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Thermal Efficiency of Cement Kiln

Heat (Energy) Balance for Cement Plant

Falling Film Evaporators in the Food Industry

Falling film evaporators are especially popular in the food industry where many substances are heat sensitive.A thin film of the product to be concentrated trickles down inside the wall of the heat exchanging tubes.   Steam condenses on the outside of the tubes supplying the required energy to the inside of the tubes.  As the water from the process stream evaporates (inside the tubes) from the solids dissolved, the product become more and more concentrated.  The evaporators are designed such that a given flow of material can be concentrated to needed solids concentration by the time the stream exits the evaporator.  The two-phase product stream is usually run through a vapor-liquid separator after exiting the evaporator.  This separator allows the vapors to be drawn off the top and the concentrated liquid to exit the bottom (most falling film evaporators are run under vacuum conditions on the process side).

Understanding the Heat Transfer

The simple heat transfer balance for falling film evaporators is:

Eq. (1)

where:

Figure 1: Typical Falling Film 

Evaporator Configuration

Q = heat duty
U = overall heat transfer coefficient
A = heat transfer area
T_S = temperature of condensing steam
T₁ = boiling point of process liquid

The overall heat transfer coefficient consist of the steam side condensing coefficient (usually about 5700 W/m² K), a metal wall with small resistance (depending on steam pressure, wall thickness), scale resistance on the process side, and a liquid film coefficient on the process side which will be extremely dependent on the viscosity of the process liquid over the concentration range.
   

The steam side coefficient can be estimated as above or it can be calculated by the following equation for laminar flow:

Eq. (2)

 and

Eq. (3)

for turbulent flow.  For the equations above,

ρ_L = liquid density (kg/m³)
ρ_V = vapor density (kg/m³)
g = 9.8066 m/s²
L = vertical height of tubes (M)
μ_L = liquid viscosity (Pa s)
μ_V = vapor viscosity (Pa s)
k_L = liquid thermal conductivity (W/m K)
ΔT = T_sat-T_wall (K)
λ = latent heat (J/kg)

All physical properties should be evaluated at the film temperature, T_f = (T_sat - T_wall)/2 except for the latent heat which is evaluated at the saturation temperature.  The resistance due to scale formation cannot be predicted and will probably have to be estimated or compensated for by added a fouling coefficient or by added 5-10% to the calculated heat transfer area (sometimes industry data is available for similar fluids to help estimate this value).  

For the process fluid, the heat transfer coefficient can be calculated with the following expression: 

Eq. (4)

where:

b = 128 (SI units) or 39 (Imperial units)
N_Pr = Prandtl number = 
D = tube diameter
N_Re = Reynolds number = 
ρ = density (subscript "L" for liquids, "V" for vapor)
μ = viscosity (subscript "L" for liquids, "V" for vapor)

Calculating pressure drops in falling film evaporators has been investigated since the late 1940's.  A universal equation is really not agreed upon.  Typically, a constant dependent on the percentage of vapor exiting the evaporator is used in a pressure drop relationship.  If your process fluid shares physical properties close to water, you may be able to accurately predict the pressure drop by using graphs and relations found in Perry's Chemical Engineers' Handbook.  

Evaporating fruit and vegetable juices presents a special challenge for chemical engineers.  Juices are heat sensitive and their viscosities increase significantly as they are concentrated.  Small solids in the juices tend to cling to the heat transfer surface thus causing spoilage and burning

Juice evaporations are usually performed in a vacuum to reduce boiling temperatures (due to heat sensitivity).  High flow circulation rates help avoid build-ups on the tube walls.

For some juices (e.g. orange), it is unavoidalbe that the flavor changes as concentration increases.  Some of the volatile, flavor-containing components are lost during evaporation.  In this case, some of the raw juice is mixed with the concentrate to replace the lost flavors.

