EP2422055B1

EP2422055B1 - Detection, measuring and separation system by stripping light fractions in diathermic oil

Info

Publication number: EP2422055B1
Application number: EP10727965.5A
Authority: EP
Inventors: Mario Gaja; Roberto Bini
Original assignee: Turboden SpA
Current assignee: Turboden SpA
Priority date: 2009-04-22
Filing date: 2010-04-21
Publication date: 2013-04-17
Anticipated expiration: 2030-04-21
Also published as: ITBS20090074A1; WO2010122591A1; IT1393625B1; EP2422055A1

Description

Field of the Invention

This invention concerns in general the processes or systems using diathermic oil as a heat vector and heat exchange with another thermovector fluid, and refers in particular to a detection, measuring and separation of light fractions in diathermic oil.

State of the Art

In the field of ORC (Organic Rankine Cycle) turbo generators, as a heat source, a flow of diathermic oil at a high temperature, heated for example in a boiler fed by biomass or in a heat exchanger provided to retrieve the heat from an industrial process, is frequently used. The diathermic oils which, furthermore, are widely used in the industry as a heat vector, are characterized in that they have a rather moderate vapour pressure at the usage temperature, often lower than atmospheric pressure. This is often specifically their characteristic that makes them preferable to other thermovector fluids, such as for example water steam or pressurized water.
In the real operation of the systems an increase in the vapour pressure of the diathermic fluid set at a given temperature may however take place, due to the fact that .in the diathermic oil, besides the components corresponding to its initial formulation, other components may also be present. In particular these are products of the thermal degradation of the oil itself, or substances entered accidentally in the oil circuit, such "as water ororganic compounds more volatile than the oil, that are used as a working fluid in the circuit where the ORC process is carried out and whit which the diathermic oil exchanges heat, or incondensable gas.
The change in the vapour pressure, plus the contribution of possible incondensable gasses, can be very damaging to the system, in that they can be the cause, for example, of cavitation phenomena in the circulation pumps of the diathermic oil, which other than damaging them can even block the circulation of the fluid provoking serious consequences.
In particular the document EP1555396 concerns a system for producing electric or mechanical energy, starting from the fumes or gasses from combustion of a biomass or from some other origin that includes a turbogenerator group operating according to the Rankine cycle and an intermediate heat exchange between the high temperature fumes or gasses and an working fluid evaporation heat exchanger feeding the turbogenerator group.
Flowing in the intermediate heat exchange circuit is a aeriform fluid that on the one hand, is heated by the high temperature gasses or fumes and on the other, releases heat to the working feed fluid of the turbogenerator group. The aeriform fluid can be an inert gas or air with a low oxygen content.
The document DE2740888 describes a method for condensation of vapors, characterized in that is used as a condensing heat exchanger unit is a plate heat, which consists of a frame, are mounted in the space between a front and a pressure plate at small internal plates.

Objective of the invention

This invention has been conceived to answer the dual requirement: i) to detect and identify the presence of volatile fractions, in particular the working fluid of the power cycle in the diathermic oil and ii) to progressively remove from the oil the volatile fractions, so as to avoid the above mentioned problems of the prior art and support the advantage of being able to measure the separated fractions.
Correspondingly, the invention proposes a detection, measuring and/or separation system of the light fraction, that is volatile, in diathermic oil circulating in an ORC unit that is based on a stripping process carried out in an optimal position of the diathermic oil circuit within the ambit of said ORC unit.
The system basically comprises a stripping receptacle positioned downstream of the thermal groups of a ORC system and designed to receive a flow of diathermic oil, means for a supply of a flow of diathermic oil from the return conduit of the thermal diathermic oil circuit to an inlet of the stripping receptacle, means for connecting an outlet of said stripping receptacle to the return conduit of said circuit, a feeder of an inert stripping gas to said receptacle to cause a separation of light fractions from the oil in the receptacle and their exit from the latter, and a drainage line connected to the top of said receptacle and configured to cool and collect the light fractions exiting from said receptacle and to monitor the presence of specific pollutants contained in it.
As a means for collecting a flow of fluid from the return conduit of the diathermic oil circuit, a pump or a dynamic fluid intake can be adopted.
The flow of oil going to the receptacle will preferably be heated in that with a higher temperature increases the partial pressure of the volatile components and thus increases the separation/removal of light fractions.
The exit of the separated fractions from the stripping receptacle towards the drainage line can be controlled by a regulating valve controlled by a level measurer of the fluid in the receptacle.

