PERFORMANCE ANALYSIS OF HEAT EXCHANGER AT PHOSPHATE ACID CONCENTRATED UNIT PT PETROCHEMICAL GRESIK

PT Petrokimia Gresik is the most complete fertilizer producing company and also produces fertilizer raw materials such as phosphoric acid. The process technology used in this company is Nissan C Process. One of the steps in this process is the phosphoric acid concentration unit. This unit consists of several steps such as evaporation, cooling, and fluorine scrubber. In the evaporation process, the most important equipment is the heat exchanger. As time goes by, it is necessary to evaluate the performance of this equipment. So, this case study aims to evaluate the performance of the heat exchanger, including the pressure of the heat exchanger the allowable pressure also to calculate the efficiency of the heat exchanger to determine whether the equipment is still suitable for use or not. Based on the calculation and analysis, obtained the pressure drop on the shell is 0.8817 psi, the pressure drop on the tube is 4.549 psi, and the efficiency of the equipment is 82.7%. So, it can be concluded that the heat exchanger used by PT Petrokimia Gresik is still suitable for use.


INTRODUCTION
PT Petrokimia Gresik is a fertilizer company in Indonesia, which produces various kinds of fertilizers such as urea, ZA, SP-36, NPK Kebomas, and organic fertilizers such as petrogenic. Besides that, it also produces various types of fertilizers, PT Petrokimia Gresik also produces raw materials for the manufacture of fertilizers, one of which is phosphoric acid. The process used in the manufacture of phosphoric acid is the Nissan C Process. The process consists of several stages, namely a grinding unit, a reaction, and hemihydrate filtration unit, a hydration and filtration unit conversion, a fluorine recovery unit, and a phosphoric acid concentration unit. These stages are very important to achieve the most optimal production.
To obtain P2O5 of 54%, the important process is the process concentration of phosphoric acid. In this process, there was an increase in P2O5 from ± 40% to concentrated phosphoric acid P2O5 of 54%. The stages in the process of concentrating this concentration are divided into 3, Evaporation Unit, Cooling Unit, and capture of fluorine gas (Fluorine Scrubber). In the evaporation process, a dilute phosphoric acid solution is introduced into a vacuum vessel to form concentrated phosphoric acid. Furthermore, the feed in the form of dilute H3PO4 from the filter will enter through the heat exchanger. In the heat exchanger, H3PO4 is heated using steam. Then the heated H3PO4 will enter the vaporizer. The bottom product that will come out of the vaporizer is concentrated H3PO4 (54%), while the top product which consists of water, a small amount of H3PO4, and fluorine in the gas are sent to a mist separator to get a product in the form of fluorine gas.
A heat exchanger (HE) is a device used to transfer heat from one system to another without mass transfer and also functions as a heater as well as a cooler [1]. Usually, the heating medium used is water which is heated as a hot fluid, and plain water as cooling water. Heat transfer occurs when there is a temperature difference between two substances, whether solid, liquid, or gas. Energy in the form of heat transfers by 3 kinds of mechanisms conduction, convection, and radiation [2]. Heat exchangers are broadly divided according to the direction of the fluid flow. Based on the direction of fluid flow, heat exchangers are divided into 3 types of flow: parallel flow, counterflow, and crossflow. Based on its use, the heat exchanger consists of a cooler, heat exchanger, reboiler, chiller, and condenser [3]. Heat exchangers are designed with the aim that heat transfer between fluids runs efficiently.
PT Petrokimia Gresik uses a shell and tube-type heat exchanger. Shell and tube heat exchangers are usually used more widely in industries where industrial production systems involve chemical processes, especially in the refining industry. The advantages of Shell and Tube Heat Exchangers are: (1) condensation or boiling heat transfer is easily accommodated, (2) pressure drop can be varied according to capacity, (3) thermal stress can be suppressed, (4) material selection can also be varied [4].
Heat exchangers are included in components or machines in general that have a useful life. The longer the heat exchanger is used, the more residue will accumulate and fouling will occur on the inside of the device. This layer of dirt can cause an increase in thermal resistance and cause the rate of heat transfer in the heat exchanger to decrease [5]. So that it will affect the performance of the heat exchanger and the productivity of the phosphoric acid concentration unit decreases. Therefore, it is necessary to analyze the heat exchanger at PT Petrokimia Gresik, especially the unit of concentration of phosphoric acid. Several previous studies have analyzed the performance of heat exchangers in various places such as in (1) PT Pupuk Sriwijaya Palembang, (2) PLTU Asam-asam, (3) Integrated Steel Mill (ISM) Krakatau, and (4) PT Semen Kupang [6], [7], [8]. In addition, the analysis of the heat exchanger has also been carried out at PT Petrokimia Gresik at the PHONSKA III Unit and Sulfuric Acid Unit [9], [10]. So there has never been an analysis of a heat exchanger in the Phosphoric Acid Concentration Unit.
At PT Petrokimia Gresik, especially the Phosphoric Acid Concentration Unit, also needs to evaluate the performance of the heat exchanger used daily to support production activities. So, based on the description of the problem, it is necessary to conduct this case study which aims to evaluate the performance of the heat exchanger including the pressure of the heat exchanger with the permissible pressure, and calculate the efficiency of the heat exchanger to determine whether the device is still suitable for use or not [11].

