Design and Simulation of a Multiple Effect Evaporator System

Raghuraman, Roll No.107CH005, for the requirements of the Bachelor of Technology degree in the Department of Chemical Engineering, National Institute of Technology Rourkela is an original work to the best of my knowledge, done under my supervision and direction. The objective of this report is to develop a model and an algorithm to design a multiple effect evaporator system. Also, it is required to be done to estimate the amount of steam saved by using vapor compression.

The use of vapor compression allows us to use the energy in the vapor that leaves the final effect. Since evaporators are an energy-intensive system, the use of vapor compression can significantly reduce steam consumption, but at the cost of the electricity required to run the compressor. For this system, the live steam requirement for the evaporator without vapor compression will first be estimated.

In order to choose the best solution, a total of 17 combinations of positioning of the compressor in the multiple effect system are identified. The cost to run the equipment without vapor compression is found to be $4009/hour.

CHAPTER ONE: INTRODUCTION

Application of evaporators

One of the most important applications of evaporation is in the food and beverage industry. Many foods that are made to last a long time, or foods that require a certain consistency, such as coffee, must go through an evaporation step during processing. It is also used as a drying process and in this way can be used in laboratories where long-term activity retention or stabilization is required (for example for enzymes).

Evaporation is also used to recover expensive solvents such as hexane that would otherwise be wasted. By law, all producers of waste must dispose of the waste in a method that complies with environmental guidelines; these methods are expensive. If up to 98% of the waste can be evaporated, the industry can greatly reduce the amount of money that would otherwise be allocated to waste management.

Evaporation is also used in the pharmaceutical industry to obtain a concentrated product and to improve the stability of products.

Problems associated with multiple effect evaporators

One of the earliest works on optimizing a multiple effect evaporator by modifying the feed flow sequence was done by Harper and Tsao in 1972 by developing a model for optimizing an MEE system by considering both forward and reverse feed flow sequences take. This work was extended by Nishitani and Kunugita (1979) in which they considered all possible feed flow sequences to optimize an MEE system for generating a non-inferior feed flow sequence. All these mathematical models are generally based on a set of linear or non-linear equations and upon changing the operating strategy, a whole new set of equations was needed to solve the new operating strategy.

They developed a generalized cascade algorithm that would be solved over and over again for the different operating strategies of a multiple effect evaporator system. The reported literature considers a number of energy reduction methods such as flashing, steam and feed splitting, steam puffing and using an optimal feed flow sequence. In connection with these, vapor compression is used in the present work on an existing industrial multi-effect evaporator, and overall cost calculations will be made.

To develop governing equations for multiple effect evaporator system with the induction of vapor compression. To define a number of combinations in multiple effect evaporator and compressor to select the best combination based on total annual cost.

CHAPTER TWO: LITERATURE REVIEW

• Different types of evaporators
• Horizontal spray film evaporators
• Basket type evaporators
• Long tube vertical evaporators
• Rising or climbing film evaporators
• Falling film evaporators
• Rising falling film evaporators
• Forced circulation evaporators
• Plate evaporators
• Mechanically aided evaporators
• Modelling of multiple effect evaporators
• Energy Conservation in Evaporators
• Flash Evaporation
• Vapour Compression
• Vapour Bleeding
• Black liquor
• Properties of black liquor
• Composition of black liquor in the current study

The temperature of the liquid in the pipes is difficult to predict because the temperature distribution is not uniform. In falling film evaporators, the liquid product usually enters the evaporator at the head of the evaporator. A heat exchanger and separator separate the concentrated product from its vapor in the lower part of the evaporator.

And to achieve this, high capacity pumps are used to maintain high velocity of the liquids in the pipes. The problem with using the steam instead of steam is that the temperature of their vapors is the same as the temperature of the liquid. Unfortunately, the temperature and pressure of steam actually lowers its latent heat per pound.

In this case, the energy input to the system is the pumping energy of the compressor. One of the most important uses of black liquor is as a liquid alternative fuel derived from biomass. The reason for this is that most of the alkalis in it are present in the form of neutral compounds.

The amount of total solids in black liquor depends on the amount of alkali charged to the digester and the yield of the pulp.

Fig. 2.2 Long tube vertical evaporators, Source [M]

CHAPTER THREE: PROBLEM STATEMENT

This difference was taken into account in the simulation because the problem statement is an actual scenario and the modeling is based on it. The temperature difference may be due to the uneven distribution of steam from the header to these effects, resulting in different pressures in the two effects. Furthermore, the cost of the compressor can be calculated using the graph below (Peters and Timmerhaus, 1991).

