IT202300006051A1 - DEVICE FOR CONTINUOUS OIL SEPARATION IN AN ORGANIC RANKINE CYCLE PLANT - Google Patents

Description

DEVICE FOR CONTINUOUS OIL SEPARATION

IN AN ORGANIC RANKINE CYCLE PLANT

DESCRIPTION

Technical sector of the invention

The present invention relates to a device for the continuous separation of oil in an organic Rankine cycle plant.

Known technique

As is known, a thermodynamic cycle is defined as a finite succession of thermodynamic transformations (for example isothermal, isochore, isobaric or adiabatic) at the end of which the system returns to its initial state.

This cycle can be direct, for example a direct Rankine cycle, in which a thermal source is used to produce mechanical/electrical energy and heat at a lower temperature than that of the thermal source; the thermodynamic cycle can also be inverse, in which the electrical/mechanical energy is used to transfer heat from a source at a lower temperature to one at a higher temperature.

In the case of direct cycles, an ideal Rankine cycle is a thermodynamic cycle composed of two adiabatic and two isobaric transformations. In the case of a direct cycle, its purpose is to transform heat into work. This cycle is generally adopted especially in thermoelectric power plants for the production of electrical energy and uses water as the driving fluid, both in liquid form and in the form of steam, with the so-called steam turbine.

More specifically, organic Rankine cycles (ORC) have been hypothesized and implemented that use high molecular mass organic fluids for the most diverse applications, in particular for the exploitation of low-medium enthalpy thermal sources. As in other steam cycles, the system 100 for an ORC cycle (with reference to figure 1) comprises one or more pumps 110 for feeding the organic working fluid, at least one heat exchanger 120 (also called preheater or evaporator, depending on the function performed) for carrying out the preheating, vaporization and possible superheating or heating phases in supercritical conditions of the same working fluid, a turbine 130 for the expansion of the fluid, mechanically connected to an electric generator 140, a condenser 150 that returns the organic working fluid to the liquid state.

? It is also known that in ORC cycles the use of high molecular mass organic fluids very often involves the need to introduce downstream of the turbine and upstream of the condenser an additional heat exchanger called a ?regenerator? 160 or ?recuperator? which recovers a good part of the sensible heat of the low pressure organic fluid vapor, heat which is used to preheat the organic working fluid in the liquid phase downstream of the cycle feed pump and upstream of the preheater.

The cycle therefore includes rotating parts for the conversion of the enthalpy of the working fluid into mechanical work, i.e. the turbine, and generally also for the transport of the liquid fluid from the condenser to the heat exchange circuit with the hot source, i.e. the pump.

The ORC system operates at pressures other than ambient pressure: to prevent contamination of the fluid in the environment, and vice versa, it is advisable to use rotating mechanical seals with the use of oil as a barrier between the internal and external environment of the component.

The contact area between the two components in relative motion requires constant lubrication and pressure to ensure perfect operation and durability over time since, in fact, it limits wear due to direct contact between the two parts. In these systems there is a control unit that supplies the sealing parts with pressurized oil. This control unit must have a significant oil storage volume, typically 100-300 l/MW of electrical power, to comply with the lubrication of the sealing means and bearings of the turbine(s).

Rotary seals are subject to physiological oil losses and, in some cases, even more serious losses may occur due to mechanical failure of the seal, following technical problems, for example, thermal or mechanical overloads. Physiological losses in an ORC plant of several MW can range from a few deciliters per day in plants with excellent sealing, up to 1 liter per day, but there may be plants with particularly worn seals that introduce into the plant even a few liters per day of lubrication oil.

To limit and minimize the entry of oil into the system, there are known systems for the physical separation of the oil, such as rotating rings, also called "oil throwers", which, inserted inside the seal, launch the oil towards the side wall of the seal itself, facilitating its collection.

Another system could be to provide labyrinths between the rotating part and the fixed part of the seal, so as to channel the flow into a tortuous path in which the oil particles are forced to hit the walls of the labyrinth itself, changing direction many times, favoring the accumulation of oil on the external walls of the labyrinth, for easier recovery.

In any case, these systems cannot guarantee complete separation of the two flows.

