Experimental model validation of an integrated process for the ...

chemical engineering research and design 8 9 ( 2 0 1 1 ) 1252–1260 

Contents lists available at ScienceDirect 

Chemical Engineering Research and Design 

j ourna l ho me pa ge: www.elsevier.com/locate/cherd 

Experimental model validation of an integrated process for 

the removal of carbon dioxide from aqueous ammonia 

solutions 

Jan F. Maćkowiak a,∗ , Andrzej Górak b 

a ENVIMAC Engineering GmbH, Im Erlengrund 27, 46149 Oberhausen, Germany 

b TU Dortmund, Chair of Fluid Separations, 44221 Dortmund, Germany 

a b s t r a c t 

In this work modelling and experimental validation of an integrated process for the removal of carbon dioxide from 

ammonia solutions – the so called decarbonisation – is presented. In this process, carbon dioxide and small amount 

of ammonia is stripped out from the solution at ambient pressure in a packed column. Recovery of the stripped 

ammonia can be reached by combining absorption of ammonia and condensation of stripping steam. The integration 

of stripping, absorption and direct-contact condensation (DCC) can be achieved in one compact unit in which 

stripping takes place in the lower part of the packed column, and the DCC and ammonia absorption in its upper part. 

This unit has been modelled in a rigorous way considering heat and mass transfer as well as reaction rates in multicomponent 

reactive stripping, absorption and direct-contact condensation in packed columns (Maćkowiak et al., 

2009). Extensive experimental investigations in a pilot scale packed column with diameters of 0.15 and 0.32 m have 

been performed for both, the stripping and for DCC. Relevant operation parameters as well as column dimensions 

were varied during the experiments in order to investigate their influence on the selectivity of the decarbonisation 

and to achieve a broad data base for the validation. Experimental validation of the two sub-processes and the entire 

decarbonisation shows good agreement between calculated and experimental values. Based on the validated model 

a successful optimisation of the decarbonisation process in industrial scale has been performed, leading to increased 

carbon dioxide removal and reduction of ammonia losses. 

© 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 

Keywords: Condensation; Absorption; Stripping; Rate-based modelling; Carbon dioxide removal; Ammonia 

1. Introduction 

Waste water containing ammonia (NH3) and carbon dioxide 

(CO2) arise in numerous processes in chemical, petrochemical, 

food and environmental industry. Recently it becomes more 

significant in biogas production from renewable resources. 

In many cases ammonia can be recovered from the waste 

water and converted into valuable products through air or 

steam stripping. Ammonia removal can also be achieved by 

biological denitrification, especially in case of low ammonia 

concentrations. An economically feasible ammonia removal 

via stripping requires a pH value higher than 10 in order 

to shift the ammonia dissociation equilibrium (Eq. (1)) to 

molecular species. 

NH3 + H2O ⇄ NH + 

4 + OH + 

In order to keep the pH value of the waste water above 

this value, alkaline solution, mostly caustic soda, is often 

added, although it is undesirable from the economic and 

environmental point of view. The presence of carbon dioxide 

leads to an even increased demand of the caustic soda 

demand because of the acidic nature of carbon dioxide. This 

often leads to economically infeasible ammonia recovery processes. 

If carbon dioxide is selectively removed from the waste 

water, the pH-value is raised due to the deacidification effect 

∗ Corresponding author. 

E-mail address: jan.mackowiak@envimac.de (J.F. Maćkowiak). 

