Design of Sugar Industry Evaporators

Evaporator Design Calculator

Input Parameters

Feed Rate (WF, Kg/hr):

Feed Concentration (xf):

Steam Temperature (°C):

Number of Effects: 4 5 6

Effect Temperatures & Pressures

Results

Steam Consumption (Kg/hr):

Capacity (Kg/hr):

Steam Economy:

Effect	Vapor Flow Rate (Kg/hr)	Product Flow Rate (Kg/hr)	Mass Fraction (xp)	Heat Transfer Area (m²)	Heat Transfer Coefficient (KJ/m².hr. °C)

Design Considerations for Evaporator Drums and Calandria

Evaporator drums are critical components in chemical evaporation systems, and their design requires careful consideration of various factors. Most evaporator drums operate under vacuum conditions, with a typical external pressure rating of 0.1 N/m2.

Evaporator Drum Design

The evaporator drum consists of several key components:

1. Conical Bottom Portion: Designed for a similar pressure rating as the drum, this section provides a smooth transition for the vapor flow.
2. Top Head Section: This section, which is preferred to be flanged or flared, dished, or conical in shape, facilitates easy access for maintenance and inspection.
3. Cylindrical Pieces: The vapor drum is constructed from separate cylindrical pieces joined by flanges, allowing for easy assembly and disassembly.
4. Large Nozzles: Manholes, sight glasses, and other large nozzles are reinforced with compensating rings to ensure structural integrity.
5. Supports: Brackets welded to the vapor drum or calandria provide additional support and stability.

Calandria Design

The calandria, a tubular heating surface, is designed based on the shell and tube heat exchanger model. Since steam under pressure is typically used as the heating medium, the design takes into account the pressure rating of the system. The calandria and vapor drum are connected by flanged joints or directly welded.

Design Factors and Heat Balance

The design factors, heating surface area, and steam requirement are derived from the overall and individual heat balance of the multiple-effect chemical evaporation system. This involves careful consideration of the heat transfer coefficients, temperature differences, and mass flow rates of the various streams involved.

By carefully considering these design factors and incorporating them into the design of the evaporator drum and calandria, engineers can create efficient and effective chemical evaporation systems that meet the required process conditions.

The nomenclature for quadruple effect calculations for the design of sugar industry evaporators :

• C_F = Specific heat of feed (KJ/Kg^oC) 
• T_F =Temperature of feed,^oC 
• W_F = Feed, Kg/hr 
• Ts = Saturation temperature of steam to first effect, ^oF 
• Ws = Steam to first effect, Kg/hr 
• W_1-4 = Total water removed by evaporation 
• m = mass flowrate
• p = product
• v = vapor 
• f = feed 
• U = heat transfer coefficient

Design of five effect evaporators system:

The overall balance of the evaporator would be: m·f = m·p₄ +m·v
Overall vapor balance: m·v = m·v₁ + m·v₂ + m·v₃ + m·v₄

	Solute balance	Energy balance equations
First effect	mf = mp1 + mv1 mfxf  = mpxp1	ms .λs = mf Cpf (T1-Tf) + mv1 .λv1
Second effect	mp1 = mp2 + mv2 mp1xp1 = mp2xp2	mv1 .λv1= mp1Cpf (T2-T1) + mv2 .λv2
Third effect	mp2 = mp3 + mv3 mp2xp2 = mp3xp3	mv2 .λv2 = mp2Cpf (T3-T2) + mv3 .λv3
Fourth effect	mp3 = mp4 + mv4 mp3xp3 = mp4xp4	mv3 .λv3= mp3Cpf (T4-T3) + mv4 .λv4
Fifth effect	mp4 = mp5 + mv5 mp4xp4 = mp5xp5	mv4 .λv4= mp4Cpf (T5-T4) + mv5 .λv5

Assuming the steam temperature is 120^oC
The energy value of steam = 2201.6 KJ/kg from steam tables
Basis: For the mass flow rate of the entering juice ‘m’= 208.33tonnes/hr.

