# Experimental Investigation of Heat Losses in a Pilot-Scale Multiple Dividing Wall Distillation Column with Three Parallel Sections

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## Abstract

**:**

## 1. Introduction

- Weak spots with high heat losses can be identified and improved in order to increase the thermal efficiency of the column.
- The quality of simulation predictions can be enhanced. On the one hand, the heat losses can be included in simulations, whereby their influence on the internal vapor and liquid flows is considered. On the other hand, the separation efficiency of the packing, and thus, the number of theoretical stages of the column, depends on the vapor load, which in turn is reduced by heat losses. Recent studies show the impact of heat losses on the separation efficiency of structured packings [14]. Including the impact of heat losses on the internal flows also enhances the prediction of the number of theoretical stages.
- The experimental measurement of the vapor split ratio at the dividing walls can be enhanced. In the case of parallel segments, the vapor flow divides in such a way that the pressure drop is identical on both sides [6,15]. The resulting pressure drop depends on the vapor load (F-factor), the liquid load in the segments and the internal column components. With knowledge about the pressure drop as a function of these three parameters in combination with an experimentally measured pressure drop and an estimated liquid load, an important, but often neglected, operational variable can be measured experimentally: the vapor split ratio at the dividing wall. Here, the estimation of the liquid load in all segments can be enhanced considering the heat losses throughout the column height.

^{−1}. Ehlers et al. [19] present results from the same column; however, the parallel segments were reconstructed as separate, cylindrical ones with an inner diameter of 54 mm that are connected to the top and bottom segments by y-pieces. They measured higher heat losses of 295 W throughout the column height (with a comparable temperature range), which was most likely caused by the higher surface area of the parallel segments.

## 2. Materials and Methods

#### 2.1. Pilot Plant

#### 2.2. Measurement of Energy Input of Heating Rods and Heat Losses of the Evaporator

_{steel}and liquid m

_{liq}, their corresponding heat capacities c

_{p}and the temperature T increase in a certain time t interval. The mass of the steel shell and the heating rods was assumed to be 28 kg, and the mass of the liquid in the evaporator was determined with the liquid level sensor in the evaporator and the geometry of the shell. The temperature dependent specific heat capacities were calculated as described in Appendix A. The error was determined by error propagation, considering the uncertainties of the masses of the evaporator itself and of the liquid (see Table 1 for values).

_{i}, their thickness and the heat transfer coefficients α

_{i}. Further, T

_{env}is the environmental temperature, and T

_{evap,m}is the average temperature inside the evaporator according to Equation (3).

#### 2.3. Measurement of Heat Losses throughout Column Height

_{v}of the evaporated component butanol is required, for which 586.1 J/g (at boiling conditions) was used in this work [24]. At the top of the column, the vapor was condensed and totally withdrawn. Its flow was measured with a mass flow meter (see Table 1) before it flowed back into the evaporator. This procedure was chosen instead of total reflux, as the reflux flow cannot be measured in the column setup due to the missing distillate cycle. The distillate stream cools down the liquid inside the evaporator slightly; however, compared to the overall mass of the boiling liquid, the distillate flow is rather small. The difference between the evaporated mass flow according to Equation (5) and the measured distillate flow was condensed inside the column due to heat losses. Thus, the heat loss throughout the column can be determined using Equation (6).

## 3. Results and Discussion

#### 3.1. Actual Energy Input of Heating Rods

#### 3.2. Heat Losses of the Evaporator

#### 3.3. Heat Losses throughout Column Height

## 4. Summary and Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

List of variables | ||

Variable | Description | Unit |

A | Area | m^{2} |

A − F | Parameters in c_{p} correlation | - |

c_{p} | Heat capacity | Jkg^{−1}K^{−1} |

k | Heat transfer coefficient | Wm^{−2}K^{−1} |

$\dot{m}$ | Mass flow | kg/m |

P | Power setting of heating rods | % |

$\dot{Q}$ | Heat flow | W |

T | Temperature | K |

t | Time | s |

Δh_{v} | Heat of vaporization | Jkg^{−1} |

List of indices | ||

Variable | Description | |

col | column | |

c | critical | |

dist | distillate | |

env | environment | |

evap | evaporator or evaporated | |

exp | experimental | |

in | input | |

i | inner | |

liq | liquid | |

loss | loss | |

m | average | |

steel | steel | |

th | theoretical |

## Appendix A

^{−1}K

^{−1}] of a liquid is calculated according to Equation (A1) [26].

_{C}is the critical temperature. The letters from A to F are parameters that are fitted to the pure component data. In this work, data from the VDI heat atlas for butanol are used [26]: A = 3.3654, B = 31.6732, C = −30.7096, D = −46.5224, E = 71.5993, F = −2.6518, R = 0.112 Jg

^{−1}K

^{−1}and T

_{C}= 563.05 K. As the specific heat capacity of iso-butanol, which was also present with a low content around 5% in the liquid, is very similar to the one of butanol, its effect was neglected.

