Novel Applications of Eutectic Freeze Crystallization

Abstract

During the last decades, the consumption of raw materials and energy in the world has faced a tremendous increase with a corresponding industrial waste volume increases, which treatment poses serious challenges. From an economical point of view, the waste has values as it contains valuable matters. Disposal of these streams without any further treatment is the environmentally unacceptable. The foreseeable upcoming depletion of metals, pure water and other raw materials forces us to find efficient technologies to recover those, and this thesis aims to investigate novel applications of a promising crystallization technology, Eutectic Freeze Crystallization (EFC) for the energy friendly (up to 90 % lower energy costs compared to evaporation) complete recovery of salt and fresh water from industrial streams. EFC operates around the eutectic temperature and composition of an aqueous solution and recovers the dissolved salts and fresh water from almost all aqueous salt or acid containing process stream, producing extremely pure ice and pure salts. The low operating temperature promotes safe and corrosion-free operation. Applications are possible in food, pharmaceutical, petrochemical and fertilizer industries etc. An introduction and the scope of this thesis are given in Chapter 1. This work focusses on the industrial application of EFC in (bio-)chemical and oil and gas industries. HrICP-MS is used as an analytical tool to assess the purity obtained using EFC. In Chapter 2, EFC was performed for an NiSO4 containing industrial stream from 1 liter scale batch set-up in the laboratory to an industrial scale continuous pilot plant (200 liter) at the production location of nickel sulfate. The heat transfer rate from the crystallizer could be maintained at 9 kW/m2 with ice and nickel sulfate production rates of 16 and 4 kg/h respectively. Since the major impurity in the starting solution is sodium, the quaternary point and the two eutectic solubility lines for the Na2SO4-NiSO4-H2O ternary system were also experimentally determined to investigate the limitations of EFC. Recrystallization of NiSO4?7H2O from produced with EFC into the NiSO4?6H2O is described in Chapter 3, and an effort of trace element analysis was performed by hrICP-MS. Within the range of 40 °C to 90 °C an optimal i.e. most pure product was found at 50 °C. Recrystallization mother liquor was recycled to assess the performance in industrial full continuous operation. It was found that the recrystallization occurs via the solution, allowing a redistribution of impurities. The results from hrICP-MS were used to calculate partition coefficients and distribution coefficients for impurity uptake in the NiSO4 crystals. Efforts to develop and perform elemental analysis of traces in the nickel sulfate system using hrICP-MS are presented in Chapter 4. First, contaminations in blanks and standard solution were assessed, and calibration curves were measured. Second, observed memory effects (i.e. the signal is influenced by the signal of the preceding sample or standard) were reduced by extensive flushing. Third, a strong decrease on the sensitivity during one single run consisting of blanks, samples and standard solutions was observed, and it was surprisingly found that this effect could not be eliminated by using internal standards. By correction of the signal decline from standard curves at different stages within one run, and by performing three runs reliable data could be collected. Chapter 5 describes how saline water from Kuwait oil production and valuable products were recovered by EFC in 1 liter batch experiment. Inclusions of mother liquor in the final ice product are below 0.05 wt%, and in NaCl?2H2O below 0.1 wt%. Then EFC was scaled up to batch 10 liter scraped wall crystallizer. A maximum ?T of 3.5 °C between the cooling liquid and the NaCl?2H2O/ice slurry could be maintained. The quality of final salt and ice products have a good reproducibility from 1 liter to 10 liter. Recrystallization were performed from NaCl?2H2O into NaCl as a further purification step , yielding more pure NaCl. Simulated shale gas water was treated with EFC (Chapter 6), and the ice products were deemed sufficiently pure to be reused in fracking fluid. By stepwise cooling down in a batch EFC process, the system of the solution with the eutectic points of BaCl2, NaCl and SrCl2 was characterized. At the end of the batch sequence the solution will consist of calcium chloride with minor impurities. The batch process was scaled up in a 200 liter scraped wall crystallizer, and a heat transfer rate of 4 kW/m2 over the heat exchangers could be maintained without excessive scaling inside the crystallizer. In Chapter 7, an industrial problem of removing Mg from hydrated nickel sulfates using state-of-the-art molecular simulations was investigated. Periodic Density Functional Theory (DFT) and cluster DFT calculations are used to study the crystal structures and phase stability of the hexahydrated and heptahydrated Ni and Mg sulfates and their mixed phases. The calculated lattice parameters of MSO4(H2O)n (M=Ni, Mg; n=6,7) crystals are in good agreement with available experimental data. The relative energy differences of the mixed phase for both hexahydrated and heptahydrated Ni/Mg sulfates obtained from both the periodic and cluster DFT calculations are generally less than kT (25.8 meV, T = 300 K), indicating that a continuous solid solution is formed.