	Pressure maintained	Temperature from steam tables	Latent heat of vaporization
First effect	1.60687atm= 755 mm Hg	99.819oC	2257.4 KJ/kg
Second effect	1.19989atm = 480.6 mm Hg	87.616oC	2289.4 KJ/kg
Third effect	0.81229atm = 302.8mm Hg	75.844oC	2319.2 KJ/kg
Fourth effect	0.44407atm = 99.6mm Hg	60oC	2379.1 KJ/kg
Fifth effect	0.09527atm	50oC

Assuming the value of  x_f =0.15

By substituting the values in the balance equations and solving the simultaneous equations we get the following values:

Type	Vapor flow rates(tonnes/hr)	Product flow rate(tonnes/hr)	Mass fractions in terms of xp
1st effect	66	142.33	0.1804
2nd effect	64	79.33	0.2313
3rd effect	17	62.33	0.32
4th effect	10	52.33	0.6
5th effect	1	51.33	0.6

Assuming the mass flow rate of steam, m_s = 66tonnes/hr
Steam balance:

• Steam consumption m_s is = 66tonnes/hr 
• Capacity m_v = 41532.3499 Kg/hr 
• Steam economy = 4.619 

The formula for design of evaporators Q=Q₁=Q₂=Q₃=Q₄
Where Q=m_s λ_s

• m_s λ_s = Q₁ = U₁A₁ΔT₁ 
• m_s λ_s = Q₂ =U₂A₂ ΔT₂ 
• m_s λ_s = Q₃ =U₃A₃ ΔT₃ 
• m_s λ_s = Q₄ =U₄A₄ ΔT₄

	ms λs=UA ΔT	U, KJ/(m2.hr. oC)
First effect	66000×2187.18 = U1×2700×(125-110)	3564. 3
Second effect	63000×2201.6   = U2×2500×(110-102)	7012
Third effect	17000×227.168 = U3×1200×(102-90)	2530.6
Fourth effect	9310×2281.6     = U4×800×(90-80)	2658.861
Fifth effect	9320×2353        = U5×300×(80-60)	3663.41

Calculation for number of tubes:  

Formula for number of tubes= n × π × D × L = A

Type	Vapor  flow rates(kg/hr)	Product flow rate(kg/hr)	U(KJ/m2.hr oC)	Area, A=m2	n, no.of Tubes
1st effect	66000	142.33	3564.3	2700	4775
2nd effect	63000	79.33	7012	2500	4421
3rd effect	17000	62.33	2530.7	1200	2122
4th effect	9320	52.33	2658.86	800	1415
5th effect	1000	51.33	3663.41	300	531

Model mechanical design of sugar industry evaporators:   

• Temperature in evaporator = 99.819 ^oC 
• Diameter of tubes: 42 ID × 45 OD 
• No. of tubes: 4775 
• Length of the tube = 4m 

Calandria: 

• Pressure = 755 mm 
• Heating surface area = 2700 m² 
• Material of construction: Stainless Steel 
• Permissible stress for low carbon steel—98 N/mm² 
• Modulus of elasticity for low carbon steel = 19.0 X 10⁴ N/mm² 
• Modulus of elasticity for SS 304= 9.5 X 10⁴N/mm²

Calandria with vertical tubes:

• No. of tubes N_t = 4775
• Pitch of the tube = Pt/D_o = 67.5mm
• Area occupied by tubes As = n X 0.866 X S_T² / β      = 18.4 m²
• Required area for central down take = 40% X Cross-sectional area of the tube = 0.4 X 2110 X π X0.045² / 4    = 3.04m²

• The actual area of downtake is 3.1m²
• The total area of the tube sheet is  = 24.03m²
• The diameter of the tube sheet is = 5.96m
• Calandria sheet thickness: t_s = pD / 2fJ – p  = 5.48mm

Tube sheet thickness:
F = √(K/[2+3K])
Where K = E_st_s(D₀-t_s) / E_tN T_i(d₀-t_t)

• E_s = modulus of elasticity of shell material
• t_s  = Sheet thickness
• t_t   = Tube thickness
• k = 0.15527
• F = 0.250936
• t = FG√(0.25p)/f  = 5.48mm

Bottom flange of calandria:

• The thickness of the flange = 40mm
• Number of bolts = 112
• Outside diameter = 3894mm
• Pitch circle diameter = 3825mm
• Size of bolts = 20M

Evaporator drum:

• Rd =     V/A   0.0172 X (ρ_L  – ρ_V / ρ_V)^1/2
• Let Rd = 0.8
• Here V = volumetric flow rate of vapor in m³/sec = 66
• A = 31.32m²
• The diameter of drum is 8m

Recompression methods

Recompression Methods: A Comprehensive Review

Recompression is a crucial process in various industries, including oil and gas, chemical processing, and power generation. It involves the compression of a gas or vapor to a higher pressure, often to facilitate transportation, storage, or utilization. In this article, we will review the latest methods and mechanisms of recompression, discuss the calculations required to perform the operation and highlight the key parameters to consider for selecting the most suitable method.

