Pervaporation as an alternative process for solvent dehydration in dry cleaningmachines

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A brief history of the dry cleaning

The dry cleaning concept is not innovative. The first mentions of dry cleaning date back to the ancient period more than 2000 years ago. It was the ancient Romans who first realized that their garments, namely their white woolen togas, were better cleaned by substances other than water (for example, clay). They have noticed that water shrinks the wool and is not as successful at stain removal as other chemicals.
Meanwhile, ancient Greeks have also done some experimenting with non-water clean-ing. It was actually them who developed the phrase « dry cleaning » as a way to describe clean-ing that was done without water. The Romans have perfected their methods of dry cleaning which remarkably remained as the best known practices until the 1800s. The Romans found that one type of clay, known as fuller’s earth, was extremely successful in removing oil, dirt and grease. It could be kneaded into textiles to draw out impurities. Then, the fuller’s earth combined with ammonia was used once those textiles became dirty from wearing.
The beginning of modern history of dry cleaning is not clear. It is sometimes considered that using of dry cleaning began with an anecdotic story about a happy accident caused by a maidservant. At some point in the 1840s, a french textile maker Jean-Baptiste JOLLY’s maid accidentally knocked over JOLLY’s kerosene lamp onto a linen tablecloth. JOLLY was sur-prised to find that the linen in that spot became much cleaner. This revelation was quickly turned into an extension of his business, so the company of Jolly-Belin in Paris, opened in 1840s, is historically credited as the first dry cleaning business, using kerosene as its primary cleaning solvent. This dry cleaning method was at the top secret for a long time and was named by Englishmen as « French cleaning ».
Later on, the experiments with kerosene and gasoline-based cleaning was continued all over the Europe through the reminder of the 19th century. The inventors and industrial-ists have then realized that the form of cleaning predicated on dousing clothes in highly flammable liquids was not a good idea. A history of fires and explosions in dry cleaning plants made that pretty clear.
Industrialists set about finding a better alternative. An American dry cleaner William Joseph Stoddard is credited as the first to develop a successful non-gasoline-based solvent in the 1920th. Cleaners soon settled on chlorine-based solvents, and found their best suc-cess with tetrachloroethylene. Tetrachloroethylene was first discovered by Michael Faraday, one of the most prominent chemists in science history. Also known as perchloroethylene, or « perc », it remained for a long time the primary solvent used by the vast majority of dry cleaners worldwide.
By the end of the World War II, a progressive development of the dry cleaning machines led to the appearance of the public dry cleaners. The time of order processing was reduced from one week to four hours and even less, depending on the type of cloth. But unfortu-nately, such a fast development led to a decreasing of the cleaning quality. In most of the cases, it was happened because of the lack of knowledge and professional skills of the indus-trial dry cleaners staff.
As the dry cleaning methods were progressing, a lot of investigation and research work was made in that field. Nowadays, the industrial dry cleaners are equipped with modern machinery which allows to perform a more efficient and delicate cleaning. However, per-chloroethylene was classified as carcinogenic to humans by the United States Environmen-tal Protection Agency (EPA, 2011) and must be handled as a hazardous waste. To prevent it from getting into drinking water, dry cleaners that use perchloroethylene must take special precautions. When released into the air, perchloroethylene can contribute to smog when it reacts with other volatile organic carbon substances (EPA, 1994). California firstly declared perchloroethylene a toxic chemical in 1991, and its use will become illegal in that state by 2023. In European Union, according to the INRS report in 2013 (INRS, 2013), perchloroethy-lene is now forbidden to use in dry cleaners and should be imperatively replaced by a safe « green » solvent by 2020.
As a response to the above problem, in France, Arcane Industries company has devel-oped a solvent called ARCACLEAN which is as highly performant in dry cleaning as perchloro-ethylene, but, contrary to it, is non toxic, non carcinogenic and biodegradable. ARCACLEAN is useful for even higher range of spots due to its compatibility with water. As a result, Ar-cane Industries, in collaboration with ILSA (Italy), has developed a new line of dry cleaning machines ARCAFLEX which are fully compatible with the ARCACLEAN solvent.

