Reactor kinetics & ; different types of reactor trays

Reactor kinetics & ; different types of reactor trays

1. 1. REACTOR KINETICS &DIFFERENT TYPES OF UREA REACTOR HIGH EFFICIENCY TRAYS Author Prem Baboo Sr. Manager (Prod) National fertilizers Ltd, India Mob. +919425735974 prem.baboo@nfl.co.in ,pbaboo@hotmail.com An Expert for www.ureaknowhow.com Fellow of Institution of Engineers (India)
2. 2. PURPOSE 1. The main purpose HET to improve the redistribution of unreacted carbon dioxide inside the liquid phase rich in free ammonia. 2. To reduce the back mixing phenomenon due to density increase of carbamate and urea solution from bottom to reactor top. 3. To reduce also channelling which has a negative effect on the solution residence time. ADVANTAGES The activity contributes to environmental and social aspects and eventually to sustainable development by: Reduction of consumption of non - renewable fuel like NG, which is a step towards conserving natural resources. Reducing steam consumption which results in reduction in energy consumption. Steam is used in the strippers and varies proportionately with the urea production. Due to the improved conversion efficiency of the process (again due to improved tray design and increased number of trays), the steam utilisation in the overall manufacturing process has reduced. Hence the parameter of the specific consumption of steam to urea gives a clear indication of the energy saved. As elaborated above, the specific consumption of the steam to urea forms the critical parameter and hence the urea production and accordingly the steam consumptions are monitored. Fig.-1 A typical reactor therefore contains a gaseous phase and a liquid phase flowing in co-current flows inside a pressurized reaction chamber. Conversion of ammonia and carbon dioxide to ammonium carbonate and ultimately urea is enhanced as fig-1, i.e. to increase urea output, using tray reactors. Urea tray reactors substantially comprise a normally cylindrical shell, which
3. 3. extends substantially along a normally vertical axis, and is fitted inside with elements, i.e. trays, defined by respective metal sections shaped and/or perforated to divide the reaction chamber into compartments and form specific paths for the substances inside the reactor. The trays are normally perpendicular to the vertical axis of the reactor, and equally spaced along the axis to the full height of the reactor. The trays are very often perforated, i.e. have holes variously arranged and possibly of different shapes and/or sizes. Fluid dynamics and its influence on heat and mass transport rates in gas–liquid reactors is, in general, an important starting point for development of a process design. Improvements in the understanding of these aspects can be particularly fruitful in the case of urea reactors where the fluid-dynamic patterns are complicated by the co-current flow of two phases and the bubbling mode of the vapours. The analysis of such systems highlighted the non-optimal design of existing reactors and led to the conception of new reactor internals. Several industrial applications demonstrate the ability of the new configuration to improve reactor efficiency. Both energy-saving and production increases were obtained. This is a further demonstration that even mature technologies can be improved, leading not only to economic advantages, but also to a reduction in their environmental impact. With CO2conversion in the reactor ranging from 56 to 70%, depending on the particular technology adopted, efforts to obtain improvements have mostly been addressed to the recycle system. The efficiency of the synthesis reactor itself has been particularly under evaluated, probably following the common conviction that the optimum performance had already been achieved. Urea reactors consist of cylindrical vessels (generally 20–40 m high), having diameters from 1 to 3 m, containing, in most cases, several trays giving rise to a stage-wise structure. The aim is to reduce axial back mixing and redistribute the vapour phase. Some-times, reactors operating at high NH3/CO2 ratios and pressures, and having relatively small diameters, are used without trays. In the most widespread process configurations, the unconverted reactants are recycled to the reactor through a series of decreasing pressure stages using heat provided by steam. The higher the CO2 conversion, the smaller the amount of heat and the size of the equipment needed to reach a certain capacity. During the formation of urea, vapour and liquid are present all along the reactor, flowing co- Currently and exchanging mass and enthalpy fluxes through their interfaces. As the process is also characterized by reversible reactions, the overall behaviour is controlled by both physical and chemical equilibrium, coupled with physical and chemical kinetics. The trays are preferably designed for insertion through the manhole reactors are normally provided with, so they can also be fitted to existing reactors and/or removed and replaced. For which reason, the trays are normally made in a number of parts that fit together. The trays have various functions, and in particular: 1. Maximize the hold time of the light (faster) phase; distribute the reactants as evenly as possible along the reactor section, to prevent back-mixing’;
