Key information

Title
Adaptive control system for a hydrocarbon-water separator
Application number
20180679
Case type
National
Status
26.01.2020 P.g.a. manglende svar. Søker har ikke begjært gjenopptakelse
Filed
14.05.2018
Effective date
14.05.2018
Publicly available
15.11.2019
Applicant
STAUPER OFFSHORE AS (NO)
Inventor
Eivind Singdahlsen (NO)
Agent
PROTECTOR IP AS (NO)

Abstract and drawing

Disclaimer: This text has been machine-scanned and may contain errors – please refer to "Publications" for legally binding content.
A control system (200) for a separator (100) comprising a closed separator tank (110) with a frustum (120). An influent line (111) leads a feed of solids, water oil and gas to a mixer (115). A primary gas supply (113) adds a primary process gas to the mixer (115). A pump (117) on an inlet line (119) drives a vortex within the frustum (120). The vortex separates the constituents according to Stoke's law and the process gas accelerates separation. Separated constituents exit through primary outlet lines (131 – 134). The control system (200) uses fast algorithms and numerical methods to minimise a concentration of oil in a water outlet line 132 or water in oil on an oil outlet line 133 as function of an inlet flow. Systems controlling several frusta side by side in one tank are also disclosed. In these embodiments, several frusta share sensors and flow regulators.

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Date
 Date Doc. No. Process Case number In/Out Journal description To/from
 04.06.2020 13-01 Saksbehandling 20180679 UT Endelig Henleggelse PROTECTOR IP AS
 31.03.2020 12-01 Saksbehandling 20180679 UT PT Varsel om betaling av første årsavgift (3317) (PT20180679)
31.01.2020 11-01 Saksbehandling 20180679 UT Henlagt manglede svar PROTECTOR IP AS
 26.11.2019 10-02 Saksbehandling 20180679 INN Hovedbrev PROTECTOR IP AS
 26.11.2019 10-01 Saksbehandling 20180679 INN Korrespondanse (Hovedbrev inn) PROTECTOR IP AS
 17.09.2019 09-01 Saksbehandling 20180679 UT Generelt brev Patent uten frist PROTECTOR IP AS
 16.09.2019 08-02 Saksbehandling 20180679 INN Hovedbrev PROTECTOR IP AS
 16.09.2019 08-01 Saksbehandling 20180679 INN Korrespondanse (Hovedbrev inn) PROTECTOR IP AS
 11.09.2019 02-01 Endring av søker/innehaver 2019/11118 UT GH Forespørsel PROTECTOR IP AS
 06.09.2019 01-01 Endring av søker/innehaver 2019/11118 INN Generell henvendelse PROTECTOR IP AS
 26.08.2019 07-01 Saksbehandling 20180679 UT Realitet patent PROTECTOR IP AS
23.08.2019 02-01 Endring av fullmektig 2019/10284 UT GH Forespørsel PROTECTOR IP AS
14.08.2019 01-02 Endring av fullmektig 2019/10284 INN Fullmakt PROTECTOR IP AS
14.08.2019 01-01 Endring av fullmektig 2019/10284 INN Generell henvendelse PROTECTOR IP AS
 20.03.2019 06-02 Saksbehandling 20180679 INN Hovedbrev APACE IP AS
 20.03.2019 06-01 Saksbehandling 20180679 INN Korrespondanse (Hovedbrev inn) APACE IP AS
08.11.2018 02-01 Endring av fullmektig 2018/14650 UT GH Forespørsel APACE IP AS
06.11.2018 01-02 Endring av fullmektig 2018/14650 INN Fullmakt APACE IP AS
06.11.2018 01-01 Endring av fullmektig 2018/14650 INN Generell henvendelse APACE IP AS
 04.10.2018 05-02 Saksbehandling 20180679 UT PT report 02:56:35
04.10.2018 05-01 Saksbehandling 20180679 UT Realitet patent TANDBERG INNOVATION AS
 09.07.2018 04-01 Saksbehandling 20180679 UT Infobrev til oppfinner TANDBERG INNOVATION AS
 06.07.2018 03-03 Saksbehandling 20180679 INN Erklæring Søkers rett til oppfinnelse TANDBERG INNOVATION AS
 06.07.2018 03-02 Saksbehandling 20180679 INN Fullmakt TANDBERG INNOVATION AS
 06.07.2018 03-01 Saksbehandling 20180679 INN Korrespondanse (Hovedbrev inn) TANDBERG INNOVATION AS
 24.05.2018 02-01 Saksbehandling 20180679 UT Formalia 1 TANDBERG INNOVATION AS
 14.05.2018 01-03 Saksbehandling 20180679 INN Patenttegninger TANDBERG INNOVATION AS
 14.05.2018 01-02 Saksbehandling 20180679 INN Beskrivelse TANDBERG INNOVATION AS
 14.05.2018 01-01 Saksbehandling 20180679 INN Søknadsskjema Patent TANDBERG INNOVATION AS

