Silver ion catalyzed cerium(IV) mediated electrochemical oxidation of phenol in nitric acid medium

Mediated electrochemical oxidation (MEO) is one of the sustainable processes for organic pollutant destruction and has been employed for organic mineralization reactions by many researchers. In the MEO a metal ion capable of exhibiting redox behavior is oxidized from lower oxidation state to higher oxidation state by an electrochemical cell and subsequently used as an oxidant for mineralizing the toxic organics into CO₂ and water. The net result is the consumption of electrical energy for organic mineralization. Therefore, the current efficiency is an important factor and maximizing the current efficiency is one of the ways of reducing the running cost of the MEO process. It has been reported in the literature that the current efficiency could be increased using a metal ion catalyst having a good redox potential. In this study Ce(IV) mediated electrochemical oxidation of phenol was carried out with silver ion catalyst. The current efficiency for the electro-oxidation of cerium(III) in nitric acid was found to be increased by the addition of silver ions. This mixed mediator system was tested for the oxidation of phenol in order to optimize the parameters for organic pollutant destruction. The mineralization efficiency calculated based on the CO₂ evolution was found to be higher for silver catalyzed Ce(IV) mediated oxidation compared to the non-silver catalyzed system.     

Electrochemical cell current requirements for toxic organic waste destruction in Ce(IV)-mediated electrochemical oxidation process

The electrochemical cell for cerium oxidation and reactor for organic destruction are the most important operation units for the successful working mediated electrochemical oxidation (MEO) process. In this study, electrochemical cells with DSA electrodes of two types, single stack and double stack connected in series, were used. The performances towards the electrochemical generation of Ce(IV) in nitric acid media at 80 °C were studied. The current-voltage curves and cerium electrolysis kinetics showed the dependence on number of cell stacks needed to be connected in series for the destruction of a given quantity of organic pollutant. The presence of an optimum region for Ce(III) oxidation with a contribution of oxygen evolution, especially at low Ce(III) concentration (high conversion ratios), was found. The cells were applied for the Ce(IV) regeneration during the organic destruction. The cell and reactor processes were fitted in a simple model proposed and used to calculate the current needed in terms of Ce(III) oxidation rate and the number of cell stacks required for maintaining Ce(IV)/Ce(III) ratio at the same level during the organic destruction. This consideration was based on the kinetic model previously developed by us for the organic destruction in the MEO process.     

Cobalt(III)-mediated oxidative destruction of phenol using divided electrochemical cell

Mediated electrochemical oxidation is one of the suitable processes for the destruction of hazardous organic compounds and the dissolution of nuclear wastes at ambient temperature and pressure. The electrochemical oxidation of Co(II) was carried out in an undivided and divided electrochemical cell. The formation of Co(III) was studied in an divided electrochemical cell by varying conditions such as temperature and concentration of nitric acid in a batch type electrochemical reactor in recirculation mode. It was found that the formation of Co(III) increased with increasing nitric acid concentration and decreased with increasing temperatures. The produced Co(III) oxidant was then used for the destruction of phenol. It was noted that phenol could be mineralized to CO₂ and water by Co(III) in nitric acid under different nitric acid concentrations and temperatures. The evolved CO₂ was continuously measured and used for the calculation of destruction efficiency. The destruction was increased with increasing nitric acid concentration as well as the temperature. The maximum efficiency was observed to be 78% based on CO₂ evolution for 5,000 ppm phenol solution at 60 °C in a continuous feed mode. The destruction efficiency was increased 28% by addition of silver at 25 °C.     