4. 4. 2. Enhance mixing of the gaseous- and liquid phases; and 3. Reduce bubble size' to improve diffusion of the ammonia in the carbon dioxide. Numerous urea reactor tray designs and configurations are known. The Principle of high efficiency trays:- 1. Mass transfer factor 2. Contact pattern of phase 3. Fluid dynamics factors 4. Interfacial surface area 5. Geometry of reactor vessel 6. Chemical kinetics factors 7. Temperature & pressure Generally speaking, known solutions fail to provide for thorough mixing of the light and heavy phases (both consisting of supercritical fluids) , which, because of the difference in density, tend to flow along separate preferential paths defined by the design and arrangement of the trays, and in particular by the shape, location, and size of the holes in the trays. This drawback also impairs final conversion of the reactants, thus reducing urea output. 1. The geometry of the reactor tray according to the present invention provides for thoroughly mixing the gaseous and liquid phases in a urea reactor and urea production process, and so greatly increasing urea output. 2. The reactor tray according to the present invention and the reactor as a whole are also extremely easy to produce and install. 3. Urea producers can reduce consumption and/or increase production of their plants by introducing the various revamping technologies developed The installation high efficiency trays are giving an increased production of urea and reduced steam consumption; the financial benefits are determined by the urea sales and energy prices. It has been demonstrated that the installation of high efficiency reactor trays in existing urea plant is very profitable. THE USE OF REACTION KINETICS TOIMPROVE THE CONVERSION IN VERTICAL UREAREACTORS The conversion of Carbamate into urea is a relatively slow reaction and requires heat. Non converted ammonia and carbon dioxide, passing the high-pressure carbamate condenser, supply the heat needed for this reaction. Because of the equilibrium
5. 5. reaction, the reaction is preferably done in a plug flow type of reactor. Installing a number of continuous stirred tank reactors in series can approach plug flow. Thus the urea reactor is divided into a number of compartments, mostly separated with sieve trays, and each compartment acts as a continuous stirred tank reactor. As a result plug flow is approached in such a cascade type reactor. Fig.-2(a) Fig.-2(b) To obtain a continuous stirred tank reactor, stirrers should be applied. However urea reactors are not equipped with mechanical stirrers. The driving force for mixing the liquid in the compartments of the reactor is the gas phase. The urea reactor is a so- called high-pressure bubble column. By adding the gas phase through the center of a compartment via carefully designed holes, a Torus circulation exists and thus the required mixing of the liquid in such a compartment is obtained, as fig. 2(a) the principle of such a Torus circulation exists and thus the required mixing of the liquid in such a way Compartments obtained. The principle of such a torus circulation is shown in the figure.Because the urea reaction is a relatively slow equilibrium reaction a relative large retention time in the reactor is needed to approach the maximum equilibrium level. Fig.-3, However an infinite large reactor volume is required to reach this equilibrium. For economic reasons the installed reactor volume in the designing of urea plants is such that the fraction approach to equilibrium (FAE) is 95 percent. The fraction approach to equilibrium is defined as: FAE = 100* ήC02 actual/ ή C02 equilibrium Fig.-3 Retention F.A.E ή co2
6. 6. The relation between the fraction approach to equilibrium and the retention is shown in Fig. In large-scale urea plants (> 1500 MTPD), equipped with reactors with large diameters and conventional type reactor trays, it is observed that the expected fraction approach to equilibrium is not reached resulting in a relative low reactor conversion. The consequence is that at a specified plant capacity the steam consumption on the high-pressure stripper is larger than expected. The reason for the observed relative low reactor conversion was a non-optimal mixing rate in the urea reactor compartments and, thus, these compartments did not act as an optimal continuous stirred tank reactor. The non-optimal mixing behavior in such reactors can be caused by: 1. Back mixing 2. Channeling (fig-5,a) 3. Stagnant zones Back mixing occurs when the liquid phase passes the sieve trays through the gas holes. This occurs when the height of the gas cushion below the sieve tray is small. In reactors with large diameters, when the reactor tray is not perfectly horizontal then the . gas holes are in contact with the liquid phase. This is illustrated in Fig.-4 Reactors with a relative large diameter are sensitive for stagnant zones. Stagnant zones are caused by poor mixing in the compartments and have a negative impact on the reactor conversion since the compartments will not optimally act as the required continuous stirred tank reactor. To avoid the negative effects of back mixing and channeling, Stamicarbon developed in the beginning of the 90’s the high efficiency trays as illustrated in the fig. Channeling occurs when the liquid phase is partly bypassing a compartment. In urea reactors, equipped with conventional sieve trays, the liquid is transported from the one compartment to the other compartment via the annular spacing between the tray and the reactor wall. In urea reactors with large diameters it appears that the mixing rate by
