Delft University of Technology
FACULTY MECHANICAL, MARITIME AND MATERIALS ENGINEERING
Department Maritime and Transport Technology
Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl
This report consists of 57 pages and 4 appendices. It may only be reproduced literally and as a whole. For commercial purposes only with written authorization of Delft University of Technology. Requests for consult are only taken into consideration under the condition that the applicant denies all legal rights on liabilities concerning
Specialization: Transport Engineering and Logistics Report number: 2015.TEL.7987
Title: Improving suction nozzle design with simulation to increase performance of packing coke removal system
Author: J. van Essen
Title (in Dutch) Het verbeteren van de vorm van een zuigmond met behulp van simulatie, om de prestatie van een cokes afzuigsysteem te verhogen.
Assignment: Master thesis
Confidential: yes, until 19 February 2021 Initiator (university): Prof.dr.ir. G. Lodewijks
Initiator (company): ir. G. Nieuwenhuis (NKM Noell Special Cranes GmbH, Hoofddorp) Supervisor: Dr. ir. D.L. Schott
T U Delft
F A C U L T Y O F M E C H A N I C A L , M A R I T I M E A N D M A T E R I A L S E N G I N E E R I N GD e l f t U n i v e r s i t y o f T e c l i n o l o g y Department of Marine and Transport Technology
Mekelweg 2 2628 CD Delft the Netherlands Phone + 3 1 (0)15-2782889 Fax +31 (0)15-2781397 www.nntt.tudelft.nl
S t u d e n t : J . v a n Essen A s s i g n m e n t t y p e : Master thesis Supervisor ( T U D ) : Dr. ir. D. Schott C r e d i t p o i n t s (EC): 36
Supervisor ( C o m p a n y ) ir. G. N i e u w e n h u i s Specialization: TEL
R e p o r t n u m b e r : 2 0 1 5 . T L . 7 9 8 7 C o n f i d e n t i a l : Yes
Subject: I m p r o v i n g s u c t i o n n o z z l e d e s i g n w i t h s i m u l a t i o n t o i n c r e a s e
p e r f o r m a n c e o f p a c k i n g c o k e r e m o v a l s y s t e m
NKM Noell Special Cranes G m b H is supplier o f special cranes a n d handling e q u i p m e n t used i n , a m o n g s t o t h e r s , t h e a l u m i n i u m p r o d u c t i o n i n d u s t r y . T h e c o m p a n y d e s i g n s , m a n u f a c t u r e s a n d c o m m i s s i o n s t h e i r p r o d u c t s . One of t h e c r a n e t y p e s , t h e Furnace T e n d i n g A s s e m b l y (FTA) c r a n e , o p e r a t e s in t h e b a k i n g f u r n a c e f o r a n o d e s . T h e s e a n o d e s a r e r e q u i r e d f o r t h e electrolytic p r o d u c t i o n of a l u m i n i u m . T h e t r o l l e y o f t h e c r a n e carries a s u c t i o n i n s t a l l a t i o n . This facility r e m o v e s residual calcined p e t r o l e u m c o k e , w h i c h is used as c o v e r i n g m a t e r i a l d u r i n g t h e baking process o f t h e a n o d e s . T h e r e q u i r e d s u c t i o n c a p a c i t y is fixed by c o n t r a c t . T h e inlet s h a p e o f t h e suction a s s e m b l y has a large influence on t h e f l o w c a p a c i t y .
T h e a i m o f t h i s a s s i g n m e n t is t o analyse t h e e f f e c t o f t h e s h a p e o f t h e inlet, t o finally m o d i f y t h i s inlet in o r d e r t o i m p r o v e t h e suction capacity. T h e f i r s t s t e p o f t h e a s s i g n m e n t consists o f p e r f o r m i n g an analysis o f t h e c o m p l e t e suction s y s t e m , t o i m p r o v e it a n d s t a t e r e c o m m e n d a t i o n s f o r it. T h e s e i m p r o v e m e n t s can c o n s i d e r t h e size of t h e s y s t e m , b u t also t h e d e s i g n o f t h e c u r r e n t s y s t e m .
F u r t h e r m o r e , t h e a s s i g n m e n t consists o f m a k i n g an i n v e n t o r y o f t h e existing t e c h n i q u e o f p n e u m a t i c t r a n s p o r t a n d p e r f o r m i n g a literature s t u d y o f t h e c u r r e n t s t a t e - o f - t h e - a r t . Besides l i t e r a t u r e , i n f o r m a t i o n f r o m e m p l o y e e s a t NKM Noell Special Cranes G m b H can be used as w e l l . T h e last s t e p o f t h e a s s i g n m e n t considers s e t t i n g up a s i m u l a t i o n o f t h e m a t e r i a l f l o w a t t h e inlet o f t h e s u c t i o n s y s t e m , in o r d e r t o p e r f o r m nozzle design i m p r o v e m e n t .
1
Summary
This study considers the suction operation of calcined petroleum coke. This is part of the production process of aluminium. NKM NOELL Special Cranes GmbH (NNSC) is supplier of special cranes and special handling equipment for the aluminium industry and initiated this study.
To get an impression on the boundary conditions the aluminium production process is described briefly. During the production of aluminium, transformation of alumina to liquid aluminium and carbon dioxide is obtained by supplying electrical energy as direct current through carbon anode blocks. This electrolytic smelting process is called the Hall-Héroult process. The anode blocks need special properties for the aluminum production. This is achieved by undergoing a heat treatment. In this case baking of anodes takes place in an open top ring type furnace. Here, a furnace is a large building with a floor area of approximately 200 x 40 m. The floor of the building is provided with pits. These pits are filled with layers of anodes and covered by packing material to protect the anodes against ambient air during baking. The baking of the anodes takes place by moving fire equipment over the pits through the furnace. Cycle time takes around 24-28 hours per section. After baking, the anodes are removed from the pits by an overhead travelling crane, called a Furnace Tending Assembly (FTA). First the packing material needs to be removed. This material, calcined petroleum coke at temperature up to 350 °C, is removed by a vacuum suction system, which is also attached to the FTA.
The latter process is the subject of this research. It is desired to perform the coke removing operation as fast as possible. Increasing the removal rate of the coke can be achieved by scaling up the complete suction system, based on the simple thought: bigger is better. However, this has a great financial impact and it has its limitations as for example: pump performance, power supply and pit dimensions. Therefore, the suction system needs to be improved in terms of conveying rate. Starting point for increasing the suction capacity is improving the shape of the suction nozzle of an existing FTA. This requires analysing the flow of the hot air and coke in a pneumatic transport system.
Many factors influence the performance of the suction system during operation. Some factors depend on suction system behavior, but operator performance has a large influence as well. To determine the suction capacity of a certain system usually a test is performed. This test is a simplified case of the operational suction mode. During the test the suction pipe is lowered into a bed of coke for a fixed timespan. Then the amount of removed material is measured. This results in a material flow rate which represents the performance of the system.