Considering that the components of juices have close boiling points, a standard, single evaporator is seldom sufficient.  Either a multi-effect evaporation system must be used (lower capital cost, higher energy costs) or a vapor recompression evaporator (higher capital cost, lower energy costs) is employed.  In a multi-effect system, the pressure is incrementally lowered in each stage, thus pushing the boiling point lower gradually.  This permits more control over the vapor products to be discarded from the system (mainly water) and the vapors to be condensed back into the system (volatile juice components).

The vapor recompression evaporator was designed for maximum efficiency.   These units generally operate at low optimum temperature differences of 5-10 °C.   This requires a larger heat transfer area than multi-effect evaporators, thus the larger capital costs.  However, the energy savings, generally make vapor recompression the evaporator of choice in the food industry.

Figure 2: Typical Vapor Recompression Evaporator Arrangement

References:

Geankoplis, Christie J., Transport Processes and Unit Operations, 3rd Ed., Prentice Hall, 1993, ISBN 0139304398, pages 263-267

Perry, Robert H., et al, Perry's Chemical Engineers' Handbook, 6th Ed., McGraw-Hill, 1984, ISBN 0070494797, pages 10-34 through 10-38

**Special thanks to Rossana Milie from the Department of Chemical Engineering, University of Pisa, Italy for supplying the idea for this article

How to install Aspen 7.3

Process Modeling (Aspen HYSYS) V7.1 Cumulative Patch 1 issued addressed

AMMONIA AND UREA PRODUCTION

MATERIAL AND ENERGY BALANCE

Risk Management for Chemical Industries

Risk Management for Chemical Industries

Chemicals have become a part of our life for sustaining many of our day-to-day activities, preventing and controlling diseases, and increasing agricultural productivity etc. An estimation of one thousand new chemicals enter the market every year, and about 100000 chemical substances are used on a global scale. These chemicals are mostly found as mixtures in commercial products. Over one million such products or trade names are available.

The chemical industrial sector is highly heterogeneous encompassing many sectors like organic, inorganic chemicals, dyestuffs, paints, pesticides, specialty chemicals, etc. Some of the prominent individual chemical industries are caustic soda, soda ash, carbon black, phenol, acetic acid, methanol and azo dyes. Chemical manufacturing sector in India is well established and has recorded a steady growth in the overall Indian industrial scenario. The Chemical and allied industries have been amongst the faster growing segments of the Indian industry. The Indian chemical industrial sector had a turnover of around Rs.1200 billion in 2001-2002. The chemical exports also accounts for more than 16.20% of the total Indian exports during 2001-2002.

The risks associated with the chemical industry are commensurate with their rapid growth and development. Apart from their utility, chemicals have their own inherent properties and hazards. Some of them can be flammable, explosive, toxic or corrosive etc. The whole lifecycle of a chemical should be considered when assessing its dangers and benefits. Though many of chemical accidents have a limited effect, occasionally there are disasters like the one in Bhopal, India, in 1984, where lakhs of people were affected and LPG explosion in Vizag refinery where huge property damage in addition to 60 deaths was experienced. Therefore chemicals have the potential to affect the nearby environment also.