Recompression Methods

Several recompression methods are available, each with its advantages and limitations. The most common methods include:

1. Isentropic Recompression: This method involves the compression of a gas or vapor through an isentropic process, where the entropy remains constant. Isentropic recompression is often used in power generation and chemical processing applications.
2. Polytropic Recompression: This method involves the compression of a gas or vapor through a polytropic process, where the relationship between pressure and volume is described by a polytropic exponent. Polytropic recompression is commonly used in oil and gas applications.
3. Isothermal Recompression: This method involves the compression of a gas or vapor through an isothermal process, where the temperature remains constant. Isothermal recompression is often used in cryogenic applications.
4. Adiabatic Recompression: This method involves the compression of a gas or vapor through an adiabatic process, where no heat transfer occurs. Adiabatic recompression is commonly used in high-pressure applications.

Calculations Required

To perform recompression, several calculations are required, including:

• Compression Ratio: The compression ratio is the ratio of the final pressure to the initial pressure. It can be calculated using the following equation:

Compression Ratio = P2 / P1

where P1 is the initial pressure and P2 is the final pressure.

• Work of Compression: The work of compression is the energy required to compress the gas or vapor. It can be calculated using the following equation:

Work of Compression = ∫PdV

where P is the pressure and V is the volume.

• Heat Transfer: Heat transfer is an important consideration in recompression, as it can affect the efficiency and safety of the process. The heat transfer can be calculated using the following equation:

Heat Transfer = Q = m * Cp * ΔT

where Q is the heat transfer, m is the mass flow rate, Cp is the specific heat capacity, and ΔT is the temperature difference.

Parameters to Consider

When selecting a recompression method, several parameters must be considered, including:

1. Pressure Ratio: The pressure ratio is the ratio of the final pressure to the initial pressure. It is an important consideration in recompression, as it affects the compression ratio and the work of compression.
2. Temperature: Temperature is an important consideration in recompression, as it affects the compression ratio, the work of compression, and the heat transfer.
3. Gas or Vapor Properties: The properties of the gas or vapor being compressed, such as its molecular weight, specific heat capacity, and viscosity, must be considered when selecting a recompression method.
4. Efficiency: Efficiency is an important consideration in recompression, as it affects the energy required to compress the gas or vapor.
5. Safety: Safety is a critical consideration in recompression, as it affects the risk of explosion, fire, or other hazards.

Latest Developments

Recent advances in recompression technology have focused on improving efficiency, safety, and reliability. Some of the latest developments include:

• Advanced Materials: New materials, such as advanced ceramics and composites, are being developed to improve the efficiency and reliability of recompression equipment.
• Digitalization: Digitalization is being used to improve the control and monitoring of recompression equipment, reducing the risk of human error and improving safety.
• Artificial Intelligence: Artificial intelligence is being used to optimize recompression processes, improving efficiency and reducing energy consumption.

Recompression is a critical process in various industries, and selecting the most suitable method is crucial to ensure efficiency, safety, and reliability. By considering the latest developments and calculations required, engineers and operators can optimize recompression processes, reducing energy consumption and improving overall performance.

Vapor Recompression: A Comprehensive Review

Vapor recompression is a technique used to enhance the efficiency of evaporation systems by reusing the thermal energy contained in the vapor evolved from a boiling solution. This process can be achieved through two primary methods: mechanical recompression and thermal recompression.

Mechanical Recompression

Mechanical recompression involves compressing the vapor evolved from the evaporator using a positive displacement or centrifugal compressor. The compressed vapor is then fed to a heater, where its saturated temperature is higher than the boiling point of the solution, allowing heat to flow from the vapor to the solution and generating more vapor.

Mathematical Modeling

To study mechanical recompression, the following mathematical models can be developed:

1. Energy Balance: ∆E = Q - W, where ∆E is the change in energy, Q is the heat added, and W is the work done.

2. Mass Balance: ∆m = m_in - m_out, where ∆m is the change in mass, m_in is the mass flow rate into the system, and m_out is the mass flow rate out of the system.