This thesis was performed to improve the suction process of calcined petroleum coke. This was done by analysing the effect of nozzle shape on suction capacity. The assignment consists of making an inventory of the existing technique of pneumatic transport and performing literature study of the current state-of-the-art. Besides literature, information about the suction system from NNSC was used as well. Furthermore, setting up a simulation of the material flow at the inlet of the suction system, in order to perform shape improvement was also part of this research.
First an analysis was performed on the total suction system. The goal was to gather information on the suction system performance parameters and compare calculated values to measured and estimated values for a typical suction system. The theoretical analysis resulted in similar performance as was determined by measuring and estimation. Only a difference in temperature was found in the suction pipe. It is expected that air temperature rises until it leaves the hopper, instead of cooling down. An important finding was that air flow velocity has a great influence on the suction performance. Furthermore, the analysis of the pressure drop showed that the flow has the potential to transfer more solids mass.
Measures to improve the suction capacity mostly emphasize on improving feeding material to the nozzle. Many patented nozzles consist of circular concentric pipes. Complex feeding aid or control measures are applied to improve the material feeding. However, due to limited room for the nozzle in the pit these aids are not convenient. As was found by NNSC, the nozzle shape and auxiliary air configuration has a great effect on the mass flow through the pipe. Modifying the nozzle is relatively easy to realize and this option was therefore investigated further. Many former researches were performed for small circular nozzles. Research on large circular vacuum nozzles is already limited, and research on relatively large rectangular suction nozzle design was not found at all. Although little
2 information is available, some clear principles were covered. The following points influence the material handling efficiency and should be considered when designing a vacuum nozzle:
• Volume of secondary air and its arrangement,
• distance between end section of internal and external tubes, • depth of nozzle in material,
• Increasing the opening of the inlet, with considering sufficient air flow velocity. Each of these can be used to improve performance.
The final part of this research considers simulation of the material flow at the inlet of the suction system, in order to perform shape improvement. By modeling the flow with a software package coupling Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM), various nozzle shapes were tested. The current design shape which is typical for a suction system was used as reference. Performance was determined by counting the amount of particles that left the system. Results learned that performance increased when the horizontal air velocity below the nozzle increased as well, while the air flow velocity in the suction pipe was not changed. Higher velocity was achieved for a basic type nozzle by increasing the main pipe size and simultaneously reduce the ratio between secondary channel width and main pipe size as much as possible. Instead of upgrading the current design with other dimensions, a different shape with deflexed bottom resulted in even higher performance.
Based on the results from this assignment better nozzle shapes were found that could improve the performance of a coke suction system. It is recommended to choose between two options to improve performance:
1. The first option is to use the current design and increase its main pipe size in a combination with reducing the ratio between secondary channel width and main pipe size as much as possible.
2. The second option is using a standard size nozzle with deflexed bottom part.
These recommendations are based on a simplified simulation model. Therefore, it is advised to perform real life tests with these modified nozzles to verify if these findings also work in reality. The company NNSC will perform these test, however the results will not be available before finalizing this report. Therefore, no verification is available yet.
Due to the simplification for the simulation the following is recommended when future research will be performed in this subject. The main issue is the transformation from a three dimensional problem to a two dimensional one. Multidirectional influences are now discarded, but can have an effect on the performance. One should keep in mind that a 3D approach requires much more calculation power and hardware should be adjusted to that. Another point of attention is the introduction of the particles. The performed simulations used an insertion rate of particles to represent a continuous feeding to the nozzle. More realistic would be using a moving nozzle that is inserted in the particle bed. However, this was not possible due to limitations of the used model and solvers. The last recommendation for improving the simulation quality is using more realistic material instead of single size spherical particles. Using a wide particle size distribution and applying different shapes are two main points of improvement. However, adding more detail to the nozzle, the operation and the material all require more computational power. When considering developments in the past on this field of engineering, it is expected that more computational power will be more accessible in the near future.
3
Samenvatting
Dit onderzoek behandelt het afzuigen van cokes. Dit is onderdeel van het productieproces voor aluminium. NKM NOELL Special Cranes GmbH (NNSC) is fabrikant van speciale hijskranen voor de aluminium industrie en heeft dit onderzoek in gang gezet.
Om een beeld te krijgen van het aluminium productieproces volgt eerst een korte uitleg hierover. Voor de productie van aluminium wordt alumina getransformeerd tot vloeibaar aluminium en koolstofdioxide door elektrische energie als zijnde gelijkstroom door koolstof anode blokken te sturen. Dit elektrolytisch smeltproces wordt het Hall-Héroult proces genoemd. De anode blokken vereisen speciale eigenschappen voor de productie van aluminium. Deze worden bereikt door het toepassen van een warmte behandeling. Het zogenoemde bakken van anodes vindt plaats in een open top ringvormige oven. De oven is in dit geval een groot gebouw met vloeroppervlak van 200 bij 40 meter. De vloer van de oven is voorzien van putten. Deze zijn gevuld met in lagen gestapelde anode blokken en voorzien van afdekmateriaal om de anoden te beschermen tegen de buitenlucht tijdens het bakken. Het bakken gebeurt door een brander installatie over de putten in de oven te laten rijden. Na het bakken worden de anode blokken verwijderd uit de put met behulp van een bovenloopkraan, hier een Furnace Tending Assembly (FTA) genoemd. Voordat de blokken verwijderd kunnen worden dienen de blokken vrijgemaakt te worden van afdekmateriaal. Dit materiaal, gecalcineerde petroleum cokes op een temperatuur van 350 °C, wordt verwijderd door middel van een zuiginstallatie, welke zich eveneens bevindt op de FTA.
Het laatstgenoemde proces is het onderwerp voor dit onderzoek. Het is gewenst om zo snel mogelijk de cokes uit de putten te verwijderen. De snelheid kan verhoogd worden door het complete zuigsysteem op te schalen met de gedachte: hoe meer hoe beter. Dit heeft echter grote financiële consequenties en het heeft zijn beperkingen zoals: pompvermogen, elektriciteitsafname en afmetingen van de put. In plaats van vervangen zou het huidige systeem verbeterd moeten worden. De eerste stap is de zuigcapaciteit verhogen door de vorm van de zuigmond aan te passen van een bestaande FTA. Dit vereist analyseren van de stroom van warme lucht met cokes in een pneumatisch transport systeem.
Vele factoren beïnvloeden de prestatie van een zuigsysteem. Sommige factoren zijn afhankelijk van het type zuigsysteem, echter hoe de zuiginstallatie bediend wordt speelt ook een grote rol. Doorgaans wordt de zuigcapaciteit van een zuiginstallatie bepaald met een zuigtest. Deze test is een versimpelde versie van operationele modus. Tijdens de zuigtest laat men de zuigpijp in een bed cokes zakken en wordt na een bepaalde tijd gemeten hoeveel materiaal uit de put is verwijderd. Dit resulteert in een maat voor de zuigcapaciteit van het systeem.
Dit onderzoek is uitgevoerd om de zuigcapaciteit te verbeteren. Hiervoor is gekeken naar wat het effect is van vorm van de zuigmond op de zuigcapaciteit. De opdracht bestaat uit het inventariseren van de huidige en bestaande techniek voor pneumatisch transport. Naast literatuur is ook gebruik gemaakt van de kennis binnen NNSC. Vervolgens is een simulatie opgezet van de materiaalstroom in zuigmond om tot inzicht te vergaren en verbeteringen aan de zuigmond te kunnen komen.