The ARCACLEAN solvent

During its development, the ARCACLEAN solvent has undergone changes in its formula, becoming more and more efficient and balanced. The last composition of the ARCACLEAN solvent by the time of writing the present Thesis comprises the following compounds:
• Dipropylene Glycol Methyl Ether (DPM):
DPM is a mid-to-slow evaporating solvent. This hydrophilic solvent has 100% water solubility and is ideally suited as a coupling agent in a wide range of solvent systems. More broadly, its hydrophilic nature makes it an ideal coupling aid in water reducible coatings and cleaning operations (BASF, 2017);
• Dipropylene Glycol n-Butyl Ether (DPnB):
DPnB is a relatively slow-evaporating solvent which is one of the most efficient coales-cents in water-borne latex systems. Provides excellent surface tension lowering ability, and is useful in cleaning products by itself or when blended with other products such as DPM. DPnB is a good solvent for removing oils and greases (BASF, 2014a);
• Propylene Glycol n-Butyl Ether (PnB):
PnB is extensively used in heavy-duty cleaning formulations. It does an excellent job of solvating and coupling hydrophobic greases and oils in household as well as indus-trial formulations. It is partly water soluble and miscible with most organic solvents. In coatings, PnB offers good coalescing ability in systems requiring fast evaporation (BASF, 2014b).
All the compounds are fully biodegradable and do not present any hazard for the envi-ronment. The last formula of the ARCACLEAN solvent is given in Table 1.2. In some cases, a very low content of water in solvent is acceptable.

Motivations and problem statement

In the frame of replacing the perchloroethylene by an alternative « green » solvent, Arcane Industries and Innovaclean companies started an R&D investigation in adaptation of their new ARCACLEAN solvent for efficient using in dry cleaning machines of a current generation ARCAFLEX320 (Figure 1.1), produced by ILSA.
During multiple dry cleaning cycles, the solvent accumulated dust and water and, thus, needs to be regenerated. Solvent regeneration comprises two main actions: removal of the dirt (mainly consisted of fibers, colorants, grease etc.) and solvent dehydration. In fact, after the dry cleaning, the solvent becomes charged in water that contains in garments due to hu-man sweating and environmental humidity. Solvent regeneration is a key task in the whole dry cleaning cycle, as the solvent is too expensive for single use.

Current regeneration process in ARCAFLEX320 machines

Process technology in dry cleaning has the target to clean garments as good as possible without damaging them, at lowest possible costs and with highest possible efficiency and safety. In ARCAFLEX320 dry cleaning devices, the solvent regeneration is performed using distillation. This classic technique is suitable for removing of both water and dirt from sol-vent. Due to its principle and operation simplicity, the distillation became extremely popular and widely used process in separation of liquid mixtures. The flowsheet diagram of solvent regeneration process implemented in ARCAFLEX320 is presented in Figure 1.2.
The whole regeneration procedure in ARCAFLEX320 is made of two steps:
• Stage 1: elimination of water from the solvent (about 20 liters) during 15 minutes un-der a pressure of 200 mbar. The initial water concentration of about 9 ¥ 10% wt de-creases to about 4% wt. The solvent in the distillate is not negligible because the gen-erated distillate (between 1 and 2 liters) contains 60% wt of solvent and 40% wt of water in total mass. After 20 cycles of cleaning, all the produced distillate is mixed and dis-tilled again (stage 1a) in order to recycle a part of solvent. The finally obtained distillate (about 12 liters containing about 78% of water) represents a waste. Both residues from stage 1 and 1a are collected together and sent to stage 2.
• Stage 2: Solvent regeneration during 45 minutes under the pressure of 50 mbar in order to eliminate fibers, dust and colorants dissolved in it.
After regeneration, the majority of dirt particles are eliminated, however, the solvent still contains about 3 to 4% wt of water.

