ADVANCED AGITATOR DESIGNS IN FERTILIZER MANUFACTURING PROCESSES

Transcription

1 ADVANCED AGITATOR DESIGNS IN FERTILIZER MANUFACTURING PROCESSES by Sebastian Abredadt Project Manager Sales EKATO FLUID Sales Lörrach, Germany Prepared for AIChE Central Florida Section Clearwater th Annual International Phosphate Fertilizer & Sulfuric Acid Technology Conference April 016

2 AIChE th International Phosphate Fertilizer & Sulfuric Acid Technology Conference ADVANCED AGITATOR DESIGNS IN FERTILIZER MANUFACTURING PROCESSES ABSTRACT There are multiple different process steps and intermediate products required in the production of fertilizers. In modern fertilizer production processes or facilities a high standard of operations and maintenance is required. Since agitation is applied in many important unit operations agitators play a key role in the successful and economic operation of entire plants. This paper will focus on different examples of agitator designs for different applications in the fertilizer production process. Examples presented include draft tube agitator designs as used in sulfur melters, reactors or inter-vessel transfer pumps. Here new impeller concepts with an increased efficiency improve the mixing process and reduce required power inputs. Other examples are the agitated reactors in the digestion of phosphate rock with sulfuric acid in the wet process route. Here quite often nonstandard types of vessel geometries (e.g. rectangular, annular) and impeller types are encountered which require specific design knowledge. Examples presented will be described and are supported by lab test results and computational fluid dynamic (CFD) simulations

3 TABLE OF CONTENTS INTRODUCTION BASIC PRICIPLES AGITATOR DESIGN REQUIREMENTS. 1 SULPHURIC ACID. Sulphur Melter.. Precoat Tank PHOSPHORIC ACID. 3 MIXING TECHNOLOGY.. 3 SLURRY CIRCULATION / BLENDING.. 3 SOLID SUSPENSION. 5 PRODUCT SURFACE MOVEMENT AND SOLID INCORPORATION HEAT TRANSFER.. 6 CONCLUSION 7 OUTLOOK.. 7 REFERENCES 8

4 INTRODUCTION Major components of fertilizers are Nitrogen, Phosphorus and Potassium. Secondary elements can be Sulphur, Calcium and Magnesium. Depending on the way the mentioned elements are bound, fertilizers are distinguished between inorganic and organic fertilizers. [5] Raw materials for the production of inorganic fertilizers can be Nitrogen (air) and natural gas (CH4), Potash rock or Phosphate Rock. [5] In line with the 40 th AICHE Clearwater Conference (016) will this paper focus on the agitator design in production of fertilizer from Phosphate Rock. Special emphasis will be laid on the design basis of sulphuric acid and phosphoric acid production, which often exceeds simple blending and solid suspension. BASIC PRICIPLES Fertilizer production from phosphate rock starts with the production of phosphoric acid. The two main commercial production routes for Phosphoric acid are a thermal (dry) or wet process. The thermal process produces a very pure and concentrated phosphoric acid, but has mostly been abandoned for fertilizer production due to the high amount of energy required. For the wet process, tricalcium phosphate in the phosphate rock is primarily treated with sulphuric acid to produce phosphoric acid and insoluble calcium sulphate, latter being separated from the phosphoric acid by filtration. [1] Ca PO4 3H SO4 3CaSO 4 H3PO4 3 Above mentioned reaction is self-limiting, since an insoluble layer of calcium sulphate is formed on the phosphate rock particles. Therefore the calcium phosphate rock is initially converted to the maximum extent possible to soluble monocalcium phosphate with recirculated phosphoric acid. The monocalcium phosphate does then react with sulphuric acid to precipitated calcium sulphate and phosphoric acid. [1] Ca Ca PO4 4H 3PO4 3Ca( H PO4 H PO4 3H SO4 3CaSO 4 6 H3PO4 3 ) AGITATOR DESIGN REQUIREMENTS Agitators are one of the key components in stirred tanks of the sulphuric- and phosphoric acid production. A misunderstanding of the given process and its mixing tasks could lead to a too weak design or geometrically wrong arrangement of the agitator setup (e.g. selection of impeller type, impeller diameter, number of impeller stages). Both could cause a loss of product, low yield, high energy consumption for heating or even total shut down of the production. 1

5 A wrong understanding of the processes could also cause an overdimensioned agitator design, not only increasing the capital investment- and spare parts cost, but also the annual electrical power consumption. Therefore it is essential for an agitator design to first analyse and understand the given process and its mixing tasks. SULPHURIC ACID Agitated applications requiring specific considerations in the agitator design are Sulphur Melting and Precoat Tanks. Sulphur Melter Special requirements on the agitator design for sulphur melting tanks are a sufficient movement at the liquid surface for the incorporation of the raw sulphur, a defined pumping rate and a sufficient movement at the heating coils for uniform heat transfer. In practice the sulphur Melter tanks are either rectangular shaped pits or round tanks. The advantage of round tanks is a reduction of possible dead zones. Rectangular Pits can be shaped with a length to width ratio exceeding one and are equipped with more than one agitator, depending on the length to width ratio. Each of the installed agitators is designed with two impeller stages to enhance the surface movement, which is especially required at the dosing point of the solid sulphur. Due to the required surface movement at the dosing point it is often practice to install an agitator with higher installed power at the raw sulphur dosing point. Below mentioned figure 1 shows an example of a rectangular sulphur melting pit with two installed agitators. Figure 1. Rectangular Sulphur Melting pit with two installed agitator

