Meeting on LNG at Hydro Oil & Energy RC Jrgen B. Jensen and - PowerPoint PPT Presentation

Meeting on LNG at Hydro Oil & Energy RC Jørgen B. Jensen and Sigurd Skogestad Department of Chemical Engineering 22th May 2006 www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

2 Outline Simple cooling cycles Ammonia cooling cycle PRICO LNG process MFC LNG process Concluding remarks www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

Simple cooling cycles Introduction Specifications in design and operation Active charge and holdup tanks Degrees of freedom for operation Discussion of some designs Conclusion www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

4 Introduction Condenser 2 1 ∆ T sub P 2 1 P h P h Q H T H W s z Q C T C P l P l 4 3 ∆ T sup 3 4 h Evaporator Coefficient of performance (COP) COP h = Q h n ( h 1 − h 2 ) COP c = Q c n ( h 4 − h 3 ) W s = ˙ W s = ˙ n ( h 1 − h 4 ) n ( h 1 − h 4 ) ˙ ˙ COP h = T H / ( T H − T C ) COP c = T C / ( T H − T C ) Theoretical limit: www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

5 Introduction Stoecker, W. F., Industrial refrigeration handbook , McGraw-Hill, 1998: The refrigerant leaving industrial refrigeration condensers may be slightly sub-cooled, but sub-cooling is not normally desired since it indicates that some of the heat transfer surface that should be be used for condensation is used for sub-cooling. At the outlet of the evaporator it is crucial for protection of the compressor that there be no liquid, so to be safe it is preferable for the vapor to be slightly super-heated. www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

6 Specifications in design and operation Given # Load (e.g. Q h ), P l , P h , ∆ T sup and ∆ T sub Design 5 W s (load), choke valve opening (z), UA in two Operation heat exchangers and ? 5 ∆ T sub P P h P h Q H T H W s z Q C T C P l P l ∆ T sup h www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

6 Specifications in design and operation Given # Load (e.g. Q h ), P l , P h , ∆ T sup and ∆ T sub Design 5 W s (load), choke valve opening (z), UA in two Operation heat exchangers and active charge 5 P h Q H T H m tot = m evap + m con + m tank � �� � W s z Active charge Q C T C Neglect holdup in compressor, valve P l and piping www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

7 Active charge and holdup tanks • The “pressure level” is indirectly given by the active charge • A liquid receiver makes operation independent of total charge • Liquid level in the receiver has an indirect steady state effect Rule 1 In each closed cycle, there is one degree of freedom related to active charge Rule 2 In each closed cycle, there is one liquid level that does not need to be controlled, because the total mass is fixed. www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

8 Adjusting holdup with extra valve High pressure receiver Low pressure reciever P h P h Q H Q H W s W s P l P l z z Q C Q C Pressure drop across the extra The extra valve gives sub-optimal valve gives sub-cooling operation! www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

9 Extra valves removed High pressure receiver • Tank and condenser may be merged together • Condenser exit will be saturated Q H liquid ( ∆ T sub = 0 ◦ C) • Disadvantage: Some sub-cooling often optimal • Have used one degree of freedom (“no valve”) to set the degree of sub-cooling to a non-optimal value www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

10 Extra valves removed • Evaporator exit will be saturated vapour ( ∆ T sup = 0 ◦ C) Low pressure receiver • Advantage: No super-heating is optimal • (Some super-heating might be necessary to avoid droplets in the compressor) • Have used one degree of freedom (“no valve”) to set the T C degree of super-heating to an Q C optimal value www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

11 Degrees of freedom for operation During operation the equipment is given. Nevertheless, we have some operational or control degrees of freedom. 1 The compression power W s . We assume that it is used to set the “load” for the cycle 2, 3 Effective heat transfer area (UA). There are two degrees of freedom related to adjusting the heat transfer, which may thought of as adjusting (reducing) the effective UA value in each heat exchanger (i.e. bypasses). However, we generally find that it is optimal to maximize the effective UA. 4 Adjustable choke valve (z) 5 Adjustable active charge www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

12 Optimal designs Optimal 1 • Liquid receiver before compressor minimize P h super-heating Q H • Choke valve may be used to W s control sub-cooling (other P l control policies also z Sub-cooling control possible) • Potential problem: Vapour “blow out” Q C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

13 Optimal designs Optimal 2 • Equivalent thermodynamically P h • High pressure receiver Q H prevents vapour “blow out” LC W s Sub-cooling • The new valve may control control P l sub-cooling (other control z policies also possible) • Need to control one liquid level according to rule 2 Q C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

14 Non-optimal designs Non-optimal 1 Two errors: P h Q H • Super-heating is not optimal. Can be controlled to a given W s value with a thermostatic P l Super-heat expansion valve (TEV) z control • There is no sub-cooling Q C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

15 Non-optimal designs Non-optimal 2 P h Q H One error: LC W s • There is no sub-cooling P l z Q C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

16 Non-optimal designs Non-optimal 3 P h Q H Sub-cooling One error: control W s • Super-heating is not optimal P l z Super-heat control Q C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

17 Internal heat exchanger (tanks removed) Q A Q H Q H P h P h z z W s W s P l P l Q A Q C Q C Sometimes beneficial No effect for pure fluids, but often thermodynamically and gives used for mixed refrigerant useful super-heating systems such as LNG processes www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

18 Conclusion • Variable active charge makes operation independent of total charge of the system • Variable active charge gives one extra degree of freedom that depending on the design might be available for control • Optimally; ∆ T sup = 0 ◦ C, but ∆ T sub � = 0 ◦ C • There are two degrees of freedom in a simple cooling cycle (given load and max effective UA in the heat exchangers) • One should be used to minimize the super-heating • The second should be used for self-optimizing control • A receiver with no extra valve consumes one dof • Optimal before compressor • Sub-optimal before choke valve www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

Ammonia cooling cycle Process description Modelling Design vs. operation Selection of CV’s Conclusion www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

20 Process description • Four constrained inputs: Ammonia case study • W s controls the load T H (with a temperature controller) P h Q H • Maximum UA: We do not manipulate flow of W s hot and cold fluid, and P l have no bypass of heat z exchangers Q C • Fixed super-heating; ∆ T sup = 0 ◦ C T H Q loss • One degree of freedom T C T s TC C • Choke valve opening z www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

21 Process description Ammonia case study T H • T C = T room P h Q H • T H = T amb W s • Q loss = UA loss · ( T H − T C ) P l z • Temperature control Q C gives Q C = Q loss T H Q loss T C T s TC C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

22 Modelling Ammonia case study • SRK equation of state T H • Cross flow heat P h exchangers with constant Q H air temperature W s • Constant isentropic P l z efficiency (95 % ) in Q C compressor • Molar flow through valve: T H � n = z · C v · ∆ P · ρ ˙ Q loss T C T s TC C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC

23 Design vs. operation Design: ∆ T min = 5 ◦ C Ammonia case study T H min ( W s ) ∆ T − ∆ T min ≥ 0 P h subject to Q H W s P l Operation: A max = A design z Q C min ( W s ) T H A − A max ≤ 0 subject to Q loss T C T s TC C www.ntnu.no Jensen & Skogestad, Meeting on LNG at Hydro Oil & Energy RC