In cases where the oil is at a higher pressure than the operating process pressure, the oil flows inside the ORC system and mixes with the working fluid, thus altering its characteristics; once inside the system, due to the drag caused by the motion of the working fluid, the lubricating oil could be found in any duct, typically in a nebulized form along the walls of the pipes where the velocity of the working fluid is high, or in a liquid phase mixed with the working fluid itself where the velocities are low (for example, in correspondence with sudden increases in section in manifolds or vessels).

The consequences of this mixing are essentially:

- the variation of the thermodynamic characteristics of the organic working fluid, which mixes to a minimal extent with oil and varies its thermodynamic quantities, including its condensation pressure, as well as, and significantly, the points of the thermodynamic cycle;

- the variation of the composition of the organic working fluid and the increase in the overall flow rate (sum of the ORC fluid and the oil): in the system, this will have consequences on the heat exchange coefficients and on the cycle pressure to the detriment of the efficiency. Furthermore, in the turbine, a fluid will be processed that does not exchange momentum with the turbine itself and does not generate power like the organic working fluid;

- in less favorable situations, the oil can also concentrate in areas that are not easily drainable, escaping the action of an in-line separator, or, especially in systems operating at high temperatures, it could undergo cracking, forming compounds that are deposited on the exchange surfaces, increasing their thermal resistance, or on the turbine blades. This phenomenon is known as fouling, or the deposit of cracked oil films. In particular, in relatively small expanders this phenomenon can reduce the working fluid passage area by up to 20%;

- furthermore, the presence of a solid deposit, even if of minimal thickness, on the turbine blades, inevitably alters their geometry, which gradually presents a shape increasingly different from the design one, designed to maximize the exchange of momentum with the working fluid vapour.

These deposits, created by oil that escapes the action of an in-line separator, are difficult to remove without invasive interventions that lead to a plant shutdown.

It has been verified in the field that the efficiency performance of an ORC system drops by up to 10% compared to the maximum efficiency, with a significant percentage (up to approximately 10%) of oil circulating in the system compared to the flow rate of the working fluid.

There is therefore a first requirement which is to separate and remove the oil from the working fluid inside the system, in order to guarantee the optimal performance of the system over time.

The undersigned's European patent no. EP3055518 presents a solution to the problem described here, describing an "in-line" device, i.e. an oil separator 170 (as shown in figure 1) that operates by separating the oil by taking and separating a flow rate at a point in the system - which may be along the bypass line 180 between a portion of the evaporator 120 and the condenser 150 (or, as in the case illustrated in figure 1, the regenerator 160) - without necessarily having to stop the system.

It is clear, however, that it is not sufficient to simply provide solutions that implement the separation of the oil once it has entered the system to resolve the problems related to its coexistence with the working fluid, but it is also essential to evaluate the position in which to insert the separator itself.

In the current state of the art, the oil is admitted into the system by agreeing to separate it with "in-line" solutions in points with a pressure significantly higher than the condensation pressure (for example, on a special by-pass line by taking the mixture at the vaporization pressure reached downstream of evaporator 120), to then rejoin the system in areas with a lower pressure, thus allowing the flow. Normally the oil separator is inserted between the evaporator and the regenerator or condenser: the flow rate needed to separate the oil, therefore, bypasses the turbine and rejoins the condenser.

Although the system correctly separates the lubricating oil from the working fluid, solving the problem described so far, this "in-line" separation has two disadvantages:

- the tapped flow does not contribute to the production of mechanical power in the turbine;

- the oil is allowed to flow into all the components of the system since it is not separated at its entrance, i.e. in the area highlighted with the reference 190 near the bearings and seals of the turbine 130, or the shaft of the turbine itself.

In this way, while separating the oil, we end up with portions of the system in which the same oil circulates together with the working fluid before being separated. These are noteworthy portions of the system: working fluid in vapor phase and oil from turbine 130 to regenerator 160 and then to condenser 150; then working fluid in liquid phase and oil from condenser 150 to pump 110, then again to regenerator 160 and evaporator 120. Only at the evaporator outlet can this mixture reach the oil separator 170 via bypass line 180. Therefore, the oil will pass through, in particular, all the heat exchangers with consequent thermal and pressure stress. Regardless of where the mixture is taken from, the separation system still acts on a small by-pass flow rate and therefore does not exclude that the lubricating oil remains in circulation for a long time.