6 Agitators in round melting tanks are either equipped with draft tube agitators or open system agitators (see figure ). Figure. Round sulphur melting tank with draft tube (left) respectively open agitator system (right) Precoat Tank In precoat tanks filter aid is added to the liquid sulphur. The added filter add is often hard to wet and tend to float near the liquid surface. Hence the agitator needs to be designed for a vortex formation, strong enough for the incorporation of the solids. PHOSPHORIC ACID The main agitated application in the production of Phosphoric acid is the Reactor which is commonly operated in a continuous process. The shape of the reactor can vary depending on the process License. Reactors can be rectangular with separate champers, round or annular. Mixing tasks in phosphoric acid reactors can be a rapid dispersion of the reactants to avoid local supersaturation, fast incorporation of the solids phosphate rock into the mixture of the flash cooler and return flow of acid, destruction of the foam by the CO and SiF emanation, cooling by spraying and a slurry circulation with defined pumping rate. MIXING TECHNOLOGY The following paragraphs describe how above main mixing tasks in the sulphuric acid and phosphoric acid production are achieved. SLURRY CIRCULATION / BLENDING Objective of any blending task is to achieve a required homogeneity throughout the vessel volume and to avoid any supersaturation. Temperature and concentration gradients in sulphuric acid and phosphoric acid plants arise through feeding of raw material, chemical reactions and through supplying heat. 3

7 The basic approach on the agitator design for blending depends on the given flow regime, which can be turbulent, transition zone or laminar. The applicable flow regime is determined by the calculation of the agitator Reynolds number (eq. 1). [3] Re = ρ n d η = product density n = gearbox speed d = Impeller diameter = dynamic viscosity (1) The given flow regime in phosphoric acid and sulphuric acid plants is mostly turbulent. Exceptions can be for example storage or surge tanks with high solid concentrations. Research and measurements taken in industrial vessels have shown with turbulent flow regimes that the blending time characteristic n θ is a constant for stirred vessels with baffles. Where n is the gearbox speed and the blend time. For vessels with a height to diameter ratio (h1/d1) of one it is possible to calculate the mixing time for agitators with a single impeller stage using below mentioned equation. Where Ne is the power number of an axial pumping impeller. [, 3] n θ = 5.5 ( d d 1 ) Ne 1/3 () Another approach for assessing the blending performance of an agitator is the comparison of circulation characteristics. For this the agitator is designed to achieve a defined primary pumping rate q (eq. 3). [] Where Q is the pumping number associated to an impeller. q = Q n d 3 (3) The pumping number Q can be determined using the vane-type current meter (see figure 3) or laser doppler anemometry. Those methods have been used to determine the pumping number for the EKATO VISCOPROP, which is a proven Impeller design for blending in sulphuric- and phosphoric acid plants. Pictures shown below (fig. 3) were taken during a pumping number determination executed for a phosphoric acid reactor project. For which the production scaled reactor included multiple eccentrically installed agitators. The tests were performed for a customer requesting re-confirmation of existing pumping numbers. Figure 3. Primary pumping number determination with van-type current meter. 4

8 The van-type current meters were therefore installed at different area ring segments Aann,i below the impeller to measure the given flow velocity. With the available results, it is possible to calculate the given pumping rate in the lab scale test with equation 4, which is then used to determine the impellers pumping number. [] q = i (w i A ann,i ) (4) SOLID SUSPENSION The physical properties of the substances taken into account while designing suspension agitators are - Density of the pure liquid liq. - Density difference between solid and liquid - Dynamic viscosity - Design particle size dk, - Solids concentration by volume cv. Of which all impact on the hindered settling velocity wss of solid particles. The settling velocity of a single particle ws is calculated by methods given in the relevant literature. The hindering effect on the settling process due to the presence of multiple particles is quantified with following relation (eq. 5). [3] w ss = w s (1 c v ) m (5) Where exponent m is a function of the Reynolds number for particles. For the energy considerations it is assumed that all solid particles are distributed uniformly and they all begin to settle under the effect of gravity simultaneously, releasing a settling power Psettle quantified by the relation (eq. 6): P settle = w ss c v ρ g V (6) The agitator must be designed to provide a power high enough to counteract this settling power, in order to maintain a defined uniformity of the suspension. This agitator power or shaft power always amounts to be a multiple of the settling power. [3] To which degree the shaft power needs to be a multiple of the settling power depends on the difficulty of the suspension task (e.g. physical properties, high h1/d1 ratio) and the installed Impeller type. Tests results from a 1 m 3 scale model comparing the EKATO VISCOPROP with the pitch blade turbine with its simple geometry show that the pitch blade turbine requires a specific power input which can be up to times higher than with the EKATO VISCOPROP. [] PRODUCT SURFACE MOVEMENT AND SOLID INCORPORATION Most open agitator systems in sulphuric and phosphoric acid production are designed with at least two impeller stages in order to achieve a sufficient movement on the product surface for a fast incorporation of liquids, solids or slurry added near to the product surface. 5

