Energy Optimisation Of Upstream Separation And Stabilisation Plant Using Pinch Technology

Transcription

1 Energy Optimisation Of Upstream Separation And Stabilisation Plant Using Pinch Technology Ritesh Sojitra Srashti Dwivedi ITM University, Gwalior, India Abstract: Energy optimisation and process integration in process industries by application of Pinch Technology simplifies the energy saving targets and easily identifies the pinch points and opportunities of saving overall utility requirements. Pinch analysis can be carried out by various techniques and methods includes graphical method, algebraic method and computational method and various technique discussed to achieve MER (Minimum Energy Requirement). In this study Upstream Separation and Stabilisation Plant of oil & gas industry used to study pinch analysis. Graphical and algebraic methods are applied to identify the optimum pinch point and to determine the external utilities requirements. In addition to that importance of Global dt MIn analysed. Keywords, Pinch Analysis, Pinch Technology, MER, Grand Composite Curve (GCC), Composite curve (CC), Revised cascade diagram, energy conservation method, energy optimisation. I. INTRODUCTION Looking to recent adverse effects of greenhouse gases and fossil fuel scarcity, it is felt that for reduction in industrial energy demand and emissions we need to remunerate traditional engineering design methodology. w and then emerges an approach to technology which is brilliant concept to address this global issue. Therefore, adoption of latest process design integration and energy optimisation technique could be best solution to this problem. Pinch analysis is one of the latest heat integration and process synthesis techniques used for energy conservation and cost reduction in hydrocarbon, chemical and petroleum industries. Pinch analysis is a very systematic energy management methodology for minimising energy consumption and total cost of process plant. Since then pinch analysis has evolved and developed explicitly in many ways. Additionally day by day its applications are widening in various industries. Out of that this methodology is useful in complex heat exchanger network (HEN) design in refineries, chemical and oil & gas processing plants. Exemplified application of this analysis can optimise the energy consumption of HEN by calculating thermodynamic feasibility [1]. II. PREVIOUS STUDIES ON THIS SUBJECT Extensive studies on similar subject have been done by various scholars and their work inspired us to continue further study on similar topic, such as Herman Rodera and Miguel J. Bagajewicz used this analysis for Targeting Procedures for Energy Savings by Heat Integration across Plants. They proposed accomplishment of heat integration across plants either directly using process fluid streams or indirectly using intermediate streams of fluids. By application of pinch analysis to the heat transfer systems of two units. It is first shown that heat transfer leading effectively to energy savings occurs at temperature levels between the pinch points of both the units, In few scenarios, heat transfer in other regions is require to achieve maximum savings. A methodical procedure for finding heat saving target is briefed and a plan to calculate heat saving is explained too. A mathematical programing example is demonstrated to calculate the optimum pinch point of HEN. is explained as well as a plant. [2] Richard Beaman and Cliff Reese worked on Energy Optimisation Using Pinch Analysis. Concept of selecting pinch temperature, utility temperature and practical effective application of pinch analysis is illustrated. [3] Semra Ozkan and Salih Dincer applied on pinch analysis using computer code. They explained optimisation tool in Application of pinch design of heat exchanger networks by use of computer code employing an improved problem algorithm table. They used Visual Basic 3.0 for coding the Page 51