Table 3.2.Geometrical parameters of the evaporator

CHAPTER FOUR: DEVELOPMENT OF A MODEL & SOLUTION TECHNIQUE

• Model for MEE system
• Empirical relation for overall heat transfer coefficient
• Modelling of MEE with vapour compression
• Algorithm

Where λ1= Hj – hj, the latent heat of vaporization of the solvent forms the thick cloth at temperature Tj and pressure Pj (j=1, 2, 3, the power number). Alternatively, the result given by equation (3) can be obtained by taking the enthalpy reference for the first effect to be the enthalpy of the thick liquid leaving the effect at temperature T1. These fourteen equations can be stated in compact form using the following matrix equation.

The subscripts k and k+1's elements of the matrices bearing these subscripts are those given by the kth and k+1st trials, respectively. The correlation given in equation 4.18 is used to evaluate the overall heat transfer coefficient of each effect of the evaporator. From the correlation it can be seen that the overall heat transfer coefficient of each effect is a function of the temperature gradient and the average values ​​of concentration and liquor flow rate obtained from the input and output parameters.

These values ​​are assumed to be the same for the first and second effects and the same for the rest of the five effects. In the simple case where BPE and ∆solvingௗ௜௟ are negligible (i.e. for inorganic colloids). enthalpy of saturated liquid for solving. Note that with each compression we can recover all the latent heat of the vapor, significantly reducing steam consumption without compression.

We may therefore find that the enterprise is profitable by comparing the money spent in compressing the vapor with the money saved in the form of fresh steam required. The vapor pressure of each effect is calculated from the pressure values ​​of the first and the seventh effect by assuming equal pressure drop across each effect. A reasonable value of the temperature difference in each effect, solvent evaporated in each effect, and steam requirement is assumed.

Now the values ​​of the individual terms in the Jacobian matrix are calculated and the Jacobian matrix is ​​completed. Both ꞌꞌꞌ다 are known, but to calculate the values ​​of the unknown variables in ௞ ௞ we need to find the inverse of 쬬௞. The values ​​of the variables are used instead of the values ​​chosen in step three, and steps 3-6 are repeated until the values ​​of two consecutive iterations differ by a value <=0.000001.

Table 4.1 Empirical constants to find out overall heat transfer coefficient

CHAPTER FIVE: RESULT AND DISCUSSION

Cost Computation without Vapour Compression

So, using the cost of steam at $0.3/¥ of steam according to Al-Sahali et al.

Table 5.2: Variables in the different effects

Solution of model with Vapour Compression

• Cost Computation with Vapour Compression

They are listed in section 4.3 and discussed in detail in the next section. When the steam from the 7th effect is compressed and sent to the 1st effect and the Jacobian matrix is ​​solved, we get the steam requirements to be 3492.18 kg/hr and the steam costs to be $1977.45/hr. But a cost is also required to compress the steam from its pressure of 0.7009 bar to 2.8195 bar, this cost can be calculated using the equation for the energy consumption of a compressor given by Al-Sahali et al.

Using this equation, we get the work done to be 8154 KWh and again using the cost of electricity calculated by Al-Sahali et al. This results in a total cost of 4423 $/hour which is greater than the cost for the operation without vapor compression. Thus, even though the required steam is greatly reduced, the high cost of compression makes this configuration unviable.

Now we compress the vapor of the 7th effect and send it to other effects and calculate the total cost. Then, from the pressure values ​​in each effect, the total compression cost is calculated and the resulting total costs are compared. Table 5.5 shows that although steam costs are drastically reduced in the first two cases, the cost of compression is high and thus no savings are achieved through vapor compression and the method is not viable.

In the fourth and fifth cases, the costs of compression are also very low, but the savings on compression are not sufficient to counteract the higher steam requirement. Similar calculations and simulations for the fifth effect lead to the following table and graph. Similar calculations and simulations for the fourth effect lead to the following table and graph.

Fig 5.2 Schematic Diagram with values with vapour compression

Payback Period

Therefore, the initial costs invested in the purchase, transportation and installation of the equipment can be recouped in almost 1 year and 8 months, after which the unit will make a profit.

CHAPTER SIX: CONCLUSION

10] Dessouky, Ettouney, Juawayhel, Multiple Effect Evaporation and Vapor Compression in Desalination Processes, Trans IChemE, Vol 78, Part A, May 2000. Mabrouk, Thermoeconomic Design of a Multi-Effect Evaporative Mechanical Vapor Compression (MEE-MVC) Desalination Process, August 2006.

APPENDIX