^{−1}K

^{−1}, and at 100 °C, it is 0.50 Jg

^{−1}K

^{−1}[26]. To include the temperature dependence of the value in the calculation, linear interpolation was performed between these two values at the corresponding temperature.

## Appendix B

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**Figure 1.**(

**a**) Scheme of the multiple dividing wall column and (

**b**) of the pilot plant during normal operation. (

**c**) Reconstruction for measurement of heat losses over column height. A–D refer to the components in the feed mixture sorted according to their boiling points, where A is the light boiler.

**Figure 2.**Experimental determination of energy input in evaporator. (

**a**) Temperature increase at different reboiler power settings. (

**b**) Resulting estimated energy input. * Note that the data at 30% was determined between 30 and 40 °C. The temperature development was shifted from 20 to 30 °C for better comparability.

**Figure 3.**Experimental determination of heat losses at evaporator. (

**a**) Observed heat input in evaporator at elevated inner temperatures (trendlines added as visual aid) and (

**b**) resulting heat transfer coefficients.

**Figure 4.**Actual energy input provided by the heating rods to evaporate butanol mixture (boiling point: 115 °C).

**Figure 5.**(

**a**) Image of the flange of the heating rods seen from top and thermal images of the heating rods after (

**b**) 5 min (T

_{i}= 40 °C), (

**c**) 15 min (T

_{i}= 80 °C) and (

**d**) 30 min (T

_{i}= 113 °C) of operation at 70% of full power. Color indicates temperature according to bar on right side.

**Figure 6.**(

**a**) Picture of the insulated evaporator with heating rods on the back and (

**b**) thermal images of the evaporator shell after 140 min (steady state, T

_{i}= 115 °C). Color indicates temperature according to bar on right side.

**Figure 7.**(

**a**) Expected distillate flow without heat losses and actually measured ones for different energy inputs and (

**b**) heat losses throughout the column height at different evaporator power and heating jacket temperature settings.

**Figure 8.**Observed heat transfer coefficient throughout the column height for different temperature difference setpoints of the heating jackets to inner column temperature.

**Figure 9.**Main sources of heat losses at the column identified with thermal imaging camera after 140 min of an experiment with heating jackets turned off. (

**a**) Flange above evaporator; (

**b**) liquid splitter; (

**c**) flange for pressure difference measurement. Color indicates temperature according to bar on right side.

Group | Description | Value |
---|---|---|

Column specifications | Material | Glass |

Insulation | Mirrored vacuum jacked and external heating (SAF Wärmetechnik KM-HJ-450) | |

Inner column diameter | 9 × 50 mm and 2 × 80 mm | |

Packing height per segment | 1.06 m | |

Inner surface area of packed parts | 2.03 m^{2} | |

Outer surface area of packed parts | 3.86 m^{2} | |

Column height | 9.6 m | |

Temperature sensors | Greisinger GTF 103-EX (accuracy class B) | |

Mass flow meters | Yokogawa ROTAMASS Nano | |

Evaporator specifications | Material | Stainless steel 1.4571 and 1.4876 |

Insulation | 16 mm Armaflex | |

Heating rods | Heatsystems F D100, max. 10 kW | |

Height | 300 mm | |

Width | 150 mm | |

Length | 1100 mm | |

Maximum filling volume | 38 L | |

Surface area | 0.93 m^{2} | |

Estimated mass | 28 ± 10 kg | |

Liquid level sensor | VEGA VEGAFLEX 81 | |

Operating conditions | Evaporated liquid | n-Butanol (impurities of i-Butanol) |

Mass of liquid | 23–27 kg ± 2 kg (see supporting information) | |

Heat of vaporization of liquid Δh_{v} | 586.1 J/g [24]. | |

Pressure | 920–960 mbar | |

Boiling point of liquid in evaporator | 115.4–116.6 K |

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**MDPI and ACS Style**

Ränger, L.-M.; Waibel, Y.; Grützner, T.
Experimental Investigation of Heat Losses in a Pilot-Scale Multiple Dividing Wall Distillation Column with Three Parallel Sections. *ChemEngineering* **2023**, *7*, 68.
https://doi.org/10.3390/chemengineering7040068

**AMA Style**

Ränger L-M, Waibel Y, Grützner T.
Experimental Investigation of Heat Losses in a Pilot-Scale Multiple Dividing Wall Distillation Column with Three Parallel Sections. *ChemEngineering*. 2023; 7(4):68.
https://doi.org/10.3390/chemengineering7040068

**Chicago/Turabian Style**

Ränger, Lena-Marie, Yannick Waibel, and Thomas Grützner.
2023. "Experimental Investigation of Heat Losses in a Pilot-Scale Multiple Dividing Wall Distillation Column with Three Parallel Sections" *ChemEngineering* 7, no. 4: 68.
https://doi.org/10.3390/chemengineering7040068