The phenol steam reforming reaction towards H2 production on natural calcite

The steam reforming of phenol towards H₂ production was studied in the 650–800 °C range over a natural pre-calcined (air, 850 °C) calcite material. The effects of reaction temperature, water, hydrogen, and carbon dioxide feed concentrations, and gas hourly space velocity (GHSV, h⁻¹) were investigated. The increase of reaction temperature in the 650–800 °C range and water feed concentration in the 40–50 vol% range were found to be beneficial for catalyst activity and H₂-yield. A similar result was also obtained in the case of decreasing the GHSV from 85,000 to 30,000 h⁻¹. The effect of concentration of carbon dioxide and hydrogen in the phenol/water feed stream was found to significantly decrease the rate of phenol steam reforming reaction. The latter was probed to be related to the reduction in the rate of water dissociation as evidenced by the significant decrease in the concentration of adsorbed bicarbonate and OH species on the surface of CaO according to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)-CO₂ adsorption experiments in the presence of water and hydrogen in the feed stream. Details of the CO₂ adsorption on the CaO surface at different reaction temperatures and gas atmospheres using in situ DRIFTS and transient isothermal adsorption experiments with mass spectrometry were obtained. Bridged, bicarbonate and unidentate carbonate species were formed under CO₂/H₂O/He gas mixtures at 600 °C with the latter being the most populated. A substantial decrease in the surface concentration of bicarbonate and OH species was observed when the CaO surface was exposed to CO₂/H₂O/H₂/He gas mixtures at 600 °C, result that probes for the inhibiting effect of H₂ on the phenol steam reforming activity. Phenol steam reforming reaction followed by isothermal oxygen titration allowed the measurement of accumulated “carbonaceous” species formed during phenol steam reforming as a function of reaction temperature and short time on stream. An increase in the amount of “carbonaceous” species with reaction time (650–800 °C range) was evidenced, in particular at 800 °C (4.7 vs. 6.7 mg C/g solid after 5 and 20 min on stream, respectively).     

Cerium(IV)-mediated electrochemical oxidation process for destruction of organic pollutants in a batch and a continuous flow reactor

The mediated electrochemical oxidation (MEO) process with Ce(IV) and nitric acid as the oxidizing medium was employed for the destruction of various model organic pollutants in batch and continuous organic feeding modes. A near complete destruction was observed for all the model organic pollutants studied. The effects of organic concentration, temperature, concentration of Ce(IV), concentration of nitric acid and feeding time on the organic destruction efficiency were investigated. Under the experimental conditions of 80 °C and 0.95 M Ce(IV) in 3 M nitric acid, nearly 90% destruction was achieved based on CO₂ production and 95% based on TOC and COD nearly for all the organic compounds studied in batch organic addition. In the case of continuous organic addition with in situ electroregeneration of Ce(IV) by the electrochemical cell a good destruction efficiency was obtained. For long term organic feeding (120 min) the destruction efficiency was found to be 85% based on CO₂ evolution and 98–99% based on TOC and COD analyses. A model was proposed for calculating the CO₂ formation constant during the continuous process of organic addition. The model predicted a steady state CO₂ evolution pattern for the destruction process during continuous organic feeding. The experimental results obtained confirmed the predicted trends for the destruction process. The changes in enthalpy, entropy, activation energy and free energy for EDTA degradation were found to be 26.7 kJ/mol, −230 J/(mol·K), 29.7 kJ/mol, and 118 kJ/mol respectively.     

Influence of sulfur on the structural, surface properties and photocatalytic activity of sulfated TiO2

TiO₂ materials were prepared by sol–gel method and then impregnated with sulfuric acid and calcined using different temperatures and atmosphere (air and nitrogen). Systematic variation of these two experimental parameters makes possible to modulate the amount of surface sulfur from the impregnation procedure. The best photocatalyst for liquid phenol degradation was obtained after calcination at 700 °C in air, while gas toluene degradation optimum performance is obtained by calcination at 700 °C in nitrogen from 500 °C. Structural analysis of these materials by XRD, micro-Raman spectroscopy and FE-SEM shows that once calcined at 700 °C the material was a well-crystallized, high surface area anatase structure in all cases. The surface characterization by FTIR and XPS confirms the presence of a higher amount of sulfur species and acidic OH groups in samples partially calcined in nitrogen, and a low XPS O/Ti-atomic ratio with the O 1s peak shifted to higher binding energies (1.8 vs. 2 ± 0.1 and 530.4 eV vs. 529.8 eV, respectively, against the reference materials) for samples calcined at 700 °C, temperature at which most of sulfate species have been evolved. The paper presents an attempt to correlate the contribution of the observed structural defects within the anatase sub-surface layers and surface acidity to the different photoactivity behaviour exhibited for phenol liquid phase and toluene gas phase photodegradation.     

Photocatalytic activity of porous titania nanocrystals prepared by nanoscale permeation process in supercritical CO2: Effects of supercritical conditions

Porous TiO₂ nanocrystals (PTN) were synthesized using activated carbon templates with supercritical CO₂ by using the nanoscale permeation (NP) process. The photoactivity of PTN was tested by methylene blue (MB) degradation. Compared with the commercially available P-25, all PTN exhibited significant photocatalytic degradation of MB mainly due to their porous structure with high surface area, high hydroxy concentration and small crystalline size. The optimum temperature and pressure are found to be 60 °C and 26 MPa, under which obtained PTN-1 shows the highest photoactivity and slow deactivation for MB degradation after 15 trials.     