• • Design and Pre-modification review : Improper layout like location of plant in down wind side of tank farm , fire station near process area , process area very close to public road and wrong material of selection had caused severe damages to the work and outside environment
  • Chemical Risk Assessment: Not assessed for new chemicals from the point of view of compatibility, storage, fire protection, toxicity, hazard index rating, fire and explosion hazards
  • Process Safety Management: HAZOP, FTA, F&E Index calculation, reliability assessment of process equipment, incorporating safety trips and interlocks, scrubbing system, etc. not done before effecting major process changes, lack of Management of Change procedure (MoC), etc.
  • Electrical Safety: Hazardous area classification , protection against static electricity , improper maintenance of specialized equipment like flameproof etc were ignored.
  • Safety Audits: Periodical assessment of safety procedures and practices, performance of safety systems and gadgets along with follow up measures were not carried out.
  • Emergency Planning: Lack of comprehensive risk analysis indicating the impact of consequences and specific written down and practiced emergency procedures along with suitable facilities had increased the severity of the emergency situations.
  • Training: Safety induction and periodical refresher training for the regular employees and contract workmen were not carried out.
  • Risk Management & Insurance Planning: Thorough identification and analysis of all risks and insurance planning were not done so that interruption risks and public liability risks could also be managed effectively.
    A. Risk Management Consultancy
    Following specialized risk management services are offered to chemical industries, considering the kind of risks that exists in these plant operations:
    1. STANDARD CONFORMANCE and PERFORMANCE EVALUATION (SCOPE)
    SCOPE would evaluate the existing measures / system based on applicable national / international standards.
    A few SCOPE reviews that we recommend for chemical manufacturing plants are:
    1.1 SCOPE-FP (Fire Protection)
    Indian Standards
    • IS 2189 - Standard for automatic fire detection and alarm system
    • IS 2190 - Code of practice for selection, installation and maintenance of first aid fire extinguishers
    • IS 3844 - Code of practice for installation and maintenance of internal fire hydrants and hose reels
    • IS 6382 - Carbon dioxide fire extinguishing system - fixed, design and installation
    TAC Standard
    Tariff Advisory Committee recommendations on hydrant and sprinkler system for fire protection.
    Oil Industry Safety Directorate
    • OISD 117 - Fire Protection Facilities for Petroleum Depots and Terminals
    • OISD 142 - Inspection of fire fighting equipment and systems
    • OISD 158 - Recommended Practices on Storage and Handling of Bulk Liquefied Petroleum Gas
    NFPA Standards
    • NFPA 12 Carbon Dioxide Fire Extinguishing Systems
    • NFPA 654 Prevention of Fire & Dust in Pharmaceutical Industries
    • NFPA 1600 Disaster Management
    • NFPA 921 Fire & Explosion Investigation
    • NFPA 45 Fire protection for Laboratories using Chemicals
    1.2 SCOPE - OHS (Occupational Health and Safety)
    • IS 14489 Code of Practice for Occupational Safety & Health Audit
    • NFPA 101 Life Safety Code 1.3 SCOPE-ER (Electrical Risk)
      • Hazardous Area Classification (base standard: IS 5572)
      • Selection of Electrical Equipment for Hazardous Areas (base standard: IS 5571)
      • Lightning Protection (base document: IS: 2309 /NFPA 780 /BS 6651)
      • NFPA 70 B Recommended Practice for Electrical Equipment Maintenance
      • NFPA 70 E Standard for Electrical Safety in Employee Work places
      2.0 PROCESS SAFETY MANAGEMENT
      • Hazard & Operability (HAZOP) studies
      • Failure Tree Analysis (FTA)
      • Event Tree Analysis (ETA)
      • Primary Hazard Analysis (PHA) using Dow Index
      • Risk Assessment (with risk ranking technique)
      3.0 ELECTRICAL RISK ASSESSMENT
      • Review of Hazardous Area Classification
      • Lightning Protection Risk Assessment
      • Identification & Control of Electro-Static Hazards
      • Review of electrical Preventive Maintenance System
      • Electrical Risk Assessment (fire, shock explosion) using Semi-Quantitative Risk Ranking (SQRR) technique
      4.0 FIRE RISK ASSESSMENT
      • Identification & assessment of fire risks during operations in receipt, storage, transfer and handling of chemicals (raw materials and finished products)
      • Identification & control of ignition sources in areas where flammable chemicals are stored / handled / transferred
      • Review of chemical compatibility in storage areas and to suggest appropriate fire loss control measures
      • Review of fire detection measures adopted in the plant & to suggest suitable improvement measures
      • Review of the various active (fire hydrant, sprinkler, portable fire extinguishers) and passive fire protection requirements for chemical storage and handling areas and to suggest improvements as necessary
      • Review of contractor safety awareness (chemical spill, fire fighting, emergency communication, knowledge of plant hazards & safety regulations) and to recommend suitable improvement measures to enhance contractor safety
      • Review of safety awareness and safety training requirements ( training identification and efficacy) of plant employees with respect to hazards present in the plant
      Fire risk assessment will be carried out based on techniques like Matrix method, Hani Raafat Risk Calculator. The consequence, likelihood and exposure of each hazard are arrived using a systematic approach and will help to determine the relative importance of hazard and focus on significant risks.
      