Het onderzoek is gestart met het analyseren van het complete zuigsysteem. Het doel hiervan was informatie te verzamelen over welke parameters de prestatie bepalen. Ook kon hiermee een vergelijking worden gemaakt tussen een theoretische benadering en gemeten en geschatte eigenschappen van het systeem. De theoretische benadering resulteerde in vergelijkbare prestatie als bepaald met de gegeven informatie. Enkel een verschil in temperatuur was gevonden in zuigpijp. De luchttemperatuur zal stijgen in de zuigpijp tot na de hopper, in plaats van dalen. Een belangrijke bevinding was dat de luchtsnelheid grote invloed heeft op de zuigcapaciteit. Daarnaast gaf de analyse van drukval over het systeem aan dat het referentie zuigsysteem potentie heeft meer massa te transporteren.
Literatuur leerde dat bij veel methoden om de capaciteit te verhogen de nadruk ligt op het verbeteren van de toevoer van materiaal naar de zuigmond. Gepatenteerde zuigmondontwerpen bestaan voornamelijk uit ronde doorsneden, gecombineerd met complexe toevoersystemen. Voor het huidige systeem zijn deze niet geschikt door de geringe ruimte voor de zuigmond in de put. NNSC weet uit eigen ervaring dat vorm van de zuigmond en methode van valse lucht toevoer effect hebben op de zuigcapaciteit. De huidige zuigmond aanpassen is relatief eenvoudig en is rede geweest voor nader onderzoek. Uit literatuur volgt echter dat bij veel onderzoek is gebruik gemaakt van een relatief kleine
4 zuigmond met ronde doorsnede. In dit geval gaat het echter om een rechthoekige doorsnede, hier is weinig over gevonden. Enkele principes zijn wel duidelijk naar voren gekomen welke kunnen resulteren in een verbeterd ontwerp.
De volgende punten hebben invloed op de zuigefficiëntie en moeten in acht genomen moeten worden bij het ontwerpen van een zuigmond:
• Volume van valse lucht en configuratie,
• de afstand tussen de rand van de buitenmantel en zuigpijp bij concentrische zuigmonden, • diepte van de zuigmond in het materiaal,
• Het vergroten van de zuigopening, met inachtneming van voldoende luchtsnelheid. Elke van deze punten kan gebruikt worden om de zuigprestatie te verbeteren.
Het laatste deel van dit onderzoek beschouwt het simuleren van de materiaalstroom in de zuigmond. Met behulp van een softwarepakket dat een koppeling maakt tussen Computational Fluid Dynamics (CFD) en Discrete Element Method (DEM), is een gevarieerde set vormen van zuigmonden getest. Het huidige ontwerp is gebruikt als referentie. De prestatie van een zuigmond is bepaald aan de hand van het aantal korrels dat uit het simulatiesysteem is verwijderd over een vast tijdsinterval. Uit de resultaten is gebleken dat zuigmonden het best presteren waarbij de horizontale luchtsnelheid onder de zuigmond hoog is, terwijl de luchtsnelheid in de zuigpijp onveranderd bleef. Bij de standaard zuigmond, gebaseerd op het huidige ontwerp, kan de hogere snelheid worden bereikt door de breedte van de zuigpijp te vergroten. Een extra verhoging wordt bereikt indien tegelijkertijd de verhouding wordt verkleind tussen afmetingen van het valse lucht kanaal en de zuigpijp. In plaats van de afmetingen van de huidige zuigmond aan te passen kan ook gekozen worden voor een ander vorm van de zuigmond. Hogere prestaties waren geconstateerd indien de onderzijde van de zuigmond naar buiten gebogen is.
Dit onderzoek heeft geresulteerd in een verbeterd ontwerp van de zuigmond wat leidt tot hogere zuigcapaciteit. Aanbevolen wordt om uit twee opties te kiezen die tot verbeterde prestaties leiden.
1. De eerste optie is om het huidige ontwerp te gebruiken, maar de afmetingen van de hoofddoorsnede van de zuigmond te vergroten en daarbij de verhouding verkleinen tussen afmeting van valse lucht kanaal en afmeting van de hoofddoorsnede.
2. De tweede optie is de huidige vorm aan te passen naar gebogen onderzijde van de zuigmond. De genoemde aanbevelingen zijn gebaseerd op een vereenvoudigd simulatiemodel. Daarom wordt geadviseerd de aangepaste zuigmonden in werkelijkheid te testen, om te verifiëren of de resultaten uit de simulatie overeenkomen met de realiteit. Het bedrijf NNSC zal deze testen gaan uitvoeren, echter zijn de resultaten nog niet bekend bij het afronden van dit rapport.
Enkele aanbevelingen zijn van toepassing voor een eventueel vervolgonderzoek. Een belangrijk punt in de vereenvoudiging bij het modeleren, is de transformatie naar tweedimensionaal probleem. Invloeden vanuit een derde richting zijn hier niet beschouwd, maar kunnen invloed hebben op de prestatie. Echter, het uitvoeren van de simulatie in drie dimensies vereist veelvuldige hoeveelheid rekencapaciteit.
Een tweede aandachtspunt is het introduceren van korrels in het simulatiesysteem. Dit onderzoek heeft gebruik gemaakt van een stroom van materiaal toevoer bij stilstaande zuigmond. Realistischer zou zijn, als een vaste hoeveelheid korrels wordt gebruikt met een bewegende zuigmond. Door beperkingen in het model was dit echter niet mogelijk.
Een laatste aanbeveling met betrekking tot modeleren is gebruik te maken van realistisch korrel materiaal in plaats van vereenvoudiging van zowel vorm als grootte. Ook hier geldt echter dat dit consequenties heeft op de vereiste rekencapaciteit. Zoals in het verleden is gebleken met verbeteringen op dit gebied, wordt verwacht dat dit in de toekomst meer rekencapaciteit toegankelijk is.
5
Contents
Summary ... 1 Samenvatting ... 3 Contents ... 5 1 Introduction ... 6 2 Suction System ... 82.1 Description and Specifications of Suction System ... 8
2.2 Material Properties ... 10
2.3 Analysis of typical coke suction system ... 10
2.4 Comparison of Typical Performance and Theory ... 17
2.5 Conclusion ... 22
3 Literature review on suction improvement ... 23
3.1 Patents ... 23
3.2 Experience by NKM Noell Special Cranes ... 26
3.3 Nozzle Design Research... 26
3.4 Conclusion ... 30 4 Suction Simulation ... 31 4.1 Simulation Software ... 31 4.2 Model Setup ... 32 4.3 Model Settings ... 34 4.4 Model Sensitivity ... 38 4.5 Conclusion ... 39
5 Experimental Plan for Simulation ... 40
5.1 Key Performance Indicators ... 40
5.2 Simulation of Basic Nozzle ... 41
5.3 Simulation of Special Nozzle Shapes ... 43
6 Simulation Results ... 44
6.1 Results basic nozzle type ... 45
6.2 Results special nozzle shapes ... 48
7 Conclusions ... 51
8 Recommendations ... 53
References ... 54 Appendix A: Scientific Research Paper ... A Appendix B: Typical System Analysis ... B Appendix C: Typical Simulation setup ... C Appendix D: Simulation results ... D
6
1
Introduction
This study considers the suction operation of calcined petroleum coke. This is part of the production process of aluminium. NKM NOELL Special Cranes GmbH (NNSC) is a supplier of special cranes and special handling equipment for the aluminium industry and initiated this study.