Received 5 October 2010; Received in revised form 21 December 2010; Accepted 25 January 2011 

0263-8762/$ – see front matter © 2011 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. 

doi:10.1016/j.cherd.2011.01.022 

(1)

Nomenclature 

chemical engineering research and design 8 9 ( 2 0 1 1 ) 1252–1260 1253 

Variables 

a specific packing surface area (m 2 m −3 ) 

dc column diameter (m) 

E heat flux (W m −2 s −1 ) 

FV gas capacity factor (Pa 0.5 ) 

h molar enthalpy (kJ mol −1 ) 

˙N i molar flow of component i (mol s −1 ) 

k reaction rate constant (m 3 kmol −1 s −1 ) 

L molar liquid flow (mol s −1 ) 

mD vapour load (kg m −2 h −1 ) 

N molar flux (mol m −2 s −1 ) 

T temperature ( ◦ C) 

u velocity (m s −1 ) 

V molar vapour flow (mol s −1 ) 

x liquid phase molar fraction (mol mol −1 ) 

y vapour phase molar fraction (mol mol −1 ) 

ı film thickness (m) 

�z height of a rate based model stage (m) 

ε void volume of the packing referred to empty 

column (m 3 m −3 ) 

D degree of desorption acc. to eq. 8 (%) 

selectivity acc. to eq. 8 (%) 

Sub- and superscripts 

C condensation 

D desorption 

exp experimental 

GB gas (vapour) bulk 

GF gas (vapour) film 

L liquid phase 

LB liquid bulk 

LF liquid film 

S stripping 

sim simulated 

V vapour phase 

without adding chemicals. Selective carbon dioxide removal 

(decarbonisation) prior to the actual ammonia stripping is 

therefore beneficial. So far, no suitable process for the selective 

ammonia recovery at ambient pressure is known from the 

literature. In some cases, high pressure processes are used 

to take advantage of the different pressure dependency of 

the gas–liquid equilibrium of carbon dioxide and ammonia 

in water to achieve a selective fractionation. At pressure of 

up to 30 bar, only carbon dioxide is desorbed from the solution 

while ammonia remains in the liquid phase (Schmidt, 

1970; Yu et al., 2010). Such processes are not recommended 

for end-of-the pipe waste water treatment in chemical and 

petrochemical industry and are not applicable in stand-alone 

environmental processing, especially in biogas production. 

The DecaStripp © -process allows a decarbonisation at 

ambient pressure. Carbon dioxide is stripped out from the 

solution simultaneously with ammonia. Recovery of the 

stripped ammonia can be reached by combining absorption 

of ammonia and condensation of stripping steam in a subsequent 

step. The interaction of the different unit operations of 

stripping, absorption and direct-contact condensation (DCC) 

can be achieved in one single packed column (see Fig. 1). 

The decarbonisation column consists of two sections which 

are directly interconnected. In the lower section, the pre- 

Fig. 1 – Decarbonisiation process for selective carbon 

dioxide removal at ambient pressure. 

heated solution is fed in on top and steam from the reboiler 

flows counter-currently from the bottom, carrying the desorbed 

components toward the upper part. In the upper section, 

the steam is condensed directly into the condensing agent and 

ammonia is absorbed simultaneously in the liquid. To avoid 

re-absorption of the carbon dioxide, the DCC-section has to 

be carefully designed to keep the low soluble carbon dioxide 

in the vapour phase. A high selectivity of the CO2 removal 

is desired in the desorption part in order to achieve a high 

degree of decarbonisation. The decarbonised solution leaves 

the column at the bottom and can be further treated in order 

to recover ammonia. The efficiency of the entire ammonia 

recovery process is significantly affected by the design of the 

decarbonisation stage. 

The modelling of this complex integrated process requires 

an adequate, experimentally validated modelling framework. 

The application of the rate-based approach for the modelling 

of multicomponent reactive separation processes has proved 

to be superior to other concepts like the equilibrium stage 

model. The rate-based approach is physically more consistent 

and overcomes the drawbacks of multicomponent efficiencies 

or HETP concept. 

The relevance of a model which shall be used for 

design purposes is highly affected by the quality of available 

experimental data used for the model validation. For 

many relevant industrial processes, data from the literature 

is scarce. The same is true for the NH3–CO2–H2O system, 

especially in connection with a selective carbon dioxide 

removal from aqueous solutions. In order to provide data 

for the presented model extensive experiments have to be 

performed. 