In systems operating at particularly high temperatures, the oil can deteriorate, forming solid compounds which, depositing on the walls of the exchanger, affect its performance.

Therefore, there is a further requirement, that of separating and removing the oil from the working fluid inside the system at the very moment in which the oil itself is introduced into circulation (reference 190, near turbine 130), in order to guarantee the correct functioning and a high overall efficiency of the ORC system over time.

This further technical problem cannot be solved with "in-line" devices that process a small by-pass flow according to known techniques: in fact, the point of entry of the lubricating oil into the system is inevitably in correspondence with the turbine seal, connected to the shaft, in the low pressure stage area. This is an unfavourable area for the insertion of an oil separation device since this area is at a pressure very close to the condensation pressure: the insertion of an oil separator in this area, which would at the same time be the most suitable for the separation of the oil itself because it would prevent it from flowing into the other components of the system, is complicated by the lack of driving force (i.e. a sufficient pressure drop) to generate the flow of the flow to be processed, since the pressure in this area, already at 100 bar, is too high. very close to that of condensation, would be further lowered by the presence of the oil separator 170 (which introduces a pressure drop, albeit minimal) and above all by the presence of the piping and valves for the conduction and regulation of this bleed in the oil separator 170 and from there to the system.

Summary of the invention

The solution to all the technical problems mentioned in the previous paragraph is obtained, according to the present invention, with a device for the continuous separation of oil in an organic Rankine cycle plant, comprising an oil separator and an ejector, downstream of the expansion turbine.

This solution allows the flow of working fluid and oil to be separated to have the driving force to rejoin the turbine exhaust, allowing continuous separation of the oil near its entrance into the system, preventing the problems that this entrance would otherwise cause.

According to one aspect of the present invention, therefore, there is described a device for the continuous separation of oil in an organic Rankine cycle plant having the features set forth in the independent product claim annexed to the present specification.

Further preferred and/or particularly advantageous ways of implementing the aforementioned system are described according to the characteristics set out in the attached dependent claims.

Brief description of the drawings

The invention will now be described with reference to the attached drawings, which illustrate some non-limiting examples of implementation of a regenerator for ORC plants, wherein:

- Figure 1 schematically illustrates an ORC plant with oil separator, according to the known technique,

- Figure 2 schematically illustrates a device for the continuous separation of oil in an organic Rankine cycle plant, according to a first and preferred embodiment of the present invention,

- Figure 3 illustrates a diagram of an ejector of the device of Figure 2, and

- Figure 4 illustrates a schematic of the oil separator of the device of Figure 2.

Detailed description

With reference to figure 2, a known turbine 1 of an ORC plant, also known and for which reference will continue to be made to figure 1, is schematized together with an admission line 9 of the saturated or superheated vapour of the organic working fluid, coming from a heat exchanger (known and therefore not shown in figure 2, but nevertheless illustrated in figure 1) and an exhaust line 10 of the expanded vapour of the working fluid which leads it to the regenerator or, directly, to the condenser (both known and therefore not shown in figure 2, but nevertheless illustrated in figure 1).

According to the present invention, the device 20 for continuous oil separation in an organic Rankine cycle plant comprises:

- a first line 2, exiting from the turbine shaft area, i.e. downstream of a seal 8 of turbine 1 and in fluid communication with it. In the first line 2, all the oil flow entering the system flows through the seal 8, positioned on the low pressure of turbine 1. Therefore, in this first line 2 there is a mixture of working fluid vapour and lubricating oil. By means of an ejector 5, as will be seen, a depression zone is created with respect to the turbine exhaust pressure, which recalls this flow precisely through the first line 2 and conducts it into an oil separator 3;

- a second line 6 for tapping a flow of vapour of the working fluid, the flow rate of which has a value between 0.1% and 0.2% of the total flow rate at the turbine inlet (i.e. by means of the admission line 9), which acts as the ?driving force? of the ejector 5 to create a low pressure zone at the outlet of one of its convergent-divergent nozzles. The sampling point of this flow 6 is the result of an optimization between the lowest available pressure level sufficient for the operation of the ejector, and the maximum tapping flow rate (which influences the diameter of this duct). In theory, the smaller the pressure drop operated by the ejector, the greater its efficiency; however, the smaller the drop, the greater the flow rate required for the same useful effect, therefore the greater the sizing of the ejector system;

