Decaffeination of Coffee and Tea

Reference ID: MET-D159 | Process Engineering Reference Sheets Calculation Guide

Introduction & Context

The calculation presented here supports the design and sizing of a batch supercritical CO₂ extraction (SFE) unit for decaffeinating coffee beans or tea leaves. It quantifies the amount of supercritical solvent required to achieve a specified caffeine removal while preserving key flavor compounds. This mass-balance based approach is essential for:

Estimating CO₂ inventory and recycle loops.
Evaluating solvent loading and process economics.
Ensuring selectivity between caffeine and desirable aroma constituents.

The methodology is applicable to batch extractors, pilot-scale runs, and scale-up studies where equilibrium solubility data are available.

Methodology & Formulas

All symbols are expressed in standard engineering notation. Masses are in kilograms (kg), pressures in bar, temperatures in degrees Celsius (°C), and solubility in kg caffeine / kg CO₂.

Step a – Initial component masses

\[ M_{caf,i} = M_{feed} \cdot \frac{w_{caf,i}}{100} \qquad M_{flav,i} = M_{feed} \cdot \frac{w_{flav,i}}{100} \]

Step b – Target removal and retention

\[ R_{caf} \ (\text{target \%}) \qquad Ret_{flav} \ (\text{target \%}) \]

Step c – Final masses after extraction

\[ M_{caf,f} = M_{caf,i} \left(1 - \frac{R_{caf}}{100}\right) \] \[ M_{flav,f} = M_{flav,i} \left(\frac{Ret_{flav}}{100}\right) \]

Step d – Extracted and lost masses

\[ M_{caf,ext} = M_{caf,i} - M_{caf,f} \] \[ M_{flav,lost} = M_{flav,i} - M_{flav,f} \]

Step e – Minimum CO₂ required

\[ M_{CO2,min} = \frac{M_{caf,ext}}{S} \]

Step f – Solvent loading (caffeine per unit CO₂)

\[ L = \frac{M_{caf,ext}}{M_{CO2,min}} \]

Step g – Selectivity of caffeine over flavor compounds

\[ \beta = \frac{R_{caf}/100}{1 - Ret_{flav}/100} \]

Empirical Validity Checks

Criterion	Requirement	Engineering Rationale
Operating pressure	\(P_{op} \ge P_{crit} = 73.8\; \text{bar}\)	Ensures CO₂ remains supercritical.
Operating temperature	\(T_{op} \ge T_{crit} = 31.1\;^{\circ}\text{C}\)	Maintains supercritical phase.
Caffeine solubility	\(0.001 \le S \le 0.05\; \text{kg kg}^{-1}\)	Based on literature for 100–300 bar and 40–80 °C with 0–10 % ethanol.
Co-solvent concentration	\(w_{cosolvent} \le 10\;\%\)	Limits excessive co-extraction of flavors.
Selectivity	\(10 \le \beta \le 50\)	Feasible range for caffeine over typical aroma compounds under optimized conditions.

Example Calculation (Idealized Batch)

Given a 100 kg feed with 2 % w/w caffeine and 1 % w/w key flavor markers, and targets of 97 % caffeine removal and 95 % flavor retention, the algebraic results are:

\[ M_{caf,i} = 100 \cdot \frac{2}{100} = 2\;\text{kg} \qquad M_{flav,i} = 100 \cdot \frac{1}{100} = 1\;\text{kg} \] \[ M_{caf,f} = 2 \left(1 - \frac{97}{100}\right) = 0.06\;\text{kg} \qquad M_{flav,f} = 1 \left(\frac{95}{100}\right) = 0.95\;\text{kg} \] \[ M_{caf,ext} = 2 - 0.06 = 1.94\;\text{kg} \qquad M_{flav,lost} = 1 - 0.95 = 0.05\;\text{kg} \]

Assuming a solubility \(S = 0.02\;\text{kg kg}^{-1}\):

\[ M_{CO2,min} = \frac{1.94}{0.02} = 97\;\text{kg} \qquad L = \frac{1.94}{97} = 0.02\;\text{kg kg}^{-1} \] \[ \beta = \frac{0.97}{1 - 0.95} = 19.4 \]

Interpretation & Design Guidance

The calculated \(M_{CO2,min}\) defines the minimum CO₂ inventory per batch; actual circulation may be higher to accommodate recycle losses.
A loading \(L\) of 0.02 kg kg⁻¹ aligns with typical operating points for 5 % ethanol co-solvent at 250 bar and 50 °C.
The selectivity \(\beta\) of 19.4 falls comfortably within the feasible range, indicating that the chosen pressure, temperature, and co-solvent level adequately discriminate caffeine from flavor compounds.
If any validity check fails, adjust the operating pressure, temperature, or co-solvent concentration before finalizing equipment sizing.

Industrial decaffeination typically relies on solvent extraction or physical separation processes. The most common methods include:

Direct solvent extraction using methylene chloride or ethyl acetate.
Indirect solvent extraction where beans are soaked in water and the caffeine is removed from the water phase.
Supercritical carbon dioxide extraction, which uses CO₂ as a selective solvent under high pressure.
The Swiss Water Process, which utilizes carbon filtration and solubility gradients without chemical solvents.

Supercritical CO₂ extraction is highly favored by process engineers because it operates at lower temperatures compared to traditional solvent methods. This preserves the volatile aromatic compounds and flavor precursors within the coffee bean. The process involves:

Pressurizing CO₂ beyond its critical point to act as a solvent.
Circulating the fluid through the green coffee bed to selectively dissolve caffeine.
Separating the caffeine from the CO₂ in a secondary vessel.
Recycling the CO₂ back into the system to minimize waste.