Description, claims and drawings

Disclaimer: This text has been machine-scanned and may contain errors – please refer to "Publications" for legally binding content. Description
BACKGROUNDField of the invention[0001] The present invention concerns an adaptive control system for a vortex separator.Prior and related art[0002] There is a general need to separate oil from water, e.g. in wastewater, process water, bilge water and other liquids with a high water content. In these applications, a primary aim is to remove oil from effluent water for environmental reasons. A secondary aim is to reclaim oil for recycling by reuse or combustion. A subsea well on a production field for oil or gas may produce a fluid with a substantially lower water cut. Separating water from such a fluid may aim to reduce energy consumption by returning effluent water directly to an injection well for increasing a reservoir pressure. The alternatives include spending energy on extra separation required to release effluent to the ocean and/or on pumping effluent water to a wellhead, which on a production field may be hundreds of metres below the sea surface.[0003] A gravity separator uses gravity to separate an influent fluid depending on the densities of the constituents. Separators of interest herein add a centrifugal force to increase separation speed. The added force reduces footprint and increases throughput, both of which are desirable features aboard a ship or elsewhere where space and/or weight is expensive.[0004] A hydrocyclone separates oil from water by entering the influent fluid tangentially into a frustoconical chamber, hereinafter 'a frustum' for short. The influent fluid enters the frustum at high speed, which creates a deep vortex with a gas-filled core. During operation, oil trickles down the inner surface of the vortex and is removed from a bottom of the frustum.[0005] The high rotational speed in a hydrocyclone implies high vorticity: A small liquid volume within a vortex rotates counter-clockwise if the vortex rotates clockwise. At high vortex speeds, a substantial vorticity causes turbulence that mixes oil and water in the effluent water. The resulting emulsion in the effluent fluids is difficult to separate.[0006] EPA (1970)1 describes modified hydrocyclones with substantially reduced rotational speeds and thereby lower vorticity and shallower surface vortices. The report recommends pumps and valves to minimise emulsification in the influent, and points out a general need for control systems to optimise operation and reduce a need for trained operators. EPA (1970) also describes a robust mode of operation in which a pocket of oil is allowed to grow until a 1 Report 15080EUU 10/70, "Vortex separation for oil spill recovery systems", the US Environmental Protection Agency, Water Quality Office, October 1970 limit, at which time a pump is started to remove the oil collected in the pocket. The pump stops at a minimum level selected to ensure that an oil intake is immersed in oil phase. This mode of operation accommodates a wide range of oil-in-water concentrations and obviates a need for precise process control.[0007] WO2010080035 and WO2013109345 provide examples of cylindrical separator tanks with vertical axes of rotation, inner frustoconical walls and a mixer for adding a process gas to the influent fluid. These and other vortex separators of interest operate in the so-called 'Stokes' regime' in which Stokes' law apply. Specifically, gravity, a centrifugal force and frictional forces act on spherical oil droplets and gas bubbles in a liquid vortex. If the oilwater mixture is not too emulsified, separation speed is proportional to the density difference between the sphere and the surrounding fluid as well as to the flow rate of a tangentially directed inlet flow. However, vorticity and turbulence increase with the angular speed.[0008] Gas has a greater affinity to oil than to water, and larger gas bubbles have a larger surface available for an oil film. Thus, larger gas bubbles also increase the separation speed. Some influent fluids contain dissolved hydrocarbons that are gaseous under operating conditions. A process gas such as N2, CO2 or air is added to influents without such dissolved hydrocarbon gases. Combustible mixtures of air and gaseous hydrocarbons are avoided. In gas terms, the centrifugal force adds a pressure gradient proportional to the distance from the rotational axis to the pressure gradient caused by gravity. The bubble size depends on a pressure drop as the dissolved gas enters the vortex, and the pressure drop is proportional to the flow rate of the inlet fluid. Too large gas bubbles increase the turbulence, and thereby inhibit separation of oil from water. 'Increasing turbulence' is equivalent to the vortex system leaving the Stoke's regime when the Reynold's numbers become too large.[0009] Separation also depends on coalescence of oil droplets and oil on the surface of gas bubbles to an oil phase. Mechanical coalescers enhance separation using boundary effects near a surface, e.g. on baffles, plates and/or grids.[0010] WO2017164747 assigned to the present assignee discloses a hydrocarbon-water separator comprising a cylindrical separator tank containing an inner frustoconical wall. A major design feature is a bottom pan closing the frustoconical wall at a wide, lower end to form a frustum. Fluid enters the frustum tangentially near the bottom pan. Solids are collected by the bottom pan and exits through a slurry outlet. A vortex of fluids is contained within the frustum and damped by a quiescer, e.g. a perforated plate and/or baffle plates at a narrow top of the frustum. A water phase descends slowly outside the frustoconical walls toward a water outlet at the bottom of the separator tank. Oil and gas phases exit through separate outlets near the top of the separator tank. An alternative separator tank comprises several stages, each stage containing one frustum. The stages are coupled in series such that the water phase from one stage provides the influent for a subsequent stage.[0011] The closed frusta in WO2017164747 precisely define the vortex volume in which the separation takes place. Further, high volumetric capacity, relatively small size and robust design makes the tank suitable for subsea use, e.g. for installation on a seafloor on a production field for oil or gas. Pre-treatment of the influent, e.g. by means of upstream filtering or sand cyclones, may be required. Post treatment downstream on outlet lines may also be required. Pre and post treatments are beyond the scope of the present invention.[0012] A control system for a vortex separator needs sensors for measuring fluid flows and concentrations as well as flow regulators such as pumps and valves to adjust fluid flows through inlet and outlet lines. We will use terms from control theory in their common meaning. In particular, a 'closed loop' is a control loop in which a controller uses sensor inputs to control fluid flows by means of flow regulators. Opposite, an 'open loop' means a control loop running without sensor input and implies fixed regulator settings. Any control system may run 'open loop' until corrective measurements become available. Further, it is common knowledge that loops may be nested. For example, classical navigation used dead reckoning in a first loop to estimate positions until a position fix could be obtained in a secondary loop, e.g. by means of a chronometer and a sextant or by triangulation of landmarks.[0013] In the proposed system, some sensors, e.g. for measuring oil content in effluent water, may require 30 seconds or more to provide a reliable and repeatable measurement. This is substantially longer than typical response times for regulators such as pumps and valves.[0014] A general objective of the present invention is to improve the operation of a vortex separator as described while retaining benefits from prior art. Specific objectives include reducing the cost of operation, improving the efficacy and the separation rate and/or maximise the throughput of the separator.SUMMARY OF THE INVENTION[0015] This is achieved by a control system for a separator according to claim 1. Further embodiments and features appear in the dependent claims.[0016] More particularly, the invention concerns a control system for a separator, wherein the separator comprises:- a closed separator tank, - a frustum within the separator tank, the frustum having a lower wide end closed by a bottom pan and an upper narrower open end,- an influent supply line leading to a mixer,- a gas supply line leading to the mixer,- an inlet line leading from the mixer to a tangentially oriented inlet within the frustum near the bottom pan,- a quiescer near the upper end of the frustum,- a slurry outlet line leading from the bottom pan,- a water outlet line leading from a water region below the bottom pan,- an oil outlet line leading from an oil region near the quiescer, and- a gas outlet line leading from a gas region above the quiescer.[0017] The control system is distinguished by:- equipment for measuring a mass flow and a flow regulator for controlling a fluid flow through each of the gas supply line, the inlet line and the gas outlet line,- a concentration sensor for measuring a first concentration C1 of either oil in a water phase within the water outlet line or water in an oil phase within the oil outlet line, and- a controller with logic foro minimising the first concentration C1 as function of a mass flow through the inlet line, o regulating a mass flow through the gas supply line such that the mass flow through the inlet line remains constant during a measurement interval, ando minimising the first concentration C1 as function of the mass flow through the gas supply line at a fixed mass flow through the inlet line.[0018] In the claims, articles 'a', 'an' and 'the' generally mean '(the) at least one' whereas 'one' would mean exactly one. For example, 'a control system … comprising a separator tank' implies that one control system may control several separator tanks. Further, the separator tank may have a non-circular cross section and contain two or more frusta side by side.[0019] The term 'for' should be construed as 'suitable for'. For example, a volume flow meter 'for' gas is not necessarily suitable for liquids and vice versa.