2 Composite Curve and Grand Composite Curve and sourced real process data from petroleum refinery to demonstrate the cost saving and energy saving by application of pinch design. VB coded DarboTEK has demonstrated improved problem algorithm table for accomplishment of retrofit design of HEN. [4] In Pinch location at heat capacity flow-rate disturbances of streams for minimum utility cost heat exchanger networks Jacek Jezowski, Roman Bochenek and Alina Jezowska briefed influence of pinch phenomenon in designing of heat exchanger network. With application of MILP optimisation model a rigorous approach was developed for locating optimum pinch point. Derived results can be applied in targeting stage and in synthesise flexible HEN. However further research are required to extend the approach for optimum flow and temperature targeting. [5} To achieve practically optimum results, design engineer should take care of following points. The temperature difference should not be less than be below ΔT Min. In addition to that there should be mitigation of CP criteria Above pinch CP HOT =< CP COLD Below pinch CP HOT >= CP COLD If above criteria couldn t met in existing design and if it is not complex HEN design then split the stream. Generally splitting the stream in equal values of CP. This splitting stream can be determined by two factors and stream data at pinch point. Number of hot streams and cold streams CP values of each stream. [6] Process parameters above the Pinch Point Process parameters below the Pinch Point III. METHODS OF PINCH ANALYSIS SHot =< SCold SHot> >= SCold Following are various heat integration application procedures which have been proven very effective. Mathematical Procedure Graphical Procedure Computer Programing Procedure As time goes, all of above techniques are evolving, expanding and developing which are helping in energy conservation and minimising negative environmental impact. Mathematical Procedure: Mostly algebraic methods used for algorithm development for this procedure. This method was used in supply chain & logistics industries for solution of transhipment problem in the past. However, now days use of mathematical procedure are rising as it can be automated and various numerical and analytical methods are readily available. Graphical Procedure: This method uses graphical representation viz. Temperature-Enthalpy (T-H) Diagram, Composite curves (CC), Grand Composite Curve (GCC) etc. This method is very easy to understand but it involves tedious work if it performed manually. Computer Programming Procedure: it can be programmed using various coding languages and can be visualised on spreadsheet or other GUI. As this method doesn t involve routine manual calculations it can minimises errors aroused due to human factor and it can be used to design, optimise or study very large complex heat exchanger network. However accuracy of results depends of concept and methodology of programming. Such as type of numerical method used, step size of iteration and tolerance threshold to converge problem. Divide Hot Compare individual matches for below condition CPHot =< CPCold Apply Matches for Pinch Analysis Divide Cold Divide Hot Compare individual matches for below condition CPHot >= CPCold Apply Matches for Pinch Analysis Divide Cold Figure 1: Process flow for Splitting at pinch point V. A GENERIC APPROACH TO ACHIEVE MER PINCH ANALYSIS To achieve minimum energy requirement (MER) sequentially, start at pinch point and gradually move away, monitor CP monitor inequality constrains for matches, maximise heat load on each match by selecting one stream. Be aware that more away from pinch will facilitate more flexibility in selection matches however it is at expense of energy. OBJECTIVE In this study a generic process flow scheme of the Upstream Separation and Stabilisation Plant was acquired from oil & gas industries and pinch analysis applied across the HEN of the plant in order to estimate the optimum energy demand as low as reasonably as practicable (ALARP). The major difference in energy consumption between old integration technique and pinch analysis are differentiated, exemplified method discussed and analysed. [7] IV. BASIC RULES OF OPTIMUM PINCH ANALYSIS During design ensure that basic rules of thermodynamics are not violated Heat Transfer across the Pinch Use hot utilities above pinch Use cold utilities below pinch Page 52

3 40 ºC Off Gas to Degasser FG network Stabilised Condensate Stabilised Condensate Pump Water Cooler Cooler bypass valve Pressure Letdown Slug Catcher Inlet Gas from Gathering Station Pressure Letdown Separator Gas KOD 3.5 ºC Stabiliser Gas to NGL Recovery Unit Filter Coalescer E-5 Gas Compressor Unit Oily Water Suct. KOD Actual Temperat ure ºC Gas Heater 40 ºC Pre- Feed Heater T(n+1)- Tn CpHot dhhot Hot C MJ/h/K MJ/h MJ/h CpCold dhcold Cold MJ/h/ K MJ/h MJ/h Water ºC LP Steam ºC ºC Reboiler 195 ºC Figure-2. Upstream Separation and Stabilisation Plant Figure 2: Process Flow Diagram SHORT PROCESS DESCRIPTION Above Upstream Separation and Stabilisation Plant contains slug catcher where reservoir fluid stratified and act as a buffer liquid storage where gas and liquid separated. Gas from slug catcher routed to KOD and filter coalescer via gas heater. Liquid stream routed to 3 phase separator where it is flashed to medium pressure where lighter hydrocarbon flashed out and free water settled in boot. Liquid condensate from separator routed to stabiliser where condensate is stabilised bring down its vapour pressure to safe level so that it can be shipped safely in atmospheric storage. Following heat transfer stream identified in above unit and analysed for further heat conservation Table B: Actual Intervals Table PINCH ANALYSIS Stre am. Strea m Fluid Type Supply Tempera ture Target Tempe rature dt Min Heat Capacity Flowrate Table A: Process Data Heat Flow Supply Shift Targe t Shift C C C kcal/ C.h MJ/h C C P-1 Cold P-2 Cold P-3 Hot P-4 Hot Figure 4: Actual Diagram Figure 3: Grid Diagram Figure 5: Composite Curve Page 53