The factors affecting the performance of activated carbon prepared from oil palm empty fruit bunches for adsorption of phenol

Powdered activated carbons (PACs) were produced from oil palm empty fruit bunches (EFB) by varying the operating parameters of temperatures, CO₂ gas flow rates and activation times using 2-level full factorial experimental design. The EFB samples were first carbonized for 30 min using nitrogen gas followed by physical activation using CO₂ to optimize best production conditions. The optimum conditions for PACs produced were investigated through adsorption tests on aqueous solution of phenol. The results of this study demonstrated that the activation temperature with the range of 800–900 °C had the most significant effect on the adsorption characteristics as well as the yield of the activated carbon produced. Based on the analysis of variance (ANOVA) and model equation developed, the optimum production conditions for the EFB PAC were found to be at the activation temperature of 900 °C with CO₂ gas flow rate of 0.1 L/min and activation time of 15 min. Characterization of PAC produced showed that the activation conditions would find good-quality adsorbent with the BTE surface area of 345.1 m²/g and well forming pores distribution.     

Phenol degradation by Fenton's process using catalytic in situ generated hydrogen peroxide

The recent reported pathway using oxygen and formic acid at ambient conditions has been utilized to generate hydrogen peroxide in situ for the degradation of phenol. An alumina supported palladium catalyst prepared via impregnation was used for this purpose. Almost full destruction of phenol was carried out within 6 h corresponding to the termination of 100 mM formic acid at the same time. In addition, a significant mineralization (60%) was attained. A simulated conventional Fenton process (CFP) using continuous addition of 300 ppm H₂O₂ displayed maximum 48% mineralization. Study of different doses of formic acid showed that decreasing the initial concentration of formic acid caused faster destruction of phenol and its toxic intermediates. The catalytic in situ generation of hydrogen peroxide system demonstrated interesting ability to oxidize phenol without the addition of Fenton's catalyst (ferrous ion). Lower Pd content catalysts (Pd1/Al and Pd0.5/Al) despite of producing higher hydrogen peroxide amount for bulk purposes, did not reach the same efficiency as the Pd5/Al catalyst in phenol degradation. The later catalyst showed a remarkable repeatability so that more than 90% phenol degradation along with 57% mineralization was attained by the used catalyst after twice recovery. Higher temperature (45 °C) gave rise to faster degradation of phenol resulting to almost the same mineralization degree as obtained at ambient temperature. Meanwhile, Pd leaching studied by atomic adsorption proved excellent stability of the catalysts.     

ZnBO-doped (Ba, Sr)TiO3 ceramics for the low-temperature sintering process

ZnBO-doped (Ba, Sr)TiO₃ ceramics were investigated for low-temperature co-fired ceramics (LTCCs) applications. Until now, B₂O₃ and Li₂CO₃ dopants have been commonly employed as the low-temperature sintering aids. In this paper, we suggest ZnBO as an alternative dopant to the B₂O₃ and Li₂CO₃. To reduce the sintering temperature of (Ba, Sr)TiO₃, we have added 1–5 wt.% of ZnBO to (Ba, Sr)TiO₃. ZnBO-doped (Ba, Sr)TiO₃ ceramics were respectively sintered from 750 to 1350 °C by 50 °C to confirm the sintering temperature with different dopant contents. By adding 5 wt.% of ZnBO to the (Ba, Sr)TiO₃ ceramics, the sintering temperature of (Ba, Sr)TiO₃ ceramics can be reduced to 1100 °C. From the XRD analysis, ZnBO-doped (Ba, Sr)TiO₃ has no pyro phase. By adding ZnBO dopants to (Ba, Sr)TiO₃ ceramics, both of relative dielectric permittivity and loss tangent were decreased. From the frequency dispersion of dielectric properties, the relative dielectric permittivity and loss tangent of 5 wt.% ZnBO-doped (Ba, Sr)TiO₃ were 1180 and 3.3 × 10⁻³, while those of BST were 1585 and 4.8 × 10⁻³, respectively.     