2. Rate-based modelling 

The core element of the rate-based model is an axial segment 

of a packed column in which simultaneous mass transfer and

1254 chemical engineering research and design 8 9 ( 2 0 1 1 ) 1252–1260 

Fig. 2 – Structure of an axial segment of the column according to the film theory. 

chemical reactions are described according to the extended 

film model (see Fig. 2). The model is based on separately formulated 

mass and heat balances for the vapour and liquid 

phases (Kenig and Górak, 1995; Kucka et al., 2003). These balances 

are linked by the interfacial heat and mass fluxes, E 

and N i, which are equal for both phases. Thermodynamic 

equilibrium between the phases is assumed at the interface. 

According to the film model, the resistance to mass and heat 

transfer is placed in thin films adjacent to the phase boundary. 

Chemical reactions in the system NH3–CO2–H2O influence 

the ammonia and carbon dioxide concentrations in the liquid 

phase and thus the gas–liquid equilibrium. Reaction equilibrium 

and kinetics are considered in both the liquid film 

and bulk phase. Usually, the gas-liquid equilibrium in the 

NH3–CO2–H2O system is determined by 5 key reactions (Eqs. 

(3)–(7), see e.g. Edwards et al., 1978). As kinetics of the reaction 

are considered as well in the developed model, the reaction of 

carbon dioxide with hydroxide ions (Eq. (2)) becomes significant 

in the studied caustic pH value range (Danckwerts, 1970; 

Pohorecki and Moniuk, 1988). 

CO2 + OH − 

kf 1 

⇄ 

krev1 HCO − 

3 

k f 2 

⇄ 

NH3 + HCO − 

3 NH2COO 

krev2 − + H2O (3) 

kf 3 

CO2 + 2H2O ⇄ HCO 

krev3 − 

3 + H3O + 

NH3 + H2O ⇄ NH + 

4 + OH − 

HCO − 

3 + H2O ⇄ H3O + + CO 2− 

3 

2H2O ⇄ H3O + + OH − 

In multicomponent mixtures the component fluxes across 

the interface are usually calculated using the Maxwell–Stefan 

equations, which consider diffusional interactions between 

the particular species, which can affect the transport rates 

(Taylor and Krishna, 1993). In a previous work (Maćkowiak 

et al., 2009), the linearized theory presented by Toor (1964) 

and Stewart and Prober (1964) has been applied to handle the 

Maxwell–Stefan equations for the diffusive mass transfer in 

the film region. Its main assumption, which has proven to be 

of excellent accuracy for the majority of mass transfer processes, 

is that diffusion coefficients remain constant in the 

film region. Several works (Huepen and Kenig, 2005; Kucka 

(2) 

(4) 

(5) 

(6) 

(7) 

et al., 2003; Thiele et al., 2007) showed that simplified models 

like the effective diffusivity method are sufficient in order to 

describe most reactive absorption processes because of negligible 

diffusional interactions between components. This is 

basically due to the diluted systems often applied in absorption 

and is as well true for (reactive) desorption processes 

(Maćkowiak et al., 2009). The complexity of the model and calculation 

time can therefore be reduced. This assumption holds 

as well for the desorption process applied in decarbonisation 

of waste water. A detailed comparison of the two models 

showed, that deviations between the two calculation methods 

can be neglected for the stripping section of the decarbonisation 

process (Maćkowiak et al., 2009). 

This does not hold for multicomponent DCC, which is used 

in the decarbonisation process. Here, the (at least) ternary 

vapour phase is usually not diluted and diffusional interactions 

may not be neglected. Deviations in mass transfer 

fluxes between the Maxwell–Stefan and the effective diffusivity 

method increase for the DCC compared to the desorption 

Fig. 3 – Test facilities consisting of various test columns for 

vapour(gas)/liquid separation processes with diameters 

from 0.15 to 0.5 m and packing height of up to 10 m.


