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Microwave and Radio Frequency Drying

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Microwave and Radio Frequency …

Microwave and Radio Frequency
Drying

0 INTRODUCTION/PURPOSE
1 SCOPE
2 FIELD OF APPLICATION
3 DEFINITIONS
4 MICROWAVES
4.1 Microwaves General
4.2 Microwaves Heating
4.3 Microwaves Generation
4.4 Microwaves Transmission
4.5 Microwaves Hazards

5 MICROWAVE HAZARDS
5.1 Microwaves Drying
5.2 Microwaves Drying Systems

6 CRITERIA FOR CONSIDERING MICROWAVE DRYING

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  • 1. GBH Enterprises, Ltd. Process Engineering Guide: GBHE-PEG-DRY-008 Microwave and Radio Frequency Drying Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 2. Process Engineering Guide: CONTENTS Microwave and Radio Frequency Drying SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 MICROWAVES 2 4.1 4.2 4.3 4.4 4.5 Microwaves General Microwaves Heating Microwaves Generation Microwaves Transmission Microwaves Hazards 2 2 3 3 3 5 MICROWAVE HAZARDS 4 5.1 5.2 Microwaves Drying Microwaves Drying Systems 4 4 6 CRITERIA FOR CONSIDERING MICROWAVE DRYING 5 Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 3. 0 INTRODUCTION / PURPOSE Microwave drying utilizes the volumetric adsorption of microwave radiation throughout the body of the wet material. The principle is that of the domestic microwave cooker. Present technology favors small-scale applications. Radio frequency drying uses the same principle, but operates at a different wave length of radiation. Its application is rarer than that of microwave drying. 1 SCOPE This Process Engineering Guide summarizes the pertinent features of microwave dryers, their range of operations and use within industry. It covers the general principles of microwave radiation and its application to drying. For more detailed information it directs the reader to the relevant external literature and summarizes its content. Radio frequency drying is referred to in the literature only. 2 FIELD OF APPLICATION This Guide applies to process engineers in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide, the following definitions apply: SPS The Separation Processes Service (SPS) is a research and consultancy organization, based in the UK. It is active in the main operations related to separation, including comprehensive coverage of drying. Microwaves These are electromagnetic waves with the electric and magnetic fields at right angles to each other and to the direction of propagation of the waves. They are a form of radiant energy similar to light, infrared and radio waves. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 4. 4 MICROWAVES 4.1 Microwaves General Microwaves have wavelengths shorter than radio waves and longer than infrared waves. UHF and VHF are in the microwave region. The frequencies allocated for industrial scientific and medical use are 896 MHz, 2450 MHz, 5800 MHz and 22125 MHz. The frequencies used for industrial heating are 896 MHz and 2450 MHz. These have wave lengths of 0.33 m and 0.122 m respectively. Microwaves are not "ionizing radiation" as the wavelengths are greater than that for visible light and the wave quantum energy is very small. 4.2 Microwave Heating Most molecules, although electrically neutral, have an asymmetrical distribution of electrons and may be electrically positive at one end and negative at the other. For example, the water molecule exists as a dipole due to the strong connection of electron pairs to the oxygen atom. In an electric field these polar molecules will align themselves in a specific direction, and when the field is removed will return to their original position. The electric field of microwave radiation changes millions of times per second and the resulting molecular agitation produces heat. Materials react differently depending on their molecular structure. Water and most solvents are heated very rapidly by microwaves whereas PTFE, polypropylene and glass are unaffected. The heating effect of microwaves on a material can be characterized by measuring its dielectric loss factor. The heating power(p) is related to the frequency (f) the electric field strength (e) and the dielectric loss factor of the material (k), by the following relationship: 4.3 Microwave Generation Microwaves are generated by vacuum oscillators, the most common being the magnetron and the klystron. The former are most frequently used for industrial heat generation while the latter are more likely to be used in communications applications. Magnetrons are only approximately 70% efficient in conversion of the electrical energy into microwaves. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 5. Cooling is therefore needed to remove the heat generated by the remaining electrical energy. At 896 MHz, magnetrons are available that can produce up to 60 kW of microwave energy while magnetrons at 2450 MHz are only capable of producing up to 5 kW of microwave energy. 4.4 Microwave Transmission Microwaves are transmitted along wave guides designed to operate above a certain cut off frequency with very low losses. Waveguides are simply rectangular pipes usually of brass or aluminium (approximately 5 cm by 10 cm for 2450 MHz and 12.5 cm by 25 cm for 896 MHz). Microwaves will radiate out from an open waveguide in a dispersed beam like a light beam from a torch. 