To get an impression on the boundary conditions the aluminium production process is described briefly. The production of aluminium starts with mining bauxite. Form this ore product aluminium oxide is separated first. This alumina, Al2O3, is then dissolved in an electrolyte at 950 °C.
Transformation of the alumina to liquid aluminium and carbon dioxide is obtained by supplying electrical energy as direct current through carbon anode blocks. This electrolytic smelting process is called the Hall-Héroult process [1].
The anode blocks need special properties for the aluminum production. This is achieved by undergoing a heat treatment. In this case baking of anodes takes place in an open top ring type furnace. Here, a furnace is a large building with a floor area of typically 200 x 40 m. The floor of the building is provided with anode pits, see Figure 1. The installation can consist of up to 72 sections of 5 to 9 parallel pits. These pits are filled with layers of anodes and covered by packing material to protect the anodes against ambient air. The baking of the anodes takes place by moving fire equipment over the pits through the furnace. Cycle time takes around 24-28 hours per section.
Figure 1, View of typical anode baking furnace.
After baking, the anode blocks are removed from the pits by a crane, called a Furnace Tending Assembly (FTA), see Figure 1. The anode blocks in the pits are covered by the packing material, so this needs to be removed first. This material, calcined petroleum coke (CPC) at temperature up to 350 °C, is removed by a suction system. This suction system is attached to the FTA as well.
FTA
7
1.1 Problem statement
This thesis focusses on a faster removal of the packing coke. Increasing the removal rate of the coke can be achieved by scaling up the complete suction system, based on the simple thought: bigger is better. However, this has a great financial impact and it has its limitations as for example: pump performance, power supply and pit dimensions. Therefore, the suction system needs to be improved in terms of conveying rate, while maintaining suction system settings. Starting point for increasing the suction capacity is improving the shape of the suction nozzle of an existing FTA. This requires analysing the flow of the hot air and the coke in a pneumatic transport system.
The following main research question was formulated for this study:
Which nozzle shape and dimensions lead to improved suction performance within a given system?
This report presents the steps taken to find an answer for this research question.
1.2 Structure of the report
This thesis was performed to improve the suction process of calcined petroleum coke. This is done by analysing the effect of nozzle shape on suction capacity.
The assignment consists of making an inventory of the existing technique of pneumatic transport and performing literature study of the current state-of-the-art. Besides literature, information about the suction system from NNSC is used as well. Furthermore, setting up a simulation of the material flow at the inlet of the suction system, in order to perform shape improvement is also part of this research. After this introduction this report continues with a general description of a packing coke suction system in chapter 2. Specifications will be given for a typical system to get familiar with which main parameters a suction system operates and with what order of magnitude these are. Thereafter, an analysis is described of a typical NNSC suction system. This was performed to support the measured and estimated operating parameters by NNSC with a deeper literature based approach. Based on the results of this section, chapter 3 continues with a literature review of possibilities to improve the suction operation, including patented nozzle designs for pneumatic conveying.
To be able to design a new nozzle, the suction process was simulated and repeated for different nozzle shapes. Chapter 4 describes how this simulation was set up, followed by chapter 5 which describes the experimental plan and which tests were performed. The results of the simulations are presented in chapter 6, and in more detail in Appendix D. Conclusions of this research are presented in chapter 7. Recommendations for future research on this subject can be found in chapter 8.
The appendices contain a scientific paper of this research, detailed calculations and results for both the system analysis and the nozzle shape experiments.
8
2
Suction System
This chapter describes the principles of an FTA suction system. Furthermore, specifications of a typical design by NNSC are given. These are used to perform an analysis of the suction system with a theoretical approach. The analysis shows how the system performs in terms of pressure drop and temperature course. The pressure drop gives insight in the losses through the system. The temperature determines the density of the air, which affects the air flow velocity through the system. These properties are required to determine the pressure drop.
2.1 Description and Specifications of Suction System
The Furnace Tending Assembly is a versatile system that operates on a gantry crane in the baking furnace. It carries the suction system and an anode handling system to place and remove anode blocks. The suction system removes and loads packing coke from and to the anode pits. Figure 2 indicates the FTA and anode pits in a typical furnace.
Figure 2, Schematic view of suction system and anode pit. Table 1, Typical pit dimensions.
Specification Value Unit
Pit length 5500 mm
Pit width 730 mm
Clearance between anode and pit wall 70 mm
2.1.1
Operation
During baking anode blocks are stacked in the anode pit. The residual volume around the blocks is filled with packing coke. The packing material needs to be removed in order to remove the anode blocks. When the packing coke and anodes are removed, they are still at a high temperature, which results in heated air around the pits and suction nozzle.
During normal operation, firstly the packing coke is removed by vertically inserting the suction nozzle in the material at the ends of the pit, between the anode blocks and the pit wall, see Figure 3a. Secondly, a horizontal movement is used to remove a layer of material on top of the anodes, see Figure 3b. These movements restrict the shape and dimension of the suction nozzle due to required clearance between the anode blocks and the pit wall and width of the pit. Protruding parts are not desired either, as these might damage the anode blocks.
9
a) b)
Figure 3, Operational movements for removal of packing cokes from anode pit
2.1.2
Performance testing
Many factors influence the performance of the suction system during operation. Some factors depend on suction system behavior, but operator performance has a large influence as well. To determine the suction capacity of a certain system usually a performance test is performed. The test is used to verify if the suction capacity meets the requirements. During performance tests there are no anodes present in the pit, and the nozzle is inserted in a pile of packing coke vertically. This test is a simplified case of the operational suction mode, see Figure 4. During the test the suction pipe is lowered into a bed of coke for a fixed timespan. Afterwards the amount of removed material is measured. This results in a material flow rate which represents the performance of the system.
10 The suction nozzle is controlled by an operator in the crane above the pits. His task is to control the nozzle in such a way that the nozzle position results in an as much as possible material flow. This is not a specific defined operation, and depends on the operator skill.
A reference design will be used for the study on improving the suction nozzle. For a typical FTA suction system the specifications are given in Table 2.
Table 2, Typical suction system specification for normal operation (Source: NKM Noell Special Cranes).
Specification Value Unit Notes
Suction mass capacity 24 [kg/s] ~ 86 tph
Coke max temperature 350 [°C]
Ambient air temperature 70 [°C]
Suction pipe internal width 150 [mm] Suction pipe internal length 490 [mm]
Air mass flow rate 9516 [kg/h]
Airflow rate in suction pipe 4.32 [m³/s] for air 270 °C, vacuum 0.06 bar Air velocity in Suction Pipe 60.0 [m/s]
2.2 Material Properties
The compound of the packing coke and size grading are usually specifically determined for a process plant. They influence the efficiency and quality of the baking process. Furthermore, the material degrades when used due to particle-particle and particle-wall impact. This means that packing coke properties in general can differ slightly. The material specification in Table 3 provided by NNSC is given for new typical coke and is used further on in this report.