Fig. 4 – Experimental set up for the investigation of desorption (steam stripping) and direct-contact condensation processes 

in the NH3–CO2–H2O system. 

process. This causes, for practical purposes even more important, 

a significant difference between the calculated vapour 

outlet concentrations of ammonia, carbon dioxide and water 

which in some cases exceeds 25%. These deviations which 

have been observed during the theoretical analysis of the 

process (Maćkowiak et al., 2009), can be regarded as critical 

for column design, especially when ammonia emission limits 

have to be obeyed. Therefore the effective diffusivity method is 

applied to calculate mass transfer fluxes in the stripping section 

and the Maxwell–Stefan equations for the DCC section in 

the decarbonisation process. 

The models have been implemented into the commercial 

simulation environment Aspen Custom Modeler © , which 

provides a fully numerical algorithm to solve the system of 

rate-based equations. The simulation tool offers a direct link 

to the software package Aspen Properties © for the calculation 

of the required physical properties. A subdivision of the 

liquid and the gas/vapour film into discrete elements allows 

the description of the nonlinear concentration and temperature 

profiles. Packing specific fluid dynamics (Maćkowiak, 

2009), relevant mass transfer correlations (Maćkowiak, 2008) 

and the component specific reaction rates are accounted for 

by adequate sub-models. The description of external elements 

like reboilers and heat exchangers complement the model. 

Detailed model equations are shown elsewhere (Maćkowiak 

et al., 2009). 

3. Experimental set-up 

In order to provide relevant data for a model validation, an 

extensive experimental investigation has been performed in 

pilot scale packed columns. Therefore semi-industrial scale 

packed columns have been used to study desorption via steam 

Table 1 – Varied parameters during the experimental analysis of the desorption and DCC process using lattice type 

McPac 1 and 2 random packings. 

Desorption and DCC Symbol (unit) Range 

McPac random packings a (m2 m−3 )/ε (m3 m−3 ) 90–185/0.97 

Column diameter dc,i (m) 0.15–0.316 

Packing height for each segment H (m) 0.9–5.3 

Liquid load uL (m3 m−2 h−1 ) 6–26 

Gas capacity factor FV (Pa0.5 ) 0.1–2.5 

Liquid feed concentrations (desorption) cin /c NH3 in 

Vapour feed concentrations (DCC) 

(mol/l) CO2 y 

(0.12–0.31)/(0.08–0.27) 

in /y NH3 in 

Liquid feed temperature 

(vol.%) CO2 T ( 

(1–5)/(1–25) 

◦C) 80–98


Fig. 5 – Simulated concentration profiles and experimental 

values of a desorption experiment with high selectivity in 

the NH3–CO2–H2O system. 

stripping and the DCC process in the system NH3–CO2–H2O 

separately. The separate investigation of the two unit operations 

avoids interactions of the varied operating parameters 

which might occur due to the interconnection of both processes 

in the decarbonisation column. 

The test columns are part of a fully automated R&D facility 

for various fluid separation processes operated by Envimac 

Engineering, see Fig. 3. The test columns have been especially 

designed for flexible adjustment to the requirements of distillation, 

absorption, stripping and direct-contact condensation. 

Both processes studied in this work have been integrated in 

one steel column with a column diameter of dc,S = 0.316 m and 

dc,C = 0.150 m and a total height of up to 10 m, see Figs. 3 and 4. 

A steam generator supplies steam for both heating and stripping, 

it can be used to heat up the reboiler or it is fed directly 

into the bottom of the multifunctional column. Above, the 

stripping section is installed. The vapour mixture of the stripping 

section is used as feed for the DCC section. 

In order to avoid interactions, the condensate from the 

upper section is collected on a collector tray and removed from 

the column via a side stream. The vapour passes the collector 

tray through chimneys. The condensing agent enters the 

column at ambient conditions on top of the column. 