[0020] The 'quiescer' in claim 1 represents known devices to reduce rotation and oscillations as well as features to enhance coalescence. For example, vertical baffles may reduce the vortex rotation and a weir may dampen surface waves on a liquid surface in embodiments with several frusta side by side. Mechanical coalescers comprising vertical or inclined plates are known in the art. A horizontal perforated plate or mesh may enhance coalescence to the oil phase from an oil droplet in water and from an oil film on the surface of a gas bubble.[0021] 'Equipment for measuring a mass flow' includes mass flow meters and load cells. Indirect measurements are also possible. For example, a mass flow of gas can be derived from the ideal gas law and measurements of pressure and temperature in a known gas volume.[0022] The 'flow regulators' are components of the control system required to adjust fluid flows through the supply lines, the inlet line and the outlet lines. In particular, each flow regulator comprises a pump and/or a valve suitable for increasing and decreasing fluid flow through a line in response to control signals from the controller. Fixed restrictions, nozzles, manually adjusted valves etc. that do not receive control signals are not part of the control system, and are not 'flow regulators' as the term is used herein.[0023] During operation, the bottom pan collects solids and separates a vortex within the frustum from the water phase below the bottom pan. The quiescer reduces vortex rotation near the upper narrower open end of each frustum. The proposed control system minimises a first concentration C1, which is generally known to decrease inversely proportional to the inlet flow in the Stoke's regime and increase with increased turbulence. The control system further reduces the first concentration C1 by finding a gas flow providing a minimum for the fixed inlet flow. Design and fluid parameters such as frustum and tank dimensions, densities, viscosities, sizes or distributions of oil droplets and gas bubbles are avoided.[0024] The most general form of the proposed control system does not include sensors and flow regulators on the outlet lines leading from the separator tank. This means that flows through the outlet lines may be set manually or implemented in separate control systems, not that the outlet lines necessarily lack sensors or flow regulators. For example, a second controller may adjust the slurry outlet line, the water outlet line and the oil outlet line to maintain a constant liquid level within the separator tank, and a third controller may maintain a constant mass of gas in the tank, e.g. by keeping a pressure/temperature ratio constant in the gas region. The second and third controllers in this example are not part of the most general form of the proposed control system. However, preferred embodiments realise several control loops within one controller to avoid unnecessary hardware costs.[0025] Preferred embodiments further comprises equipment for maintaining a constant mass of gas in the gas region. Here and elsewhere, functional features that do not require inventive effort replace structural features that would be unreasonably limiting and/or reduce clarity. For example, it is generally known that the ideal gas law is a good approximation to practical gases and that the mass of an ideal gas is proportional to pV/T where p is pressure, V volume and T temperature of the gas. Hence, the equipment for maintaining a constant mass of gas may include sensors for measuring pressure, volume and temperature in the gas region. In addition or alternatively, the equipment might comprise a mass flow meter on the gas outlet line. Further, a suitable flow regulator on the gas outlet line is obviously required to maintain the mass of gas or the equivalent pressure.[0026] Preferred embodiments of the control system further comprise a volume flow meter on the inlet line as well as a volume flow meter and a flow regulator on each of the slurry outlet line, the water outlet line and the oil outlet line.[0027] Since the most general form of the control system comprises a flow regulator on the inlet line, preferred embodiments comprise volume flow meters and flow regulators on the inlet line and the outlet lines for slurry, water and oil. During operation, these lines contain incompressible fluids or liquids with unknown densities, so regulating volume flows rather than mass flows makes sense. It is possible to maintain a constant liquid level in the separator tank, and thereby a constant gas volume, by controlling liquid flow. These embodiments may comprise a check valve on the gas outlet line to ensure a constant pressure in the gas region. Hence, embodiments controlling liquid flows are alternatives or additions to embodiments with equipment for maintaining a constant mass of gas in the gas region.[0028] For a preferred mode of operation, the controller comprises logic for collecting a volume of oil phase in the separator tank during an oil collection interval, then expelling the volume of oil phase during a subsequent oil disposal interval while maintaining a constant liquid level in the separator tank.[0029] As described in EPA (1970), this mode of operation reduces the need for precise control and permits limited tilting of the separator tank without affecting separator efficiency. Again, the control system is described by functions that can be realised without inventive effort. The constant liquid level in the separator tank implies that the sum of the volumes of oil and water must be constant during the oil collection interval and the oil disposal interval.[0030] Embodiments with a flow regulator on the slurry outlet line may further comprise logic for collecting solids in the bottom pan during a solid collection interval, then expelling the solids together with fluid from the bottom of the frustum during a subsequent solid disposal interval. The solid collection and disposal intervals may be adapted to an assumed maximum content of solids in the influent fluid such that sensor feedback is not required. This is an example of a control system running 'open loop', i.e. without sensor feedback. It is to be understood that any part of the present control system may run open loop permanently or until a measurement becomes available. [0031] The proposed control system may further comprise a mass flow meter and a flow regulator on the influent line and/or on a water supply line leading to the mixer. The water supply line is optional and adds water to the influent fluid if desired. The sum of mass flows through the influent line, the optional water supply line and the gas supply line equals the fixed mass flow through the inlet line during the measurement interval specified above.[0032] The control system may further comprise an oil phase detector located such that it is immersed in oil phase during operation. In these embodiments, the controller is configured to reduce the mass flow through the inlet line whenever the oil phase detector indicates a gas phase. This happens if a surface vortex becomes too deep due to a large rotational speed, so the oil phase detector provides a cut-off value for the fluid flow through the inlet line.[0033] The control system may further comprise a load cell for measuring a total mass of the content within the separator tank during operation. Subtracting the mass of an empty separator tank from a measured mass is trivial. Likewise, the skilled person knows how to compensate for heave and other forces aboard a ship by means of a reference mass subjected to the same forces as the separator tank. The measurement accuracy may conveniently be of order 10-3, which corresponds to kilograms for separator contents of a few thousand kg. The load cell combined with controlled inlet and outlet flows enables measurements of, for example, the mass of solids collected in the bottom pan.[0034] Some embodiments have a mass flow meter on the slurry outlet line. These embodiments provide an alternative or addition to the load cell for measuring the mass of solids collected in the bottom pan. Since any part of the control system may run open loop, a mass flow meter on the slurry outlet line is optional.[0035] Similarly, the water and oil outlet lines may be equipped with mass flow meters. In some embodiments, mass flow meters on the inlet and outlet lines enable control of fluid flows according to a mass balance that includes liquids and gas.[0036] Embodiments with volume flow meters and mass flow meters on the water and oil outlet lines may further comprise logic for calculating densities of fluids in the water outlet line and the oil outlet line. Since these lines lead from the water and oil regions, respectively, the densities correspond to the density of water phase in the water region and oil phase in the oil region. Embodiments with a load cell operating in the preferred mode of EPA (1970) may measure an increase in oil volume by a mass decrease proportional to the density difference.[0037] Embodiments with mass flow meters on the water and oil outlet lines may further comprise:- a second concentration sensor for measuring a second concentration C2 being o the water concentration Cw on the oil outlet line if the first concentration C1 is the oil concentration Co on the water outlet line, ando the oil concentration Co on the water outlet line if the first concentration C1 is the water concentration Cw on the oil outlet line,- logic for computing mass fractions of water and oil in a fluid entered through the inlet line.[0038] Mass fractions oil in water and water in oil combined with masses of oil phase and water phase enables calculation of the masses of water and oil entering the separator tank during a time interval comparable to the response time for pumps and valves. As the most general form of the proposed control system measures the mass flow through the inlet line, mass fractions of water and oil in the inlet fluid are readily computed. If the mass flows through the supply lines leading to the mixer are known, then similar mass fractions for the influent fluid are trivially computed. Mass fractions may of course be stored for later use.BRIEF DESCRIPTION OF THE DRAWINGS[0039] The invention will be described in greater detail below be means of an exemplary embodiment with reference to the accompanying drawings, in which:Fig. 1 illustrates a preferred embodiment of the invention,Fig. 2 illustrates a steady state during separation,Fig. 3 illustrates an inlet flow as a function of time,Fig. 4 illustrates a numerical method for finding a minimum concentration,Fig. 5 illustrates a mass fluctuation due to solids in a bottom pan,Fig. 6 illustrates a mass fluctuation due to a variable volume of oil phase,Fig. 7 illustrates an embodiment with several frusta within one separator tank, and Fig. 8 is a schematic top view of the embodiment in Fig.7.DETAILED DESCRIPTION[0040] The drawings are schematic and not to scale. Several details known to the skilled person are omitted from the drawings for clarity of illustration.