Table 3, Packing coke specification for typical suction system.
Specification Value Unit
Particle density 2080 [kg/m³]
Size grading distribution: • >5 mm • 1-5 mm • <1 mm < 1 > 90 <9 % % %
Calcined petroleum coke is a coarse granular material. The idealized shape of the grain is spherical, but can be non-spherical as well due to degrading during transport and cohesion of particles during the baking process.
2.3 Analysis of typical coke suction system
In order to gain better understanding of the suction performance, a typical suction system was analysed. It consists of five main subsystems connected by four pipe sections as depicted in Figure 5:
From pump to inlet: 1. Pipe section
2. Filter, removes fine particles from the air stream 3. Pipe section
4. Cooler, necessary to reduce temperature to acceptable level for blower 5. Pipe section
6. Cyclone separator, to remove small size particles in the air stream 7. Pipe section
8. Storage hopper, where medium and large particles are removed from the air stream
9. Suction pipe, with an inlet at the bottom and a reverse head and outlet to the storage hopper on top.
11 The previously listed parts can be considered as the parts required for the suction operation. Other parts of the system are the dust bin and discharge pipes. These are not relevant for the suction performance.
Figure 5, Schematic view of typical FTA suction system by NKMNoell Special Cranes (NNSC).
The suction system operates as a vacuum conveyor. To explain how this works the principles of vacuum conveying are discussed first.
2.3.1
Pneumatic transport for bulk solid handling
Vacuum conveying is a form of pneumatic transport, which works using pressure difference. Pneumatic transport can be performed by moving a gas (often air) under high pressure (> 1 bar gauge), low pressure or negative pressure (vacuum). The choice of what pressure to use depends on the distance and the material to convey.
In vacuum conveying in particular, a pump or often a Roots blower, displaces gas from a pipeline to the environment thereby creating flow. By feeding material particles at the entrance of the pipeline the air picks up these particles and transports it through the pipeline. A discharge point is positioned between the entrance and the pump. The particles are collected in for example a hopper. The air flows further through the pipeline and is discharged through the blower after filtering.
Dilute and dense phase
In pneumatic transport of particulate solids classification two flow regimes are distinguished: • Dilute phase flow
• Dense phase flow.
Multiphase flow is generally characterized by three specifications: • Gas velocity,
• Volumetric solids concentrations • Pressure drop over distance.
The differences between dilute and dense phase are presented Table 4 below.
1
2
5
6
4
3
7
8
9
12
Table 4, Flow Classification [3].
Dilute phase flow Dense phase flow
Gas velocity (m/s) >20 1-5
Volume concentration (%) < 1 >30
Pressure drop per unit length (mbar/m) <5 >20
The flow regimes are divided in several sub regimes, as presented below. The dilute side shows continuous flow, while the dense side shows plug flow.
Figure 6, Flow patterns for vertical gas-solid flow [2].
Figure 7 below represents the relationship between gas velocity and pressure gradient ∆p/∆L for a vertical transport line. Three different feed rates of solids are shown, G=0 for no solids, G1 for low solid feed and G2 for high solid feed. Solids concentration decrease with increase of gas velocity from left to right. The choking velocity, UCH, represents the lowest possible velocity at which the dilute
phase transport line can be operated for the given feed rate. For vertical pneumatic transport, the choking velocity marks the boundary between dilute phase and dense phase flow.
Figure 7, Phase diagram for dilute-phase vertical pneumatic transport [3].
The suction system is considered to be operating in the dilute flow regime, because of high air velocity and relatively low solids volumetric concentration. In practice this concentration is much smaller than 1%.
No solids Low solid feed High solid feed
13
Feeding systems
Various pipeline feeding devices are available. Each with a different operating pressure range. A summary is given in Figure 8.
Figure 8, Various pipeline feeding devices with operating pressure range [4].
For this research, the considered vacuum system operates with a suction nozzle. This is required, as the material has to be drawn from an open storage.
2.3.2
Mechanics of flow of particles
This section describes the basic principles and properties for combined particle and gas flow [3].
Gas and particle velocity
The following basic relationships govern the flow of gas and particles in a pipe. Indices used:
• f for gas,
• p for particles (solids).
First the theoretical value of superficial gas velocity, defined as:
(Eq. 1)
And the superficial solids (particles) velocity is defined as:
(Eq. 2)
The fraction of pipe cross-sectional area available for the flow of gas is usually assumed to be equal to the volume fraction occupied by gas. It is called voidage or void fraction Ɛ.
(Eq. 3)
This results in fraction of pipe area available for the flow of solids of (1 - Ɛ). Instead of the theoretical velocities actual gas velocity can then be defined as:
(Eq. 4)
=
Ɛ =
=
!
" #$ $! $ % %
=
=
&$
!
" #$ $! $ % %
=
14 and actual particle velocity,
(Eq. 5)
The relation between superficial velocities to actual velocities is found by rewriting the equations above:
(Eq. 6) (Eq. 7)
The symbol G is used to denote the mass flux of solids, based on the mass flow rate of solids (Mp) and
cross sectional area (A):
(Eq. 8) Continuity
Continuity counts for mass flowrates of particles and gas, Mp and Mf respectively. Mass flow for air and
solids is constant at a continuous section without leaks. Below the mass flows are written in terms of superficial velocities.
(Eq. 9) (Eq. 10)
The mass flow equations give an expression for the ratio of mass flow rates. This ratio is known as the solids loading [3]:
(Eq. 11)
This equation shows that the average voidage Ɛ at a particular position along the length of the pipe, is a function of the solids loading and the magnitudes of the gas and solids velocities for given gas and particle density.
'
' =
(1 − +,
,
' =
,
' = (1 − +,
- =
'
= 1 −
=
= (1 − +
15
Momentum
The momentum equation for a section of pipe is written below, in order to obtain an expression for the total pressure drop along a section of transport line.
Figure 9, Section of conveying pipe: basis for momentum balance [3].
When considering a section of pipe of cross-sectional area (A) and length (δL) inclined to the horizontal at an angle (Ɵ) and carrying a suspension of voidage (Ɛ). The momentum balance equation is:
With momentum determined by mass flow rate times the velocity. Describing all acting forces and momentum results in:
or
(Eq. 12)
In this formula from Rhodes [3], Ffw and Fpw are the gas-to-wall friction force and solids-to-wall friction
force per unit volume of pipe, respectively.
