Development of a mathematical model for blowdown of vessels containing multi-component hydrocarbon mixtures
This thesis describes the development of a mathematical model, BLOWSIM, for simulating vapour space blowdown of an isolated vessel containing single (vapour) or two-phase (vapour and liquid) hydrocarbon mixtures based on three Cubic Equations of State (CEOS). These include Soaves Redlich-Kwong (SRK), Peng- Robinson (PR) and the recently developed Twu-Coon-Cunningham (TCC) CEOS. The performances of the above equations are first evaluated by comparing their predictions for a range of important thermophysical properties (including vapour/liquid equilibrium data, speed of sound and fluid densities) with experimental data for single and multi-component hydrocarbon systems. These data are reported as a function of reduced pressures and temperatures in the ranges 0.00053 - 43.41 and 0.33 - 2.09 respectively. Typical systems tested include pure alkanes as well as mixtures containing methane, ethane, propane, H₂S, CO₂, N₂ and trace amounts of heavy hydrocarbons. The above is then followed by applications of all three equations in the blowdown model and comparing the results with those obtained from a number of experiments relating to the blowdown of the various hydrocarbon systems from a maximum pressure of 120 atm and ambient temperature. Typical output include the variations of fluid pressure, temperature (both liquid and vapour), discharge rate as well as the wetted and unwetted wall temperatures with time. Another major part of the study includes investigating the effects of different assumptions relating to the estimation of the liquid/wall heat transfer coefficient, the thermodynamic trajectory of the fluid in the vessel as well as the fluid phase at the orifice on blowdown predictions. We find that in general all three CEOS provide a similar level of accuracy with TCC CEOS providing the best performance in terms of predicting vapour speed of sound at Pr > 3. However, the equation gives rise to relatively large errors in predicting liquid speed of sound at Tr \leq 0.6. Typical accuracy of the blowdown model in terms of predicting fluid and wall temperatures during depressurisation are ±7 and 5K respectively.