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Dynamic Steady State

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Continuous Discrete

Deterministic Stochastic

Dynamic Steady State

Needs

Synthesis

Heat and Material Balances

Size and Costing

Economic Evaluation

Parameter Optimization

Structual Optimization

Final Flowsheet

Flowsheet

Analysis

Problem:

Compute Material Balance

Compute Energy Balance

How to compute?

Sequential Modular Approach

Equation Solving Approach

Simultaneous Modular Approach

Aspen & ChemCad use the sequential modular approach

Specify flowsheet (topology) as series of process blocks connected by process streams

You specify inputs, process conditions and how to compute physical properties

System determines a computational sequence -- you can override

Simulator computes output flows from these data.

Simulator also computes energy usage, etc.

Simple Example

Flash a water stream

Flow: 1000 lb/hr

Pressure: 200 psia

Temperature: 300 F

Composition: 100% water

Given Inputs

Operating Conditions

Flash vessel is well insulated

Flashed to 20 psia

All flows

Temperature

Pressure

Desired

Flow: ?

Temperature: ?

Pressure: 20 psia

Vapor

Q = 0

20 psia

Flow: 1000 lb/hr

Pressure: 200 psia

Temperature: 325 F

Flow: ?

Temperature ?

Pressure: 20 psia

Liquid

F = V + L

Mass Balance:

F*HF = V*HV + L*HL

Energy Balance:

Note: Since vapor and liquid, system is at saturation, so temperature is known (from steam tables)

Mass Balance:

1000 lb/hr = V + L

Energy Balance:

1000lb/hr*296 Btu/lb = V*1156.2 Btu/lb+ L* 196.25 Btu/lb

Note: Since vapor and liquid, system is at saturation, so temperature is known (from steam tables = 228 F and get enthalpy data from steam tables)

L = 896.1 lb/hr

V = 103.9 lb/hr

T = 228 F