- an oil separator 3, preferably of coalescing type, in fluid communication with the first line 2. In the separator (with reference also to figure 4) the flow of working fluid, contaminated by oil, enters by means of a nozzle 41. A first separation of the oil occurs due to the sudden drop in flow speed caused by the large difference in section between the inlet nozzle 41 and a calm zone 40 of the separator inlet; a second separation occurs due to the change in direction of the flow, and to a minimal extent a separation occurs due to the collision of the same with the walls 42 near the inlet. The separated oil is collected in the lower part of the separator, while the working fluid in the gaseous state continues through the duct 43 which puts the inlet 41 in communication with the outlet 45 by means of a coalescing filter 44. The oil drops collected by the filter membrane descend by gravity, accumulate in the lower part of the filter and from there pass into a drainage line 11, by means of which, optionally, they can return to the lubrication circuit of the turbine 1. The purified flow, instead, is expelled from the outlet nozzle 45. Advantageously, the oil separator 3 is heated with a thermal blanket or other heating means to avoid condensation of the working fluid, which must remain in a gaseous state;

- downstream of the oil separator 3, a discharge line 4 leads to the ejector 5, to which two flows of vapour of the working fluid are connected: a first low pressure flow, the one exiting the separator which runs along the discharge line 4 and a second flow at higher pressure which derives from the bleed of superheated and pressurised working fluid from an intermediate stage of the turbine 1 and which flows along the second bleed line 6; line 4 is conveniently equipped with a non-return valve 46, to prevent the flow from returning to the separator 3 if transitory conditions occur which allow it;

- ejector 5 which, as is known, is a static compressor that exploits the Venturi effect produced by a convergent-divergent nozzle to convert the energy given by a high-pressure driving fluid into an increase in the speed of the same, so that it can suck in a second fluid, mixing it with the driving fluid, and compress it so that at the exit from the ejector this second fluid will have a higher pressure than the suction pressure. For convenience, the innovative features of ejector 5 are described below;

- finally, a third line 7 downstream of the ejector 5 brings the overall steam flow, the sum of the flows coming from the turbine bleed and from the oil separator 3, back to line 10 which carries the working fluid steam from the turbine to the regenerator or condenser.

As an example, the pressures in the different lines under design conditions could be:

- line 6: 10 bars

- turbine outlet, start of line 2: 1 bar

- exhaust line 4, ejector inlet: 0.7 bar

- start line 7: 1.3 bar

The head required for the flow of exhaust line 4 to return to the turbine exhaust is generated by ejector 5.

In an ideal system, all the oil flow entering the seal is conveyed from the first line 2, exiting the seal 8 of the turbine 1, to the oil separator 3. However, in some system configurations, it is not certain that all the oil from the seals contaminating the working fluid will be intercepted by the system according to the present invention. In this case, it will be acceptable to admit a small percentage of lubrication oil into the system, then separating it with an appropriate in-line separator, for example with the system described in the European patent of the undersigned cited above. Even in this case, however, the advantages described here would not be lost, since the oil flow entering the system would still be minimal compared to a scenario with equal boundary conditions but without an ejector.

The low pressure flow (for example, 0.7 bar), after passing through the separator 3, enters the ejector 5 together with the flow coming from the bleed of a high pressure zone of the turbine 1 (for example 10 bar).

The flow of the third line 7, which is at a higher temperature than the condenser 150, can conveniently be used to heat the separator 3, maintaining it at a temperature sufficient to avoid condensation of the working fluid inside it.