The pilot plant is equipped with state-of the art measurement 

devices and a highly automated Siemens S7 process 

control system. This ensures high accuracy and reproducibility 

of the experiments, shown by the adequate relative 

deviation in total mass balance of maximal ±4% and ±8% 

for both the carbon dioxide and ammonia liquid side balance. 

The ammonia concentrations in the liquid phase have 

been detected by a photometer, the carbon dioxide contents 

by titration. 

As a random packing, McPac high performance packings 

of two different sizes have been chosen for the experiments 

(Maćkowiak, 2001). These packings have a very low tendency 

for fouling and thus are especially suitable for “dirty” liquids 

as they often occur in industrial separation processes. 

Fig. 6 – (a) Experimental degree of desorption of ammonia 

and carbon dioxide as a function of the vapour load. On the 

right axis additionally the resulting selectivity is shown. (b) 

Experimental and simulated degrees of desorption for both 

components as well as corresponding selectivity as a 

function of liquid feed temperature. Error bars indicate 

±10% deviation. 

The size of the test columns requires an appropriate 

amount of feed solution, which therefore was produced by dissolving 

NH4HCO3 in a 10 m3 feed tank. The feed concentration 

of ammonia cNH varied between 2 and 5 g/l, which is typical 

3 

for many kinds of waste waters, e.g. from biogas or coking 

industry. The main objective of the desorption part is a high 

selectivity of carbon dioxide compared to ammonia removal 

efficiency. The selectivity CO2,NH is calculated based on the 

3 

removal efficiencies i 

D as follows: 

CO2 D 

CO2,NH = 3 − NH3 D 

CO2 D + NH3 D 

with 

i 

D = 1 − 

˙N L 

i,out 

˙N L 

× 100% (8) 

i,in 

The subscript D denotes that efficiency refers to the desorption 

process. 

In order to analyse the separation efficiency of the stripping 

part, the influencing parameters like liquid and vapour load,


packing height and type, feed concentration and temperature 

were varied over a wide range (Table 1). The experiments in 

the condensing section are used to gain detailed understanding 

of the condensation process accompanied with absorption 

in packed columns. DCC in the system NH3–CO2–H2O can be 

regarded as condensation in the presence of inerts, as the solubility 

of carbon dioxide in water is small enough under the 

studied conditions to be neglected. Literature about multicomponent 

condensation processes with inerts in packed columns 

is very scarce, what justifies extensive experimental studies. 

This leads to a database consisting of 50 data records for the 

desorption process and 39 for the DCC process. 

4. Process analysis and model validation 

Based on the experimental results data reconciliation has 

been performed for both the stripping and the DCC process 

in order to provide consistent input data for the simulations. 

Therefore the measured flow rates and concentrations were 

varied within their specific tolerances until consistent component 

balances were achieved. The analysis of the experiments 

is based on the consideration of removal efficiencies for both 

ammonia and carbon dioxide and the resulting selectivity. The 

following chapters describe the process analysis and the validation 

of the rate based models separately, according to the 

performed experiments. 

4.1. Stripping section 

The presented rate-based model allows for the description of 

concentration and temperature profiles along the packed column. 

A comparison of calculated concentration profiles and 

experimental output at reconciled input values for a desorption 

experiment is shown in Fig. 5. It shows that most of the 

carbon dioxide is already desorbed near the top of the column. 

Even low carbonate concentrations cause an increased chemical 

demand in the following ammonia removal steps, thus 

the bed length should be long enough to achieve maximum 

removal efficiency. Furthermore, it shows that the desorption 

rate of ammonia can be held to a small value, when operating 

conditions are chosen appropriately. Thus, high selectivities 

of around 90% can be achieved. 

