Two examples of steady state simulation with HYSYS at GALPenergia Sines Refinery José Egídio Fernandes Inverno 1*, Eurico Correia 2, Pablo Jiménez-Asenjo 3 and Josep A. Feliu 3 1 Galpenergia, Refinaria de Sines, 7520-952 Sines, Portugal 2 Intituto Superior Técnico, Lisboa, Portugal 3 Aspentech Inc, Passeig de Gràcia 56, 08007 Barcelona, Spain Abstract This document shows two examples of aplication of a simulation software, in this case HYSYS, to a real situation in an oil refinery: a crude destillation model and a sour water stripper model. Keywords: simulation software, oil refining, crude destillation, sour water stripping 1. Introduction The purpose of this paper is to present the work developed at Sines Refinery in cooperation with the University (Instituto Superior Técnico, Lisbon) regarding the development of models of some process units of GALPenergia SINES Refinery. The work was done during six months and HYSYS was used as simulation tool. Sines Refinery has been using this software for the last 4-5 years with success in solving some small operational problems. The chalenge here was to model a complete process unit in order to allow the optimization of the operation. 2. Crude destillation Unit The crude destillation unit at Sines Refinery is the process unit where the crude oil is first processed in the refinery and is composed by the following equipment: 2 pre-heat trains (each one composed by several heat exchangers, a desalter and a furnace) 1 pre-flash drum 1 main destilation column (a very complex column with several pumparounds and side strippers) 4 secundary fraccionation columns (Deetanizer, debutanizer, naphta splitter and a deisopentanizer) * Author to whom correspondence should be adressed: josé.inverno@galpenergia.com
The original model was initially developed in close collaboration with Hyprotech (now Aspentech) some years ago. The objective of the present works was to validate the model, checking all the settings and calibrations, to verify the answer of the model to real operating conditions. The first step for a successfull simulation is a correct choose of the thermodynamic method that will be used in the calculations of the state variables and the physical properties. For the family of compounds that are present in the crude distillation unit process streams, the Peng-Robinson equation of state is normally accepted as adequate. The second step is to correctly characterize the feed composition. Refineries can be feeded with a multiplicity of crude oils and usuallly the feed is not a crude of only one origin but a crude mix that can vary from 3 to 10 diferent crude types. Simulation packages are able to properly characterize petroleum fluids if data from laboratory assays (distillation curves, light end analysis, bulk properties, etc.) is available. How to do it if we don t have this type of data available? Ready-to-use characterized oil crudes from a BP crudes database are available in another commercial software (KBC s Petrofine with CAMS) leased by GALPenergia. This software doesn t use rigorous thermodynamic models but is used to simulate all the refinery for schedulling and blending proposes.the results of generating a crude slate with this software is a huge table with all types of data (SG, S, V, Ni, UOPK, viscosities, etc.) for 75 different components (22 pure light components and 53 heavier pseudo components separated trough different cut points). All this data can be used to feed the model developed in HYSYS and check the correct characterization of the quality of the feed. However, using a database and not current crude data, has the inherent problem of ignoring the properties variability of the real crude extracted from fields. When observing mismatches between simulated and real plant data there will always remain the doubt if the cause is a modelization mistake in the process or a wrong (outdated) feed characterization. A back-blending procedure would have been as well possible in case assays from the distillation column products whould have been available. Using the distillation curves, the bulk properties and the light end analysis obtained from the column products, HYSYS could have back-generated the original crude entering the column by leveraging the properties as a function of the products flows leaving the column. In any case, using the crude database, we have obtained a good agreement between the crude column simulation and the real data obtained from the process. At the end of the paper, the Figure 1 shows the general arrangement of the unit operations in the simulation of this unit. In the Table 1 we can compare the results of the simulation with the real data obtained. For the case study we have used the 10 of June 2003 operating data. In this day the refinery treated a crude mix composed by 4 different crudes (Es Sider, Qua Iboe, Sahran Blend and Marlin). We can compare the column temperature profile and also some products properties (SG Specific gravity and 5 an 95 v/v % ASTM D86 distillation temperatures). From the table we can see that we have obtained a good agreement between the crude column simulation and the real data obtained from the process.