Direct synthesis of mesoporous silicalite-1 supported TiO2–ZrO2 for the dehydrogenation of EB to styrene with CO2

Direct synthesis route was developed to support TiO₂–ZrO₂ binary metal oxide onto the carbon templated mesoporous silicalite-1 (CS-1). Metal hydroxide modified carbon particles could play a role as hard template and simultaneously support metal components on the mesopores during the crystallization of zeolites. Such supported TiO₂–ZrO₂ binary metal oxides (TZ/CS-1) showed better resistance to deactivation in the oxidative dehydrogenation of ethylbenzene (ODHEB) in the presence of CO₂. These catalysts were found to be active, selective and catalytically stable (10 h of time-on-stream) at 600 °C for the dehydrogenation of ethylbenzene (EB) to styrene (Sty).     

Synthesis of yttrium aluminum garnet (YAG) powder by homogeneous precipitation combined with supercritical carbon dioxide or ethanol fluid drying

YAG precursors were synthesized by the urea method in aqueous solution using supercritical carbon dioxide and ethanol fluid drying technique, respectively. The composition of the precursors, the phase formation process and the properties of the calcined powders were investigated by means of XRD, IR, TG/DSC, BET, TEM and SEM. Compared with the classically prepared powders at room temperature in air, the amorphous precursor dried by supercritical CO₂ fluid was loosely agglomerated and directly converted to pure YAG at about 900 °C. The resultant YAG powders showed good dispersity with an average crystallite size about 20 nm and specific surface area of 52 m² g⁻¹. However, the precursor dried by supercritical ethanol fluid was crystalline. Extensive phase segregation occurred during the drying process and resulted in the formation of separate phases such as monoclinic Y(OH)₃ and pseudoboehmite. YAM and YAP phases appeared in the calcination process and phase pure were not detected until 1200 °C.     

Characterization of sepiolite as a support of silver catalyst in soot combustion

Turkish sepiolite–zirconium oxide mixtures were applied as a support for the silver catalyst in a soot combustion. Sepiolite–Zr–K–Ag–O catalyst was characterized by XRD, N₂ adsorption, SEM, TPR-H₂ and EGA-MS. The combustion of soot was studied with a thermobalance (TG-DTA). The modification resulted in a partial degradation of the sepiolite structure, however, the morphology was preserved. The adsorption of N₂ of the modified sepiolite is a characteristic for mesoporous materials with a wide distribution of pores. The specific surface area S_BET equals 83 m²/g and the pores volume is 0.23 cm³/g. The basic character of the surface centers of sepiolite is indicated by CO₂ desorption (TPD-MS) at 170 °C and at about 620 °C due to a surface carbonates decomposition. The thermodesorption of oxygen at 650–850 °C indicates the decomposition of AgO_x phases at the surface. The presence of AgO_x phases is also confirmed by TPR-H₂ spectrum (low temperature reduction peak at 130 and 180 °C). The high-temperature reduction at about 570 °C is probably related to Ag–O–M phases on the support.The soot combustion takes place at T₅₀ = 575 °C. Without silver (sepiolite–Zr–K–O) T₅₀ = 560 °C but sepiolite modified with silver (sepiolite–Zr–K–Ag–O) undergoes the same process at T₅₀ = 490 °C.     

Reaction mechanism and electrochemical performance of LiFePO4/C cathode materials synthesized by carbothermal method

Spray drying and carbothermal method was employed to investigate reaction mechanism and electrochemical performance of LiFePO₄/C cathode by using different carbon sources. Micro-structural variations of LiFePO₄/C precursors using different carbon sources were studied by Thermo-gravimetric (TG)/Differential Thermal Analysis (DTA). The LiFePO₄/C samples were characterized by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) absorption spectroscopy. The results indicated that the crystallization temperature of LiFePO₄ was 453 °C, while the transform temperature was 539 °C from Li₃Fe₂(PO₄)₃ to LiFePO₄. At 840 °C, LiFePO₄/C sample with an excess of impurity phase Fe₂P gave much poorer electrochemical performance. The severe decomposition of LiFePO₄/C happened at 938 °C and generated impurity phases Li₄P₂O₇ and Fe₂P. The clear discharge platform of Fe₂P emerged at around 2.2 V.     