[0041] Fig. 1 illustrates a control system 200 for a separator 100. The separator corresponds to the hydrocarbon-water separator of WO2017164747 and comprises a closed separator tank 110 with an inner frustum 120. A controller 210 receives measurements from various sensors and provides control signals to flow regulators comprising pumps and/or valves. [0042] During operation, an influent supply line 111 contains an influent fluid with solids, water, oil and perhaps an amount of dissolved gas. An optional water supply line 112 supplies water, e.g. seawater, if desired. A gas supply line 113 supplies a controllable amount of gas, which comprises process gas and optionally gas from a gas region 4 in the upper part of separator tank 110 supplied through a gas return line 114. The influent supply line 111, optional water supply line 112 and gas supply line 113 lead to a mixer 115. The mixer 115 produces an inlet fluid comprising solids, water, oil and gas. In the present specification and claims, we distinguish between 'influent fluid' in the influent supply line 111 and 'inlet fluid' in an inlet line 119.[0043] An inlet pump 116 and/or an inlet valve 117 control(s) a flow of inlet fluid through the inlet line 119. The inlet pump 116 primarily provides a pressure, and may hence be replaced by a column of liquid. Alternatively, the inlet pump 116 may control the flow through inlet line 119 without the valve 117.[0044] An inlet assembly 118 provides a controllable flow of inlet fluid through the inlet line 119. Thus, the inlet assembly 118 represents all suitable combinations of known mixers 115, pumps 116, valves 117 etc. The inlet assembly 118 further contains flow meters and other sensors as desired. It sends measurements from sensors to the controller 210 and receives control signals from the controller 210 through a two-way communication line.[0045] Sensors, communication lines and pump/valve assemblies are present on outlet lines 131 – 134 as well. For consistency and ease of understanding, reference numerals 2xx refer to equipment on line 1xx, letter 'a' refer to sensors, 'b' to a communication line to/from the controller 210 and 'c' to a flow regulator.[0046] As explained in the summary section, the term 'flow regulator' means devices receiving control signals from the controller. For short, we use the term 'regulator' in the same sense. Lines with manual valves are of course possible – manual valves are simply outside the proposed control system. As noted, we use 'for' in the meaning 'suitable for'.[0047] Using these conventions, 219a may represent, for example, a volume flow meter and/or a mass flow meter for measuring flows through the inlet line 119. The communication line between the controller 210 and inlet line 119 is denoted 219b. An inlet regulator 219c denotes pumps or valves designed to decrease or increase the flow through the inlet line 119.[0048] Similarly, a double headed arrow 213a-c between gas supply line 113 and a circled '210' represent a suite of sensors 213a, e.g. flow meters, a communication line 213b to and from the controller 210 and a regulator 213c for controlling the flow of gas through line 113. If a water supply line 112 is present, sensors 212a provide measurements to the controller 210, and a regulator 212c may receive control signals from the controller. A communication line 212 b is implied.[0049] Double headed arrows 231a-c to/from a slurry outlet line 131, 232 a-c to/from a water outlet line 132, 233a-c to/from an oil outlet line 133 and 234a-c to/from a gas outlet line 134 represent sensors, communication lines and regulators in a similar manner.[0050] In principle, a volume flow rate and a mass flow rate for an incompressible fluid or liquid may be measured by collecting the liquid flow in a box for a short interval, e.g. δί = 2 s. The measured volume flow rate would be the collected volume AV divided by the interval åt, i.e. AY/åt. Similarly, the measured mass flow rate would be AM I åt where AM is the mass collected in the box during åt.[0051] Using dot notation for time derivatives, we define a 'volume flow' vx= VxΔt where Vx is the volume flow rate and Δt an interval comparable to response times for pumps and valves. That is, the 'volume flow' is actually a small volume. Similarly, we define a 'mass flow' mx= mxΔt where mx = dmx/d is a mass flow rate assumed to be constant during Δt. The 'mass flow' is actually a small mass, and vx and mx are simply convenient notations that will be further explained with reference to Fig. 2.[0052] In practice, flow meters measure mass and volume flow rates on supply lines 111-113 leading to the mixer 115, the inlet line 119 from the mixer 115 to the frustum 120 and on the outlet lines 131-134. Suitable sensors or sensor types follow from common design criteria such as physical properties and total cost of ownership. For example, Coriolis mass flow meters provide accurate and repeatable measurements over wide ranges of viscosity and temperature. Moreover, there are several vendors of Coriolis flow meters, installation is convenient and the cost of operation is relatively low. Similar design criteria obviously apply to volume flow meters and other sensors.[0053] In Fig. 1, the sensor suite on water outlet line 132 contains a sensor for measuring a mass ratio or concentration Co of oil in a water phase. Similarly, the sensor suite on the oil outlet line 133 contains a sensor for measuring a mass ratio or concentration Cw of water in an oil phase. A real embodiment of the control system 200 may measure one or both of these mass ratios or concentrations. This will be further explained with reference to Fig. 4.[0054] The inlet assembly 118 supplies inlet fluid with a known flow rate and a pressure po through the inlet line 119 to the frustum 120. A reduction valve, e.g. implemented as one or more nozzles in a tangentially oriented inlet 123, causes a pressure drop po -p1 such that gas dissolved in the inlet fluid forms bubbles at the bottom of the frustum 120. [0055] The frustum 120 comprises a frustoconical wall 121 and a bottom pan 122 closing the frustoconical wall 121 at a lower, wide end. The tangential inlet 123 is located near the bottom pan 122 and induces a vortex 1 within the frustum 120. Preferably, the inlet 123 comprises several tangentially oriented pipes distributed around the circumference of the frustum near the bottom pan 122. Alternative embodiments may comprise deflectors or baffles redirecting the inlet flow. In these embodiments, the deflectors should be long enough to redirect the inlet flow completely, because a partially redirected inlet flow has a radial component that reduces the efficiency of the separator 100.[0056] Solids 11 in the inlet fluid settle on the bottom pan 122. The solids 11 and some influent fluid can be expelled or removed through the slurry outlet line 131.[0057] Water migrates to the periphery of the vortex 1 and spills over the top of the frustum to a water region 2. During operation, this region contains 'water phase' meaning water with the reduced oil concentration Co. The water phase descends outside the frustum 120 to and through the water outlet line 132, which extends from the bottom of separator tank 110. The bottom pan 122 prevents the inlet fluid from mixing with the water phase 2.[0058] Oil migrates toward a rotational axis z of the vortex 1 and enters an oil region 3 containing oil with the reduced water concentration Cw. The oil phase in region 3 floats on top of the water phase and may extend to the inner wall of the separator tank 110. The intake to the oil outlet line 133 is located near the rotational axis z and relatively low in the oil region 3. This enables embodiments in which the oil phase does not extend to the inner wall of the separator tank 110 and a variable volume of oil phase above and/or immersed in the water phase in the water region 2.[0059] An oil phase detector 135 near the intake to oil outlet line 133 detects whether the intake to line 133 is immersed in oil phase or not. The oil phase detector 135 may for example measure the electric conductivity between two electrodes attached to the inner end of the oil outlet line 133. Seawater has a significantly greater conductivity than oil, and the conductivity of gas is much lower than that of oil.[0060] A quiescer 124 at the top of the frustum 120 dampens oscillations in the liquid. The quiescer 124 preferably comprises a perforated plate and/or vertical baffles to reduce rotation and induce laminar, radial flows. A perforated plate in the quiescer 124 also aids coalescence of oil droplets and oil on the surface of gas bubbles to the oil phase in the oil region 3. The quiescer 124 may be located above or below the intake to the oil outlet line 133. [0061] Baffles 125 within the frustum 120 illustrate a mechanical coalescer and may be located anywhere within the frustum. Any vertical baffles near the top of frustum 120 must be designed to avoid turbulence and remixing or re-entrainment of oil and water.[0062] Released gas enters the gas region 4 in the uppermost part of the separator tank 110. The optional return line 114 extends from this region. In real embodiments, the gas phase comprises gas with a small concentration of liquids, e.g. a mist of water and oil droplets. Coalescers and demisters known in the art may reduce the liquid concentration. In the present specification, we assume that the concentration of liquid in the gas region 4 is negligible.[0063] According to Henry's law, the amount of a gas dissolved in a liquid is proportional to the partial pressure of the gas above the liquid. A downstream degasser may reduce the partial pressure of process gas and/or hydrocarbon gas, and thereby the amount of dissolved gas. Here, we disregard the amount of gas dissolved in water and oil in the liquid regions 2 and 3.