多管程换热器网络的最小温差分析与夹点设计

在实际生产过程中,多管程换热器广泛应用,其中既存在逆流换热也存在并流换热。针对多管程换热器网络中并流换热与逆流换热时换热温差对夹点特性尤其是换热网络综合的影响,定量分析最小温差的选择与夹点特性及其总费用的关系。兼顾管程数目的选择以及换热网络总费用两方面因素,以年度化总费用最低为目标,优化求解多管程换热器网络的最小温差。然后,将求得的最小温差作为夹点温差,遵循夹点设计原则综合换热网络。同时,计算得出多管程换热器网络的F_T修正因子,并对结果进行分析。最后,结合示例对比分析求解结果,证明方法的有效性。     

分流式换热器的性能比较和理论分析

基于对<2-4>及<2-6>型分流式换热器基本方程的解析解,给出了温度效率P和对数平均温差修正系数F的计算公式。在热容量比R及传热单元教NTU的实际有意义的范围内,比较了<1-n>(n=2,3,4)及<2-n>(n=4,6,8)型分流式换热器的P值。对结果进行了有益的讨论。     

Heat exchanger network synthesis with detailed exchanger designs—2. Hybrid optimization strategy for synthesis of heat exchanger networks

We propose a new strategy to synthesize heat exchanger networks with detailed designs of individual heat exchangers. The proposed strategy uses a multistep approach by first obtaining a heat exchanger network topology through solving a modified version of the mixed integer nonlinear programming (MINLP) stage-wise superstructure of Yee and Grossmann, which includes a smoothed LMTD approximation and pressure drops. In a second nonlinear programming (NLP) suboptimization step, we allow for nonisothermal mixing to solve problems with or without exchanger bypasses. The selected heat exchangers along with the mass and energy balances obtained are then used to design the network with detailed exchanger designs through solving a sequence of NLPs for individual heat exchanger designs. The NLPs are based on the detailed discretized optimization models of Kazi et al., which solve quickly and reliably to obtain heat exchangers based on rigorous, first-principles derived coupled differential equations. These models solve a differential algebraic equation system and do not rely on usual assumptions associated with other heuristic-based exchanger design methods, such as log mean temperature difference and F_T correction factors. These detailed exchanger designs are then used to update the network optimization model through sets of correction factors on heat exchanger area, number of shells, heat transfer coefficients, and pressure drops of each exchanger design, in a method based on that of Short et al. The method solves reliably, guaranteeing feasible exchangers for every potential network generated by the shortcut models, through validation with rigorous heat exchanger models at every iteration. In addition, the method does not increase the nonlinearity of the MINLP model, nor does it require any manual intervention or initialization from the user. Three examples are solved and the results are compared to those obtained in the literature.     

Highly active TiO2−x−yNxFy visible photocatalyst prepared under supercritical conditions in NH4F/EtOH fluid

A novel N and F co-doped TiO₂ (TiO_2−x−yN_xF_y) photocatalyst is prepared by treating the TiO₂ precursor in NH₄F/ethanol fluid under supercritical conditions. During photocatalytic degradation of methylene blue under visible light irradiation, the as-prepared TiO_2−x−yN_xF_y exhibits higher activity than the undoped TiO₂, N-doped TiO₂ (TiO_2−xN_x), and F-doped TiO₂ (TiO_2−yF_y). Based on the characterizations including XRD, Raman, FTIR, TEM, PLS, UV–vis DRS, N₂ adsorption–desorption isotherms, XPS and NH₃-TPD, the synergetic promotions of N- and F-dopants incorporated into the TiO₂ lattice are discussed based on the enhanced spectral response in visible region, oxygen vacancies, and surface acidic sites. Meanwhile, the supercritical treatment also promotes the activity owing to the increase in both the surface area and the crystallization degree of anatase, and the enhanced incorporation of N- and F-dopants into the TiO₂ lattice.     

Explosive Hazard of Vapor–Gas Mixtures of Fluoride‐Containing Saturated Hydrocarbons with Fluoride and Chlorine Trifluoride

The limits of explosion of the CF₃CFH₂–F₂–N₂, C₃F₈–F₂–N₂, C₄F₈–F₂–N₂, C₄F₈–ClF₃–N₂, C₄F₁₀–ClF₃–N₂, and F₂ClCF₂Cl–F₂–N₂ mixtures are determined. For some compositions of the first two mixtures, the maximum pressure of explosion and the maximum pressuregrowth rate are measured. The transition from deflagration to detonation is observed in undiluted mixtures close to stoichiometric compositions. The burning rates of the mixtures examined are comparable with the burning rates of oxygen–hydrogen mixtures.     

