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Flash Operations

Overview

A flash operation is a rapid phase separation process in which a liquid mixture is introduced into a flash drum, resulting in the formation of two distinct phases: a vapor phase and a liquid phase. This operation occurs rapidly and is based on the principle of phase equilibrium, where the vapor and liquid phases reach thermodynamic balance at a given temperature and pressure.

flash_image

Simple flash drum example.

Flash Operation Logic

In the code, two sequential flash operations are performed:

1. First Flash Operation

The first flash is executed using the temperature and pressure specified by the user.

2. Second Flash Operation

After the first separation, a second flash operation is performed. In this step, the system conditions are changed to ambient temperature and ambient pressure.

Flash Data Processing

The flash operation results, including phase compositions and thermodynamic properties, are extracted directly from DWSIM after each flash operation.

However, two important properties — Burn Rate and Evaporation Rate — are not directly available from DWSIM. These properties are instead calculated within the code using:

  • A heat of combustion database; and

  • Additional thermophysical property calculations.

This ensures that all necessary parameters are available for further analysis even when DWSIM does not provide them directly.

Burn Rate Calculation

The burn rate represents the rate at which a fuel combusts from a surface, expressed in units of \((\text{kg/m²s})\). It quantifies how much mass of fuel burns per unit area per unit time.

The burn rate is calculated using the following equation: $$ \text{Burn Rate} \left(\frac {kg} {m²s}\right) = 1.27 \cdot 10^{\text{-6}} \rho_f
\left[ \frac{\Delta H_{\text{c}}} {\Delta H_{\text{v}} + c_p (T_{\text{b}} - T_{\text{f}})} \right] $$

Where:

  • \( \rho_f \) = Density of the fuel \((\text{kg/m}^3)\)
  • \( \Delta H_{\text{c}} \) = Heat of combustion \((\text{kJ/kg})\)
  • \( \Delta H_{\text{v}} \) = Heat of vaporization \((\text{kJ/kg})\)
  • \( c_p \) = Specific heat \((\text{kJ/kg·K})\)
  • \( T_{\text{b}} \) = Boiling point temperature of the fuel \((\text{K})\)
  • \( T_{\text{f}} \) = Actual fuel temperature \((\text{K})\)

Evaporation Rate Calculation

The evaporation rate represents the mass of fuel evaporating per unit area per unit time, expressed in \((\text{kg/m²s})\). It provides a measure of how quickly the liquid fuel transitions into the vapor phase under given conditions.

The evaporation rate is calculated using the following equation: $$ \text{Evaporation Rate} \left(\frac {kg} {m²s}\right) = k \cdot \left( \frac{T_{\text{b}} - T_{\text{f}}}{\Delta H_{\text{v}}} \right) $$

Where:

  • \( k \) = Constant (for average soil and concrete; k = 10.5)
  • \( T_{\text{b}} \) = Boiling point temperature of the fuel \((\text{K})\)
  • \( T_{\text{f}} \) = Actual fuel temperature \((\text{K})\)
  • \( \Delta H_{\text{v}} \) = Heat of vaporization \((\text{kJ/kg})\)

\( \Delta T \) Calculation

The temperature difference, \( \Delta T \), plays a critical role in both the Burn Rate and Evaporation Rate calculations, as it represents the temperature for both processes.

In both cases, \( \Delta T \) is the difference between the boiling point temperature of the fuel (\( T_{\text{b}} \)) and the actual fuel temperature (\( T_{\text{f}} \)).

1. Initial Temperature Loop

The Initial Temperature Loop iteratively adjusts the temperature difference (\( \Delta T \)) to ensure the system reaches a stable state. Starting with an initial fuel temperature, a flash calculation is attempted at \( T_{\text{release}} \) + \( \Delta T \). If the flash succeeds and the vapor fraction is less than 1, (\( \Delta T \)) is increased by 50 K, and the process repeats. If the flash fails, tolerances are adjusted, and the calculation is retried. This loop continues until the vapor fraction is 1.

Here’s a flowchart illustrating the Initial Temperature Loop:

  flowchart TD
    A[Start] --> B[Save state & 
    set vapor_fraction = 0];
    B --> C[Loop: vapor_fraction < 1];
    C --> D[Try flash calculation at 
    T = release + ΔT];
    D --> E{Flash succeeds?};
    E -->|Yes| F{vapor_fraction < 1?};
    F -->|Yes| G[Increase ΔT by 50 K] --> C;
    F -->|No| H[Go to refinement loop];
    E -->|No| I[Adjust tolerances 
    and retries] --> D;

Flowchart illustrating the initial steps of the iterative temperature adjustment process in a flash operation.

2. Refinement Temperature Loop

The Refinement Temperature Loop is a more detailed iteration process that follows the initial temperature loop. It begins by checking if the vapor fraction is greater than or equal to 1. If true, \( \Delta T \) is decreased by 1 K, and a flash calculation is attempted. If the flash succeeds but the vapor fraction remains 1 or above, the loop continues. If the flash fails, tolerances are adjusted, and the calculation is retried. This process repeats until the vapor fraction is less than 1, and a refined temperature is found. Finally, the system returns the \( \Delta T \).

Here’s a flowchart illustrating the Refinement Temperature Loop:

  flowchart TD
    A[Refinement loop] --> B[Loop: vapor_fraction >= 1];
    B --> C[Decrease ΔT by 1 K];
    C --> D[Try flash calculation];
    D --> E{Flash succeeds?};
    E -->|Yes| F{vapor_fraction < 1?};
    E -->|No| G[Adjust tolerances 
    and retries] --> D;
    F --> |No| B[Loop: vapor_fraction >= 1];
    F --> |Yes| H[Refined temperature found];
    H --> I[Return ΔT];

Flowchart illustrating the refinement steps of the iterative temperature adjustment process in a flash operation.