[0064] Gas from the gas region 4 may leave the separator tank 110 through the gas outlet line 134. The gas outlet line 134 and the other inlet and outlet lines 113, 119, 131- 133 may be described by function. For example, closing or opening the gas outlet line 134 obviously involves the regulator 234c as defined. As noted in connection with the gas supply line 113, sensors and regulators for liquids are not necessarily suitable for gas.[0065] We assume that the ideal gas law is a good approximation to the state in gas region 4. For this, a pressure sensor 141 input a pressure p4 through a communication line 241 to the controller 210. Similarly, a level sensor 142 transmits a signal representing a gas volume V4 through a communication line 242 to the controller 210, and a temperature sensor 143 sends a signal representing a temperature T4 through a communication line 243 to the controller 210.[0066] Effluent from any line 131-134 may be post-processed if desired. For example, water effluent from line 132 may be supplied to a subsequent stage or system 200 as described in WO2017164747. Pre- and post-processing are outside the scope of the present invention.[0067] A support stand 150 carries and supports the separator tank 110. Figs. 1 and 2 show the support stand 150 unrealistically small compared to the separator tank 110. The support stand 150 may be gimbaled, e.g. for use on a ship, in which case the inlet line 119 and outlet lines 131-134 comprise flexible tubes rather than rigid pipes.[0068] A load cell 151 provides a mass Ms of the content within the separator tank 110. A second accelerometer with a separate reference mass is required to compensate for heave, i.e. acceleration in the z-direction, and other forces acting on the separator tank aboard a ship. This is known in the art so the extra mass and accelerometer are omitted from Fig.1. Example: Relative volumes and masses in the separator tank[0069] As noted, Figs.1 and 2 are not to scale. In order to illustrate relative volumes and masses, we define a reference cylinder with cross sectional area 1 m2, liquid volume 1.0 m3 and 10 molar volumes (10Vm) of ideal gas on top of the liquid. The volumes and masses in the reference cylinder are not necessarily representative for a real separator.[0070] Vm is 24.5 l at 25°C and 1 atm, so the cylinder has V4,ref =245 l or a height 24.5 cm of gas on top of a 1 m high column of liquid. The mass of 10Vm is in the range 160 - 440 kg where the minimum corresponds to pure methane (CH4) and the maximum to pure CO2.[0071] Referring to Fig.2, the volume V1 of the frustum 120 is:where (z2-z1) is the height, r1 is the largest radius and r2 is the smallest radius of the frustum. The last term is the volume of a conical bottom pan 122 with height zb.[0072] Separation efficiency and a desire for a compact separator design implies that the separation volume V1 should be as large as possible. However, water phase must be able to pass the frustum 120, and there must be a minimum amount of liquid above the frustum to keep the oil phase detector 135 immersed in oil phase. The cross section of the reference cylinder πr2 =1.0 m2 implies a radius r = 56.4 cm. A frustum with largest radius r1 = 50 cm, smallest radius r2 = 25 cm and height 80 cm has an inclination arctan[(50-25)/80] = 17.4° to the vertical. Plugging these values and zb =2 cm into (1) yields V1, ref = 365 l. A steady volume flow of 10 m3/h replaces this volume in just over 2 minutes, so we expect a characteristic time constant for the system to be in the order of minutes or tens of minutes. The volume (V2 + V3) = (1.0 – 0.365) m3 = 635 l.[0073] Assuming an inlet fluid with 5% by weight of oil with density 900 kg/m3 in seawater with density 1025 kg/m3, the reference volume 635 l would contain 38 l oil. Thus, a layer of oil floating on top of the water would be 3.8 cm thick. Such a thin layer would require precise control to keep the oil phase detector 135 immersed in oil phase, especially at sea.[0074] The density of seawater differ up to 5 kg/m3 from the mean value 1025 kg/m3 in the temperature range 0 – 30 °C. In the reference volume, this corresponds to about ±3 kg, which is less than 10% of the mass of oil and much less than the minimum mass of gas, 160 kg. The mass ratios in this example are valid for all oil densities, which typically are 800 – 900 kg/m3 for hydrocarbon oils. Figure 2: Steady state and notation[0075] Fig. 2 illustrates a steady state of the separator 100. The liquid surface has a shallow surface vortex caused by the rotating liquid. The depth Δzl of the surface vortex grows with the rotational speed and provides an absolute limit for the inlet flow. A second level indicator 136 in combination with the level indicator 142 measures the depth Δzl . If Δzl or the pressure p4 becomes too large, the oil phase detector 135 will indicate gas phase near the oil intake. Then, inlet flows should be reduced and an alarm may be raised.[0076] Fig. 2 also illustrates a notation that will be used in the following:Mx is a mass within the separator tank 110,ΔΜx is a change of mass within the separator tank 110 during a time interval Δt or Δτ, Vx is a volume within the separator tank,ΔVx is a change of volume within the separator tank 110 during the time interval Δt or Δτ, mx is a mass flow rate, for instance measured in SI units kg/s,Vx is a volume flow rate, for instance measured in SI units m3/s,Δt is a time interval comparable to the response times for pumps and valves, e.g. Δt = 5 s, and Δτ represents a measurement interval, e.g. Δτ = 10Δt.Subscriptx = 'S' refers to the separator tank 110,x = 'a' refer to gas added through gas supply line 113,x = 'd' refers to gas dissolved in the influent fluid entered through line 111, x = 'in' refer to inlet fluid in the inlet line 119,x = 1-4 refer to regions 1-4 and corresponding outlet lines 131-134,x = 's', V, 'o' and 'g' refer to 'solids', 'water', 'oil' and 'gas', respectively,[0077] As noted, we use the shorthand notation mx = mxΔt and vx = VxΔt for an incremental mass and volume passing through a line during At. Further, we will use a super minus for the start of an interval At and integers i,j, k, n, e.g. as general counting variables. Other parameters will be explained when needed.Separator volumes and volume flows[0078] Using the definitions and conventions above, the volume of separator tank 100 is:where Vs and V1 are constants, V2 + V3 is a volume of liquids and V4 is a gas volume. [0079] The net changes in volume during At are:[0080] Vs and V\ do not change during Δt so ΔV1 = 0 is omitted from (3). Summing (3) over k intervals, we obtain:[0081] We want to keep the liquid level within the separator tank 100 constant to keep the quiescer 124 near the liquid surface and thereby aid coalescence to the liquid oil phase in the oil region 3. A constant liquid level implies that the gas volume V4 and the liquid volume V2 V3 are constants, and therefore that[0082] Further, we consider the fluid in lines 119, 131, 132 and 133 incompressible. Thus, the change of liquid volume in regions 2 and 3 depend on the input volume flow vin and the liquid outlet flows v1-v3, not on the volume flow v4 of gas:[0083] For a constant liquid volume, ΔV2 ΔV3 should be close to zero. Setting the left hand side of (8) to zero yield an expression for v2 as function of other liquid volume flows:[0084] In words, the volume flow of liquid through the water outlet 132 depends on liquid volume flows through lines 119, 131 and 133, not on the gas flow through line 134.Separator masses and mass flows[0085] The mass Ms = M1 M2 M3 M4 is the sum of masses in regions 1-4 and does not include the mass of the empty separator tank. The net change of mass during Δt is:[0086] We define the mass of inlet fluid passing through inlet line 119 during Δt as:[0087] In a real embodiment, normalised parameters would be preferred in the calculations for numerical reasons. Specifically, a digital processor can represent more numbers in the range [0, 1] than in any other range, so the RHS of equation (11) might advantageously be rewritten (fs +fw +fo + fg)min where all fractions within the parentheses are in the range [0, 1] and thus suitable for numerical calculations. Further, the sum of fractions is always 1, which permits normalisation of at regular intervals. In the present specification, we continue to use small masses mx for ease of understanding. Similar arguments apply to other parameters.[0088] Substituting (11) into (10) and summing over k intervals, each of duration Δt, yields:whereMsk is the mass of the content of separator tank 100 at interval number k,M ° is an initial value for the mass of the content of separator tank 100,ms = ΔM1 is the mass of solids collected in the bottom pan during Δt,mg = ΔM4 is the mass of gas entering the gas region 4 during Δt.[0089] The first sum on the right hand side (RHS) of (12) concerns solids. It has terms in which ms > m1 such that Ms increases and terms in which ms < m1 such that Ms decreases. Thus, estimates of ms can be obtained by controlling the flows through lines 119 and 131 — 134, then measuring the increase and decrease of the mass Ms of the separator tank 100.[0090] The sums containing ΔΜ2 and ΔM3 yield masses M2 and M3 in regions 2 and 3, respectively. The water phase contains a small mass of oil δm0 drawn from pure oil, and the oil phase contain a small mass of water δmw drawn from pure water:where M2 + M3 = Mw + M0 is the total mass of liquid in regions 2 and 3.[0091] Next, we define the mass ratios C0 = δmo/M2 and Cw = δmw/M3. The fluids in outlet lines 132 and 133 are equal to the fluids in regions 2 and 3, so we may rewrite (13) and (14):[0092] Solving (15) and (16) with respect to mw and m„ yields:[0093] Equations (17) and (18) show that mw and m„ can be obtained from measurements of m2, m3, Co and Cw on outlet lines 132 and 133. Thus, if a mass flowmeter and a concentration sensor are present on each line 132, 133, then the control system 200 can provide estimates of mw and m„. However, only one concentration sensor measuring either Co or Cw and mass flow meters on inlet lines 113 and 119 are required to operate the separator system 100 optimally.