Effects of baffle inclination angle on flow and heat transfer of a heat exchanger with helical baffles

Numerical simulations were carried out to study the impacts of various baffle inclination angles on fluid flow and heat transfer of heat exchangers with helical baffles. The simulations were conducted for one period of seven baffle inclination angles by using periodic boundaries. Predicted flow patterns from simulation results indicate that continual helical baffles can reduce or even eliminate dead regions in the shell side of shell-and-tube heat exchangers. The average Nusselt number increases with the increase of the baffle inclination angle α when α < 30°. Whereas, the average Nusselt number decreases with the increase of the baffle inclination angle when α > 30°. The pressure drop varies drastically with baffle inclination angle and shell-side Reynolds number. The variation of the pressure drop is relatively large for small inclination angle. However, for α > 40°, the effect of α on pressure drop is very small. Compared to the segmental heat exchangers, the heat exchangers with continual helical baffles have higher heat transfer coefficients to the same pressure drop. Within the Reynolds number studied for the shell side, the optimal baffle inclination angle is about 45°, with which the integrated heat transfer and pressure drop performance is the best. The detailed knowledge on the heat transfer and flow distribution in this investigation provides the basis for further optimization of shell-and-tube heat exchangers.     

Quantum-Chemical Justification of the Structure of Fluorozirconate Glasses1

A number of fluorozirconate clusters of the general formula [Zr _m F _n ]^{(4m– n)}and barium fluorozirconate clusters [BaZr₂F _n ]^{(10 – n)}are studied by the quantum-chemical discrete variational X method. It is shown that the mean energies of fluorine ion addition for all structural units observed in real fluorozirconate crystals lies in a narrow range. The vibrational spectra are analyzed within the energy approach. It is demonstrated that the coordination number of zirconium in a number of barium fluorozirconate glasses is equal to seven or eight. The possible mechanism of structural transformations in binary fluorozirconate glass with a variation in its composition is proposed. The rigidity of different bonds in fluorozirconate clusters is investigated qualitatively. It is established that the rigidities of zirconium bonds with bridging and terminal fluoride ions level off in polymers, which is a prerequisite for the glass formation in fluorozirconate systems.     

Effect of Polyoxyethylene Chain Length on the Physicochemical Properties of N,N‐Dimethyl‐N‐dodecyl Polyoxyethylene Amine Oxide Hybrid Surfactants (C12EOnAO,with n = 1–4)

In order to determine the structure‐performance relationship of nonionic‐zwitterionic hybrid surfactants, N,N‐dimethyl‐N‐dodecyl polyoxyethylene (n) amine oxides (C₁₂EO_nAO) with different polyoxyethylene lengths (EO_n, n = 1–4) were synthesized. For homologous C₁₂EO_nAO, it was observed that the critical micelle concentration (CMC), the maximum surface excess (Γ_m), CMC/C₂₀, and the critical micelle aggregation number (N_m,c) decreased on going from 1 to 4 in EO_n. However, there were concomitant increases in surface tension at the CMC (γ_CMC), minimum molecular cross‐sectional area (A_min), adsorption efficiency (pC₂₀), and the polarity ([I₁/I₃]_m) based on the locus of solubilization for pyrene. The values of log CMC and N_m,c decreased linearly with EO_n lengthening from 1 to 4, although the impact of each EO unit on the CMC of C₁₂EO_nAO (n = 1–4) was much smaller than that typically seen for methylene units in the hydrophobic main chains of traditional surfactants. Compared to the structurally related conventional surfactant N,N‐dimethyl‐N‐dodecyl amine oxide (C₁₂AO), C₁₂EO_nAO (n = 1–4) have smaller CMC, A_min, and CMC/C₂₀, but larger pC₂₀, Γ_m, and N_m,c with a higher [I₁/I₃]_m. This may be attributed to the moderately amphiphilic EO_n (n = 1–4) between the hydrophobic C₁₂ tail and the hydrophilic AO head group.     