4 Table 3: Actual Intervals Table Figure 8: Shifted Composite Curve Shifted Tempe rature T(n+1) -Tn CpHot Figure 7: Shifted dhhot Hot CpCold dhcold Cold C MJ/h/K MJ/h MJ/h MJ/h/K MJ/h MJ/h Table C: Shifted Intervals Table Figure 9: Grand Composite Curve VI. RESULTS Grid Diagram (Figure 1), Actual Diagram (Figure 2), and Composite Curve Diagram (Figure 3) are prepared based on Process stream data (Table 1), Actual Interval Table (Table B ) of Upstream Separation and Stabilisation Plant. Pinch analysis applied across the thermal design of the plant cascade diagram prepared using algebraic procedure and shifted obtained Shifted Diagram (Figure 7), Shifted Composite Curve (Figure 8), Shifted Interval Table ( Table C),) and Cascade Diagram (Figure 9). If we see Cascade Diagram indicate pinch point and deficit of Hot duty of MJ/h, in Revised Cascade Diagram by supplying the same amount of Hot Utility it made the feasible system. However pinching on hot side is no more feasible except deviation in the specification of dt Min.. Hence Minimum Page 54

5 amount of LP steam to stabilizer boiler will be required of heat duty MJ/h. Process Flow Diagram indicates the trim cooler which cools stabilised condensate to 40 ºC, however pinch analysis shows that there will be no cold utility requirement hence we can eliminate Gas cooler and it will save significant capital cost and operating cost. Min Hot Utility MJ/h Min Cold Utility 0 MJ/h During Problem formulation Global dt Min is kept 10 ºC. High specification of dt Min Minmay lead to excess consumption of utilities and lower may lead to unpractical. Hot Pinch C Cold Pinch 3.51 C VII. CONCLUSION Application of Pinch Analysis on thermal design of Upstream Separation and Stabilisation Plant has determined requirements of both the utilities, hot pinch and cold pinch. Process flow diagram shows Gas Cooler which was added as a part of traditional design but on flip side pinch analysis indicates that it can be eliminated and we can cave of MJ/h energy plus capital cost of cooler. Global dt Min has very significant effect on the pinch point and minimum utilities requirement. It should be fixed at optimum otherwise design can lead to either high energy consumption or lead to under design hence system will never satisfy the required target Although it is very simple in terms of technical computation but it is very powerful tool for process optimisation and integration. REFERENCES [1] Smith, Chemical Process Design and Integration, 8th edition, [2] [2] Herman Rodera and Miguel J. Bagajewicz Targeting Procedures for Energy Savings by Heat Integration across Plants. [3] Richard Beaman and Cliff Reese Energy Optimisation Using Pinch Analysis. [4] Semra Ozkan and Salih Dincer Application of pinch design of heat exchanger networks by use of computer code employing an improved problem algorithm table. [5] Jacek Jezowski, Roman Bochenek and Alina Jezowska Pinch location at heat capacity flow-rate disturbances of streams for minimum utility cost heat exchanger networks. [6] Linhoff March Introduction to Pinch Technology, [7] Ian C. Kemp Pinch Analysis and Process Integration, 2nd Edition, Page 55