[0094] If both concentration sensors are present, a measurement of the mass flow m,„ on line 119 enables calculations of fw = mw/min and/0 = mo/min. The mass fractions fw,f0, Cw and Co vary 'slowly' and may be approximated by constants during a measurement interval Δτ. For example, a concentration sensor may require 30 seconds or more to provide a reliable and repeatable Co or Cw, so Δτ =10Δt might be appropriate in this case. In a real embodiment, a minimum measurement interval Δτ is of course determined from a sensor's data sheet.[0095] Mass densities p2 = M2/V2 = m2/v2 and p3 = M3/V3 = m3/v3 can also be measured on the outlet lines 132 and 133. If V4 is kept constant, equation (6) implies thatwhere the density difference (p3 - p2) is assumed to be constant during kAt.[0096] A volume flow meter on the oil outlet line 133 conveniently measures a varying volume of oil phase as described in EPA (1970), e.g. V3, max - V3, min predetermined to 100 litres. Equation (19) enables a redundant measurement of the volume V3 by measuring Ms. For example, a constant volume V2 V3 = Vs - V 1 - V4 and a volume flow v3 = 0 for an interval may increase the oil volume from Vs, min to Vs, max and correspond to the separator mass Ms decreasing from Mmax to Mmin because (p3 - p2 ) < 0.[0097] The last term of (12) concerns gas. In the steady state, the mass of gas entering the frustum 120 equals the mass of gas leaving the frustum during each interval At. That is:whereAMA is the mass of gas entering the gas volume VA during At andmg is the mass of gas in the inlet mass flow min according to equation (11).[0098] From the ideal gas law, the number of moles in V4 is n = p4V4 / RT4 where R = 8.314 kgm2/(K· mol· s2) is the universal gas constant. Further, the molar mass of the inlet gas and the temperature T4 are expected to change slowly compared to Δt = 5 s. We may control the flow through gas outlet line 134 such that MA remains 'constant', or more precisely such that ≈ 0 , by keeping p4 and V4 constant. As noted, V4 should be kept constant bymanipulating liquid volume flows, not gas flows. [0099] As indicated in Fig. 2, the mass of inlet gas is mg = md + ma where md is the mass of gas dissolved in the influent fluid within line 111 and ma is the controllable mass of added gas. Since md is expected to change slowly compared to Δt,where md is constant and p4 and V4 are controlled to be constants such that mg = m4.[0100] Alternatively, (21) yields md = m4 - ma where m4 and ma are measurable. In practice, md may be known in advance. For example, bilge water may contain a negligible amount of dissolved gas such that md ≈ 0. In this case, the added gas may be pure process gas with a known molar mass, e.g. N2 with molar mass 28.0. In this example, M4 = 28p4V4 / RT4 where all quantities on the RHS are known constants or measurable.[0101] Alternatively, the gas may be regarded as a mixture of process gas such as N2 or C02 and gaseous hydrocarbons from line 111 and/or the return line 114. That is,wheremHC is the mass of gaseous hydrocarbons andmp is the mass of process gas, both entered during the interval At.[0102] Since mg = m4 and hence measurable when p4 and V4 are constant in the steady state and since the molar mass of the process gas is known, the mass mHC follows from (22).Start-up period[0103] Fig. 3 illustrates an incremental inlet mass μ = min as a function of time t. In a first time interval 0 to τ0, min increases to a first target value μο. The time το is the time required to start separation, for instance the time at which the oil phase detector 135 starts detecting oil phase rather than water phase.[0104] Initially, we want to measure concentration(s) Co and/or Cw as often as possible, so we use a minimum measurement interval AT during start-up. In a previous example, a concentration sensor had a minimum measurement interval AT = 10Δt ~50 s. In Fig. 3, a similar Δτ is a fixed interval between times τ0 to τ3, i.e. the minimum Δτ = τi - τi-1 where i = 1, 2, 3, .... Specifically, a first sample is taken at TO and the concentration becomes available at τ1. A second sample is taken at τ1 and the result becomes available at τ2, etc. Measurements at τn and τn+1 show a short sampling interval δτ, e.g. a few seconds, which is generally negligible compared to the minimum measurement interval Δτ for most sensors. [0105] Concentration sensors for measuring Co and Cw may have different designs and hence different minimum times to provide reliable and repeatable results. Further, mass flow meters such as Coriolis flow meters have other measurement intervals Δτ.[0106] The gradually reduced slopes of lines 301-303 illustrate a gradual reduction from an initial rapid inflow rate to a steady rate that maintains a constant angular velocity in the vortex within the frustum 120. In Fig.3, the slopes are (µi – µi-1)/Δτ where i = 1, 2, 3 and Δτ the minimum measurement interval discussed above. Hence, a gradual reduction of slope corresponds to a gradual reduction of intervals (µi – µi-1). In Fig.3, each slope and each interval is half the previous slope and interval, so (µ2 -µ1) = ½(µ1 - µ0), etc.[0107] Next, we define a minimum mass flow increment Δµ greater than uncertainties and measurement accuracies related to min. Starting from an initial value µ0 and dividing by 2 in each interval, we arrive at Δµ in j steps, after which each increment of min is at least ±Δµ. For example, j = 4 or 6 correspond to minimal Δµ = µ0/16 or µ0/64, respectively.[0108] Opposite, a minimal Δµ provides τ0 = 2jΔµ in the present example. Since the sum (1/2 1/4 … 2-j) = (1 – 2-j), the inlet flow at τj is min(τj) = µ0 + (1 - 2-j)·α0Δτ where α0 is an initial slope. Any fraction less than 1 may replace ½ in the example above.[0109] The dashed line 304 illustrates the control system 200 running open loop with a slightly different slope than the desired slope of line 301. A measurement of µ1 at time τ1 allows a correction of flow rates. In the next interval, the control system may run open loop with the corrected rates, which may correspond to a slope slightly different from the desired slope of line 302 and so forth.[0110] Line 305 illustrates a steady state in which Δτ = (τn+1 – τn) is significantly longer than the minimum measurement interval described previously. If the minimum Δµ and the slope of line 305 are known, then a suitable Δτ = (τn+1 – τn) can be calculated.Separation in the steady state[0111] Separator efficiency measured by Co or Cw depends on min, which affects the vortex speed within the frustum 120, and mg, which affects the size of gas bubbles. Co and Cw are inversely proportional to min in the Stoke's regime, then increases with min as vorticity induces turbulence. Similarly, an increasing mg in the Stoke's regime increases bubble sizes, thereby enhancing oil-liquid separation and reducing Co and Cw. At some point, mg and the associated bubble size increases the turbulence such that Co and Cw starts to increase. [0112] Separator efficiency may be described by a function C(µ, γ); µ = min and γ = mg. According to the previous paragraph, C(µ, γ) is expected to have a minimum C(µm, γm) in the Stoke's regime near the state where turbulence becomes significant.[0113] Numerical methods for finding a minimum of a multivariate function such as C(µ, γ) are well known and include methods of steepest gradients. However, many of these methods depend on derivatives in a Jacobian, which may be difficult or time consuming to derive from finite differences involving concentration measurements taken once or twice per minute.[0114] A minimum (µm, γm) approximate to within measurement accuracies and minimal increments Δµ and Δγ can be obtained by finding µm independently of γm. In particular, the mass flow through line 111 may be controlled according to min – ma. Thereby, the flow through line 111 decreases when the mass ma of added gas increases and vice versa such that min remains constant during a measurement period Δτ. A minimum of a function C(µ) of one independent variable is expected to be close to the minimum of C(µ, γ), specifically to within one or a few minimal increments Δµ. Similarly, a minimum of a one dimensional function C(γ) is expected to approximate the minimum of C(µ, γ) to within one or a few minimum increments Δγ for the mass flow of gas.Estimating an optimal inlet mass flow[0115] Fig. 4 shows a solid curve 401 representing a function C1(µ) where C1 is Co or Cw, and µ represents the inlet mass flow min. In the Stoke's regime, C1(µ) is inversely proportional to µ as illustrated by a dashed curve 402, which in Fig.4 starts diverging from the curve 401 near the data point (µi-1, C1i-1). As µ continues to increase, turbulence grows such that the mass ratio C1(µ) starts increasing with µ. Thus, the function C1(µ) has a minimum (µm, C1m), which in Fig. 4 is near the data point (µi, C1i).[0116] The interval µ0 to µj corresponds to the start-up period τ0 to τj, where we expect slopes to be negative. A positive slope in the start-up period indicates that τ0 is set too small. After the j first intervals, each lasting one Δτ and several Δt, the slopes decrease toward a small negative value as illustrated by the secant 403. At data point (µi, C1i), the slope αi = (C1i+1 – C1i)/(µi+1 – µi) becomes positive as illustrated by the secant 404. A cut-off value µmax illustrates the value of min at which the oil phase detector 135 starts to detect gas phase due to the surface vortex shown in Figs. 1 and 2.[0117] The values µm of the minima for Co(µ) and Cw(µ) are not necessarily identical, so trying to minimise Co and Cw simultaneously may lead to an unstable solution. We select one of them, e.g. Co, as the value to minimise. Selecting one variable also means that we avoid or reduce matrix manipulation, and thereby reduce at least some numerical error sources.[0118] Specifically, we want to determine a point at which C1'(µ) = 0 to within µm ± Δµ, where Δµ is the minimal value above noise and measurement accuracy. Recall the previous example in which Δµ was µ0/16 or µ0/64. To find the minimum, we prefer the 'false position method' applied to slopes αi(µ) = (C1i+1 – C1i)/(µi+1 – µi) and define initial values f(a0) < 0 as the slope of secant 403 and f(b0) > 0 as the slope of secant 404. Accordingly, a0 is anywhere in the interval [µi-1, µi> and b0 is anywhere in the interval <µi, µi+1].