4.5 Microwave Hazards The information below is given to provide general guidance. However, users of this Guide should ascertain the current state of advice on prevention of exposure to hazards, including any National, European and International Guides and Standards. There are two basic hazards of microwaves: (a) Tissue damage due to exposure to high levels of microwave energy: Several criteria exist, e.g. 100 Wm-2 at 50 mm from the microwave source, maximum current densities in the body of f/100 mA/m-2 (f in Hz), specific absorption rates of 0.4 W/kg body weight. Users should ensure they are using the latest recommended criteria. Note also that people carrying metallic implants such as cardiac pacemakers may be at risk. (b) Electric field effects causing corona discharge and arcing: Problems with corona discharge are not usually present on industrial heating applications because the field strengths are much lower than those needed to cause breakdown. The field strength for a particular application should, however, be calculated to confirm this. Arcing can be eliminated by good equipment design with effective earthing of all components. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 6. 5 MICROWAVE DRYING 5.1 General For a water wet system, the microwaves will selectively heat up the water and at atmospheric pressure the product mass will heat up to 100°C. To prevent overheating of the product, it is necessary to operate under vacuum and so suppress the liquid boiling point. Once all the free moisture has been evaporated, the bound water will be heated and even under vacuum the product temperature will rise. Ultimately, the product will be overheated and even burnt unless microwave heating is stopped. 5.2 Microwave Drying Systems Microwave equipment can be divided into two broad types: (a) belt - where continuous operation is the norm; and (b) oven - where batch processing is usual. In a microwave oven, the microwaves will bounce about by reflection and set up a field pattern which may or may not be uniform depending on the relationship between the microwave frequency and the cavity dimensions and shape. The more patterns of reflection which can be set up within the cavity, the more uniform will be the microwave heating. 6 CRITERIA FOR CONSIDERING MICROWAVE DRYING When microwave drying is being considered a number of factors should be taken into account. These are: (a) Scale: Microwave drying is not ideal for large tonnage applications purely on cost grounds - both the cost of the installation and electrical running costs. For this reason, the applications tend to be in the fine Chemical / pharmaceutical industries where scale tends to be small and products have a high added value. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 7. (b) Start moisture: Microwaves are not economic for the drying of materials with a high moisture content as conventional drying methods are more cost effective for this duty. (c) Final moisture: Microwaves are not effective in drying product down to less than 1.0% as when the solvent has been removed the microwave energy will tend to be absorbed by the product causing it to heat up. (d) Temperature sensitivity: Microwaves will heat up the product to be dried to the boiling point of the solvent being evaporated. If the product needs to be kept below that temperature then operation under vacuum is necessary. This is the case with most pharmaceutical applications. As an alternative to vacuum operation, a high gas flow through the product will allow the heat generated by the air to be transferred away. (e) Microwave absorbance: Microwaves are only effective for drying if the solvent which needs to be evaporated absorbs microwave energy. The microwave absorbance is proportional to the "loss Factor". The ideal system for microwave drying should have a product (solid) with a low loss factor and a solvent with a high loss factor. Products which have a high loss factor can be prone to heating by microwave absorbance when the moisture content is low, i.e. as the product becomes dry. Hence the comment on final moisture in (c). Therefore if the product is manufactured on a small scale, has a start moisture less than 20%, a final moisture greater than 1%, has a low microwave absorbance and is being dried from a solvent with high microwave absorbance it will be suitable for microwave drying. If it is temperature sensitive, vacuum operation will be needed. If the product does not meet all of the above criteria but there is no other alternative, then microwave drying may still be a viable option. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com
  • 8. Refinery Process Stream Purification Refinery Process Catalysts Troubleshooting Refinery Process Catalyst Start-Up / Shutdown Activation Reduction In-situ Ex-situ Sulfiding Specializing in Refinery Process Catalyst Performance Evaluation Heat & Mass Balance Analysis Catalyst Remaining Life Determination Catalyst Deactivation Assessment Catalyst Performance Characterization Refining & Gas Processing & Petrochemical Industries Catalysts / Process Technology - Hydrogen Catalysts / Process Technology – Ammonia Catalyst Process Technology - Methanol Catalysts / process Technology – Petrochemicals Specializing in the Development & Commercialization of New Technology in the Refining & Petrochemical Industries Web Site: www.GBHEnterprises.com