Brief Description of the Drawings

Further details of the invention will be evident from the continuation of the description made in reference to the enclosed purely indicative drawings, in which:

Fig. 1 shows a general diagram of the system of the invention;
and
Figs. 2 and 3 show two variations in the form of derivation of a flow of fluid from the thermal oil circuit.

Detailed Description of the Invention

In said drawings is represented a general diagram of a ORC unit generically indicated by the number 10, fed with diathermic oil through a circuit 11 in which the arrows F, F1 indicate the flow directions along feed and return conduits 12, 13, respectively. In a part upstream of its circuit 11, the diathermic oil is heated to a high temperature in a boiler provided for the combustion of a biomass, or by a group of solar collectors, or by a heat recuperator of an industrial process, not shown, or by other sources.
Consequently, by means of the delivery conduit 12, the oil is delivered to an ORC unit 10. Here the hot oil causes evaporation of an organic working fluid in an evaporator 14 and, in succession, a preheating of the liquid working fluid in a preheater 15 to then exit from the ORC unit through the return conduit 13.
The ORC unit 10 comprises, however, additional components, among which at least: a turbine 16 in which the expansion of the vapour of the organic working fluid takes place; an electric generator 17, preferably operated directly by the turbine, or by a reduction unit 18, suitable for transforming the mechanical power of the turbine into electric power; a condenser 19 in which the vapour of the working fluid returns to the liquid state releasing heat to a current of cooling water 19'; a feed pump 20 that brings the pressure of the working fluid to a pressure compatible with that of the evaporation. Often, between the turbine and the condenser there is also made available a regenerator 21 that provides the liquid to be evaporated with the sensitive heat of the expanded vapour coming from the output of said turbine.
The ORC 10 unit is typically like an integrated and compact system, in which the various components are assembled on board one or more opportune platforms ("skids") complete with connecting tubes and wirings.
In the delivery conduit 12, which is for feeding the oil, a three-way valve 22 is inserted in the known way, carrying out the function of distributing. Otherwise, with the same result, a mixing valve 23 is inserted on the return conduit 13, so as to achieve an adjustment of the capacity of hot oil delivered to the heat exchange groups 14, 15 of the ORC unit 10.
Now, according to an embodiment of the invention, in the circuit 11 of the diathermic oil a recycling pump 24 is also inserted with the dual function
of maintaining a recycling capacity of the oil, once cooled, by means of the abovementioned heat exchange groups 14, 15, and
of reducing and standardizing the temperature of the oil fed to the groups, in particular in the phases in which the delivery of oil is firmly divided by the three-way valve on valves 22 or 23.
Preferably, the pump 24 will be the variable speed type, to intensify the capacity of fluid recycled in the starting phases with a low capacity through the three- way valves 22 or 23.
More precisely, the recycling pump 24 is connected, in the inlet, to the return conduit 13 of the diathermic oil circuit and therefore is upstream of the ORC 10 unit. It is operated to extract a flow of diathermic oil from said return conduit 13 and to convey it, in output, through a stripping group with an inert gas. This group comprises:
a stripping receptacle 25, fed by oil with continuity from said pump 24 through a check valve 26 and a register valve 27,
an inert gas feeder 28, such as nitrogen, argon, etc. coming from a relative source 29 in the form of a cylinder or another source,
a drainage conduit 30 positioned at the top of said receptacle and provided with a modulation valve and adjustment of the delivery valve 31, with a cooler/condenser 32, with a receiver tank 33, with detectors 34 and/or 35 of the presence of specific contaminants (in particular, detectors of the presence of compounds attributable to an input of the working fluid in the diathermic oil) from the return conduit 13.
The tank 33 will preferably be provided with at least a transparent window 36 to see the content and the level of liquid inside. Furthermore, said tank can comprise one or more level measurers 37 so as to monitor the quantity of liquid gathered and possibly to differentiate non mixable liquids of different densities, such as water and oil, as well as a rotative bottom outlet 33'.
On the fluid line from the recycling pump 24 to the stripping receptacle 25 an eventual heater 38 can be provided to heat the oil flowing to said receptacle and to contribute in this way to improving the stripping process. This heater 38 can thus advantageously use the hot oil taken directly from the delivery conduit 12 through a line 39 as a heat exchange vehicle and then sent again to the return conduit 13 of the circuit 11. In addition, the pump output 24, better downstream of the relative check valve 26, is preferably connected to the delivery conduit 12 of the diathermic oil circuit 11 by means of a line 40 including an adjustable valve 41.