Electrochemical promotion of the CO2 hydrogenation reaction using thin Rh, Pt and Cu films in a monolithic reactor at atmospheric pressure

A monolithic electropromoted reactor (MEPR) with up to 22 thin Rh/YSZ/Pt or Cu/TiO₂/YSZ/Au plate cells was used to investigate the hydrogenation of CO₂ at atmospheric pressure and temperatures 220–380 °C. The Rh/YSZ/Pt cells lead to CO and CH₄ formation and the open-circuit selectivity to CH₄ is less than 5%. Both positive and negative applied potentials enhance significantly the total hydrogenation rate but the selectivity to CH₄ remains below 12%. The Cu/TiO₂/YSZ/Au cells produce CO, CH₄ and C₂H₄ with selectivities to CH₄ and C₂H₄ up to 80% and 2%. Both positive and negative applied potential significantly enhance the hydrogenation rate and the selectivity to C₂H₄. It was found that the addition of small (0.5 kPa) amounts of CH₃OH in the feed has a pronounced promotional effect on the reaction rate and selectivity of the Cu/TiO₂/YSZ/Au cells. The selective reduction of CO₂ to CH₄ starts at 220 °C (vs 320 °C in absence of CH₃OH) with near 100% CH₄ selectivity at open-circuit and under polarization conditions at temperatures 220–380 °C. The results show the possibility of direct CO₂ conversion to useful products in a MEPR via electrochemical promotion at atmospheric pressure.     

An effect of iron(III) oxides crystallinity on their catalytic efficiency and applicability in phenol degradation—A competition between homogeneous and heterogeneous catalysis

Catalytic efficiency, stability and environmental applicability of five iron(III) oxide nanopowders differing in surface area and crystallinity were tested in degradation of concentrated phenolic aqueous solutions (100 g/L) at mild temperature (30 °C), initially almost neutral pH and equimolar ratio of hydrogen peroxide and phenol. The catalyst properties were easily controlled by varying in reaction time during isothermal treatment of ferrous oxalate dihydrate in air at 175 °C. Although the catalytic efficiency clearly increases with the surface area of the nanopowders, it is not due to the solely heterogeneous catalytic mechanism as would be expected. The amorphous Fe₂O₃ nanopowders possessing the largest surface areas (401 m² g⁻¹, 386 m² g⁻¹) are the most efficient catalysts evidently due to their highest susceptibility to leaching in acidic environment arising as a consequence of phenol degradation products. Thus, these amorphous samples act partially as homogeneous catalysts, which was confirmed by a high concentration of leached Fe(III) ions in the solution (19 ppm). The crystalline hematite (α-Fe₂O₃) samples, varying in surface area between 337 m² g⁻¹ and 245 m² g⁻¹, are generally less efficient when compared to the amorphous powders, however their catalytic action is almost exclusively heterogeneous as only 3 ppm of leached Fe(III) was found in the reaction systems catalyzed by nanohematite samples. A significant difference in relative contributions of heterogeneous and homogenous catalysis was definitely established in buffered reaction systems catalyzed by amorphous Fe₂O₃ and nanocrystalline hematite. The nanohematite sample exhibiting the highest heterogeneous action was tested at decreased initial phenol concentration (10 g/L), which is closer to the real contents of phenol in waste waters, and at different hydrogen peroxide/phenol molar ratios to consider its environmental applicability. At the hydrogen peroxide/phenol ratio equal to 5, no traces of the leached iron were detected and the phenol conversion of 84% was reached. Moreover, such a high degree of conversion is accompanied by a decrease of the chemical oxygen demand (COD) from the initial value of 11.23 g/L to 4.22 g/L after 125 min. This fact indicates that the considerable fraction of primary reaction products was totally degraded.     

The role of particle size on the electrochemical properties at 25 and at 55 °C of the LiCr0.2Ni0.4Mn1.4O4 spinel as 5 V-cathode materials for lithium-ion batteries

The role of the particle size on the electrochemical properties at 25 and at 55 °C of the LiCr_0.2Ni_0.4Mn_1.4O₄ spinel synthesized by combustion method has been determined. Samples with different particle size were obtained by heating the raw spinel from 700 to 1100 °C, for 1 h in air. X-ray diffraction patterns revealed that all the prepared materials are single-phase spinels. The main effect of the thermal treatment is the remarkable increase of the particles size from 60 to 3000 nm as determined by transmission electron microscopy. The electrochemical properties were determined at high discharge currents (1C rate) in two-electrode Li-cells. At 25 and at 55 °C, in spite of the great differences in particle size, the discharge capacity drained by all samples is similar (Q_dch ≈ 135 mAh g⁻¹). Instead, the cycling performances strongly change with the particle size. The spinels with Φ > 500 nm show better cycling stability at 25 and at 55 °C than those with Φ < 500 nm. The samples heated at 1000 and 1100 °C, with high potential (E ≈ 4.7 V), elevate capacity (Q ≈ 135 mAh g⁻¹), and remarkable cycling performances (capacity retention after 250 cycles >96%) are very attractive materials as 5V-cathodes for high-energy Li-ion batteries.     