−. /%0 − 1
2/3 − 1
2/3 − 4 .1 − 0, /35& sin 9 − 4 , /35& sin 9
= ,
/
: , (1 − + /
;%! ! < − =! &$ $ ! " # ! > − = ! " # ! $ > − ?&!$ "$" #$! @ − ? &$ &!$ "$" #$ ! @ = = # ! $ ##" "A &$ > : = # ! $ # #" "A >B " ! $ " #& # % % #" #" = C$" # ! $ #
#" #" #"
16 Rearranging and integrating assuming constant gas density and voidage results in
(Eq. 13)
The equation above indicates that the total pressure drop along a straight length of pipe carrying solids in dilute phase transport is made up of six terms:
(1) pressure drop due to gas acceleration (2) pressure drop due to solids acceleration (3) pressure drop due to gas-to-wall friction (4) pressure drop related to solid-to-wall friction (5) pressure drop due to the static head of the gas (6) pressure drop due to the static head of the solids
Rhodes [3] states that some of these terms may be ignored depending on circumstance. If the gas and the solids are already accelerated in the line, then terms (1) and (2) should be omitted from the calculation of the pressure drop; if the pipe is horizontal, terms (5) and (6) can be omitted. The unknowns are the solids-to-wall friction, and whether the gas-to-wall friction can be assumed independent of the presence of the solids.
2.3.3
Required air velocity in pipe
In vertical transport of particles, flow of the particles occurs when drag force overcomes gravity. For a single particle this occurs when the air velocity is higher than the terminal velocity. The relative velocity between particle and fluid Urel is defined as:
(Eq. 14)
Also referred to as the "slip velocity" USLIP. It is often assumed that in vertical dilute phase flow the
slip velocity is equal to the single particle terminal velocity UT.
Terminal velocity of a sphere in presence of a buoyancy force depends on: • d the diameter of the object
• g the gravitational acceleration (9,81 m/s2) • ρ the density of the fluid
• ρs the density of a single particle
• Cd the drag coefficient
The relation can be described by [3]:
(Eq. 15)
As particles are often not spherical, the diameter can be replaced by the sphere equivalent diameter of the object if its volume is known.
DEF
= −
(1) (2) (3) (4) (5) (6)
%
G− %
H=
1
2 ,
H:
1
2 (1 − +,
H: 1
23 : 1
23 : , 3 & sin 9 : , 3(1 − +& sin 9 :
17
2.3.4
Summary
Clearly the air flow velocity is an important parameter in pneumatic conveying. For vertical dilute transport, a minimum superficial air velocity is required to convey a material. This minimum velocity is related to the terminal velocity and is often referred to as choking velocity. Below this velocity solids near the pipe wall begin to flow downwards. This increases solids concentration which results in choking of the air flow. It is a phenomenon not properly understood [3]. A large number of empirical equations is available in literature that predict choking velocity. These differ for various material conveying systems. Klinzing [5] describes reviews of ten of these correlations and concludes three are most reliable. A safety margin of 50 % is recommended, which means a safety factor of 1.5 should be applied on the obtained minimum air velocity. Furthermore, the correlations are determined for round small (<80 mm) bore pipes. It was not found what the effect is of larger or rectangular pipes. Therefore the exact minimum velocity cannot be determined accurately. An estimation is possible and then by experimenting a more accurate value should be obtained.
2.4 Comparison of Typical Performance and Theory
The typical suction system was analysed to verify if the NNSC performance data matches the corresponding theory. The system analysis was divided into two physical parameters, temperature and pressure. First the temperature of all system parts was estimated. With the known temperature an estimation of the air flow velocity is possible. With these parameters and known dimensions of the sections it is possible to calculate the pressure drop between inlet and outlet of each section. The results provide information on which section has the most pressure loss. Furthermore, the results should give insight in what options are available to reduce this loss and to improve the suction system.
2.4.1
Performance of typical suction system
The performance analysis of the typical suction system was done by determining the pressure drop of the subsystems. That is a typical approach for analyzing pneumatic transport systems, because it represents loss due to friction forces of the flow. One of the main variables is the flow velocity. An important aspect of the considered pneumatic system is that it operates with an air flow with high temperature at the inlet, 200 to 300 °C, and reduced temperature at the outlet, to 70 °C. This temperature difference affects the density of the air. When the air mass flow is considered constant, it is expected that the difference in density results in high air velocity at the high temperature side and lower velocity at the low temperature side.
NNSC has its own performance data for the suction system. It is based on few measurements and estimations. This data is transformed into change of pressure and temperature over each section of the system. Typical operating values are presented in the graphs below. Especially the shape of the graphs is interesting because the absolute values differ for each facility.
Figure 10, Graph of air temperature change over typical FTA by NNSC.
0 50 100 150 200 250 300
Air T(°C)
18 The largest temperature drop according to the graph in Figure 10 occurs at the cooler. Another significant drop in temperature occurs at the filter. Possibly due to its large surface area. The further change in temperature occurs with a relatively small slope.
Figure 11, Graph of pressure change over typical FTA by NNSC.
The graph of the pressure drop, Figure 11, shows a relatively constant pressure change. The jump at the right side is the pressure difference between the inlet and outlet of the blower itself.
2.4.2
Temperature change
The temperature change was determined by calculating heat transfer in each main part of the FTA. In each system section hot air enters while at the same time cooling occurs by the surrounding air outside of the system. This paragraph elaborates on the performed calculations. For each calculated section first a heat transfer coefficient was determined. Next, the amount of heat transfer to the environment was estimated. Finally that amount was used in a basic heat transfer formula as cooling rate of the internal air flow. This is described by the following heat transfer formula [6]:
(Eq. 16)
Detailed calculations and references can be found in the Appendix B.
First the temperature below the suction pipe was calculated. This is the starting temperature which will be used for determining the temperature throughout the other FTA-sections.
The following applies for determining the temperature below the suction pipe: • Heat transfer from hot petcoke in anode pits to air flow.
• Combination of natural convection, forced convection and radiation.
• Heat transfer depends on the surface area of the pits, its temperature, the temperature of the oncoming surrounding air, and its velocity.
Calculation of this section was performed according to example 4.10 from AF Mills [6].
The following applies for determining the temperature in the first stage of the suction path, the suction pipe:
• In the suction pipe air is heated further by the hot solids. A heat transfer calculation was performed according to example 4.4 from [6]. It considers heat transfer from a hot particle falling through air.
• Assumed cooling occurs from the pipe to its surroundings by natural convection.
This results in a net temperature difference of the air depending on heating by particles and cooling through the pipe wall.
70 75 80 85 90 95 100 105
Pressure (abs) (kPa)
19 The next stage of the air flow is the hopper. The following applies for determining the temperature:
• Air flow velocity is reduced by entering a large chamber with a large mass of hot coke stored in it.
• Air temperature is assumed to be still increasing, by a combination of natural convection, forced convection and radiation.
The system consists of four connecting pipe sections. Temperature change was calculated similarly for all four sections. The following applies for determining the temperature change:
• Flow of hot air enters the pipe section.
• Assumed cooling occurs from the pipe to its surroundings by natural convection.
The temperature change for the cyclone separator and the cooler are not calculated but assumed to be typical values retrieved from manufacturer specifications, see appendix B.
The remaining part of the system for determining temperature change is the air filter. The following applies for determining the temperature change:
• Cooling is assumed to occur by conduction through the wall of the filter.