With reference to figure 3, the ejector comprises:

- a first section 4e for the inlet of the working fluid flow at a lower pressure, for example 0.7 bar. In practice, the first section 4e constitutes the terminal portion of the discharge line 4 which conveys the first flow of vapour of the working fluid from the outlet of the oil separator 3 to the ejector 5;

- a second section 6e for the inlet of the working fluid flow at a higher pressure, for example 10 bar. This second section 6e constitutes the terminal portion of the second line 6, the one that conveys the second flow of vapour of the working fluid, tapped at an intermediate stage of the turbine 1;

- a nozzle 31 which is only affected by the second flow of the second inlet section 6e. This flow, in the nozzle 31, converts its pressure energy into kinetic energy, creating a region with high-speed motion downstream of it, with a Mach number greater than 1 and a turbulent regime, which promotes mixing. The nozzle 31 can assume the convergent - divergent shape if one wishes to operate with pressure jumps greater than the critical one: in the example case in fact the pressure in the second section 6e is much higher than the pressure in the first section 4e, at least ten times higher. For the correct functioning of the ejector 5, downstream of the nozzle, where the flows of the first section 4e and the second section 6e rejoin, an area with a lower pressure than the pressure in the first section 4e must be created, therefore the pressure jump that the nozzle 31 must operate is much greater than the critical one. The nozzle 31 in the convergent-divergent shape allows the critical pressure ratio to be exceeded and for the fluid, downstream of the nozzle, to reach a lower pressure than that of the first section 4e;

- a third section 32, substantially cylindrical and almost isobaric except for the pressure drops, having the function of mixing and exchanging the quantity of motion between the flows coming from the first section 4e and the second section 6e which, substantially, form a third flow of steam, the sum of the first and second flows of steam,

- a fourth section 33, divergent, which carries out the reconversion of kinetic energy into pressure energy, so that the third flow of vapour of the working fluid arrives at the outlet 7e of the ejector 5 and, therefore, at the third line 7, with a pressure greater than the condensation pressure. In this way the flow exiting from the ejector 5, by means of the third line 7, can rejoin the flow of expanded vapour of the exhaust line 10 from the turbine 1.

The device 20 for continuous oil separation, according to the present invention, has the following advantages:

- by separating it immediately downstream of the turbine, the oil is not affected by the thermal and pressure stress it undergoes in the heat exchangers; therefore, it does not deteriorate and does not form compounds harmful to the proper functioning of the ORC system;

- in practice, the oil enters almost entirely into the ORC system and, therefore, does not influence the thermodynamic cycle;

- the oil, not undergoing any thermal stress, once separated can be recovered by returning it directly to the lubrication circuit of the ORC system;

- the high pressure flow is not tapped upstream of the first stage of the turbine but in an intermediate region of the expansion and, therefore, after having already partially contributed to the generation of power in the turbine.

The device 20 according to the present invention can therefore conveniently be used in an organic Rankine cycle system, for example as illustrated in figure 1, comprising at least one feed pump, at least one heat exchanger, at least one expansion turbine and at least one condenser. Advantageously the ORC system can also be provided with a regenerator.

The device according to the present invention may also be conveniently used in points other than the one previously described (downstream of the turbine). For example, the ejector may also suck from other low pressure points of the system in which there may be a mixture of working fluid and lubricating oil: speed reducers communicating with the turbine via labyrinth seals, lubrication control units, etc.

In particular, in systems with turbines, the turbine seal may not be mechanical but labyrinth-type. Even in this variant, the solution of the device 20 for the continuous separation of oil comprising the ejector, as previously described, remains valid in all its elements. The only variation is the suction point, which will no longer be in correspondence with the turbine shaft but near the lubrication unit. In this case, it is the working fluid, which is normally at a higher pressure than the oil, that overcomes the labyrinth and contaminates the oil, contrary to what happens in the case previously described. The function of the separation and the ejector, although physically the same, in this application has the function of recovering the working fluid that would otherwise remain in the lubrication unit.

Similarly to the use of the ejector, a compressor could be used for the same purpose, with the same purpose. The ejector, however, unlike the compressor has the notable advantage of being a static component and not requiring any seal, adjustment, or mechanical power supply. Furthermore, for the working conditions considered, the compressor would have to be placed in positions that are difficult to access, making maintenance activities complicated. The advantages of the ejector therefore appear evident: it is smaller in size than the compressor, it is a static component, therefore more reliable, it is cheaper.