      

      5.0 RISK ANALYSIS & EMERGENCY PLAN

      • Identification of scenarios of potential disasters / emergencies leading to loss of life , property damage etc. and qualitative assessment of their likelihood.
      • Quantitative risk assessment for selected scenarios of major credible events.
      • Recommendations for risk control measures wherever applicable.
      • Preparation of onsite emergency preparedness plan
        

      
      

      6.0 RISK MANAGEMENT & INSURANCE PLANNING
      
      

      • Identification of all major internal and external pure risks including the natural risks and analysis of the impact of above risks
      • Review of existing risk control measures and offering comments
      • Scrutiny of all existing major insurance policies in respect of:
      • Rationalization of basic rate of premium and widening of covers
      • Applicability / eligibility of discounts in premium
      • Application of suitable clauses, warranties and conditions
        
        
      • Identification of possible areas for refund of premium and suggestions regarding procedure for the same
      
      
      • Selection of insurance coverage on the basis of risk analysis
        
        
      • Providing guidelines for fixation of sum insured and illustrate the same on a selected equipment
      • Evaluation of business interruption exposure due to identified risks
      • Providing guidelines on documentation requirements, procedures for claims under various policies, evaluation of insurers

      B. Risk Management Training

      Specialized and focussed training, if imparted effectively, can contribute significantly to Risk Management. Expert faculty, carefully selected training module, interactive and participate approach, useful training material, case studies and syndicate exercises could help in having effective risk management system in place. The training topics for bulk drug industry could be:

      • Chemical Safety
      • Safety with Compressed gases
      • Solvent Safety
      • Hazard Identification Techniques
      • Industrial Risk Management
      • Fire Prevention and Protection
      • Electrical Risk Management
      • Emergency Preparedness
      • Safety Management system
      • Accident Prevention
      • Personal Protective Equipment

Distillation in ASPEN HYSYS

Distillation in HYSYS

Introduction:

In this tutorial, you will be walked through a simple distillation process in HYSYS.  A binary liquid stream containing 70 mass percent Benzene and the rest Toluene entering at 30ºC and 1 atm flowing at a rate of 1 kg-mol/sec will be distilled to ultra pure distillate and bottoms streams.

Initial Setup:

1.      Start a new case in HYSYS

2.      Select Benzene and Toluene for the components.

3.      Use the Peng-Robinson Fluid Package.

Setting up the Distillation Column:

1.      Since this is a simple distillation case, 3 process streams are needed.  Place 3 material streams for the feed, the distillate, and the bottoms, and 2 energy streams for the re-boiler and condenser.

2. In order to make the simulation easy to follow, rename the material streams to feed, distillate, and bottoms.  Rename the energy streams to condenser energy, and re-boiler energy

3.      Now a distillation column should be placed.  Click on the Distillation Column button in the Simulation Toolbar, and then place it on the simulation window.  

4.      Now the column is ready to be hooked up to the process and energy streams.  To do this, double click on the Distillation Column to bring up the “Distillation Column Input Wizard”.  Hook up the streams to their appropriate locations.  

5.      Now the type of condenser must be specified.  HYSYS simulates 3 different types of condensers; a partial, full reflux, and total.  Select a total condenser.  Click “Next >” to proceed.

6. The next step is to specify the pressures of Distillation column.  There are no pressure drops in the condenser and the whole process is run at 101.3 kPa.  Type those into the respective fields and click “Next >”.

7.  In the next step, HYSYS prompts for temperature estimates for the top stage, condenser, and re-boiler.  Since these are optional fields, they can be ignored and HYSYS will calculate them when the simulation is run.  Click “Next >” to proceed to the final step.

8.      The last step to fully configure the column is to specify the reflux ratio and distillate flow rateA refux ratio of 3 and a distillate flow rate of 2700 kgmol/hr have been specified.  Click “Done…” to complete the distillation column connection and specifications.   

