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MÔ HÌNH HÓA CÁC

QUÁ TRÌNH CÔNG NGHỆ

HÓA HỌC CƠ BẢN

2017

MASS CONSERVATION

CSTR

Consider the CSTR — the reaction is now exothermic and heat is removed by a jacket containing a vaporizing

medium, so an energy balance is necessary

Assumptions:

• vessel contents well mixed

• species A and B present in dilute solution

SYSTEM IN WHICH PRESSURE

AND VOLUME VARY

EQUILIBRIUM SYSTEM —

FLASH DRUM

Derive an alternative model for the equilibrium flash vessel

in which separate species and energy balances are written for both liquid and vapour phases With reference to the

discussion of the index of differentialalgebraic equations : (a) what are the mathematical properties of this model?

(b) what additional information does this model calculate? (c) from a practical point of view, which of the two models

is appropriate for implementation in SpeedUp?

Hint: it is useful to introduce variables in the balance

equations representing the flow of energy and each

chemical species between the two phases

EQUILIBRIUM SYSTEM —

DISTILLATION TRAY

Consider the following abstraction of a distillation tower:

Each stage has the following regions of material and energy accumulation:

having identified this decomposition, we can focus on modelling each region in isolation

Here, we will derive a model for the froth on the plate, which is

presumably designed to approach phase equilibrium as closely as

possible

Note trays are numbered from the top of the column downwards

Froth on plate:

Control volume: the froth on the plate (e.g., encompassing both phases)

Assumptions:

• phases at phase equilibrium

• individual phases well mixed

• adiabatic

• both phases present at all times — e.g., model valid

while T BUBBLE ≤ T ≤ T DEW

• number of moles in vapour phase negligible in

comparison to liquid phase

• PV work terms in energy balance negligible

NON EQUILIBRIUM SYSTEM -

FLASH DRUM

Consider a model of a flash drum in which it cannot be assumed that the vapour and liquid phases are in phase equilibrium:

The vessel can be abstracted as two well-mixed phases constrained within a fixed volume Material and energy fluxes exist between the phases:

NON EQUILIBRIUM SYSTEM - FLASH DRUM

Control volumes: control volume for gas phase and control

volume for liquid phase Both will vary in size and shape as the liquid level rises and falls

Assumptions:

• individual phases are well-mixed

• vessel adiabatic

• liquid feed stream well-mixed with the liquid phase

• both phases present at all times

• bulk flows into and out of vessel specified by user or

determined by upstream and downstream models

These assumptions are the simplest assumptions that can be made It is interesting to note that to assume anything else

would require a detailed knowledge of the internal

topology of the vessel and physico-chemical discontinuities (e.g

the level of the feed pipe relative to that of the liquid level, the area of heating coil/jacket immersed in the liquid phase, etc.)