[0119] Using the general counting variable k, a secant through (ak, f(ak)) and (bk, f(bk)) crosses the tangent through (µm, Cm) at[0120] We want to maintain different signs on each side of the minimum, i.e. to keep µm bracketed. Thus, if f(ck) and f(ak) have the same sign, then ak+1 := ck and bk+1 := bk. Otherwise, f(ck) and f(ak) have opposite signs and we set ak+1 := ak and bk+1 := ck.[0121] Equation (33) is usually rewritten as[0122] Since f(ak) and f(bk) have opposite signs, the 'differences' in the numerator and denominator of (34) are in fact sums. Thereby, we avoid differences close to 0 and division by very small values, both of which may cause numerical errors and loss of significant digits. In the present application, the smallest (bk – ak) is about the size of Δµ, the smallest valid change of min, so the risk of losing digits due to small (bk – ak) is limited. Equation (34) is preferred over (33) due to the fraction of sums.[0123] For comparison, the secant method uses equation (33) but does not keep µm bracketed. Hence, the secant method may fail to converge in situations where the false position method converges. Both methods may be regarded as finite difference versions of Newton's method, which is excluded because we lack an expression for C1'(µ) =dC1/dµ. Further, we expect the false position method to converge faster than the bisection method, in which (33) is replaced by ck = ½(ak + bk). However, the bisection method provides an alternative if the false position method fails to converge, e.g. within 10 minutes after start-up.[0124] The function C(γ) describes mass ratio Co or Cw as a function of gas mass γ = mg. As before, we select one parameter, e.g. Co, to minimise. Since we aim to find an approximate minimum close to the real minimum of C(µ, γ), we may use the false position method or the bisection method to find γm from C(γ) without regards to C(µ). In other words, one process may execute a numerical method for finding µm and another process may execute the same or a different numerical method to find γm. The two processes may use the same measurements of Co. Recall that the mass flow through line 111 should be controlled according to min – ma.Controlling the slurry outlet[0125] The mass flow m1 contains solids and a small mass of fluid from V1. The slurry outlet 131 may allow a continuous mass flow or be controlled by an open loop with a fixed solid collection interval and a fixed solid disposal interval. In the following, we describe an alternative in which solids are collected in the bottom pan 122 until a maximum mass is reached, then expelled through the slurry outlet 131 together with a small amount of liquid.[0126] Fig. 5 illustrates the first term on the RHS of equation (12), i.e. the sum over Δt concerning solids. Time t10 marks a time at which all solids are removed from the bottom pan 122 such that the mass M1, min within the frustum 120 is minimal. During the solid collection interval t10 to t11, the slurry outlet 131 is closed such that During this interval, solids collect in the bottom pan 122 contributes to the sum and increases M1. The increase is illustrated by line 501, which has a constant slope (M1,max – M1,min)/(t11 – t10).[0127] The next interval t11 to t14 illustrates a measurement interval in greater detail.Specifically, the slurry outlet line 131 starts opening at t11 such that m1 gradually becomes larger than ms. A maximum mass flow rate m1 through slurry outlet 131 is achieved at t12, maintained until t13 and the mass flow stops at t14. At t14, all the solids added from t10 to t14 are removed, so M1 is back to M1, min, and the process starts over. A solid polyline 502 illustrates the resulting M1(t) from t11 to t14, and a dashed line 503 illustrates its linearization.[0128] The difference between polyline 502 and its linearization 503 represents process errors and unmodelled features. Specifically, we assume that the effects of opening and closing line 131 are relatively small and compensate for each other as illustrated by 'bumps' on either side of line 503. Thus, the interval starts at t11 when line 131 begins to open and ends at t14 when line 131 is fully closed. If the sum of squared differences becomes too large, a different linearization with measurements at t12 and t13 may be required.[0129] As before, the control system runs open loop during all or part of the interval [t10, t11] and is corrected by a measurement at the end of a measurement interval Δτ11.[0130] The linear relationship represented by line 503 implies that a minimum Δτ can be determined by a measurement accuracy. For example, the outlet lines 131-134 may be controlled such that ΔM1 ≈ ΔMS. Assuming that the load cell 151 is accurate to 3 digits, then a mass difference to 2 digits of solids in the bottom pan 122 is measurable and accurate. In the reference cylinder of a previous example, we may assume a separator mass MS = 1000 kg. Then, the mass of solids in the bottom pan might be (5.0 ± 5.0) kg, which complies with a measurement accuracy ±1.0 kg and a total mass difference 10 kg = 10-2MS.Controlling the liquid outlets[0131] Fig. 6 illustrates the method of EPA (1970) in which oil is collected in a pocket, then expelled in a subsequent period. It is possible to keep V2 constant and allow V3 to increase and decrease as shown in Fig.6. However, such an embodiment would require a rather large gas volume on top of the oil phase to accommodate the variable V3, and therefore a relatively large separator tank 110. Instead, preferred embodiments keep V2 + V3 constant. In other words, the liquid level and hence the gas volume V4 are constants to within the uncertainties of measurements and estimates. A constant liquid level has the additional benefit that the quiescer 124 may be located permanently close to the liquid surface and thereby aid coalescence to the oil phase in region 3 and gas phase in region 4. As mentioned, a perforated plate or horizontal grid in the quiescer 124 is known to enhance this coalescence.[0132] In the preferred separator system, the oil outlet line 133 is closed from t30 to t31 and open from t31 to t32. Opposite, the water outlet line 132 is open from t30 to t31 and closed from t31 to t32. Vmin is a minimum volume, e.g. the volume of oil required to keep the oil phase sensor 135 immersed in oil phase plus a safety margin.[0133] From t30, the control system 200 runs open loop with an estimated v2 and computes a sum ΔV from v2 and the number of intervals of duration Δt. A volume flow meter on line 132 provide a measurement of v2 at any time Δτ in the interval t30 to t31. This enables the controller 210 to improve an estimate of the slope ΔV/Δτ, and a 'best measurement' (Vmax – Vmin)/Δτ30 is achieved when Δτ30 and (Vmax – Vmin) are as large as possible. Specifically, the liquid volume V2 + V3 is preferably kept constant such that the volume (Vmax – Vmin) of water phase exits through the water outlet line 132 during the interval t30 to t31. This volume is predetermined, e.g. 100 litres as in a previous example.[0134] Similarly, a mass flow meter on line 132 provides a sum ΔM2 of increments m2. By definition, ρ2 = ΔM2/ΔV2, so estimates of ρ2 can be obtained at any Δτ in the interval. Again, the best estimate of ρ2 is obtained when ΔV = (Vmax – Vmin) and Δτ = (t31 – t30) = Δτ30. A sample of water phase taken in this interval provides a measurement of Co. [0135] Similar to the previous example with the slurry outlet 131, we may close the oil outlet 133 and maintain flows such that ΔMS ≈ ΔM2 + ΔM3 in the interval t30 to t31. Then, (19) enables a redundant estimate for (ρ3 – ρ2). If ( p3 - p 2) =(900 -1025) = -125 kg/m 3 , then a 100 l increase in V3 and a constant V2 + V3 corresponds to 12.5 kg decreased separator mass.[0136] Initial estimates of Co and ρ2 may be obtained during an initial interval (τ30 ± ½Δτ30)0 shortly after a first measurement of the solid mass flow ms. At the beginning of this interval, we have only initial values, e.g. ρ2 = 1025 kg/m3 and ρ3 = 900 kg/m3 as in previous examples. At the end of this interval, we have a better estimate for ρ2 and may improve our estimate of ρ3 by measuring MS and applying (19) as described in the previous paragraph.[0137] During a subsequent interval t31 to t32, the water outlet 132 is closed and oil phase exits through line 133. This is a measurement interval (τ31 ± ½Δτ31) providing measurements of ρ3 and Cw. The form (τ ± ½Δτ) is useful and customary on a larger time scale than the scale illustrated in Figs. 5 and 6. Specifically, it can be shown2 that the errors in an average measurement f(τ) = ½[f(τ–½Δτ) f(τ+½Δτ)] at the central value τ is bounded by (½Δτ)2, which becomes very small when Δτ becomes small on the larger time scale.[0138] For example, as the separator system 100 approaches the steady state, we expect Co, ρ2 and the interval Δτ30 = (t31 – t30) needed to increase V3 by (Vmax – Vmin), e.g.100 litres, to approach constant values such that the slope of line 601 becomes constant. Thus, an insignificant change of Co, ρ2, Δτ30 or slope ΔV/Δτ from measurement interval (τ30 ± ½Δτ30)n to the next interval (τ30 ± ½Δτ30)n+1 indicates that the separator system is in the steady state. In the steady state, the time between subsequent central values τn, τn+1 may be increased as illustrated by line 305 in Fig.3.[0139] Similarly, an insignificant change of ρ3, Δτ31 or a slope ΔV/Δτ of line 602 from one measurement period (τ31 ± Δτ31)n to the next (τ31 ± Δτ31)n+1 indicates that the separator system 100 is in the steady state. In the steady state, measurements of Cw or ρ3 at τi may alternate with measurements of ρ2 such that τi is somewhere between τn and τn+1.[0140] Equations (17) and (18) provide expressions for mw and mo, and thereby enable an estimate of min = ms + mw + mo + mg. Dividing all flows by min, we obtain the sum of mass fractions (fs + fw + fo + fg) = 1 briefly described above. 