Low temperature deposition of titanium oxide containing thin films in trench features from titanium diisopropoxide bis(dipivaloylmethanate) in supercritical CO2

We studied supercritical carbon dioxide fluid deposition of titanium oxide (TiO₂) in trench features on Si substrates using a flow-type deposition apparatus from titanium diisopropoxide bis(dipivaloylmethanate), aiming at fabricating conformal films at a relatively low temperature. We investigated the deposition rate and step coverage under a fluid temperature from 40 to 60 °C, a pressure from 8.0 to 10.0 MPa, and a substrate temperature from 80 to 120 °C. They were dependent on the fluid density, indicating that the solubility difference between the bulk fluid and the neighborhood of the substrate surface plays a decisive role for the deposition. An excellent conformal filling of the trench features was achieved from the fluid of 60 °C under 8 MPa on the substrate kept at 80–100 °C. The XPS spectra of the deposited film suggested partial formation of TiO₂, and the XRD spectra showed the existence of some crystalline TiO₂ (anatase).     

Feasibility of Na-based thermochemical cycles for the capture of CO2 from air—Thermodynamic and thermogravimetric analyses

Three Na-based thermochemical cycles for capturing CO₂ from air are considered: (1) a NaOH/NaHCO₃/Na₂CO₃/Na₂O cycle with 4 reaction steps, (2) a NaOH/NaHCO₃/Na₂CO₃ cycle with 3 reactions steps, and (3) a Na₂CO₃/NaHCO₃ cycle with 2 reaction steps. Depending on the choice of CO₂ sorbent – NaOH or Na₂CO₃ – the cycles are closed by either NaHCO₃ or Na₂CO₃ decomposition, followed by hydrolysis of Na₂CO₃ or Na₂O, respectively. The temperature requirements, energy inputs, and expected products of the reaction steps were determined by thermodynamic equilibrium and energy balance computations. The total thermal energy requirement for Cycles 1, 2, and 3 are 481, 213, and 390 kJ/mol of CO₂ captured, respectively, when heat exchangers are employed to recover the sensible heat of hot streams. Isothermal and dynamic thermogravimetric runs were carried out on the pertinent carbonation, decomposition, and hydrolysis reactions. The extent of the NaOH carbonation with 500 ppm CO₂ in air at 25 °C – applied in Cycles 1 and 2 – reached 9% after 4 h, while that for the Na₂CO₃ carbonation with water-saturated air – applied in Cycle 3 – was 3.5% after 2 h. Thermal decomposition of NaHCO₃ – applied in all three cycles – reached completion after 3 min in the 90–200 °C range, while that of Na₂CO₃ – applied in Cycle 1 – reached completion after 15 min in the 1000–1400 °C range. The significantly slow reaction rates for the carbonation steps and, consequently, the relatively large mass flow rates required, introduce process complications in the scale-up of the reactor technology and impede the application of Na-based sorbents for capturing CO₂ from air.     

Simultaneous desorption–crystallization of CO2–CaCO3 in multi-stage flash (MSF) distillers

In this work, the desorption–crystallization of CO₂–CaCO₃ in MSF distillers was simulated by coupling mass transfer with chemical reaction and correlating the CaCO₃ crystallization rates to the CO₂ release rates. The model was applied to two 20-stage reference MSF once-through and recycles distillers. The CO₂ release rates decreased exponentially from the first stage to the last stage. The CO₂ release rates increased with increasing top brine temperature (TBT) and so CaCO₃ deposition rates did. The CaCO₃ deposition rates increased by 76.9, 102.5 and 123.0 g per ton distillate at 90, 100 and 110 °C, respectively. This corresponded to fouling resistance of 0.64, 0.83 and 1.00 m²K/kW, respectively. The results were fully discussed and interpreted.