7. 7. the Torus circulation may not be large enough to avoid these channeling effects. The channeling effect is shown in Fig. Fig.-5(a) Fig-5(b) HIGH EFFICIENCY TRAYS, HET These high efficiency reactor trays are equipped with liquid risers where the liquid enters the following compartment. By staggering the liquid risers, the liquid is forced into the Torus circulation and channeling is eliminated. To avoid back mixing, the gas cushions were increased and this makes the trays less sensitive to horizontal variations of the tray. Because the conversion of carbamate into urea is an equilibrium reaction, the reaction is preferably done in a plug flow type of reactor; that is to say, one in which the flow of reaction medium is uniform and non-turbulent over the entire cross-section of the reactor interior. It is difficult to prevent turbulence and back mixing in a large unconfined body of fluid; however, an approximation to overall plug flow can be attained in a number of continuous stirred tank reactors arranged in series. Therefore the urea reactor is divided into a number of compartments, separated from one another by sieve trays, and each compartment emulates a continuously-stirred tank reactor. The driving force for mixing the liquid in the compartments of the reactor is the gas phase. By forcing the gas phase to pass through the centre of a compartment via carefully designed holes, a Torus-shaped circulation prevails and thus the required mixing of the liquid in such a compartment is obtained. However, in larger reactors non-optimal mixing behaviour has been identified and investigated. Identified causes were back mixing, channelling and stagnant zones. To address that problem, Stamicarbon has developed a new generation of high efficiency trays known as Siphon Jet Pump trays. The compartments, separated by sieve trays, are equipped with a draft tube. Inside the draft tube there is a two-phase flow of gas and liquid. The effective density of this two- phase flow is considerably lower than the liquid density on the outside of the draft tube, and the density difference further enhances liquid circulation, promoting mixing. The deflector plates in the pool reactor and pool condenser, which work on the same principle as the draft tube, have amply proved this effect. Using Siphon Jet Pump trays provides the closest approach to a continuous stirred tank reactor without necessitating
8. 8. any mechanical agitation. The mixing rate is increased significantly and the negative effects of back-mixing and channelling are avoided. The first Siphon Jet Pump trays were installed at SKW Piesteritz, and the result was so satisfactory that Siphon Jet Pumps have been installed in all three plants and are currently in operation. They have had the effect not only of making operations very smooth and raising the capacity of the existing plants, but also of reducing the HP steam requirement of the HP stripper. Amongst others, Fauji Pakistan, ABF Malaysia and Qafco Qatar have also installed iphon Jet Pumps in their Although the high efficiency trays 'improved the reactor efficiency significantly, they did not improve the mixing rate. The mixing is still reliant upon the Torus circulation. In practice it appeared to be difficult to keep the strict tolerances for the gap between the reactor tray and the reactor wall because of the no roundness of the reactor. To improve the mixing rate in the reactor compartment sand to avoid strict Mechanical tolerances, Stamicarbon recently developed a new generation of H.E.T. Known as siphon jet pumps Fig.-6 The compartments, separated by sieve trays, are equipped with a draft tube. Inside the draft tube there is a two-phase flow with the density of this two-phase flow being considerably less than the liquid density at the outside of the draft tube. By this density difference liquid circulation is enhanced further stimulating the mixing. The deflector plates in the pool reactor and pool condenser, in which the deflector plates have a similar function as the proposed draft tube, have proved these phenomena
9. 9. Because of this heavy circulation effect and thus improved mixing rate it is no longer necessary to equip the reactor trays with liquid risers. The liquid can enter the following Compartment via the annular spacing between the tray and the reactor wall in a similar fashion as the conventional Reactor trays. The strict tolerance regarding the gap between The tray and the reactor wall for the new generation HET is No longer required The first Siphon Jet Pumps were installed in one of the plants of SKW Piesteritz. Because the trays were operating very satisfactory, two other reactors of SKW Piesteritz are now also operating with Siphon Jet Pumps. In the following table the current references for Siphon Jet Pumps are presented. Table-1 Client Capacity Year in Number of New/Modified (MTPD) operation trays trays SKW Piesteritz 3 1050 2001 11 New SKW Piesteritz 1 1050 2002 11 New SKW Piesteritz 2 1050 2003 11 New Fauji Fert.Pakistan 1670 Completed in 2004 10 Modified ABF Malaysia 2250 Completed in 2004 11 New Qafco II 1400 Completed in 2005 11 New Daqing 2300 Completed in 2005 11 New Qafco III 3000 Completed 11 New The gas holes in the tray are more centered than in the conventional tray design to improve the driving force and the tray is equipped with a ring that acts as a Venturi to improve the mixing rate,as fi 8 & 9.By installing these siphon jet pumps all aspects to approach the continuous stirred tank reactor are included. The mixing rate is increased significantly and the negative effects of back mixing and channeling are avoided.
