Embodiments of the subject matter disclosed herein generally relate to systems comprising turboexpanders and driven turbomachines and methods for operating the same.
Turboexpanders are widely used for industrial refrigeration, oil and gas processing and in low temperature processes. In some known applications turboexpanders are used in heat recovery cycles to drive an electric generator. US 2011/0305556 discloses a system and method for power generation including a turboexpander with at least two expansion stages for heat recovery and mechanical power generation to drive an electric generator. In this known application the turboexpander is introduced in a Rankine cycle.
EP 2400117 discloses the application of a turboexpander-compressor system according to the prior art, wherein the same fluid is processed in the turboexpander and in the compressor. FIG. 1 illustrates the turboexpander-compressor system of the prior art. The system is labeled 200. A turboexpander 210 has a turboexpander impeller 212. The turboexpander 210 receives an inlet gas flow at 214. Inside the turboexpander 210 the gas may expand and thus cause rotation of the turboexpander impeller 212. The expanded gas exits the turboexpander 210 at 216 enabled by a separator (Sep) 220. When the turboexpander-compressor system 200 functions at design conditions, a pressure p1 and a temperature T1 measured respectively by a sensor Sp1 235 and sensor ST1 236 of the inlet gas flow at 214, as well as a pressure p2 and a temperature T2 measured respectively by a sensor Sp2 237 and sensor ST2 238 of the gas flow at the exit side 216 have values close to predetermined values. However, in some situations the turboexpander-compressor system operates in off-design conditions. When off-design conditions occur, the pressure p1 of the incoming gas flow at 214 may be adjusted to become again close to the respective rated value, using, for example, a first set of moveable input guide vanes (IGV1) 218. The first set of moveable input guide vanes 218 are located at an inlet of the turboexpander 210.
In the turboexpander-compressor system 200 illustrated in FIG. 1, a compressor 224 has a compressor impeller 226. The compressor 224 receives the gas flow from the turboexpander 210 and delivers a compressed gas flow at the delivery side 228. However, between the turboexpander 210 and the compressor 224, the pressure of the gas flow may be altered due to other process components (e.g., separators, coolers, valves) and pressure losses, so that the gas flow at 216 has pressure p3 when entering the compressor 224 measured by sensor (Sp3) 239.
The mechanical work generated by the expansion of the gas in the turboexpander rotates the turboexpander impeller 212. The turboexpander impeller 212 is mounted on the same shaft 230 as the compressor impeller 226. The compressor impeller 226 therefore rotates due to the mechanical work generated during the expansion of the gas in the turboexpander 210. The rotation of the compressor impeller 226 provides energy used to compress the gas in the compressor 224. The mechanical work necessary to rotate the compressor impeller 226 affects the rotating speed u of the shaft 230 measured by speed sensor (Su) 234 and, thereby, indirectly affects the process of expanding the gas inside the turboexpander 210.
The turboexpander efficiency is related to a ratio of the rotating speed u of the shaft 230 and the enthalpy drop ΔH across the turboexpander 210. The gas expansion in the turboexpander 210 may be considered approximately an isoentropic process.
The characteristic parameters (i.e., p1, T1, p2 and T2) of the gas expansion in the turboexpander 210 and the rotating speed u of the shaft 230 may not vary independently. Therefore, in off-design conditions, in order to maximize the turboexpander efficiency, the pressure p3 of the gas flow at the inlet 216 of the compressor 224 may be controlled, for example, by a second set of moveable inlet guide vanes IGV2 232 provided at the compressor inlet. By modifying the pressure p3 of the gas flow 216 input in the compressor 224, the rotating speed u of the shaft 230 is modified and, therefore, the efficiency of the turboexpander 210 can be maximized.
A controller 240 receives information regarding the pressure p1 and the temperature T1 of the gas flow at the inlet side 214 of the turboexpander 210, the pressure p3 of the gas flow at the inlet 216 of the compressor 224, and the rotating speed u of the shaft 230, by suitable sensors. The controller 240 may send commands C1 to IGV1 218 in order to adjust the pressure p1 of the gas flow at the turboexpander inlet 214 to be within a predetermined range. Based on monitoring the acquired information, the controller 240 determines when the turboexpander-compressor system 200 functions in off-design conditions. When the controller 240 determines that the turboexpander-compressor system 200 functions in off-design conditions, the controller 240 sends commands C2 to the second set of IGV2 232 to adjust the pressure p3 of the gas input into the compressor in order to maximize a ratio R between the rotating speed u of the shaft 230 and the enthalpy drop ΔH across the turboexpander 210.
In this known embodiment, the same controller controls the moveable inlet guide vanes of the turboexpander and the moveable inlet guide vanes of the compressor to optimize the efficiency of the system, based on the assumption that the same fluid is processed in the two turbomachines.