9.      Clicking done will bring up a window displaying all of the data and options of the distillation column.  In order to achieve the ultra pure distillate stream that is needed, it has been estimated that the column needs 23 trays with the feed entering at tray 7.  HYSYS sets the default number of trays at 10 with the feed entering at tray 5, so 13 trays need to be added and the location of the feed needs to be changed. 

Note:  By default HYSYS numbers the trays in numerically ascending order, with tray 1 being at the top.

10.      To add the extra trays, Simply click on “n = ” tab under “Num of Stages” on the drawing of the column, and simply enter in 23.

11.            To change the Tray location from tray 5 to 7, Click the arrow that appears by “5_Main TS” and select “7_Main TS” to change the inlet feed location.

Specifications of the Feed Stream:

After completing the steps above, the Distillation column should be fully specified for most simple distillation cases.  To complete and run the simulation, the feed stream needs to be specified.

1.      Double click on the Feed stream.  This will open its specification menu.  Select the appropriate tab and simply type in the desired values.  For this example, an inlet temperature of 30 C, a pressure of 101.3 kPa, a flow rate of 1 kg-mol/sec, and a feed with 70 mass % benzene have been specified.  

Running the Simulation with a Distillation Column:

Now the distillation column and feed stream are properly specified and the simulation is ready to run.

1.      To get HYSYS to start actively running the simulation and solving for unknown properties, the solver must be active.  To activate the solver, simply click on the green light  in the upper toolbar.

2.      When a distillation column is present in a simulation, it must be started separately.  Double click on the column to bring up its main window.  Click on the “Run” button to simulate the distillation process.  Close the window, and you will see that the distillate and bottoms streams have turned from turquoise to dark blue, signifying that HYSYS has successfully solved for those streams unknown properties, and the simulation is successful.

Note:  After this stage, if any of the specifications on the distillation column are changed, the “Reset” button must be pressed, and then the “Run” button pressed again to rerun the distillation simulation with the new specifications.

Advanced Specifications of the Distillation Column:

HYSYS has the ability to use different criteria other then the reflux ratio and distillate flow rate to simulate the column.  For example, the bottoms flow rate or the composition of the distillate at any given tray can be specified, and HYSYS can simulate the column based on those new specifications. 

1.      To change the specification criteria of the distillation column, double click on the column, and bring up the main window.  Then Click on “Specs” to bring up the advanced specification window. 

Note:  Notice that there are 4 column specifications present; 2 are active and 2 are not.  When a specification is active, that means HYSYS is using it to simulate the distillation.

2.      Say that the specification has changed to a bottoms flow rate of 650 kgmol/hr.  To avoid over specifying the distillation column, the Distillate flowrate must be inactive.  To do that, select “Distillate Rate” and uncheck “Active” to make it inactive.

3.      To make the bottoms flow rate of 650 kgmol/hr specification, select the “Btms Prod Rate” and check “Active” activate it.  Type in 650 in the “Specification Value” column.  Then click “Reset” and then “Run” to simulate the column using the new specifications.

4.  HYSYS has the ability to simulate the column based on different specifications not currently seen.  For example, a composition of 80 mass % benzene is needed in the distillate.  To use that specification, click “Add” in the Column Specifications section and select “Column Component Fraction” from the list.  Then Click “Add Spec(s)…

5. Clicking Add Specs will automatically bring up a window asking for the composition specifications.  Since    the composition is specified at the Distillate stream rather then a specific stage, check “Stream”.  Then          under “Draw” select “Distillate @COL1”.  Since the specification was given in Mass percent, select “Mass Fraction” in the “Basis” category.  Then enter .8 for the Spec Value.  Finally select Benzene as the component for calculation and close the window.

6.      To avoid conflicts and over specification, the bottoms flow rate specification must be de-activated, and the mass fraction specification activated.  Do so by the procedures outlined above.   Then click “Reset” and “Run” to simulate the process using the distillate composition as the basis for calculation.