• It is divided in two sections with height of 1.5 m to account for different temperature sections in the filter.
Results of the temperature change calculations are presented in Table 5 and Figure 12 below.
Table 5, Results of temperature change through the system.
Section Tin (°C) dT Tout (°C) Taverage (°C)
Blower 84 0 84 84 Pipe 85 0.2 84 85 Filter 144 59 85 114 Pipe 145 1.2 144 144 Cooler 245 100 145 195 Pipe 248 3.3 245 246 Cyclone 263 15 248 256 Pipe 267 4.3 263 265 Hopper 243 24 267 255 Suction pipe 171 72 243 207
The values from the theoretical approach for determining the temperature change are compared with the operational data determined by NKM.
Figure 12, Graph of calculated air temperature change compared to NNSC data.
0 50 100 150 200 250 300
Air T(°C)
NNSC Data Theory data20 The performed calculation has the purpose to verify that theoretical results are in the same order of what experience shows. The results cannot be adopted as exactly true due to simplifications of the system. The shape of the graphs above are more important than the actual values.
Clearly the temperature at the intake is lower, but eventually rises when the air flow passes through the hopper. The graphs show similar slope for the further sections of the system. Although cooling in the piping system has more effect according to the calculations than is expected by NNSC.
2.4.3
Pressure drop
Equation 13 describes the six terms that contribute to pressure drop of a flow in a pipeline. For each section of the system the pressure drop was determined by these six terms of the equation.
The voidage, Ɛ, at the inlet was predetermined, and estimated further in the system based on given separation performance.
Pressure drop due to gas to wall friction was determined by the following equation [7]:
(Eq. 17)
The friction factor ‘f’ was determined by the common used Darcy-Weisbach equation [7], which depends on surface roughness of the pipe, the diameter and the Reynolds number. Included in the friction loss term are the minor loss coefficients for pipe components like, entrance, exit and bends. These coefficients are based on experimental data retrieved from literature.
Pressure drop due to friction of solid to wall was determined for the sections between inlet and filter. Assumed is that no particles are present in the air flow after the filter.
Rhodes [3] provides different solid-to-wall friction equations for the vertical and horizontal sections.
, for horizontal sections
(Eq. 18)
and
, for vertical sections
(Eq. 19)
Details can be found in the calculation in the Appendix B.
The relation between pressure, volume, mass flow and temperature of a gas is described by the ideal gas law:
(Eq. 20)
The relation is used by interpreting the volume as volume flow rate and mass as mass flow rate accordingly. The mass flow rate is assumed to be known and to be constant. The volume flow rate is determined by the density, which is estimated at section inlet pressure and the earlier calculated temperature. The volume flow rate is required to derive the superficial air velocity, which is an important parameter in determining the pressure loss; it is found as squared in the pressure drop terms.
The calculation starts with the blower operating pressure as input. A typical operating value was used in this case. The system works with negative pressure. This means that it operates below atmospheric pressure. For each pipe section the pressure loss is calculated. In this case loss means reducing negative pressure. In absolute terms this means that the pressure rises. Therefore, every calculated loss is actually added positively to the absolute pressure for each section. The increased pressure is then used as input for the next section to calculate the pressure loss there. A given amount of
%O = CM
1
23 = 0,057 ∗ - ∗ 3 ∗ U
VW1
23 = H∗ ∗X∗Y ∗ZW=
H∗ [∗(G \+∗] ∗ ^∗Z W∆% = ∗ _
`
3
a: Ʃcd ∗
1
2 ∗ , ∗ O
H21 pressure loss was given for the filter, the cooler and the cyclone separator based on typical component specifications.
Results of the pressure change calculations are presented in Table 6 and Figure 13 below.
Table 6, Results of pressure change through the system.
T_average (K) Section # ∆p Tot (mbar) p_Out (mbar)
Pipe bot 480 10 23.9 1013 Pipe 480 9 125.4 989 Rev head 480 8a 0.0 864 Hopper 528 8 0.0 864 Pipe 538 7 2.5 864 Cyclone 529 6 10.3 861 Pipe 519 5 1.8 851 Cooling 468 4 17.0 849 Pipe 417 3 2.2 832 Filter 387 2 15.0 830 Pipe 358 1 2.0 815
The values from the theoretical approach for determining the pressure change are compared with the data determined by NKM.
Figure 13, Graph of calculated pressure change compared to NNSC data.
The performed calculation has the purpose to verify that theoretical results are in the same order of what experience shows. The results cannot be adopted as exactly true because of simplifications of the system. The shape of the graphs above are more important than the actual values. Furthermore, the NNSC data was given for maximum blower performance, while the calculation was performed for normal operation performance. This results in higher pressure at the blower inlet for the NKM data. The pressure graphs do show different shapes. The calculation shows the largest pressure drop at the suction pipe, and is relatively constant further in the system. Data provided by NKM shows a more constant stepwise slope of the pressure through the system. The large pressure drop in the suction pipe is a reason to take a closer look at the suction pipe.
70 75 80 85 90 95 100 105
p (kPa)
NNSC Data Theory Data22 The contribution of each term of the pressure drop formula, equation 13, for the suction pipe is given in Table 7.
Table 7, Pressure loss contribution in suction pipe (section 9).
Pressure term 1 2 3 4 5 6
Contribution 12% 15% 42% 22% 7% 1%
Clearly, the 3rd term contributes the most. This term strongly depends on the air flow velocity. By
decreasing the velocity one would expect that pressure loss is decreased and the material flow would improve. However, the air flow velocity has a lower boundary. A minimum value is required for transporting material. For vertical flow this is called the choking velocity [3] as explained in section 2.3.3. The considered equations determine the voidage and related air flow velocity at which dilute flow becomes dense flow. The following equations require iterative solving to find matching values for voidage ƐCH and air flow velocity UCH.
(Eq. 21)
(Eq. 22)
Where UT is terminal velocity of a particle.
The choking velocity was calculated for a typical suction system to investigate what minimum air flow velocity was required and what would be the corresponding particle mass flow see appendix B for detailed calculation. For a 5 mm spherical particle the choking voidage was 0.99041, which corresponds to almost 1 % volume concentration of particles. A corresponding air flow velocity was calculated as 40 m/s. A safety factor of 1.5 [3] prescribes to use 60 m/s. With a corresponding particle velocity of 24.5 m/s, see appendix B for calculation, this results in a possible mass flow of 29 kg/s. This indicates that by reducing the air flow velocity from 70 m/s to 60 m/s the system should be capable of transporting 29 kg/s instead of the 22 kg/s used as typical value in the suction system analysis.
2.5 Conclusion
The theoretical analysis results in similar performance as was determined by measuring and by estimation. A difference in temperature was found in the suction pipe. It is expected that air temperature rises until it leaves the hopper, instead of cooling down. An important finding was that air flow velocity has a great influence on the suction performance. Furthermore, the analysis of the pressure drop showed that the flow has the potential to transfer more solids mass. This is the goal of the overall study as well. Further analysis is required to understand how the particle mass flow through the pipe can be improved. This will be discussed later in this report.
ea ea
−
f=
-, (1 −
ea+
,
g,hh=
2250`(
ea i,h− 1+
;
ea ea−
f<
H23
3
Literature review on suction improvement
Upscaling the complete suction system can be used to increase the suction rate of the packing coke. This considers for example applying a blower with more power or increase pipe dimensions. However, these are radical measures with high financial and operational impact. The system analysis in the previous section learned that the system has the potential to transfer more material with the same air flow settings.
