*Calculations are based on the Mackay and Matsugu correlation for mass transfer coefficients.
How to Calculate the Evaporation Rate of a Solvent
In industrial chemistry and laboratory safety, determining how quickly a solvent evaporates is critical for assessing exposure risks, drying times for coatings, and fire hazards. The evaporation rate is influenced by the chemical's intrinsic properties and environmental conditions.
Key Factors Influencing Evaporation
Vapor Pressure: This is the most significant factor. Chemicals with high vapor pressure (like Acetone) evaporate much faster than those with low vapor pressure (like Water or Mineral Spirits).
Molecular Weight: Heavier molecules generally evaporate more slowly than lighter ones under identical conditions.
Temperature: As temperature increases, vapor pressure rises exponentially, significantly accelerating evaporation.
Airflow (Wind Speed): Moving air removes the saturated vapor layer from the liquid's surface, maintaining a steep concentration gradient and increasing the rate.
Surface Area: Evaporation is a surface phenomenon; doubling the exposed surface area doubles the evaporation rate.
The Mathematical Formula
A widely used scientific model for calculating mass evaporation rate (E) is based on the mass transfer coefficient (k):
E = (k × A × M × P) / (R × T)
Where:
k: Mass transfer coefficient (influenced by wind speed).
A: Exposed surface area.
M: Molecular weight of the solvent.
P: Vapor pressure.
R: Ideal gas constant.
T: Absolute temperature in Kelvin.
Practical Example
If you have 0.5 square meters of Acetone (M = 58.08 g/mol) exposed at 25°C where the vapor pressure is approximately 231 mmHg, and a slight breeze of 0.5 m/s is present:
Vapor Pressure: 231 mmHg
Molecular Weight: 58.08
Area: 0.5 m²
Calculated Rate: Approximately 1,250 grams per hour.
What is Relative Evaporation Rate?
In many Safety Data Sheets (SDS), you will see a value for "Relative Evaporation Rate." This is a dimensionless number where n-Butyl Acetate is assigned a value of 1.0.
Fast Evaporators: > 3.0 (e.g., Acetone, Methyl Ethyl Ketone).
Medium Evaporators: 0.8 to 3.0 (e.g., Ethanol, Toluene).
function calculateEvaporation() {
var p = parseFloat(document.getElementById('vaporPressure').value);
var mw = parseFloat(document.getElementById('molWeight').value);
var tC = parseFloat(document.getElementById('tempC').value);
var area = parseFloat(document.getElementById('surfaceArea').value);
var wind = parseFloat(document.getElementById('windSpeed').value);
var resultDiv = document.getElementById('results');
if (isNaN(p) || isNaN(mw) || isNaN(tC) || isNaN(area) || isNaN(wind)) {
alert("Please enter valid numeric values for all fields.");
return;
}
// Convert Celsius to Kelvin
var tK = tC + 273.15;
// Mass transfer coefficient calculation (Simplified Mackay and Matsugu)
// k (m/s) = 0.0173 * (u^0.78) * (MW^-0.33)
// We ensure wind speed is at least 0.01 to avoid zero issues
var effectiveWind = Math.max(wind, 0.01);
var k = 0.0173 * Math.pow(effectiveWind, 0.78) * Math.pow(mw, -0.33);
// Gas constant R in (mmHg * m3) / (K * mol) is ~62.36
// Rate (g/s) = (k * A * MW * P) / (R * T)
var ratePerSec = (k * area * mw * p) / (62.36 * tK);
// Convert g/s to g/hour
var ratePerHour = ratePerSec * 3600;
// Relative Evaporation Rate Calculation (Approximation relative to n-Butyl Acetate)
// n-BuAc: P ~ 10 mmHg at 20C, MW 116.16
// This is a complex ratio, standard simplification: RER = (P_solvent * MW_solvent) / (P_buac * MW_buac)
// Usually, the ratio of (P*MW) is the primary driver
var pBuAc = 10;
var mwBuAc = 116.16;
var relRate = (p * Math.sqrt(mw)) / (pBuAc * Math.sqrt(mwBuAc));
// Display results
document.getElementById('massRate').innerText = ratePerHour.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
document.getElementById('relRate').innerText = relRate.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
resultDiv.style.display = 'block';
resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}