To ensure consistent product quality and regulatory compliance, process engineers must strictly control the following parameters:

Solvent-to-bean ratio to ensure efficient mass transfer.
Temperature profiles, as excessive heat can degrade the bean structure.
Contact time, which directly impacts the residual caffeine levels.
Post-extraction drying stages to ensure moisture content returns to the target range of 10 to 12 percent.

Worked Example: Batch Decaffeination of Coffee Beans Using Supercritical CO2

Consider a batch process for decaffeinating 100 kg of coffee beans using supercritical CO₂ extraction enhanced with 5% ethanol as a co-solvent. The beans have an initial caffeine content of 2.0% w/w and key flavor compounds at 1.0% w/w. The operational targets are to remove 97% of the caffeine while retaining 95% of the flavor compounds, under conditions of 250 bar and 50°C, where the equilibrium solubility of caffeine in the scCO₂ mixture is known.

Knowns (Input Parameters):

Feed batch mass, \( M_{\text{feed}} = 100.0 \, \text{kg} \)
Initial caffeine content, \( w_{\text{caf,i}} = 2.0\% \) w/w
Initial key flavor compounds content, \( w_{\text{flav,i}} = 1.0\% \) w/w
Target caffeine removal percentage, \( R_{\text{caf}} = 97.0\% \)
Target flavor retention percentage, \( \text{Ret}_{\text{flav}} = 95.0\% \)
Equilibrium solubility of caffeine in scCO₂ with co-solvent, \( S = 0.02 \, \text{kg caffeine/kg CO}_2 \)
Operating pressure, \( P = 250.0 \, \text{bar} \)
Operating temperature, \( T = 50.0 \, ^\circ\text{C} \)
Co-solvent (ethanol) concentration, \( 5.0\% \) by mass

Step-by-Step Calculation:

Determine the initial masses of caffeine and flavor compounds in the feed.
- \( M_{\text{caf,i}} = M_{\text{feed}} \cdot \frac{w_{\text{caf,i}}}{100} = 100.0 \, \text{kg} \cdot \frac{2.0}{100} = 2.0 \, \text{kg} \)
- \( M_{\text{flav,i}} = M_{\text{feed}} \cdot \frac{w_{\text{flav,i}}}{100} = 100.0 \, \text{kg} \cdot \frac{1.0}{100} = 1.0 \, \text{kg} \)
Compute the final masses in the beans after extraction based on target percentages.
- \( M_{\text{caf,f}} = M_{\text{caf,i}} \cdot \left(1 - \frac{R_{\text{caf}}}{100}\right) = 2.0 \, \text{kg} \cdot \left(1 - \frac{97.0}{100}\right) = 2.0 \, \text{kg} \cdot 0.03 = 0.06 \, \text{kg} \)
- \( M_{\text{flav,f}} = M_{\text{flav,i}} \cdot \frac{\text{Ret}_{\text{flav}}}{100} = 1.0 \, \text{kg} \cdot \frac{95.0}{100} = 1.0 \, \text{kg} \cdot 0.95 = 0.95 \, \text{kg} \)
Calculate the mass of caffeine extracted into the solvent.
- \( M_{\text{caf,ext}} = M_{\text{caf,i}} - M_{\text{caf,f}} = 2.0 \, \text{kg} - 0.06 \, \text{kg} = 1.94 \, \text{kg} \)
Estimate the minimum mass of CO₂ required using the equilibrium solubility.
- \( M_{\text{CO2,min}} = \frac{M_{\text{caf,ext}}}{S} = \frac{1.94 \, \text{kg}}{0.02 \, \text{kg/kg}} = 97.0 \, \text{kg} \)
Derive the solvent loading, defined as caffeine mass extracted per kg of CO₂.
- \( L = \frac{M_{\text{caf,ext}}}{M_{\text{CO2,min}}} = \frac{1.94 \, \text{kg}}{97.0 \, \text{kg}} = 0.02 \, \text{kg caffeine/kg CO}_2 \)
Check the process selectivity, comparing caffeine removal to flavor loss.
- \( \beta = \frac{R_{\text{caf}}/100}{1 - \text{Ret}_{\text{flav}}/100} = \frac{0.97}{1 - 0.95} = \frac{0.97}{0.05} = 19.4 \) (dimensionless)
Verify operating conditions: Pressure \( P = 250.0 \, \text{bar} \) and temperature \( T = 50.0 \, ^\circ\text{C} \) are above the critical point of CO₂ (73.8 bar, 31.1°C), ensuring supercritical state. The co-solvent concentration is within the typical limit of 10%.

Final Answer:

For this batch, the decaffeination process achieves:

Final caffeine mass in beans: \( M_{\text{caf,f}} = 0.06 \, \text{kg} \)
Final flavor mass in beans: \( M_{\text{flav,f}} = 0.95 \, \text{kg} \)
Minimum CO₂ mass required: \( M_{\text{CO2,min}} = 97.0 \, \text{kg} \)
Solvent loading: \( L = 0.02 \, \text{kg caffeine/kg CO}_2 \)
Process selectivity: \( \beta = 19.4 \)

The calculated selectivity of 19.4 falls within the empirical range of 10 to 50 for optimized conditions, indicating effective separation of caffeine from key flavor compounds.

"Un projet n'est jamais trop grand s'il est bien conçu." — André Citroën

"La difficulté attire l'homme de caractère, car c'est en l'étreignant qu'il se réalise." — Charles de Gaulle