Design and Cost Optimization of Heat Exchangers Network System in a Typical Brewery Plant

DOI: http://dx.doi.org/10.24018/ejers.2020.5.2.1748 245  Abstract—Heat exchanger design and cost optimization had been carried out for Pabod Brewery, Port Harcourt, Rivers State, Nigeria using Pinch Technology as process application method. The gross energy expenditure by the plant is 10.44MW at production capacity of 400,000 liters of beer per day. On quantitative aggregate 6.157MW goes for heating and 4.267MW for cooling. A temperature pinch or minimum approach temperature (ΔTmin) of 100C, minimum heating utility of 5.04MW and cooling utility of 3.09MW were recorded. Energy upturn of 1.08MW and 1.23MW for the hot and cold flows were measured. This finding correlates to energy conservation of 18% for hot utility and 21% for the cold utility. Overall improvement in capital and annualized costs of 39% was achieved for the hot and cold utilities. The researchers strongly recommend the outcome of this research to process applications in brewery, chemical, petrochemical, oil and gas industries.


I. INTRODUCTION
In the paper titled: "Heat Exchanger Process Optimization In A Typical Brewery Plant" [1], heat exchanger process and heat optimization had been carried out for Pabod brewery, Rivers State, Nigeria. The system heat swap integration between the cold and hot utilities streams was carried out applying Pinch Technology [2,3,4,5,6,7,8,9,10]. The minimum approach temperature or pinch temperature (∆Tmin) was fixed at 100C. The heat exchangers network system in view is counter current flow heat exchanger type.
The system design targets certain paramount design parameters such as heat exchanger minimum surface area (Amin) and minimum units of heat exchangers (Nmin). Area or space distribution for the installation process might also be object of concern. Optimal total cost targeting and annualized costs for the heat exchangers network system are vital issues. These costs related problems are predicated on the same aforementioned two critical parameters [11,12] for maximum heat recuperation and recovery.

II. LITERATURE REVIEW
Pinch technology and heat process integration, provides a procedural approach for reduced energy consumption in processes. The line of attack is footed on thermodynamic rules; precisely the first and secondly laws of thermodynamics, the change in the enthalpy of the streams is taken care of by the first law whereas, the second law is used for the determination of the course of heat transfer, that is, heat energy is transferred due to potential difference, that is, from hot spots to cold areas. The process analysis starts on the balance in the material and heat [8]. Pinch technology enabled processes identify the right changes in the process conditions that can impact positively on energy economy [8].
Targets can be established for energy cutback early before designing the heat exchange system, after putting in place the bits and pieces of the material and heat balance. It is common with this approach that these targets are achieved during the design at these utility levels; goals can also be set for the utility loads [8]. Pinch technology in summary, is a dependable method that saves energy from heat, material balance and even up to entire location utility arrangement [6. 7]. Energy efficiency is awfully essential for production plants, because it is one of the deciding factors for final product price and increasing of incomes. There is more or less a stream that contains heat and need heating. Combining of all that heat energy between selected streams doubles energy value of plants and decreases utility need, this kind of connection of energy content between streams with a plant is referred to as heat integration [14,15].
Pinch technology uses the following principles to give guidance in designing a feasible and optimal Heat Exchange system The constraints are: Heat ought not to be transferred over the pinch No hot Utility should be placed beneath the pinch point cold utility should not be administered over the pinch point The temperature of the process to process heat exchanger shall not come near the stipulated ∆ in the exchanger [4,13] During matching of the streams, these provisions are worthy of note.
Above pinch, with reference to the heat capacity of the streams, , the stream needing to be cooled have to be a lesser amount of or equal to the cold stream heat capacity ℎ ≤ Below Pinch the cold stream heat capacity must obey this constraint ℎ ≥ (Wunsch, 1998). These two guidelines are used when putting in place heat exchanger network to match streams to know if they can match better. The streams are merged to make the most of the potential loads on the exchanger Assumptions on heat exchanger All exchangers are without phase change and without any Heat exchangers are considered to be in indirect contact and counter-current flow.
The research targets the amount of energy conserved in the hot and cold utilities and streams. The overall improvement in capital and annualized costs are object of great concern.
III. RESEARCH SIGNIFICANCE Pinch Technology essentially applies fundamental thermodynamics based models to address process heat swap in heat exchangers network system to enhance minimum energy requirements and maximum heat recovery in process applications. This concept application to Pabod Brewery plant will lend itself to optimal minimal energy requirement for heating and cooling utilities in the case study brewery plant. It could also lead to tremendous cut back in the in capital and annualized costs for the establishment.

A. Preamble
The design approach breaks the entire system into two distinct thermodynamics regions at pinch temperature of 100C. The two regions are heat sink (above pinch) where hot utility supplies heat and heat source (below pinch) where heat flows into the cold utility. In the process of heat swap, integration, recuperation and recovery no heat flows across the pinch subject to the condition of minimum energy requirement for heating and cooling and maximum energy recovery. The brewery process diagram is as shown in Fig.  1.

