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Psychrometry

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Psychrometry …

Psychrometry

0 INTRODUCTION / PURPOSE
1 SCOPE
2 FIELD OF APPLICATION
3 DEFINITIONS
4 PSYCHROMETRIC CHARTS
5 EXAMPLE CALCULATION
6 CHARTS FOR SPECIFIC SYSTEMS
7 BIBLIOGRAPHY


FIGURES

1 GROSVENOR CHART (Humidity vs. Temperature)
FOR AIR-WATER VAPOR AT 1.0133 bar

2 MOLLIER CHART (Enthalpy vs. Humidity) FOR
NITROGEN-TOLUENE VAPOR AT 100 kPa


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  • 1. GBH Enterprises, Ltd. Process Engineering Guide: GBHE-PEG-DRY-003 Psychrometry 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: Psychrometry CONTENTS SECTION 0 INTRODUCTION / PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 PSYCHROMETRIC CHARTS 3 5 EXAMPLE CALCULATION 3 6 CHARTS FOR SPECIFIC SYSTEMS 4 7 BIBLIOGRAPHY 4 FIGURES 1 2 GROSVENOR CHART (Humidity vs. Temperature) FOR AIR-WATER VAPOR AT 1.0133 bar 5 MOLLIER CHART (Enthalpy vs. Humidity) FOR NITROGEN-TOLUENE VAPOUR AT 100 kPa 6 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 Psychrometry is concerned with the science of solvent vapor/gas mixtures. Where the solvent is water, psychrometry is often called hygrometry. The usefulness of psychrometry in the area of drying is that it gives the relationship between the Enthalpy of a vapor/gas mixture and the composition of that mixture, where the maximum concentration of the vapor in the gas is limited by its saturation vapor pressure at a given temperature. Psychrometry therefore permits calculation of the change in solvent vapor concentration, gas temperature, limiting gas volume, heat input etc. during a drying operation, essential for the proper design of a drying system. The science is normally used in the form of two types of psychrometric chart (see Clause 4). 1 SCOPE This Guide explains the use of psychrometric charts and provides an example calculation. 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 Mollier Chart A plot of Enthalpy versus solvent vapor concentration (i.e., humidity for water). 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. Grosvenor Chart A plot of solvent vapor concentration (humidity for water) versus temperature. Dry Bulb Temperature The temperature of a gas. Wet Bulb The steady state temperature reached by a small amount of liquid evaporating into a large amount of unsaturated vapor/gas mixture. Dew-point Temperature The temperature at which the vapor pressure of the liquid vapor in the gas is equal to the saturation vapor pressure of the liquid. Taken in practice to be the temperature at which condensation of the liquid begins to occur on cooling the gas mixture at constant pressure. Absolute Humidity The mass of liquid vapor per unit volume of the gas mixture, measured e.g. in kg/m3. Relative Humidity The ratio of the actual vapor pressure of the liquid to the saturation vapor pressure over a plane surface of the liquid at the same temperature, expressed as a percentage. Enthalpy The heat content of the material on a relative basis, usually to a reference temperature of 0°C, and equal to the product of the temperature, T, and mean specific heat at constant pressure of the material over the temperature range 0 to T°C, normally evaluated at T/2. Since the specific heat of dry air is approximately 1 kJ/kg/K, its Enthalpy in kJ/kg is numerically equal to its temperature in °C. 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. 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. 4 PSYCHROMETRIC CHARTS The two types of psychrometric charts are the Grosvenor Chart, shown in Figure 1 for air-water, and the Mollier Chart, shown in Figure 2 for nitrogen-toluene. In the Grosvenor Chart the sloping dotted lines join points of equal Wet Bulb Temperature and also Enthalpy, the slopes of which happen to coincide for the air-water system. The value of the Wet Bulb Temperature is given by that temperature at which the dotted line intersects the curve of 100% Relative Humidity. The Mollier Chart is more complex. In the example shown in Figure 2 the Y-axis is humidity. Temperature is shown along the X-axis, but the isotherms slope slightly to the right of vertical. The Enthalpy is shown by a series of solid lines sloping downwards from left to right, the values being shown on the right hand Yaxis, continuing on to the X-axis below the temperature values. The dotted lines extending down from left to right are points of equal Wet Bulb Temperature, their values again being those temperatures at which the intersect with the 100% Relative Humidity curve. Note: In this system the wet bulb isotherms have a different slope from the lines of constant Enthalpy. These charts can be used to calculate one of the following, given any two of the others: (a) Dry Bulb Temperature. (b) Wet Bulb Temperature. (c) Dew-point Temperature. (d) Absolute Humidity. (e) Relative Humidity. (f) Enthalpy. 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 EXAMPLE CALCULATION Take the Take the case where air at 27°C and 40% Relative Humidity is heated indirectly to 130°C. This hot air is then contacted with a water-wet product and exits the dryer at 50°C. To avoid the risk of condensation immediately downstream of the dryer, it was decided that for this example the exit air should have a Relative Humidity no greater than 60% at 50°C, and that the maximum weight of water that could be evaporated per unit weight of air was to be determined. Examining the Grosvenor Chart in Figure 1, start at point A and heat the air to point B. The air's Relative Humidity is now very low, much less than 2%. The air now loses heat in evaporating water and its Relative Humidity increases. Assuming the cooling is adiabatic, the changing conditions of the air are shown by a line parallel with the sloping dotted lines shown in the Figure. It is seen that at point C, the temperature of the air is 323 K (50°C) and its Relative Humidity is about 50%. The water that can be evaporated per kg of air can be read off the X-axis, being the difference between points C and B, i.e. 0.042 - 0.009, = 0.033 kg water per kg air. If the moist air now loses heat to its surroundings, condensation of water will occur at the dew point, D, at a temperature of 311 K (38°C). In this way, the minimum quantity of hot air required to dry a charge of wet material, or the minimum flow rate of hot air in a continuous dryer, can be estimated. The effect of mixing of gases with different temperatures and Relative Humidifies can also be found from the charts by joining the points corresponding to the conditions of the two gases by a straight line and finding the conditions for the mixture on that line using the lever rule. 6 CHARTS FOR SPECIFIC SYSTEMS The charts for any particular solvent/gas system vary with atmospheric pressure, and appropriately modified charts have to be used for those cases where the prevailing pressure is different from normal, e.g. in designing for plants at high altitude. Grosvenor Charts and Mollier 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. For further details on psychrometry and the use of psychrometric charts see Refs. [1] and [2]. Ref. [1] in particular contains numerous worked examples. Ref. [3] is a users guide to the SPS software. 7 BIBLIOGRAPHY [1] Perry R H, Green D W; Perry's Chemical Engineers Handbook (6th Edition), Chapter 12 (1984) [2] Coulson J M, Richardson J F; Chemical Engineering (4th Edition) Vol 1: Fluid Flow, Heat Transfer and Mass Transfer, Chapter 11 (1990) 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. FIGURE 1 GROSVENOR CHART (Humidity vs. Temperature) FOR AIR-WATER VAPOR AT 1.0133 bar 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. FIGURE 2 MOLLIER CHART (Enthalpy vs. Humidity) FOR NITROGEN-TOLUENE VAPOUR AT 100 kPa 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
  • 10. 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