In addition to the modes of implementation of the invention, as described above, it is to be understood that numerous further variations exist. It is also to be understood that said modes of implementation are only exemplifying and do not limit the object of the invention, nor its applications, nor its possible configurations. On the contrary, although the description reported above makes it possible for a man of skill to implement the present invention at least according to an exemplifying configuration thereof, it is to be understood that numerous variations of the described components are conceivable, without thereby departing from the object of the invention, as defined in the attached claims.

Claims (10)

1. Device (20) for the continuous separation of oil suitable for an organic Rankine cycle plant, in which - the system is operated by an organic working fluid, and - the system comprises at least one turbine (1), equipped with low pressure sealing means (8) and a discharge line (10) of expanded vapour of the organic working fluid, the device (20) comprising: - a first line (2), downstream of the sealing means (8) of the turbine (1) and in fluid communication with it, in which there is a mixture of vapour of the organic working fluid and lubricating oil of the turbine (1), - an oil separator (3), downstream of the first line (2) and in fluid communication with it, which separates the lubricating oil from the vapour flow of the organic working fluid, and - a discharge line (4) downstream of the oil separator (3) and in fluid communication with it, in which there is a first flow of vapour of the organic working fluid, deprived of the lubricating oil, the device (20) being characterised by the fact that it is positioned downstream of the turbine (1), and from the fact of understanding: - a second tapping line (6), in which there is a second flow of vapour of the organic working fluid, tapped from an intermediate stage of the turbine (1) and at a higher pressure than the pressure of the first flow of vapour of the organic working fluid, - an ejector (5) downstream and in fluid communication with the discharge line (4) and with the second line (6), in which the first flow of vapour of the organic working fluid is the flow that the ejector (5) aspirates and the second flow of vapour of the organic working fluid is the driving flow of the ejector (5), - a third line (7), downstream and in fluid communication with the ejector (5), in which there is a third flow of vapour of the organic working fluid, the sum of the first and second vapour flows, and in which the third line (7) is also in fluid communication with the discharge line (10) of the expanded vapour in the turbine (1). 2. Device (20) according to claim 1, wherein the ejector (5) comprises: - a first section (4e) of inlet of the first flow of vapour of the organic working fluid, - a second section (6e) for the inlet of the second flow of vapour of the organic working fluid, - a nozzle (31) in fluid communication with the second section (6e), in which the second flow of vapor of the organic working fluid increases its kinetic energy, reducing its pressure energy, - a third section (32), substantially cylindrical and almost isobaric, configured so that the first flow of steam and the second flow of steam mix, generating the third flow of steam of the organic working fluid, the sum of the first and second flows of steam, - a fourth section (33), divergent, configured so that the third flow of vapour of the organic working fluid reduces its kinetic energy and increases its pressure energy until it reaches a pressure greater than the condensation pressure of the expanded vapour in the turbine (1). 3. Device (20) according to claim 1 or 2, wherein the oil separator (3) is of the coalescing type. 4. Device (20) according to claim 3, wherein the oil separator (3) comprises: - an inlet nozzle (41) in fluid communication with the first line (2), - a calm zone (40) at the entrance of the oil separator (3), in which an initial separation of the lubricating oil from the vapour of the organic working fluid occurs, - a duct (43) comprising a coalescence filter (44), on whose membrane the lubricating oil is collected, - an outlet (45) in fluid communication with the discharge line (4), - a drainage line (11) for the lubrication oil. 5. Device (20) according to claim 3, wherein the oil separator (3) comprises a thermal blanket that prevents condensation of the vapour of the organic working fluid. 6. Device (20) according to claim 3, wherein the third flow of organic working fluid vapor of the third line (7), is used to heat the separator (3), preventing condensation of the organic working fluid vapor. 7. Device (20) according to one of the preceding claims, wherein the discharge line (4) is provided with a non-return valve (46). 8. Device (20) according to one of the preceding claims, wherein the second flow of vapour of the organic working fluid, drawn from an intermediate stage of the turbine (1) has a weighted flow rate varying between 0.1% and 0.2% of the total weighted flow rate of vapour entering the turbine (1). 9. Organic Rankine cycle system comprising at least one feed pump, at least one heat exchanger, at least one expansion turbine and at least one condenser, characterised in that it is equipped with a device (20) for the continuous separation of oil, according to one of the preceding claims. 10. A system according to the preceding claim, further comprising a regenerator.