2 e.g. by adding the Taylor series of f(τ–½Δτ) and f(τ+½Δτ), which have terms f''(τ)(½Δτ) with opposite signs, to yield f(τ) = ½[f(τ–½Δτ) f(τ+½Δτ)] – ½f''(τ) (½Δτ)2 – O(Δτ4), then applying Taylor's remainder theorem Alternative embodiments[0141] Fig. 7 is a side view and Fig.8 is a top view of an essentially rectangular separator tank 110 with two frusta 120 side by side. In general, Figs. 7 and 8 illustrate embodiments with two or more similar frusta 120 in one tank 110. We expect similar frusta 120 receiving the same inlet fluid at the same mass flow min to achieve similar separation efficiencies, e.g. measured by Co. Two or more frusta sharing one separator tank 110 and one control system 200 reduces the need for sheet metal, sensors, regulators etc., and thereby manufacturing costs and costs of operation. In particular, the frusta 120 share the inlet and outlet lines 113, 119; 132-134 with associated sensors and regulators. Slurry outlets from each frustum 120 may be controlled individually or by a shared regulator 231c on a main slurry outlet 131 to expel solids collected in each bottom pan 122.[0142] In Fig.7, a symbolic input of p4, V4 and T4 to the controller 210 replaces the sensors 141-143 in Fig.1. Symbolic references to 213a-c, 219a-c and 231a-c represent sensor suites, communication lines and regulators relating to the inlet lines 113, 119 and the slurry outlet line 131, respectively. Other labels and reference numerals are the same as in Fig.1 and need no further explanation. Figs 7 and 8 do not show all components in Fig.1, for instance the gimbaled stand 150 and the load cell 151. However, embodiments illustrated by Figs.7 and 8 may include any component and feature shown in Figs. 1 and 2 and/or described above.[0143] Fig. 7 shows an optional weir 126 extending across the separator tank 110 to dampen oscillations in the liquid surface similar to the quiescer 124 and baffles 125. The weir 126 is unneeded in a cylindrical tank, and is omitted from Fig.8 for clarity of illustration.[0144] The liquid volume (V2 + V3) around and above the frusta in Fig.7 is greater than the sum of the liquid volumes (V2 + V3) of two comparable cylindrical separator tanks, e.g. with equal frusta volumes. The larger liquid volume permits a larger volume flow in the Stoke's regime than the combined volume flows of two cylindrical tanks 110 as in Fig.1. Further, the rectangular shape fills a space along a wall more efficiently than two cylinders side by side. Similarly, a separator tank with a triangular base and three frusta 120 might fit into a corner.[0145] WO2017164747 assigned to the present assignee describes a separator tank with two frusta coupled in series such that water phase from the first frustum is fed to the second frustum. In contrast, the frusta 120 in Fig.7 are coupled in parallel such that both receive the same inlet fluid through line 119. WO2017164747 also describes a tank with two concentric frusta mounted one above the other. The present invention include versions with two or more frusta with a common rotational axis and receiving the same inlet fluid. [0146] In general, designs with two or more frusta 120 mounted in one separator tank 110 may provide space efficient alternatives to several cylindrical tanks with one frustum each. However, separator efficiency depends on separator design, in particular the design of each frustum 120, the quiescer 124, baffles 125, weirs 126 and the number of frusta per tank 110. As an example of the latter, diameters and lengths of internal pipes affect the mass flows to individual frusta, and thereby the overall response to a shift in mass inflow through a shared inlet line 119. If the individual mass flows vary widely, then at least the time needed to find a minimum according to Fig.4 and the associated description increases.[0147] Fig. 8 is a schematic top view of the separator tank 110 in Fig.7. The shared influent supply line 111, inlet assembly 118 and inlet line 119 are similar to those described above. A gas supply line corresponding to line 113 in Fig.1 is understood. Internal piping to inlets 123 and to the second frustum are omitted from Fig.8 for clarity of illustration. A cut-through quiescer 124 illustrates the perforated plate of preferred embodiments. The weir 126 shown in Fig. 7 is omitted from Fig.8 to present a clear view of an intake to line 132.[0148] The slurry outlet line 131 is located above the bottom of separator tank 110 and connected to slurry outlets in the bottom pan of each frustum 120. The water outlet 132 is located below the bottom of separator tank 110 as illustrated by a dashed line. A circle without a separate reference numeral represents the intake to line 132, i.e. a water outlet through the bottom of separator tank 110.[0149] Likewise, the dashed circle representing the oil phase detector 135 marks the intake to oil outlet line 133. The location of the oil phase detector 135 at the rotational axis of the rightmost frustum 120 indicates that the rightmost frustum is controlled as described, whereas the similar left frustum 120 is assumed to achieve maximum separation efficiency at the same total mass flow min and flow of added gas ma as the controlled frustum 120.[0150] While the present invention has been illustrated by examples, the scope of the present invention is set forth in the accompanying claims.
Claims
Claims1. A control system (200) for a separator (100), the separator (100) comprising:- a closed separator tank (110),- a frustum (120) within the separator tank (110), the frustum (120) having a lower wide end closed by a bottom pan (122) and an upper narrower open end,- an influent supply line (111) leading to a mixer (115),- a gas supply line (113) leading to the mixer (115),- an inlet line (119) leading from the mixer (115) to a tangentially oriented inlet (123) within the frustum (120) near the bottom pan (122),- a quiescer (124) near the upper end of the frustum (120),- a slurry outlet line (131) leading from the bottom pan (122),- a water outlet line (132) leading from a water region (2) below the bottom pan (122), - an oil outlet line (133) leading from an oil region (3) near the quiescer (124), and - a gas outlet line (134) leading from a gas region (4) above the quiescer (124); wherein the control system (200) is characterised by- equipment (213a, 219a, 234a; 141-143) for measuring a mass flow and a flow regulator (213c, 219c, 234c) for controlling a fluid flow through the gas supply line (113) and the inlet line (119),- a concentration sensor for measuring a first concentration C1 of either oil in a water phase Co within the water outlet line (132) or water in an oil phase Cw within the oil outlet line (133), and- a controller (210) with logic foro minimising the first concentration C1 as function of a mass flow (min) through the inlet line (119),o regulating a mass flow (ma) through the gas supply line (113) such that the mass flow (min) through the inlet line (119) remains constant during a measurement interval, ando minimising the first concentration C1 as function of the mass flow (ma) through the gas supply line (113) at a fixed mass flow (min) through the inlet line (119).2. The control system according to claim 1, further comprising equipment (141-143; 234ac) for maintaining a constant mass of gas (M4) in the gas region (4).3. The control system according to claim 1 or 2, further comprising- a volume flow meter (219a) on the inlet line (119) as well as- a volume flow meter (231a – 233a) and a flow regulator (231c – 233c) on each of the slurry outlet line (131), the water outlet line (132) and the oil outlet line (133).4. The control system according to claim 3, wherein the controller (210) comprises logic for collecting a volume of oil phase in the separator tank (110) during an oil collection interval, then expelling the volume of oil phase during a subsequent oil disposal interval while maintaining a constant liquid level in the separator tank (110).5. The control system according to claim 3 or 4, wherein the controller (210) comprises logic for collecting solids (11) in the bottom pan (122) during a solid collection interval, then expelling the solids (11) together with fluid from the bottom of the frustum (120) during a subsequent solid disposal interval.6. The control system according to any preceding claim, further comprising a mass flow meter (211a, 212a) and a flow regulator (211c, 212c) on the influent line (111) and/or on a water supply line (112) leading to the mixer (115).7. The control system according to any preceding claim, further comprising an oil phase detector (135) located to be immersed in oil phase during operation, the controller (210) being configured to reduce the mass flow (min) through the inlet line (119) whenever the oil phase detector (135) indicates a gas phase.8. The control system according to any preceding claim, further comprising a load cell (151) for measuring a total mass (MS) of the content within the separator tank (110) during operation.9. The control system according to any preceding claim, further comprising a mass flow meter (231c) on the slurry outlet line (131).10. The control system according to any preceding claim, further comprising a mass flow meter (232c, 233c) on each of the water outlet line (132) and the oil outlet line (133).11. The control system according to claim 3 and 8, further comprising logic for calculating densities (ρ2, ρ3) of fluids in the water outlet line (132) and the oil outlet line (133).12. The control system according to claim 10 or 11, further comprising:- a second concentration sensor (233a, 232a) for measuring a second concentration C2 beingo the water concentration Cw on the oil outlet line (133) if the first concentration C1 is the oil concentration Co on the water outlet line (132), ando the oil concentration Co on the water outlet line (132) if the first concentration C1 is the water concentration Cw on the oil outlet line (133).- logic for computing mass fractions of water (fw) and oil (fo) in a fluid entered through the inlet line (119).
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Patent classes

IPC classesB01D 17/00B01D 17/02B04C 11/00B03B 13/00E21B 43/00CPC classesB01D 17/00B01D 17/02B04C 11/00B03B 13/00E21B 43/00

Citations

WO 2017/164747 A1 (A1 )WO 8904725 A1 (A1)US 2014/175028 A1 (A1)

Applicants, owners and inventors

Applicants
STAUPER OFFSHORE AS
Rådhusgata 15 3211 SANDEFJORD NO (SANDEFJORD Municipality, Vestfold county)Org. number: 813164132Inventors
Eivind Singdahlsen
Høyåsveien 19 3216 SANDEFJORD NO (SANDEFJORD Municipality, Vestfold county)

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PROTECTOR IP AS
PROTECTOR IP AS

Org. number: 979409524

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Org. number: 979409524
Reference: P5886NO00-BGC

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