On the other hand, the stripping receptacle 25 has an outlet that connects up to the return conduit 13 of the circuit 11 by means of a line 42 including a check valve 43. It will also be provided with a bottom outlet with a relative valve 44.
In a construction variation, that furthermore enables the elimination of the re cycling pump 24, the intake of a flow of diathermic oil from the return conduit 13 of the relative circuit 11 for the feed of the stripping receptacle 25, always carried out downstream of the ORC unit 10, can be the dynamic type, carried out (Fig. 2) by means of a drawing tube 45 and a return tube 46: the first connected to said conduit 13 upstream of a neck, fixed for example by means of a nozzle 47, along the same and directed to the receptacle 25; the second extends from said receptacle 25 and connects to the conduit 13 downstream of said neck, and each one of them is provided with a respective valve 45', 46'.
In another variation as shown in Fig. 3, however similar to the one just described and in which the same references are used to indicate the parts which are the same or similar, the neck along the return conduit 13 is made up of a section of a Venturi tube 47' which connects up to the return tube 46 from the stripping receptacle 25.
Basically, the diathermic oil is collected by the pump 24 (Fig. 1) or the dynamic drawing tube 45, 46 (Figs. 2, 3) from the return conduit 13 of the diathermic oil from the heat exchange groups 14, 13 of the ORC 10 and sent at least in part to the receptacle 25, subject to possible heating by means of the exchanger 38. The part of the oil sent to the receptacle 25 can be varied and, however, caused by the level of throttling of the valves 27, 41 or 47/47'. The oil therefore crosses the receptacle 25 and returns to the return line through line 42.
During its permanency in the receptacle 25, the oil is crossed by the flow of inert gas fed under control by the respective feeder group 28, 29. Passing in the receptacle 25, the inert gas removes the volatile fractions from the oil it has to cross, in relation to the respective partial pressure inside the gas bubbles.
The gas accumulates in the top part of the receptacle 25 and is separated by the liquid phase by means of the separator provided, schematized as a filter 48. The level of the liquid in the receptacle 25 is maintained by means of the modulation of the valve 31 under the control of a level calculator 49. For a better modulation, the valve 31 can comprise a play on valves of a different diameter in parallel with each other.
The flow of the fluid removed by the valve 31 feeds the detection instruments 34 and/or 35 of the presence of volatile fractions (in particular compounds present in the working fluid of the ORC unit), for example by the amount of absorption of infrared radiation, depending on the whether the position is upstream or downstream of the cooler/condenser 32. The condensed volatile fraction is gathered in the tank 33; the amount not condensed exits from the tank by means of a vent 33", possibly provided with further detectors and/or filters.
When the pump 24 is present, the fraction of the oil sent to be delivered and which is not admitted to the receptacle 25 can reach, by a line 40, the delivery conduit 12 upstream of the heat exchange groups, therefore contributing in maintaining an increase in delivery compared to that delivered starting from said delivery conduit.
Altogether the system guarantees, therefore, a gradual extraction of the light fractions present in the oil, whatever their origin may be, but above all allows a prompt signalling of the presence of said volatile fractions. As regards to the detection and measuring of the compounds coming from the working fluid of the ORC, the system arranges for the withdrawal of the oil immediately downstream of the point of possible contamination (typically represented by the heat exchange groups of the ORC). The collection in the tank 33 then enables possible analysis of the separated fluids to be carried out.
A further safety signal can also be integrated into the system by inserting it on the return conduit 13 of the diathermic oil 11 circuit of a receptacle 50, crossed by the oil coming from the heat exchange groups 14, 15, provided with a level and pressure calculator 51. Should a catastrophic breakage take place, for example a break in an evaporator tube, the sudden drop in the level and/or increase in the pressure in said receptacle 50 causes an immediate alarm allowing an abbreviation in the intervention and possible stoppage of the system.
The line 52, will allow the discharge to the outside of possible slow accumulation of incondensables, or their conduction to an opportune low pressure point of the circuit, in order to avoid false signals.
Within the ambit of the assembly, the signalling of the presence of extraneous compounds in the work fluid can be detected and elaborated by the control equipment and the management of the ORC unit, causing, in general, the machine to stop and the consequent reduction of the pressure in the heat exchange groups.