Synthesis and structural characterization of two novel 2D coordination polymers [PbCl(atza)]n and [PbCl(a4-ptz)]n

Reactions of Katza (atza⁻ = 5-aminotetrazole-1-acetato) or Ka4-ptz (a4-ptz⁻ = 5-[N-acetato(4-pyridyl)]tetrazole) with PbCl₂in aqueous solution, produced two new Pb(II) compounds, [PbCl(atza)]_n (1) and [PbCl(a4-ptz)]_n (2). Both compounds were structurally characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction. In 1 or 2, the Pb(II) centers are connected through the atza⁻ or a4-ptz⁻ bridging ligand to form a two-dimensional layered structure. The luminescence properties of 1 and 2 were investigated at room temperature in the solid state.     

AIRSIDE PERFORMANCE OF STAGGERED TUBE BUNDLE HAVING SHALLOW TUBE ROWS

Airside performance for tube bank having N = 1 ~6 are reported in this study. Test results indicated that the heat transfer coefficients for N = 1 and N = 2 are comparable and the heat transfer coefficients increase with the number of tube row. For the same P _t, P _t the heat transfer coefficients for the tube bank containing smaller diameter tube are higher than those of larger tube diameter for N≥2 and is comparable for N = 1. Comparisons of the heat transfer performance were made with several correlations. It is found that the Gnielinski correlation gives the best predictive ability. For multiple-row heat exchangers, the predictive ability of the well-known ?ukauskas correlation is also good. But the correlation under-estimate the present I-row heat exchangers. As a consequence, a slight modification to the ?ukauskas correlation is proposed that can extend its applicability in N = 1 and Re_D < 1000. In addition, modification to the frictional performance of ?ukauskas correlation is also made. The mean deviation of the proposed frictional correlation is 11.2%.     

Effect of amino acids containing sulfur on the corrosion of mild steel in phosphoric acid solutions containing Cl<Superscript>−</Superscript>, F<Superscript>−</Superscript> and Fe<Superscript>3+</Superscript> ions: Behavior under polarization conditions

The effect of cysteine (RSH), methionine (CH₃SR), cystine (RSSR) and N-acetylcysteine (ACC) on the corrosion behavior of mild steel in 40% H₃PO₄ solution without and with Cl⁻, F⁻, Fe^3‰+ and their ternary mixture was studied using both potentiostatic and electrochemical impedance (EIS) techniques under anodic and cathodic polarization conditions. The inorganic additives stimulate the overall corrosion reaction while the amino acids inhibit it with a predominant effect on the dissolution of iron. Both RSH and ACC are adsorbed according to Temkin’s isotherm while adsorption of RSSR and CH₃SR follows Frumkin and Langmuir isotherms respectively. The standard free energy of adsorption (ΔG ) was found to be in the order: RSSR > RSH ≅ ACC > CH₃SR. The binary mixtures of Cl⁻ or F⁻ with RSH or CH₃SR are the best inhibitors (IE > 90%) while those containing ferric ions or blend I and amino acids are not good corrosion inhibitors. EIS measurements showed that the cathodic reaction, hydrogen evolution, is charge transfer controlled while the anodic one, iron dissolution, is a complex process.     

Investigation of the phase composition and stability of the α-SiAlONs by the Rietveld method

Different compositions of Y- and Nd-α-SiAlONs were sintered at 1825 °C and analysed by the XRD method. The Rietveld technique was used for characterisation of the diffraction pattern and the x value of α-SiAlON, which has a composition of R_x^+vSi_12−(m+n)Al_m+nO_nN_16−n (R=Y, Nd). It was shown that the method yields reliable data for the content of the rare earth cations in the α-SiAlON phase, even in mixtures of α-SiAlONs with β- or α-Si₃N₄, but cannot be used for the determination of the Al and/or O contents of the α-SiAlONs. On the basis of these data, the stability regions of Y- and Nd-α-SiAlONs were established. Furthermore, it was found that the solubility area of the α-SiAlONs depends on the size of the additives. It was also shown that the solubility area of the Y-α-SiAlON is extended to lower X values than those suggested in the literature.     

Electronic conductivity and structure of DMSO-solvated A - and NR4-graphite intercalation compounds

Highly oriented carbon fibers have been intercalated with DMSO-solvated alkali- and substituted ammonium-cations by cathodic reduction. The resulting decrease in resistance was measured and correlated with values of other solvated cation- and anion-intercalation compounds of graphite. The findings indicate similar mobilities of electrons and holes in C₂₄⁻- and C₂₄⁺-salts, respectively.Basal spacings of 1st–4th stage K⁺(DMSO)_yC_n⁻ and NMe₄⁺(DMSO)_yC_n⁻, which were determined with samples prepared from natural graphite, agree with the model of octahedral solvation of the cations.     