10. 10. Fig.-7 CASALE TRAYS Fig.-8 1. Inverted ‘U’ type , better mixing due to generation of smaller bubbles increasing interfacial surface area and improving the contact pattern causing higher CO2 conversion F.A.E. A- convention trays B- high eff. Trays C -Siphon jet pump
11. 11. Fig.-9 2. Small perforation at top and sloping area for vapour space and large perforation for liquid at bottom area 3. But CASALE trays suffers from corrosion due to sharp configuration Fig.-10 In combination with other Casale technologies such as the High Efficiency Trays, the Split Flow Loop/ Full Condenser configuration is applied for increasing the capacity of CO2 stripping plant with very low investment. High Efficiency reactor trays, the HP loop is drastically debottlenecked even for a large capacity increase (Up to 50% over its original design. Casale, therefore, foresaw to install the Casale Dente High Efficiency Trays in order to debottleneck the HP synthesis section.
12. 12. TABLE.-2 - Plant performance after Casale trays installation Plants Country Year Process No. of Trays CO2 conversion Increase(%points) MP Steam Consumption reduction(kg/MT) Capacity increase (%) Togliatti Azot Russia 1993 NH3 Stripping 14 6.4 300 17 Togliatti Azot Russia 1993 NH3 Stripping 14 4 200 17 Arcadian Trinidad 1994 NH3 Stripping 14 2.8 183 9 Yuman Chem(*) China 1994 CO2 Stripping 10 3.5 148 3 Agrium Can Canada 1994 CO2 Stripping 10 5 65 - Chemco Bulgaria 1995 NH3 Stripping 14 NA 170 6 CFI(**) USA 1995 CO2 Stripping 10 3.5 70 10 Agrium USA USA 1995 NH3 Stripping 10 5.3 251 9 Amonil Romania 1996 CO2 Stripping 11 5 178 8 NFCL India 1996 NH3 Stripping 14 4.5 95 3 Shriram India 1996 Total Recycle 14 6 >100 - NFL,Nangal India 2001 Montedition 14 6 110 6 Note.-(*) only 5 HET installed (**) Data after trays installation based on Casale Survey Casale has been, in the last decades, very active in revamping existing plant and has extensive experience in the design and implementation of complete plant revamping projects, including major modifications to key equipment. Casale’s plant revamp strategy has always been to develop and apply new, advanced technologies to obtain the best possible improvement in plant performance at the minimum cost; with the aim of reducing the energy consumption and/or increasing the capacity The Casale-Dente High Efficiency Trays (HET) are the most efficient trays available on the market and are also an essential element in making the Split-Flow-Loop as efficient as it is. The improved geometry of these trays has a profoundly beneficial effect on the mass transfer efficiency of NH3 and CO2 from the vapours into the liquid phase where urea is formed. The new trays are designed in such a way that: · Vapours and liquid follow separate, but adjacent cocurrent paths through the space between the trays. This guarantees stable flow of the two phases and a better approach to an even uniform flow of the two phases throughout the whole reactor. · These separated paths through the tray are chosen so that very efficient mixing takes place between vapour and liquid. Consequently there is a very high degree of both mass and heat transfer within the liquid phase is realised. · It is possible to generate vapour bubbles with a far smaller diameter than with any previous design. As a consequence, the interfacial surface, for mass and heat transfer, is increased. · There is also a much larger interfacial surface for exchange between the
13. 13. vapour bubble emulsion and clean liquid. · The relative short path length of the recirculation streams into the emulsion phase significantly decreases transfer resistances. The trays are plates corrugated into a series of parallel linear ridges and troughs. The ridges are flattened at the top and the troughs are similarly flattened at the bottom. Large perforations are provided in the trough bottoms for liquid to pass through and there are small perforations in the tops of the ridges for gases accumulating beneath them to pass This unique design produces extremely small bubbles and, as a consequence, a very high specific surface area for mass and heat transfer enhancing the highly efficient mixing between vapours and liquid mentioned above. SNAMPROGETTI (SAIPEM) SUPERCUPS TRAYS 1. The innovative M/S. Saipem Super Cups design for Urea reactor trays has been conceived and developed by Saipem with the support of Engin Soft by means of CFD(Computation Fluid dynamics) simulation. Latest super cup trays the third generation of high efficiency trays recently invented and patented by Saipem 2. Computational Fluid Dynamics (CFD) provides a qualitative (and sometimes even quantitative) prediction of fluid flows by means of mathematical modelling (partial differential equations) 3. The computer code (software) which embodies this knowledge and provides detailed instructions (algorithms) for the computer hardware which performs the actual calculations. CFD is a highly interdisciplinary research area which lies at the interface of fluid dynamics. 