Issues and limitations of distillation

Despite numerous advantages of distillation as a separation process, it has some issues and limitations as well. These issues become even more visible when applied to solvent de-hydration in dry cleaning devices.
First of all, these issues are represented by a high energy consumption by distiller and auxiliary equipment related to it. In ARCAFLEX320, the distiller is not equipped by electri-cal resistance heater. Instead, it is heated by the means of water steam produced in steam generator. Such a solution is used due to the risk related to the presence of sludge in solvent-to-regenerate. This sludge can provoke the resistance fouling, in case if the latter would have been used.
Currently, the distillation system consumes about 20 kW of heat per distillation cycle, which yields in huge number in annual perspective. Moreover, our previous study (Guichar-don and Dimitrov, 2018) showed that the distiller is not heat consumption optimized and is enough to be heated 60% less, meaning 8 kW per distillation cycle.
In addition, the heat exchangers used in distillation, are cooled by the tap water from network, resulting in a high water consumption per dry cleaning cycle.
Second issue of distillation lays in its separation limitations in terms of thermodynam-ics. Indeed, the separation quality in distillation is completely vapor-liquid equilibrium de-pendent. Depending on the equilibrium curve of a mixture, it can be either easy or diffi-cult to separate by distillation, especially if the boiling points of components are close or an azeotrope is occurred in that mixture. Moreover, if the boiler is heated too much, the high boiling fraction can mechanically entrain the vapor of the low boiling fraction, resulting in unfavourable decreasing of distillate purity and solvent losses. While on the subject, the solvent losses in a currently implemented distillation are very important. The distillate pro-duced during the stage 1a (Figure 1.2) and considered as a waste, contains 22% of solvent.
The last issue is represented by the fact, that the solvent regeneration in ARCAFLEX320 is performed in a simple one-stage distillation which is, probably, enough to eliminate the solid dirt particles but does not offer a satisfactory separation of solvent and water.
The dehydration is a crucial step in solvent regeneration. When containing the water, the ARCACLEAN solvent becomes aggressive to textiles, provoking their shrinking and fiber degradation. As mentioned before, after the regeneration cycle, the solvent still contains 3 to 4% of water. Therefore, on of the most important tasks of the present work is to reduce the water amount in solvent as much as possible. Generally, Innovaclean recommends the water content to be less than 1%, which is practically unachievable by current distillation. In order to achieve such a separation, one should definitely prefer the fractional distillation over the one-stage distillation. However, the fractional distillation is not very favourable to implement in a dry cleaning machine due to eventual fouling.
Instead, Innovaclean was interested in implementing another dehydration technique, which will be able to completely replace the current distillation. The new process should demonstrate higher energy and separation efficiency, being at the same time environmen-tally friendly. In other words, the new dehydration process should be designed with the re-spect to the ecodesign principle.

Dry cleaning devices without distillers

The problem of replacing the distillation by other processes has been already studied by several companies. Nevertheless, the overall number of distillation-free devices is currently very limited. Solvent regeneration in these devices is mostly based on decantation, filtration, adsorption and even bio-treatment techniques. It is in some point difficult to understand the working principle of these devices as very few information is available on the manufacturer’s Internet resources due to confidential information and commercial secret. We will try to give several examples of the devices currently available in the market and to describe them in terms of solvent regeneration, basing on the information that could be retrieved.

« IPURA » by ILSA

IPURA (Figure 1.3) is the first attempt of ILSA company to set aside the solvent regenera-tion by distillation. Instead, the decantation and filtration techniques are used. IPURA uses aliphatic hydrocarbons as dry cleaning solvents.
The solvent treatment cycle in IPURA is divided into 4 principal stages. After dry clea-ning, the solvent is passed through a centrifugal separator in the bottom of which the heavy dust particles are separated. The lighter particles are then retained by a mechanical filter installed right after the centrifugal separator. The solvent dehydration is performed by de-cantation using the fact that water is non-miscible with hydrocarbons. The dehydrated sol-vent is then passed through an adsorber where light particles (most probably colorants) are eliminated.
The overall cost of this device is relatively low. To dry the garments after cleaning, a heat pump is used instead of a supplementary electric heater. However, the dehydration process efficiency and especially the process duration (generally, the decantation is not a quick pro-cedure) remain fairly unclear for us.

« NEBULA » by Renzacci

NEBULA (Figure 1.4) is an energy efficient dry cleaning machine, in which the distillation has been replaced by a double filtration of solvent (using one mechanical and one chemical filter). The following solvent purification systems are implemented: solvent purification « at a continuous independent flow » using a filter cartridge group (« HFC » and « HFD »); solvent filtration « No Flex Ultra Micron ».
NEBULA is a multi-solvent device. It uses the « COMBICLEAN » system, allowing to choose multiple solvents for a better cleaning performance depending on textile type. The DSFTM (Dynamic Saving Flow) allows to reduce at 45% the energy consumption. The manufacturer also declared a reduced cleaning cycle time.