Exploiting anionic and cationic interactions with a new emissive imine-based β-naphthol molecular probe

A new emissive molecular probe derived from 1,7-bis(2′-formylphenyl)-1,4,7-trioxaheptane and 2-hydroxy-1-naphthaldehyde has been synthesized by a Schiff-base condensation method. Its sensor capability towards cations such as Cu²⁺, Zn²⁺, Cd²⁺ and Hg²⁺, and anions such as halides (F⁻, Cl⁻, Br⁻ and I⁻) and CN⁻ was explored in DMSO solution. The geometry was optimized using density functional theory (DFT). The probe showed remarkable selectivity for Cu²⁺ and interaction with the more basic anions CN⁻ and F⁻.     

Effect on discharge of the heat treatment of graphite fluoride under a hydrogen atmosphere

Thermal decomposition of two types of graphite fluorides (CF) _n and (CF) _n , has been carried out in a hydrogen atmosphere at several temperatures between 100 and 500°C, with the object of improving the initial discharge behaviour of the Li/graphite flouride cell. The main reaction was the C-F bond rupture to form graphite-like carbon around the particle surface. The drop in cell voltage at the beginning of discharge could be minimized, and the polarization during discharge reduced by heat treatment under a hydrogen atmosphere. (CF) _n , heat treated at 400°C for 1 h, yielded a discharge capacity of 730–800 mA h per g of active material, corresponding to the discharge efficiency of 8390% at 25°C, and (C₂F) _n , heat treated at 350°C, for 10h, gave 670 mA h g^–1, corresponding to 91 % at 25°C.     

Electron transport and back reaction in dye sensitised nanocrystalline photovoltaic cells

The transport and back reaction of electrons in dye sensitised nanocrystalline solar cells (DSNC) has been studied by frequency resolved optical perturbation techniques. Intensity modulated photocurrent spectroscopy (IMPS) has been used to obtain values of the electron diffusion coefficient, D_n, as a function of illumination intensity. It was found that D_n increased with intensity (D_n∝I^−0.5). Intensity-modulated photovoltage spectroscopy (IMVS) has been used to measure the electron lifetime, τ_n, which is determined by the rate of back reaction with I₃⁻ ions in the electrolyte. It was found that τ_n decreased with light intensity (τ_n∝I^−0.5). The electron diffusion length, L_n=(D_nτ_n)^1/2, is therefore only weakly dependent on light intensity. The values of L_n were used to calculate the theoretical IPCE of the cell. Experimental measurements confirmed the prediction that the IPCE should remain almost constant over five orders of magnitude of light intensity. Possible reasons for the opposite trends in D_n and τ_n are discussed and related to the fundamental processes taking place in the DSNC.     

Synthesis and Properties of Alkylbetaine Zwitterionic Gemini Surfactants

A series of alkylbetaine zwitterionic gemini surfactants, 1,2-bis[N-methyl-N-carboxymethyl-alkyl-ammonium]ethane (C _n Ab, n = 8, 10, 12, or 14), were synthesized by alkylation of N,N′-dimethylethylenediamine with an alkyl bromide, followed by reaction with sodium 2-bromoacetate. Their solution properties were characterized by surface tension, steady-state fluorescence, and rheological measurements. Surface-tension measurements showed C _n Ab had lower critical micelle concentration (CMC) and higher efficiency in lowering the surface tension than their corresponding monomeric surfactants. The logCMC of C _n Ab decreased linearly with increasing chain lengths up to 12. Viscosity measurements showed only C₁₀Ab could enhance aqueous solution viscosity at high concentrations.     

Microporous materials of the nth-order: new classes of poroates

The systematic classification of and formula notation recently suggested for microporous and mesoporous materials with inorganic hosts (poroates) covers materials in which the pores of a host are either empty or occupied by non-porous guest species (first-order poroates). This classification and formula notation is extended, here, to cover also nth-order poroates with n>1, i.e. poroates which contain (n−1) generations of porous guest species in addition to the non-porous first-order guest species. The concept is explained with the aid of several recently discovered examples.