The previous chapter of this report showed that the total system does not need to be modified. Increased rate of material removal can be achieved by improving the material feeding to the suction nozzle.
3.1 Patents
Possibilities to improve the material feeding are modifying the nozzle or adding feeding devices to the nozzle. Several patents exist that have this purpose. Vacuum suction nozzles are used for different materials and material sizes, i.e. from cotton to fine dust. Patents for suction nozzles exist for these different fields of operation. The patents concern nozzle designs that should improve the feeding capability of the material to the suction nozzle and/or improve the flow in the nozzle itself. Some designs are rather simple, while others consist of a complex and extended apparatus.
Finnegan [8] patented a tapered nozzle with rather rectangular inlet connected to a circular pipe, Figure 14. The connection contains an adjustable annular opening for bleeding atmospheric air.
Figure 14, Finnegan’s suction nozzle for pneumatic grain conveyor
Combined with an additional opening the air flow can be controlled to a certain limit. This results in control of the grain material flow as well. This design is rather simple. Its principle is controlling the air flow by adjustable bleeding of air. Fully adjusting is actually not possible during operation.
A second patent with auxiliary air was developed by Okano [9]. His invention is a nozzle with variable total opening area by a sling cap, Figure 15. This controls the velocity to the hopper in order to control possible damage of the conveyed material.
Figure 15, Nozzle design patented by Okano.
In addition, Okano added pressurized air injection to prevent blockage and as auxiliary air supply. Kalsiak [10] developed a method and arrangement for pneumatic conveying of granular material, Figure 16. The principle is that the material is aerated, and therefor fluidized, before entering the suction pipe. This is done by introducing compressed air under a porous partition bottom.
24
Figure 16, Patented design by Kalsiak of pneumatic conveyor.
In addition, this design contains valves that regulate the pressure in the nozzle. This is required for the operation of the system, not to improve the flow itself. This example operates with the principle of improving flow. By pre-fluidizing the material improved flow is achieved further on in the conveying line.
A vacuum pick-up nozzle with air boost manifold was patented by Malugani [11]. His design consists of a concentric tube of which its exhaust air is returned to the inlet as boost, see Figure 17. It creates a vortex/swirl air input which results in controlled air flow.
Figure 17, Patented design by Malugani of vacuum pick-up nozzle.
Several patents exist for suction dredges. These are used for removing soil to clear pipes or cables. One design by Rinker [12] consist of a round nozzle with cutting head, see Figure 25. The nozzle height is axially controlled by hydraulic/pneumatic cylinders.
Figure 18, Patented design by Rinker of suction dredge.
Another design with nozzle movement control was patented by Shapunov [13]. It is meant for reloading bulk material. Material concentration is controlled by controlling the intake nozzle traveling speed. Signals from a vacuum sensitive element are used for an electric driven nozzle transfer, see Figure 19.
25
Figure 19, Patented design by Shapunov of bulk reloading system.
Briggs invented a method and apparatus for conveying solids using a high velocity vacuum [14]. As for most of the nozzles it consist of a concentric tube, see Figure 20. This example has a spoon like inlet. It considers a complex air flow drive. A supersonic jet of gas creates a vacuum at the inlet and vanes create a swirling motion of air.
Figure 20, Patented design by Briggs of vacuum nozzle.
Many patented nozzles consist of circular concentric pipes. Complex feeding aid or control measures are applied to improve the material feeding. However, as mentioned in section 2.1, due to limited space for the nozzle in the pit these aids are not convenient.
26
3.2 Experience by NKM Noell Special Cranes
As was found by NNSC, the nozzle shape and auxiliary air configuration has a great effect on the mass flow through the pipe. For example, changing the nozzle shape and the ratio in cross section area of secondary air to main air, made a difference in performance of 15%.
Modifying the nozzle, pointed out in Figure 21, is rather easy to realise and this option will therefore be elaborated further.
Figure 21, Schematic view of suction system
3.3 Nozzle Design Research
Before modifying the nozzle design in order to improve it, knowledge is required of former research on this subject. This section recites some of the former research performed on suction nozzle design in pneumatic conveying.
An important application of pneumatic transport is for unloading grain from ships. Systems to unload grain ships are operational for over 100 years. Many studies were performed in the past on this subject and can be useful for the research in this report. Although the material to be conveyed is very different from the here considered petroleum coke, comparison can be useful.
Books on pneumatic conveying were found, written by Mills [4] and Klinzing [5]. They provide experience and research based information about design of pneumatic transport systems. They both mention only little research has been documented about suction nozzle design. They refer to a small set of rather old research on suction nozzle design, mostly for conveying grain.
One topic of the gathered results from research is about the suction nozzle. In Figure 22 a schematic view is presented of a typical suction nozzle for vacuum pick-up system. The scheme shows besides material inlet, that also additional air inlet, primary and secondary, is required. Research showed that for grain material nozzle design can be influenced by modifying the sleeve position of a concentric suction nozzle. Varying the length “a” depends on buried depth of the nozzle, as the primary air inlet should not be blocked.
27
Figure 22, Suction nozzle for vacuum pick-up systems [4].
Dimension “b” depends on material, but has influence at the flow rate and efficiency of picking up material. Effect of some variations were investigated and are presented in Figure 23 below.
Figure 23, Suction nozzle showing typical modes of operation [4].
The test was performed with a 50 mm bore pipeline. Results are presented in Figure 24 below with a graph.
28 An important result of their research was that with a certain limit of retracted position of the outer sleeve, large improvement resulted in the material flow rate. The most right configuration shows that the air falling through the outer tube digs in to the material, and allows more material to flow into the suction pipe. While in the left configuration the air blocks the flow of material. The retracted height of the outer sleeve was not the basic cause of the improvement. It was rather the angle between the inner and outer tubes. It is optimal when the material flows into the nozzle under gravity, with avoiding an overfeeding situation which could result in pipe blockage. Especially the efficiency was improved. The retracted outer tube resulted in lower superficial air velocity at which more material was conveyed. Less air at lower velocity means less power required. So a combination of less power required and more material conveyed leads to higher efficiency. It should be noted that performance is also material dependent and can be influenced by environmental conditions like temperature. Variations of these for the experiments were not found, so it is impossible to conclude this retracted mode always leads to better performance.
Klinzing [5] mentions work about performance of different vacuum nozzle designs. It considers a constant Kn, determined as:
Kn = ṁs / Anv (Eq. 23)
• ṁs the solids mass flow rate in t/h,
• v is the nozzle superficial gas velocity in cm/s, • An is the nozzle area in mm2
This nozzle constant can be related to the mass flux. In order to maximize the mass flux the nozzle constant should be maximized. Experimenting with different nozzle designs resulted in the constants as shown in Figure 25.