Claims

A detection, measuring and separation system of light fractions in diathermic oil feeding in particular an ORC system (Organic Rankine Cycle), where the diathermic oil is distributed in a circuit having a delivery conduit (12) from a heating source towards the thermal groups of said system and a return conduit (13) from said thermal groups to said heating source, comprising
a stripping receptacle (25), positioned downstream of the thermal groups of the ORC system and designed to be crossed by a flow of diathermic oil means (24; 45) for an inlet of a flow of diathermic oil from the return conduit (13) of said circuit (11) to an in.let of the stripping receptacle,
means (42, 46) connecting an outlet of said stripping receptacle to the return conduit (13) of said circuit (11), a feeder (28) of an inert stripping gas in said receptacle to cause a separation of light fractions from the oil in the receptacle and their exit from the latter,
a drainage conduit (30) connected to the top of said receptacle and configured to cool and collect the light fractions exiting from said receptacle - and to monitor the presence of specific pollutants contained in it, wherein the means for an inlet of a flow of diathermic oil to the inlet of the stripping receptacle (25) comprise a pump (24), along a line that extends from the return conduit (13) to said receptacle (25) and is provided with a check valve (26) and a register valve (27) of the flow of fluid upstream of said receptacle, said pump being in particular with variable speeds, and
characterized in that
the outlet of the pump (24) can be connected to the delivery conduit (12) of the diathermic oil circuit (11) by means of a line (40) comprising an adjustable valve (41).
System according to claim 1, wherein said pump (24) is run to maintain an inlet delivery of diathermic oil to the stripping receptacle, both to reduce and equalize the temperature of the oil delivered to said thermal groups.
System according to claims 1-2, wherein a heater (38) is provided along the flow line of said pump to said stripping receptacle for the diathermic oil, said heater using, as a heat exchange means, hot oil taken directly from the delivery conduit (12) and then delivered to the return line (13) of the diathermic oil circuit (11).
System according to claim 1, wherein the means for an inlet of a flow of diathermic oil to the inlet of the stripping receptacle include, in connection with the return conduit (13) of said circuit, a ramming intake comprising a drawing tube (45) connected to said conduit (13) upstream of one its choke (47, 47') for a flow of oil to said receptacle and a second tube (46) that extends from said receptacle (25) and connects to said conduit (13) downstream of said choke, each of said conduits being provided with a respective valve (45', 46").
System according to claim 4, wherein said choke is made up of a nozzle (47) or a length of tube with Venturi effect (47').
System according to any of the previous claims, wherein the drainage conduit (30) is provided with a modulation and adjustment of the delivery valve (31), with a cooler/condenser (32), with a tank (33) designed to receive the separate light fractions coming from the stripping receptacle, with detectors (34 and/or 35) of the presence of contaminants.
System according to claim 6, wherein said modulation and adjustment valve (31) of the delivery is controlled by a fluid level detector (49) in said receptacle.