Table 1 Differences between the real operation and the simulation. Real Simulation Units (1) (2) (1)-(2) Column temperature profile Overhead Temp ºC 120.7 120.3 0% Tray 03 Temp ºC 137.7 140.8-2% Tray 15 Temp ºC 189.9 190.5 0% Tray 19 Temp ºC 203.0 213.0-5% Tray 26 Temp ºC 245.1 243.2 1% Tray 30 Temp ºC 265.6 272.8-3% Tray 35 Temp ºC 300.6 294.7 2% Tray 39 Temp ºC 311.7 319.7-3% Tray 43 Temp ºC 348.8 355.0-2% Bottoms Temp ºC 343.7 351.6-2% T Pumparound Naphtha ºC -72.21-68.7 5% T Pumparound LGO ºC -91.05-97.7-7% T Pumparound HGO ºC -73.01-71.4 2% Products properties Naphtha 95% ºC 163.7 161.5 1% Naphtha SG kg/m3 717.6 718.7 0% Light Gasoil 95% ºC 310.5 325.0-5% Light Gasoil 5% ºC 241.2 235.5 2% Light Gasoil SG kg/m3 848.4 848.1 0% Heavy Gasoil 95% ºC 376.5 384.0-2% Heavy Gasoil 5% ºC 306.5 296.2 3% Heavy Gasoil SG kg/m3 883.3 884.3 0% Atmospheric Residue 5% ºC 339.6 348.5-3% Atmospheric Residue SG kg/m3 978.8 977.1 0% 3. Sour Water stripper In many refinery units water or steam is injected to facilitate the process. This water is usually separated and collected in the boots of the overhead receivers of the destillation columns or in other type of receivers where the separation between water and hydrocarbons can be enhanced.the drawback of injecting water into the refinery processes is that it strips the acid soluble gases, NH 3 and H 2 S, generating sour water streams The objective of the sour water stripper is to reuse the recovered water in other refinery processes and to remove almost all the H 2 S and NH 3 present, mainly because environmental reasons.
If by any reason the water cannot be reused, and these two gases are not substantially removed the sour water needs to be sent to a wastewater treatment plant where the treatment to remove this type of contaminants can be finally performed but with a very high cost. With the information that in the existing installation the NH 3 removal was not effective, we used the simulation software to simulate the column (Sour Water Stripper) to find out what would be the best operating conditions in terms of column temperature, pressure, and stripping steam in order to maximize the removal of NH 3 from water. For this simulation, and because the type of compounds present is not the same than in the crude distillation column, it is necessary to use another thermodynamic method. For sour systems, due to the presence of H 2 S and NH 3 there are some adequate thermodynamic methods present in HYSYS database (the Sour options of the SRK and PR equations of state as well as the Stryjek Vera modification of the PR). Finally, for data availability, it was decided to use the Peng Robinson Stryjek Vera [PRSV] equation of state The tests performed using the column model in HYSYS have concluded that to have a better NH 3 removal the operating temperatures should be increased, pressures decreased and the stripping steam flow optimised. Graphs 1 and 2 show the relationship between the pressure, temperature and steam flow in the level of NH 3 in the stripped water. 160 140 120 137 Overhead P: 2bar Overhead P: 2.5bar Ammonia (ppm) 100 80 79 60 51 45 40 33 26 20 21 15 10 9 7 0 4 6 6.5 7 7.5 8 8.5 9 Steam flow (t/h) Graph 1- Ammonia in stripped water as a function of stripping steam flow
120 100 114 98 Overhead P: 2bar Overhead P: 2.5bar 84 Ammonia (ppm) 80 60 40 20 40 34 29 71 61 26 23 53 21 0 75 80 85 90 95 100 105 Column overhead temperature (ºC) Graph 2 - Ammonia in treated water as a function of the overhead temperature The real plant column has been adjusted using the results obtained trough HYSYS simulation. The increase in column performance obtained has supposed company savings both in raw water consumption and in the waste water treatment plant. Figure 2 shows the general arrangement of the unit operations in the simulation of this unit 4. Conclusions Simulation software is a very good tool for the process industry, not only at the level of conceptual design but also during the entire lifecycle of the equipment, were it can be very useful for performance, debottlenecking and process studies. The presented results have been obtained using the steady state version of the software, but all the simulations done in steady state can have an evolution to a dynamic simulation (for example to build process simulators for operator training or to study the behaviour of the process units in transient conditions), or to a real time optimisation system where, together with the advanced process control tools, can be very profitable in the optimisation of the operation in real time.
Figure 1 Crude distillation column diagram Figure 2 Sour water stripper diagram