3. Momentum Balance: ∆p = ρ * ∆V, where ∆p is the change in pressure, ρ is the density of the fluid, and ∆V is the change in volume.

Thermal Recompression

Thermal recompression involves compressing the vapor using a steam jet ejector. The high-pressure steam draws and compresses the vapor, which is then fed to a heater, where its saturated temperature is higher than the boiling point of the solution, allowing heat to flow from the vapor to the solution and generating more vapor.

Mathematical Modeling

To study thermal recompression, the following mathematical models can be developed:

1. Energy Balance: ∆E = Q - W, where ∆E is the change in energy, Q is the heat added, and W is the work done.

2. Mass Balance: ∆m = m_in - m_out, where ∆m is the change in mass, m_in is the mass flow rate into the system, and m_out is the mass flow rate out of the system.

3. Momentum Balance: ∆p = ρ * ∆V, where ∆p is the change in pressure, ρ is the density of the fluid, and ∆V is the change in volume.

Optimization and Limitations

While vapor recompression offers several benefits, including improved efficiency and reduced energy consumption, there are limitations to its application. These include:

• Temperature Drop: A minimum temperature drop of 10°F is required to make compression economical.
• Equipment Cost: The cost of compression equipment can be prohibitively expensive.
• Standby Steam Capacity: Standby steam capacity is required to supply heat for starting the system.
• Boiling Point Elevation: If the boiling point elevation of the solution is appreciable, the energy needed for compression increases rapidly.

By developing mathematical models to study vapor recompression, engineers can optimize the design and operation of these systems, minimizing limitations and maximizing efficiency.

Forced circulation evaporators

In natural circulation evaporators, the liquid enters with velocity 0.3 to 1 m/s and generally, the heat transfer coefficients are very low and particularly with viscous liquids. So, whenever we are dealing with a concentration of highly viscous and scale forming solutions there is no alternative but to use forced circulation evaporators. By increasing the velocity of the liquid flow (generally 2 to 6 m/s) through the tubes heat transfer coefficients increases enormously and high liquid velocity which is resulting from the pumps prevents the scale formation on heating surfaces.

Two types forced circulation evaporators.

(i)  horizontal heating type

(ii) vertical heating type

Construction:

They consist of a centrifugal pump, a 1-2 shell & tube heat exchanger on the case of the horizontal heating element and 1-1 shell & tube heat exchanger in the case of vertical heating element, an evaporator body with vapor outlet at the top, deflector plate, an outlet for product discharge at the bottom.

Working:

Centrifugal pump forces the liquid through the tubes at high velocity and gets heated by condensing steam on the shell side. The solution becomes superheated and flashes into a mixture of vapour and liquid in the evaporator body. Deflector plate separates the vapour and liquid in the vapour space, and vapours are removed at the top, concentrated liquid comes out at the bottom.

Advantages:

1. High transfer coefficients obtained even with viscous solutions.

2. Whenever we are dealing with the concentration of highly viscous and scale forming solutions forced circulation evaporators prevents the scale formation on heating surfaces

3. Residence times are low so that heat sensitive viscous materials also can be used.

Disadvantage:

The main disadvantage of forced circulation evaporators is high pumping cost.

Applications:

To handle high viscous solution in continuous operations external pumping system is provided by forced circulation evaporators. Salt, plastic, polymer, and pharmaceutical industries use it for crystallizing, concentrating and thickening of the products.

Long vertical tube evaporator

These are two types.
(i) Long vertical tube falling film evaporator
(ii) Long vertical tube rising film evaporator

(i)Long tube vertical (climbing film) rising film evaporator:

Construction:
This consists of a long tubular heating element incorporating the tubes 25 mm to 50 mm and 3 to 10 meters in length. And also it contains at the top a separator or vapour space for removing entrained liquid from the vapour. The deflector is provided just above the tube sheet.
Working:
The feed at near boiling is fed to the bottom of the evaporator. It is then pumped inside the tubes. Steam is provided on the shell side. Liquid and vapour are separated in the vapour separator at the top. Deflector acts as both primary separator and foam breaker. Multiple effects are used to achieve higher steam economy.
Applications:
Its counter heat exchange design gives the ability to handle foam and forth forming type materials.