Table of contents :

1 Context and problemstatement 
1.1 A brief history of the dry cleaning
1.2 The ARCACLEAN solvent
1.3 Motivations and problem statement
1.3.1 Current regeneration process in ARCAFLEX320 machines
1.3.2 Issues and limitations of distillation
1.3.3 Dry cleaning devices without distillers
1.3.3.1 « IPURA » by ILSA
1.3.3.2 « NEBULA » by Renzacci
1.3.3.3 UNISEC
1.3.4 Other classic dehydration processes
1.3.4.1 The extractive distillation
1.3.4.2 The adsorption
1.3.4.3 Membrane processes
1.4 Thesis workflow
2 Thermodynamics of solvents: theoretical and experimental study 
2.1 Experimental measurements of solvents properties
2.1.1 Experimentalmaterials and methods
2.1.1.1 Materials
2.1.1.2 Working principle of a static apparatus
2.1.1.3 Treatment of experimental points
2.1.2 Sample analysis by the gas chromatography
2.1.2.1 Importance of the quantitative sample analysis
2.1.2.2 Analysis performing
2.1.2.3 Peaks identification and calibration
2.1.3 Experimentalmeasurements workflow
2.1.3.1 Pure compounds vapor pressure
2.1.3.2 VLE and VLLE of aqueous binary mixtures
2.1.3.3 VLE of organic binary mixtures
2.1.3.4 VLE of the quaternary mixture ARCACLEAN + water
2.2 Thermodynamic modelling of phase equilibria occurring inH2O+ARCACLEAN mixtures
2.2.1 Pure compounds parameters estimation
2.2.1.1 Definition of the equation of state parameters
2.2.1.2 Application of the Peng-Robinson EoS to glycol ethers
2.2.1.3 Fitting of estimated pure compound properties
2.2.2 Prediction of low-pressure phase equilibria using the NRTL-PRmodel
2.2.2.1 Generalized NRTL model
2.2.2.2 Generalized reference state at constant volume GRS-CV
2.2.2.3 Association of the generalizedNRTL modelwith the Peng-Robinson EoS
2.3 Results and discussion
2.3.1 VLE of all binary mixtures
2.3.2 VLE of the quaternary mixtures
2.4 Chapter conclusion
3 Pervaporation as an alternative process for solvent dehydration in dry cleaningmachines
3.1 Literature review
3.1.1 Pervaporation overview
3.1.2 Materials of pervaporation membranes
3.1.2.1 The organic membranes
3.1.2.2 The inorganicmembranes
3.1.2.3 The hybrid membranes
3.1.3 Dehydration of organic solvents by pervaporation
3.2 Experimental study of solvent dehydration by pervaporation
3.2.1 Materials
3.2.2 Experimental unit description
3.2.3 Analysis by gas chromatography
3.2.3.1 GC device
3.2.3.2 GC operating parameters and setup
3.2.3.3 GC Calibration
3.2.4 Results and discussion
3.2.4.1 Membrane Sultzer PERVAP 4102-3184
3.2.4.2 Membrane Sultzer PERVAP 4510-2898
3.2.4.3 Membrane Sultzer PERVAP 4100-3239
3.3 Modelling of solvent dehydration by the solution-diffusion model
3.3.1 General approach to mass transfer modelling in polymeric membranes
3.3.2 The solution-diffusion model
3.3.2.1 Brief mathematical description
3.3.2.2 Reporting the pervaporation data
3.3.3 Determining the activity coefficients for solution-diffusion model
3.4 Results and discussion
3.4.1 Influence of temperature
3.4.2 Influence of feed concentration
3.4.3 Prediction of membrane performance in batch dehydration at different temperatures
3.5 Chapter conclusion
4 On the design of the pervaporation unit for solvent dehydration and its integration in the dry cleaning machine 
4.1 ARCAFLEX320 distillation system
4.1.1 Overview and working principle
4.1.2 Devices that can be reused
4.1.3 Process constraints
4.2 Design of the pervaporation unit
4.2.1 Membranemodule
4.2.2 Vacuum system
4.2.3 Permeate condenser
4.2.4 System layout
4.2.5 Operational costs estimation and comparison with distillation
4.3 Chapter conclusion
Conclusions and directions for future research 
Bibliography 
Appendix A Literature reference states and expressions for g E
A.1 Zero pressure reference state (Michelsen, 1990)
A.2 Infinite pressure reference state (Huron and Vidal, 1979)
A.3 Constant packing fraction (Peneloux et al., 1989)
A.4 Generalized reference state at constant volume (Neau et al., 2011)
Appendix B Pure compounds group contribution parameters for the estimation of critical properties
AppendixC Measured experimental vapor pressures and VLE data for purecompounds, binary and quaternary mixtures
C.1 Pure compounds
C.2 Aqueous binary mixtures
C.3 Organic binary mixtures
C.4 Quaternarymixtures
Appendix D GC setup and operating conditions used in Chapter 2
Appendix E Membrane cell used in experiments for the laboratory scale pervaporation

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