Flash drum modeling

The product stream from the reactor (Bl) was flash cooled using a flash drum (B3) to separate vapors from the liquid phase. The flash models available in ASPEN-Plus determine the thermal and phase conditions of a mixture with one or more inlet streams. A separator (B6) was employed to separate C02 and steam. The resulting recycle streams no. 10 and no. 9 were sent to B13 and B7, respectively. The liquid stream (no. 14) from the flash drum was sent to a Pneumapress filter (B10), where it was separated into filter cake (stream no. 13) and filtrate (streamno. 8). This separation was done to facilitate heat extraction from the product stream for heat exchanger Bll. 

For example, when we consider the design of specialty chemical, polymer, biological, electronic materials, etc. processes, the separation units are usually described by transport-limited models, rather than the thermodynamically limited models encountered in petrochemical processes (flash drums, plate distillations, plate absorbers, extractions, etc.). Thus, from a design perspective, we need to estimate vapor-liquid-solid equilibria, as well as transport coefficients. Similarly, we need to estimate reaction kinetic models for all kinds of reactors, for example, chemical, polymer, biological, and electronic materials reactors, as well as crystallization kinetics, based on the molecular structures of the components present. Furthermore, it will be necessary to estimate constitutive equations for the complex materials we will encounter in new processes. 

The thermodynamic equilibrium between the vapor and liquid phases [imposes certain restrictions on the state variables of the system, and I must be included in the mathematical model of the flash drum if it is to be consistent and correct. These equilibrium relationships, as known from chemical thermodynamics, are ... 

Let us first determine the degrees of freedom for the flash drum. The modeling equations are ... 

Determine the number of controlled and manipulated variables for the flash drum (Example 23.1) assuming steady-state operation. Why are the results different from those of Example 23.1 State the danger involved when we consider steady-state models to design a MIMO control system. 

Modelling a single tray is similar with a dynamic flash discussed before. The solution of the assembly of trays, increased with condenser, flash drum and reboiler, is a much more difficult problem, however. The equations presented below (for notations see Fig. 4.6) are known as MESH equations for modelling distillation columns at steady state enlarged with left hand terms for accumulation. 

In this chapter, the user was asked to find the flowrate of the liquid and vapor outlet streams of the flash separator. The vapor and liquid in the flash drum are allowed to reach equilibrium, before they are separated. HYSYS separator was used to model the flash separation process. 

The flash drum in Figure 20.9 illustrates a situation where a stream containing a binary mixture of two components, A and B, is flashed through a valve and separated in a flash drum into an overhead vapw stream and a residual liquid product stream. The hquid in the drum is cooled by external heat exchange with hquid recycle. This process is modeled with 11 variables F,-, T, Q, F, Pf h, Tf, Fv, y, Fi, and x. Two variables are considered to be extemaUy defined, T and C. The model involves five equations a... 

We could, for example, consider the mathematical modeling of a flash drum separator or a distillation column. The stoichiometric equations (order of magnitude 1) stay with the enthalpy balance equations (order of magnitude 10 -10 ) and significant differences in orders of magnitude are present in the resulting nonlinear system. 

Application Remarks You do not need to feel intimidated with the set of equations above. You will find the task simple when you follow the top-down approach to establish steam balances from boilers to each steam header, to steam turbines and letdown valves, and to deaerator and blowdown flash drum. The top-down approach for steam balance is discussed in detail in Chapter 16. During the process of setting up the steam balances, you could apply some of the equations above for modeling the equipment and subsystems. The steam balances can be conducted readily in a spreadsheet environment. 

Because benzene and toluene are common solvents one can find in the literature the thermodynamic data needed to model the flash drum. However, it is instructive to consider how one might measure the requisite data. We need a map of the phases (vapor, liquid, or two phase) for benzene/toluene mixtures. The coordinates of the map should be the composition of the mixture (jc axis) and the pressure of the system (y axis). We dust off the device we used for benzene, diagrammed in Figure 4.3, and measure the phase of the system as a function of pressure, at constant temperature. 

Rigorous, flow-sheet-based models for hydrocrackers include sub-models for furnaces, pumps, compressors, reactors, quench zones, flash drums, recycle gas scrubbers, fractionation towers, and - importantly - economic data. As discussed in Chapter 23 by Mudt, et al., such models can comprise hundreds of reactions and hundreds of thousands of equations. The model grows when inequalities are included to ensure a feasible solution that honors process constraints. To solve such models in real time (i.e., in less than an hour), open-equation mathematics and high-powered solvers are used. 

An Aspen Flash model is used for the reflux drum with pressme set at 1 bar and design specification of a vapor fraction of 10 , which makes the drum essentially adiabatic. A small vapor flow rate is necessary so that the control valve in this vent line can be sized. In the Aspen Dynamics simulation, the valve is completely closed. The liquid holdup in the drum is set to give 5 min at 50% full (diameter 3 m and length 6 m). 

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