The gas and liquid load has significant influence on the 

selectivity of ammonia and carbon dioxide removal from the 

liquid. Mass transfer of NH3 in the binary NH3–H2O system is 

known to be controlled by gas and liquid side resistance, thus 

vapour load has a larger influence on ammonia removal compared 

to the system CO2–H2O, which is basically controlled 

by the liquid side resistance. This effect is shown in Fig. 6a, 

the degree of desorption of ammonia is more sensitive to 

changes of the vapour load than the corresponding value for 

carbon dioxide. Fig. 6b shows the influence of the liquid feed 

temperature on the desorption process. Beside the experimental 

values the simulated values are displayed additionally as 

lines in this diagram. High temperatures close to the boiling 

point of the solution thus enhance both, ammonia and carbon 

dioxide desorption. The different dependency on temperature 

changes leads to an optimal value in respect of the selectivity 

of around 90 ◦ C under the studied conditions. The simulated 

values agree well within ±10% with the experiments. 

During the experiments a dependency between the desorption 

of carbon dioxide and ammonia, and the inlet 

concentration ratio of both components has been observed. 

Fig. 7 – (a) Parity plot for all performed desorption 

experiments compared to the rate-based model based on 

removal efficiencies according to Eq. (2). (b) Corresponding 

parity plot for the selectivity of the carbon dioxide 

stripping. 

Increasing ratio of carbon dioxide to ammonia inlet concentration 

leads to higher selectivity of the separation process due to 

increased carbon dioxide and decreasing ammonia removal. 

The observed dependency cannot be explained by consideration 

of only binary mass transfer effects. As a consequence, the 

carbamate-reaction describing chemical interaction between 

ammonia and carbon dioxide (Eq. (3)) should not be neglected 

even at high temperatures of up to 100 ◦ C. 

To summarise all performed experiments, Fig. 7a shows a 

parity plot for all desorption experiments based on degrees 

of desorption for both ammonia and carbon dioxide and the 

simulated values. It can be stated that the model reproduces


Fig. 8 – Flow profile of NH3 and CO2 along the packed bed 

in the DCC section. 

the experiments within a maximum deviation of ±20% for the 

conditions shown in Table 1. The average deviation between 

experimental and simulated values is 7.4% for the carbon dioxide 

values and 12% for the ammonia values. This difference is 

caused by the different accuracy of the applied specific analytic 

method. The model can therefore be used for design 

as well as for optimisation of desorption processes. Fig. 7b 

displays the resulting selectivities during the experiments 

compared to the simulated values. 

Fig. 7b shows, that during the performed experiments not 

only high selectivites of up to 90% have been achieved, but 

not appropriate operating conditions may lead to very low 

selectivities, mainly caused by to high ammonia desorption 

rates. Even negative values of the selectivity were observed, 

in this case the degree of desorption of ammonia exceeds 

the value for carbon dioxide. This fact underlines that selective 

desorption of carbon dioxide from aqueous ammonia 

solutions is possible, but operating conditions have to be 

carefully designed because of various interacting parameters, 

for this purpose the developed rate based model can be 

used. 

4.2. Direct contact condensation section 

Fig. 8 shows a typical flow profile for the total condensation 

of steam and absorption of ammonia, where steam and 

ammonia concentration in the vapour phase is reduced to a 

minimum, in comparison the carbon dioxide amount remains 

almost constant. This proves the assumption of carbon dioxide 

being a quasi-inert in the studied vapour mixture. 

Molar flow rates are chosen to display the condensation 

and selective absorption along the packed bed because molar 

overflow of the vapour decreases significantly during the 

condensation process and thus concentration profiles have 

limited meaning. 

Compared to the DCC of pure vapours, which has been 

experimentally investigated in a different work, the presence 

of NH3 and CO2 affects noticeably the condensation of 

Fig. 9 – (a) Parity plot for liquid outlet molar flow of 

ammonia in the direct contact condensing section. (b) 

Corresponding parity plot of experimental and simulated 

vapour outlet temperatures. 

steam in the multicomponent mixture. Carbon dioxide can 

be regarded as a quasi-inert, which increases the mass transfer 

resistance in the vapour phase compared to the pure 

vapour. Therefore, the condensation efficiency decreases with 

increasing inert concentration what has to be considered 

during the design of the packed bed condenser. For the condensation 

of vapour from a mixture containing 10 vol.% of 

an inert, den necessary condenser area might be up to 80% 

larger than for the condensation of the same amount of pure 

vapour. This results in a 80% higher packing height in case 

of packed bed condenser with constant column diameter. 