4. The Reactor trays that prevent back-flow of the heavier solution from the upper part downwards and favour the gas absorption in the liquid phase. 5. The support of a systematic plan of fluid-dynamic simulations gave a significant contribution to the development of the innovative design. 6. The proprietary M/S Saipem Super Cups (“New Design”) greatly increases the mixing of the liquid and gaseous phases, respectively ammonia and carbamate, and carbon dioxide, thus optimizing the product conversion rate in the reactor. The immediate benefit is the lower specific steam consumption requirement to decompose carbamate to CO2 and NH3 in downstream sections. 7. This represents a further step ahead to get closest to the theoretical equilibrium conversion in the reactor. In fact, the increase in the reaction conversion is strictly dependent on the mixing conditions of ammonia, carbamate and carbon dioxide through the reactor so that the main purpose of these innovative trays is to further improve the contacting conditions among the reagents. 8. The peculiar behaviour of the Super Cups is characterized by a triple fluid-dynamic effect – Gas Equalizer, Mixer Reactor and Gas Distributor. 9. The first effect of Super Cups is to uniformly distribute the concentration of the gaseous phase reagent on the entire section of the tray. In this way, the gas bubbles moving upward “lose the memory” of the non-uniformity of the previous reaction stage and the non-reacted CO2 can be evenly fed to each cup of the tray. Figure shows the formation of the “gas-cushion” (blue area) just below the tray externally to the cups. The cups behave as multiple confined reaction volumes in which the
14. 14. reagents - gaseous CO2 and liquid ammonia & carbamate – heavily swirl inside, thus reaching a high mixing degree. Each cup performs as a static mixer where the phases are strongly contacted. 10. In this way the Super Cups Trays do not simply behave as gas distributors – as in other commercial designs. But perform as additional active reaction stages which can be modelled as a Continuous-Stirred-Tank Reactor (“CSTR”),as fig 12. The CSTR behaviour (ideal perfect mixing) of each single tray can be clearly observed by the comparison of RTD curves for the new and standard designs. 11. The mean residence time increases by about 70% with respect to the standard design, thus strongly improving the urea formation yield. 12. The CO2 gaseous phase forming the gas-cushion below the tray can be partially streamed inside the cups to create a mixer reactor and partially distributed on the upper stage. This split range is one of the most critical design parameter since it allows the customization of the RTD curve of each reactor stage and the increase or decrease of the CSTR (perfect mixing) or PFR (plug flow) behaviour according to the composition of each stage. 13. The Super Cups Trays permit an increase in the urea reactor efficiency with consequent beneficial effects in terms of higher return on investment, lower energy consumptions and reduced environmental impact. 14. The CFD study of the traditional perforated plate vs. the innovative tray facilitated the ability to compare the fluid dynamic behaviour of several designs in terms of mixing performance of the reactants, flow patterns, pressure drops and residence time. Fig.-11
15. 15. Fig.-12
16. 16. CONCLUSIONS With the combination of skilful modelling and original design, the possibility was proven of increasing the efficiency of urea reactors, which were considered for a long time to be operating close to their optimum. This new tray design represents a significant upgrade of the urea reactors and, by consequence, of the whole plant. The net improvement of the CO2 conversion in an existing plant has, in fact, The following advantages: 1. The reduction of the energy consumption and of recycle. 2. The possibility of a sensible increase of the production with the same reactor. The development and successful design of the High Efficiency trays in the reactor was possible through a very accurate fluid dynamic simulation of the system combined with the modelling of the chemical-physical equilibriums and of the heat transfer phenomena. The most important of these consists of a sharp reduction in specific steam consumption. This feature was con-firmed by a number of test run results carried out in the field. Reductions of specific steam consumption up to 250–300 kg per ton of urea have been obtained and capacity increases up to 10–20 % .