Intensification of Operation of Inclined Plate Settler at Fertilizer Plant

Fertilizer plant’s process waters contain high concentrations of nitrogen compounds, such as ammonium and nitrate. Phosphorus and fluorine, which originate from phosphoric acid and rock phosphate (apatite) used in fertilizer production, are also present. Phosphorus and nitrogen are the primary nutrients causing eutrophication of surface waters. At fertilizer plant process waters are held in closed internal circulation. In a scrubber system process waters are used for washing exhaust gases from fertilizer reactors and dry gases from granulation drums as well as for cooling down the fertilizer slurry in neutralization reactor. Solids in process waters are separated in an inclined plate settler by gravitational sedimentation. However, the operation of inclined plate settler has been inadequate. The aim of this thesis was to intensify the operation of inclined plate settler and thus the solids separation e.g. through coagulation and/or flocculation process. Chemical precipitation was studied to reduce the amount of dissolved species in process waters. Specific interest was in precipitation of nitrogen, phosphorus, and fluorine containing specimens. Amounts of phosphorus and fluorine were reduced significantly by chemical precipitation. When compared to earlier studies, annual chemical costs were almost eight times lower. Instead, nitrogen compounds are readily dissolved in water, thus being difficult to remove by precipitation. Possible alternative techniques for nitrogen removal are adsorption, ion exchange, and reverse osmosis. Settling velocities of pH adjusted and flocculated process waters were sufficient for the operation of inclined plate settler. Design principles of inclined plate settler are also presented. In continuation studies, flow conditions in inclined plate settler should be modelled with computational fluid dynamics and suitability of adsorbents, ion exchange resins, and membranes should be studied in laboratory scale tests.

Sellumassan varastosäiliön rakenneanalyysi

Tämä työ käsittelee valkaistun ja valkaisemattoman sellumassan varastosäiliön tärinää ja värähtelyä. Värähtelyn seurauksena säiliön seinämän jäykisterenkaan hitsausliitokseen on syntynyt särö. Työn tavoitteena on selvittää, mikä johtaa särön syntyyn ja miten kestäväm-piä varastosäiliöitä voidaan rakentaa materiaali-, valmistus- tai rakennemuutoksien avulla. Työ alkoi tutkimalla rakennemateriaalina olevan duplex-teräksen mikrorakenteen ominai-suuksia, sekä sen hitsattavuutta ja seostamista kirjallisuustutkimuksena. Kirjallisuustutki-musta jatkettiin selvittämällä mahdollisia vaurion syntymekanismeja seinämän särölle. Työssä analysoitiin myös tehtaalla mitattuja värähtelyarvoja. Lopuksi laskettiin FE-analyysillä tyhjän varastosäiliön ominaismuodot ja -taajuudet moodianalyysillä, sekä selvi-tettiin harmonisella analyysillä pinnankorkeuden vaihtelun vaikutus siirtymävasteeseen ja kriittisiin värähtelytaajuuksiin. Varastosäiliöön kohdistuvaa värähtelyä ei ole mahdollista poistaa kokonaan, mutta väräh-telyn aiheuttamia seurauksia kyetään rajaamaan useilla keinoilla. Toimenpiteinä voivat olla ainakin seinämän materiaalin paksuuden lisääminen, jäykisteripojen lisääminen ja kriittisten sellun pinnankorkeuksien välttäminen. Kriittiseksi pinnankorkeudeksi havaittiin 40–45 %:n täyttöaste ja turvalliseksi korkeudeksi 35–38 %:n täyttöaste. Varastosäiliölle kriittisen ominaistaajuuden katsotaan syntyvän taajuuksilla 3,3–3,8 Hz ja 5,8–6,4 Hz. Sellumassa putoaa varastosäiliöön noin 2 Hz taajuudella.

Convenience of scallops in plate structures

In this thesis was performed comprehensive study about the convenience of scallops in plate structures. A literature review was performed and lack of knowledge was fulfilled with fatigue tests performed in the laboratory of Steel Structures at the Lappeenranta University of Technology and with finite element method. The aim of this thesis was to produce design guidance for the use of scallops for different structural details and different loading conditions. An additional aim was to include more precise instructions for scallop design to produce good fatigue resistance and appropriate manufacturing quality. The literature review was performed searching bridge engineering and maritime standards and design guides and studies from scientific databases and reference lists from the literature of this field. Fatigue tests were used to research the effect of using scallops or not using scallops to fatigue strength of bracket specimen. Tests were performed on three specimens with different scallop radii and to five specimens without scallops with different weld penetration depths. Finite element method using solid elements, symmetry and submodels was used to determine stress concentration factors for I-beams with scallops. Stresses were defined with hot spot stress method. Choosing to use a scallop or not in the structure is affected by many factors, such as structural and loading conditions and manufacturability. As a rule of thumb, scallops should be avoided because those cause stress concentration points to the structure and take a lot of time to manufacture. When scallops are not used, good quality welding should be provided and full weld penetration is recommended to be used in load carrying corner weld areas. In some cases, it is advisable to use scallops. In that case, circular scallops are recommended to be used and radius should be chosen from fatigue strength or manufacturing point of view.