Such effects often cannot be described accurate enough using 

simple, basically empirical design correlations, because dif-

Fig. 10 – Typical temperature profile of vapour and liquid 

phase along the packed bed in the DCC section compared 

to the experimental values. 


fusional interactions, simultaneous absorption of ammonia 

with its significant enthalpy of solution and design parameters 

influence the condensation process simultaneously, what 

can be considered using a rate-based model. 

For the DCC in packed columns, so far no reliable data 

for multicomponent condensation has been published. The 

parity plot in Fig. 9a shows that the experiments performed 

in this work can be reproduced with a similar precision as 

the desorption experiments. In condensation processes the 

correct calculation of the temperatures plays a major role 

in condensation processes compared to the steam stripping 

process, where usually no significant temperatures gradients 

occur along the column. Therefore, beside the mass transfer 

related comparison Fig. 9b shows a parity plot of experimental 

and simulated vapour outlet concentrations, which again 

show a satisfying accuracy. 

The presented rate-based model has therefore been validated 

for the first time for direct contact condensation 

processes characterised by large temperature gradients along 

the column. A typical temperature profile for the studied condensation 

process is shown in Fig. 10. 

Due to the inert, the vapour temperature decreases during 

the condensation process, leading to smaller driving forces for 

heat transfer compared to the condensation of pure vapours. 

For design purposes the rate based model is strongly recommended. 

5. Conclusions 

A process for the selective removal of dissolved carbon dioxide 

from ammonia solutions (decarbonisiation) has been presented. 

The process consists of two steps, a desorption part, 

where carbon dioxide is stripped by means of steam and a 

DCC part, where the steam is condensed and gaseous ammonia 

is absorbed in a packed column. To describe the reactive 

multicomponent separation processes a rate-based model for 

both steps has been developed based on previous works. The 

model has been implemented into Aspen Custom Modeler 

and features Maxwell–Stefan equations to handle multicom- 

ponent mass transfer for the DCC process as well as the 

simplified effective diffusivity method which can be applied 

with high accuracy for desorption processes in the system 

NH3–CO2–H2O. An extensive experimental analysis of both 

desorption and the DCC identified the sensitive operating 

parameters influencing the selectivity of carbon dioxide to 

ammonia removal for the desorption and the DCC. The experiments 

have been performed in pilot scale packed columns 

with diameter from 0.15 to 0.316 m and a total column height 

of up to 10 m. Input concentrations of ammonia and carbon 

dioxide and the vapour and liquid load have great influence 

on the selectivity of carbon dioxide removal. In contrast to the 

binary system CO2–H2O, in which transfer resistance is located 

mainly the liquid phase, the vapour load affects the ammonia 

removal significantly. The ammonia removal efficiencies thus 

vary from 3 to 80% depending on the operation point of the 

column. As a result, selectivities of up to 90% can be achieved 

in the decarbonisation process, resulting in an increase of the 

outlet pH-value of up to 10.5, which is high enough to reduce 

the chemical demand in the following ammonia removal step 

by 90%. 

The experimental data has been successfully used to validate 

the models, which is characterised by a satisfying 

accuracy under the studied conditions of max. ±20%. The 

range of operating conditions and the number of 50 data sets 

for the desorption and 39 sets for the DCC allow a most convincing 

validation for reactive stripping and direct contact 

condensation in packed columns resulting in a powerful tool 

for optimisation and design purposes. 

Acknowledgements 

We highly appreciate the support of ENVIMAC Engineering 

GmbH as well as the Ziel-2-Projekt of Nordrhein-Westfalen 

and the European Union. 

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Experimental model validation of an integrated process for the ...

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