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Equilibrium Moisture Content

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Equilibrium Moisture Content …

Equilibrium Moisture Content

0 INTRODUCTION/PURPOSE
1 SCOPE
2 FIELD OF APPLICATION
3 DEFINITIONS
4 THEORY
5 IMPORTANCE OF EQUILIBRIUM MOISTURE CONTENT
6 MEASUREMENT OF EQUILIBRIUM MOISTURE CONTENT
7 BIBLIOGRAPHY

FIGURES

1 DYNAMIC METHOD : EQUILIBRIUM MOISTURE
CONTENT RIG
2 DESORPTION ISOTHERMS

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  • 1. GBH Enterprises, Ltd. Process Engineering Guide: GBHE-PEG-DRY-002 Equilibrium Moisture Content 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: Equilibrium Moisture Content CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 THEORY 3 5 IMPORTANCE OF EQUILIBRIUM MOISTURE CONTENT 3 6 MEASUREMENT OF EQUILIBRIUM MOISTURE CONTENT 3 7 BIBLIOGRAPHY 4 FIGURES 1 DYNAMIC METHOD : EQUILIBRIUM MOISTURE CONTENT RIG 4 2 DESORPTION ISOTHERMS 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 The drying of solids is a complex process. The drying rate depends on the nature of the material in question, the particular mechanism (or mechanisms) involved in the moisture transfer process and the external conditions to which the solids are exposed. Knowledge of the drying process is a prerequisite to dryer design and cannot be calculated with any degree of confidence using existing theoretical models. In practice the drying rate should be measured under conditions approximating as closely as possible to those envisaged for the full scale dryer. The parameters which need characterizing are: (a) Drying kinetics; (b) Equilibrium Moisture Content; (c) Physical, thermal and chemical properties of both the feed and the dry product. 1 SCOPE This Guide describes the significance of the Equilibrium Moisture Content in understanding the drying of solids. 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 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. Equilibrium Moisture Content The Equilibrium Moisture Content (Xe) is the moisture content of a product when at equilibrium with the partial water pressure of the surroundings. Free Moisture Content The Free Moisture Content is the moisture in excess of the Equilibrium Moisture content. With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary of Engineering Terms. 4 THEORY The moisture in a material to be dried can be present in two forms, bound and unbound. The nature of the solids and their accompanying moisture determines the drying characteristics. The bound moisture in a solid exerts an equilibrium vapor pressure lower than that of pure water at the same temperature. Water retained in small capillaries in the solid, adsorbed at solid surfaces, as solutions in cell or fibre walls, or chemically combined with the solids (water of crystallization) falls in the category of bound moisture. The unbound moisture in a hygroscopic material is that moisture in excess of bound moisture. All water in a non-hygroscopic material is unbound water which exerts an equilibrium vapor pressure equal to that of pure water at the same temperature. The Free Moisture Content consists of unbound and some bound moisture. Only Free Moisture Content can be evaporated. A useful measure of bound moisture is the Gibbs free energy change ΔG (J/mol) which occurs when a mole of water is adsorbed or desorbed from the solid. The Gibbs free energy change is defined as: 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. Some typical values for ΔG are: 5 IMPORTANCE OF EQUILIBRIUM MOISTURE CONTENT The Equilibrium Moisture Content of a solid is an important consideration in the understanding of drying processes because: (a) When drying a solid there is a limit to the extent of water removal. (b) If the gas contacting the solids has a humidity greater than zero there will be a certain solids moisture content below which no further drying will take place, this is the Equilibrium Moisture Content. (c) The moisture content will never fall below the Equilibrium Moisture Content however long the drying time. In addition, the material's characteristics with respect to the nature of the residual moisture will provide a better understanding of final product quality. 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. 6 MEASUREMENT OF EQUILIBRIUM MOISTURE CONTENT There are two basic experimental techniques for determining Equilibrium Moisture Content: (a) Constant Volume (or static) Method The constant volume or static method involves placing a sample of solid in a constant volume system containing gas at fixed conditions. The normal technique for maintaining the gas humidity constant is to place a saturated salt solution in the system. The main disadvantage of this method is that it is time consuming and at high humidifies can take several weeks to reach equilibrium. (b) Dynamic Method The other technique involves a dynamic method where gas at a fixed temperature, humidity and flow is drawn over the solids until equilibrium is attained. This technique overcomes many of the disadvantages of the static method. The main features of the apparatus are shown schematically in Figure 1. Such a rig is situated at the SPS in the UK and is available for in-house experimentation (cost details can be obtained upon enquiry). Measured Equilibrium Moisture Content values are normally plotted in the form of isotherms against relative humidity. Depending on whether the humidity is increasing (adsorption) or decreasing (desorption) slightly different isotherms are found. Figure 2 shows desorption isotherms for a range of different materials and indicates the nature of the bound moisture and the difficulty of the drying task. The effect of temperature on sorption isotherms is not large and for design purposes can be ignored for variations up to 10°C from the experimental temperature. 7 BIBLIOGRAPHY [1] Spray Drying Handbook 4th edition p76, K Masters, George Godwin [2] SPS Industrial Drying Technology - Short Course Data 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. FIGURE 1 DYNAMIC METHOD: EQUILIBRIUM MOISTURE CONTENT RIG FIGURE 2 DESORPTION ISOTHERMS 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
  • 9. 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

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