additionally and in parallel to the steps discussed A) and B), the method performs a stop control of the flare gas recovery system in a predictive way, wherein if it is identified that system depressurization is not supported by recovery, the system is forcibly stopped.