(ii)Long tube vertical falling film evaporator:

Construction: This consists of a long tubular heating element incorporating the tubes 50 mm to 250 mm and 3 to 10 meters in length.
Working:
The feed to the evaporator is fed to the top of the evaporator through efficient liquid distributors. Steam is given on the shell side. The concentrate is collected at the bottom. The same operation is repeated in multiple effects to achieve steam economy.
The main problem in this type of evaporator is that of distributing the liquid uniformly as a film inside the tubes. This is done by a set of perforated metal plates above a carefully levelled tube sheet.
Applications:

• Used in desalination industries to separate water and salts in seawater. Optimized models of the multi-effect system having smooth or double fluted tubes are used to obtain better performance ratio. Submerged vertical tube evaporator developed from the classic falling film evaporators are used to provide fresh water with better thermal efficiency.
•  It is employed in a high capacity refrigeration system that uses propane as the refrigerant. 
• Used widely in liquid food evaporation.

Evaporation and Types of Evaporation Equipments

The objective of Evaporation: To concentrate a dilute solution consisting of non-volatile solute and volatile solvent.

In this unit operation, the solvent to be evaporated is generally water and concentrated solution is a product. The vapour generated usually has no value, it is condensed and discarded.

Types of evaporation:

1. Single effect evaporation: The solution to be concentrated flows inside the tubes.The heating medium is steam condensing on metal tubes.Usually, the steam enters at 3 atm abs. and boiling liquid is under moderate vacuum.This increases the temperature difference between the steam and boiling liquid.When a single evaporator is used, the vapour from the boiling liquid is condensed and discarded. This is called single effect evaporation.It is simple but utilizes steam ineffectively. To evaporate 1 kg of water from the solution we require 1-1.3 kg of steam.
2. Multiple effect evaporation: Increasing the evaporation per kg of steam by using a series of evaporators between the steam supply and condenser is called multiple effect evaporation

Properties of evaporating liquids that influence the process of evaporation: 

1. Concentration: As the concentration increases, the viscosity and density increase thereby the boiling point of solution increases. The increase in boiling point and viscosity reduces the rates of heat transfer in evaporators.
2. Foaming: Solutions like organic compounds tend to foam during vaporization.The foam is carried away along with vapour leading to heavy entrainment.
3. Scale: Solutions deposit scales on the heating surface .overall heat transfer coefficient drastically decreases and leads to shut down of the evaporators for cleaning of the tubes.
4. Temperature sensitivity: Pharmaceuticals products, fine chemicals and foods are damaged when heated to moderate temperatures for relatively short times. So special techniques are employed to reduce the temperature of the liquid and time of heating.
5. The material of construction: Evaporators are made of some kind of steel. however many sols attack ferrous metals and are contaminated by them. Copper, nickel, stainless steels can also be used. 

Classification of evaporators:

Evaporation is a process of vaporization of a volatile solvent from a solution in order to increase the concentration of the solute. The types of evaporation equipment are:

(1) Natural circulation evaporators

     (a) Long tube vertical falling film evaporator

     (b) Long tube vertical rising film evaporator
 These are used generally for simple evaporation operations either as the single effect or multiple effects.

(2) Forced circulation evaporators

    (a) Forced circulation evaporator with a horizontal heating element

    (b) Forced circulation evaporator with a vertical heating element 

These are used generally for salting, viscous and scale forming solutions.

(3) Agitated film evaporator

The main advantage of this type is to give high heat transfer coefficients even with very high viscous liquids. This is a modified falling film evaporator. Mechanical agitation is used to reduce the viscosity of very high viscous solutions.

Industry applications of various evaporators:

Long tube vertical rising film evaporator:

These types of evaporators are widely used in industries for the handling of foaming, frothy liquors, and used for the production of condensed milk

Long tube vertical falling film evaporator:

These types of evaporators are widely used in industries for concentrating highly heat-sensitive materials such as orange juice, food materials etc. which require short residence times. And used for the production of condensed milk.

Forced circulation evaporators:

These types of evaporators are widely used in industries for salting, viscous and scale forming solutions, and used in crystallizing operations in which it is necessary for the solids to be in suspension.

Agitated film evaporator:

The main advantage of this type is to give high heat transfer coefficients even with very high viscous and heat sensitive liquids such as gelatin, rubber latex, antibiotics and fruit juices. But these are very costly and will be having smaller capacities.
Recompression methods used for evaporators