University of Groningen
Absorption of formaldehyde in water
Winkelman, Jozef
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to
cite from it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2003
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Winkelman, J. G. M. (2003). Absorption of formaldehyde in water Groningen: s.n.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 18-06-2017
Chapter 1: Introduction
Chapter 1
Introduction
This thesis describes theoretical and experimental work on the absorption of formaldehyde
in water and the development of chemical engineering models for the description and
optimization of industrial formaldehyde absorbers. This introduction gives a short description of
the industrial formaldehyde production process, and of the formaldehyde absorption step therein.
The introduction ends with an outline of the other chapters and appendices.
Formaldehyde is an important base chemical in the process industry with a world
production rate of approximately 10 million metric tons annually (Weirauch, 1999). Historically,
one of the first important applications was in the production of artificial indigo. Nowadays, its
main applications are in the production of engineering plastics and resins, especially urea, phenol
and melamine resins, of which large quantities are used in the plywood and particle board
manufacturing industry, and also in the manufacturing of rubber, paper, fertilisers, explosives,
preservatives, etc. (Walker, 1964; Cancho et al., 1989).
Formaldehyde is industrially produced from methanol. The production is perfomed at
approximately atmospheric pressure. Three major steps can be identified, see Fig 1. In a first
step, the liquid methanol is vaporized into an air stream, and steam is added to the resulting
gaseous mixture. In a second step, the gaseous mixture is lead over a catalyst bed, where the
methanol is converted to formaldehyde via partial dehydrogenation and partial oxidation. The
temperature of the gaseous product increases to approximately 870 K because of the highly
exothermic character of the conversion of methanol to formaldehyde.
tail gas
water
steam
air
methanol
55 wt% formalin
VAPORIZER
REACTOR
ABSORBER
Fig. 1. Simple scheme of the formaldehyde production process.
1
Chapter 1: Introduction
To prevent thermal decomposition of formaldehyde, the gas stream is cooled directly after
passing over the catalyst. In a third step, the formaldehyde is absorbed in water in an absorption
column, because formaldehyde in its pure, gaseous form is highly unstable, and also because the
reactor product stream contains the formaldehyde diluted in other gases, mainly nitrogen. From
the absorber, the commercial product is obtained: an approximately 55% by weight solution of
formaldehyde in water, or formalin.
The design, operation and optimization of formaldehyde absorbers is complicated by a
number of factors, of which two important ones are the reactions in the liquid phase and the
exothermicity of the processes in the absorber. Formaldehyde absorbers operate less efficient
than could have been expected based on the good apparent solubility of formaldehyde in water.
The reason is that, in aqueous solutions, formaldehyde reacts with water to methylene glycol and
higher poly(oxymethylene) glycols via a series of reversible reactions
CH 2 O + H 2 O
(1)
CH 2 (OH) 2
CH 2 (OH) 2 + HO(CH 2 O) n −1 H
HO(CH 2 O) n H + H 2 O .
(2)
The good apparent solubility of formaldehyde in water is actually the good solubility of
methylene glycol, and the capacity of the solution to accommodate poly(oxymethylene) glycols.
Formaldehyde itself, like most gases, is only sparingly soluble in water.
The rate of the hydration reaction (1) is relatively fast, causing chemical enhancement of the
gas-to-liquid transfer of formaldehyde. The formation rate of the higher poly(oxymethylene)
glycols is low, with reaction times in the order of tens of minutes to hours, depending on the
temperature.
The absorption of formaldehyde, and its consequent hydration, as well as the condensation
of steam are exothermic processes. Therefore, the temperature of the liquid increases as it flows
down the absorber.
Because of factors such as the ones mentioned above, formaldehyde absorbers generally are
divided into different absorption sections. Each of the absorption sections, or beds, is provided
with a relatively large, externally cooled liquid recirculation stream. A typical example of a
formaldehyde absorber is shown in Fig. 2.
This thesis
The major aim of this work is the development of reliable models that are capable, first, of
accurately describing the performance of current formaldehyde absorbers, second, of predicting
the influence of changing various operating parameters, and third, of optimising the performance
of the absorber columns towards formaldehyde absorption efficiency and capacity. To achieve
this goal, knowledge of the kinetics of the principal reaction (1) and the consecutive
polymerisation reactions (2) is of major importance. The kinetics of the latter, the formation of
the poly(oxymethylene) glycols, have been investigated extensively by other research groups
(p.e. Dankelman et al., 1988; Hasse & Maurer, 1991; Hahnenstein et al., 1994, 1995).
2
Chapter 1: Introduction
tail gas
water
feed
product
Fig. 2. Scheme of a typical formaldehyde absorber.
The investigations into the kinetics of the principal reaction are treated in the next three
chapters. Following these are two chapters on the development of chemical reaction engineering
models for formaldehyde absorbers, a concluding chapter, and some additional material.
Chapter 2 describes the experimental work on the determination of the dehydration rate of
methylene glycol, the reverse of reaction (1). Using a traditional wet chemistry methodology, the
dehydration rates where obtained from the measured formation rates of hydroxymethane
sulphonate from the reaction of formaldehyde with SO 32 - , at various temperatures. The results
could be correlated to an Arrhenius type expression.
3
Chapter 1: Introduction
In Chapter 3, a theoretical treatment is presented of the problem of simultaneous
absorption and/or desorption of two components, accompanied by a first-order reversible liquid
phase reaction among the two. The analytical solutions developed here for the concentration
profiles in the mass transfer film and for the enhancement factors are used in the next chapter.
Chapter 4 describes the experimental determination of the kinetics of the hydration of
formaldehyde in water. The measurements are based on the chemically enhanced absorption of
formaldehyde gas into water in a stirred cell and mathematical modelling of the transfer process.
The temperature dependency of the hydration rate constant correlates well to an Arrhenius type
expression. From the results of this chapter, combined with those of chapter 2, the equilibrium
constant of the hydration of formaldehyde is obtained.
In Chapter 5 a model is developed for formaldehyde absorbers, based on differential
equations for the mass and energy balances in each phase. The resulting set of coupled boundary
value problems is solved by a semi-transient method.
In Chapter 6 the absorber model is extended with a description of the behaviour of
unconverted methanol that enters the absorber. Also modelled now are vaporisation and
reabsorption of methylene gylcol and hemiformal, the principal reaction products of
formaldehyde with water and methanol. The modelling is based here on a non-equilibrium stage
model.
Concluding remarks can be found in Chapter 7.
In the appendices, some additional material can be found: Appendix A presents the results
of experimental work on the determination of the viscosity of aqueous formaldehyde solutions
and correlations for the density and viscosity of aqueous formaldehyde solutions as a function of
the temperature and the strength of the solution; Appendix B elucidates some calculation
methods to obtain the equilibrium composition of solutions containing formaldehyde. Some
additional material to chapter 4, on the determination of the reaction order of formaldehyde in the
hydration reaction, is included in Appendix C.
4