9 Figure 4 shows a CFD study for a multiple staged agitator system with EKATO VISCOPROP, showing how the product in flow velocity is transported from each impeller stage to the next in an axial direction. Figure 4. CFD study, speed lines of axial product flow induced by EKATO VISCOPROP For some application it is furthermore necessary to form a vortex to incorporate solids from the liquid surface. A number which can be used for the prediction of a vortex formation is the dimensionless Froude number Fr (eq. 7): Fr = n d g (7) Where g is gravity. [3] Tests with the EKATO VISCOPROP showed that there is a possibility to predict the vortex formation. Therefore a dimensionless number describing the agitator to tank geometry is plotted against the Fr number required to create a vortex. In order to be able to predict the vortex formation for various applications, EKATO has carried out a large amount of tests with different geometrical ratios, viscosities and for centrically and eccentrically installation. HEAT TRANSFER The heat flux Q is normally supplied or removed via the tanks jacket or inner coils. Q is obtained with equation 8. Q = k A (θ i θ a ) (8) Where k is the heat transition coefficient, A the heat transfer surface area, θ i θ a the temperature difference between product (i) and cool/heat fluid (a). [3] The heat transfer coefficient k is calculated from the product- and inner heat transfer coefficients, α a and α i respectively, and the resistance of the vessel wall (wall thickness s, thermal conductivity) (see eq. 9). [4] 1 k = 1 α a + s λ + 1 α i (9) 6

10 The only number which can be influenced by agitation is the inner heat transfer coefficient α i, which can be determined using the Nußelt equation (eq. 10). [4] Nu = α i d i λ = C Re a Pr b ( η η w ) 0,14 (10) Where C is a constant dependent on the geometrical arrangement of the agitator inside the vessel (e.g. impeller type, number of impeller stages, d/d1 ratio) and the type of the heat transfer elements (e.g. jacketed tank, coils). The parameters a and b primarily depend on the given flow regime and are for a turbulent flow regime a = /3 and b = 1/3. η w is the dynamic viscosity at the heat transfer area, which can be especially of interest for cooling applications. If the general power equation for agitators is combined with the heat Nußelt equation, the following relation (eq. 11) results for the power per unit volume input (P/V). [] P/V Ne/(C 4,5 d /d 1 ) (11) Figure 5 below shows a comparison of the relative values for various agitator impellers, the two-stage EKATO VISCOPROP having a particularly high efficiency. [] Figure 5. Comparison of Impeller efficiency for heat transfer in stirred tanks. CONCLUSION The paper highlights that EKATO has the requisite knowhow for the process design of agitators for stirred applications. EKATO is aware of the special mixing tasks given in the production of sulphuric and phosphoric acid and has the required literature and experimental based knowledge to fulfil those requirements. On request EKATO is also able to do various experimental trials in their more than 1000 m big technology centre or perform application oriented CFD studies. OUTLOOK In phosphoric acid reactors we do often face special vessel geometries (e.g. annular reactor) in which multiple agitators are installed in one tank. For those tanks it is often a 7

11 requirement that phosphoric acid is recirculated within the tank in order to react the calcium phosphate rock as far as possible to soluble monocalcium. EKATO is highly interested in cooperation with users to gain more knowledge on the flow patterns in such tanks using CFD studies. Often we face deposits of CaSO4 influencing the shaft safety and critical speed. EKATO by now knows how to handle those deposits for their mechanical design but would be interested to study different materials or linings and their behaviour in the reactor. After an extensive experimental study on impeller efficiency combined with CFD investigations EKATO has developed the high-efficient TORUSJET for application in DTB crystallizers. Combined with a corresponding set of guide vanes this impeller provides very high axial flow velocities and therefore significantly increases the pumping efficiency in mass crystallizers. In a next step it is our aim to adapt this new developed impeller type for other applications apart from crystallizers. REFERENCES [1] Best Available Techniques for Pollution Prevention and Control in the European Fertilizer Industry, Booklet No. 4 of 8: PRODUCTION OF PHOSPHORIC ACID, European Fertilizer Manufacturers Association, 000. [] EKATO Rühr- und Mischtechnik GmbH 000. EKATO Handbook of Mixing Technology, ISBN Schopfheim: H. Strütt + D. Rünzi. [3] EKATO HOLDING GmbH 01. EKATO. THE BOOK, ISBN Emmendingen: Druckerei Hofmann. [4] Matthias Stieß Mechanische Verfahrenstechnik 1, ISBN Berlin: Springer-Verlag. [5] April