B. Applicable Thermodynamics Models
The applicable models for computational analysis in the design process of the proposed heat exchangers network system HENs) are as follows [1] : The thermal load, Q, for utilities and streams is expressed as, Where, Q, ΔH-thermal load or enthalpy change of the streams (W) Cp-heat capacity flow rate (W/0C) Ts-source temperature (0C) Tt-target temperature (0C) The hot stream thermal energy need is expressed as: The cold stream thermal energy need is expressed as: m-mass flow rate of streams (kg/s) Th,in-inlet or source temperature of the hot stream ( 0 C) Th,out-outlet or target temperature of the hot stream ( 0 C) Tc,in-inlet or source temperature of the cold stream ( 0 C) Tc,out-outlet or target temperature of the cold stream ( 0 C) The exit temperatures of the hot and cold streams are determined by expressions in (4) and (5).
Heat exchanger surface area, A, per match is expressed as: , where ∆TLMTD is the logarithmic mean temperature difference expressed as: U-overall heat transfer coefficient (W/m 20 C) ΔTLMTD-logarithmic mean temperature difference ( 0 C) h1, h2-convective heat transfer coefficients of the fluid at inner and outer walls of the heat exchanger tubes (W/m 20 C) The shifted temperature for hot stream is expressed as, Heat transfer coefficient for shell heat exchanger is given as, Heat transfer coefficient for tube heat exchangers is expressed as, The minimum number of heat exchangers units for maximum heat recovery, Nmin MER, is expressed as : Nh-Number of hot streams Nc-Number of cold streams Nu-Number of utility stream AP/BP-Above/Below pinch The different cost related parameters in the cost estimation process are as follows: = ( ) + Where: t-years The following inequalities rules must be observed for heat exchangers design above and below pinch region. Above pinch region constraints: ≤ implying the number of heat exchangers for the hot flows should be less or equal to that of cold streams Below pinch region constraints: ≥ implying the number of heat exchangers for the hot flows should be greater than or equal to that of cold streams. Heat balance analysis is done on streams falling within temperature intervals.
Where, ∆ --the heat needed within the n th gap Σ --summation of the heat capacity flow rate of the cold flow within the gap Σ ℎ --summation of the heat capacity flow rate of the hot flows within the gap ∆ --shifted temperature difference Table VI shows the brewery plant operation data. The shifted temperature for the hot and cold streams are as shown in Table II and III respectively.  Secondary data for the network cost calculations are in Table IV and V below:  Where a, b and c are the cost law constants, which are dependent on the heat exchanger type, the pressure ranking and construction materials, PL is the plant life and rate of return or interest rate. Table I to V were integrated at pinch temperature of 10 0 C to generate the cascade table in Fig. 2.  Table Cascade The Composite curve in Fig. 3 shows where pinch occurs while Fig. 4 displays the hot and the cold energy expectations.

E. Design procedure
The design procedure takes cognizance of heat capacity flow rate of the streams in stream splitting and heat exchangers network designs above and below pinch [1]. By designing above and below pinch the proposed heat exchanger network system shown in Fig. 5 was developed. For optimal heat exchanger network design the minimum number of heat exchanger units is nine, the proposed system design operates with eight heat exchangers units. The projected heat exchanger units with calculated heat loads, surface area requirements and the logarithmic mean temperature difference (∆TLMTD) for the heat exchanger units are as shown in Table X. Table IX shows the inlet and outlet temperatures for the respective heat exchangers. Results in Table X were obtained by applying (6) to find the logarithmic mean temperature difference of the respective heat exchangers in Table IX. To calculate the logarithmic mean temperature for Heat Exchanger HX1:  Equations (15), (16), (17) and (18) were applied to determine the heat exchangers capital and annualized cost. The minimum surface area for each of the heat exchangers (Amin) was computationally determined keeping in view that the minimum number of heat exchanger units (Nmin) is eight (8). The computational results are in Table IX. The heat for each heat exchanger was determine by subtracting the minimum heat from the maximum heat load.
Heat load for exchanger HX1: The approach applies for heat exchangers HX3 to HX8. Certain valid assumptions are assumptions are required in heat exchangers network cost determination. The assumptions relate to certain specific constants as in Table  XI and XII. Where a, b and c are the cost law constants, which are dependent on the heat exchanger type, the pressure ranking and construction materials, PL is the plant life, ROR is the interest rate. The minimum number of heat exchangers units for maximum heat recovery, Nain MER, is expressed as: Operating cost = Σ( ℎ ( ℎ . )) + Σ( ( . )) Formula for annualized cost = = ( ) + To compute the value of λ for the costing process    V. ANALYSIS OF RESULT The original layout of the plant consumes energy aggregate of 6.157MW for heating and 4.267MW for cooling. In this research a temperature pinch or minimum approach temperature (ΔTmin) of 100C was used in the pinch analysis of the heat exchangers performance. The research findings confirmed minimum heating utility of 5.04MW and cooling utility of 3.09MW with energy upturn of 1.08MW and 1.23MW for the hot and cold flows respectively. This correlates to energy conservation of 18% for hot utility and 21% for the cold utility [1]. On the basis of capital and annualized costs aspect this translates to 39% savings in energy consumption. More efficiently eight heat exchangers instead of nine were integrated into the system design. This is a pointer to lesser capital and installation costs.

VI. RECOMMENDATION
Pinch analysis and heat integration should be applied to the Pabod Brewery process design using diverse temperature difference (ΔTmin) for energy targets. It should be noted that reduction in the minimum approach temperature (ΔTmin) improves upon energy recovery increases the thereby declining the utility spending and cost, with a payoff of decreased heat exchanger size and purchase cost.

VII. CONCLUSION
Heat exchangers process and design optimization had been carried out for a typical brewery plant using Pinch Technology at pinch temperature of 100C. Energy recovery of 18% discovered for hot utility and 21% for the cold utility. Overall improvement in capital and annualized costs of 39% was attained for the hot and cold utilities. More so, the total cost for the proposed heat exchanger units $753,575.78 while the annualized cost is $5,579,121.9. This translates to substantial savings in capital and operation costs of the establishment in view, if the findings are implemented in their process plant.