RESEARCH METHOD
The efficiency and pressure of the heat exchanger are calculated by conducting a case study by looking for equations from the literature study. After that, the field study of the concentration unit of PT Petrokimia Gresik to collect the data that will be needed. After obtaining these observational data, analyze the pressure and efficiency of the heat exchanger. From problem formulation to decision making, it is modeled in the form of a flow chart (Figure 1) which is expected to be able to explain the steps to be taken.

RESULTS AND DISCUSSION
Based on the results of field studies, to determine the feasibility and calculate the efficiency of the heat exchanger, the following data in Table 1 were obtained: In the data processing, analysis, and evaluation of the heat exchanger design, the results were obtained:

Heat Balance Analysis
In the heat balance calculation analysis, first analyze the LMTD (Log Mean Difference Temperature), which is used to determine the temperature driving force for heat transfer in flow systems, especially in heat exchangers. The LMTD is the logarithmic average of the temperature difference between the hot and cold streams at the end of the exchanger. The larger the LMTD, the more heat is transferred. The use of LMTD is obtained from the analysis of heat exchangers with constant flow rates and fluid thermal properties. The results of the LMTD calculation analysis on the heat exchanger can use in Equation 1.  The heat balance calculation analysis is intended to find the working heat of the tool [12]. In the heat balance, Equation 2 is used as shown [13]:

Shell Pressure Drop Analysis
Pressure drop is the maximum pressure drop that is allowed in the heat exchanger when a fluid passes through it [14]. The pressure drop will be greater with the increase in the fouling factor in the heat exchanger because it is used for too long. The pressure drop can be calculated using Equation

Pressure Drop Tube Analysis
The pressure drop on the tube side heat exchanger has been formulated [6].
Based on the results of the calculation of the pressure drop tube using this equation, a value of 4,591 psi is obtained. Meanwhile, previous research was conducted by Ali Imron (2018) at the Sulfuric Acid Unit of PT Petrokimia Gresik obtained a value of 0.2988 psi [9].

Analysis of Efficiency
The efficiency of the heat exchanger is calculated from the ratio of the actual heat transfer rate in the heat exchanger to the possible heat exchange rate [15].
Where: Q tube = Balance heat in the tube ( BTU/hr) Q shell = Balance heat in the shell (BTU/hr) Based on the results of calculations using this equation, the efficiency of the heat exchanger is 82.7%. Therefore, the heat exchanger is still feasible to use because the efficiency value is above 75%.
In previous studies, the efficiency of the heat exchanger was calculated by comparing the actual heat transfer rate with the maximum heat transfer rate. The results obtained by Jajat Sudrajat who conducted research at the Integrated Steel Mill (ISM) Krakatau were 37.4% in the first period and 33.7% in the final period [8]. While the research results of Irwin Bizzy and Setiadi who conducted research at PT Sriwijaya fertilizer obtained an efficiency of 79% [6].
Different from others, Yohanes Lebo et al researched the performance of heat exchangers at PT Semen Kupang to calculate efficiency by comparing Cmin/Cmax and the results are 28-29% [11].

CONCLUSION
Based on the results of the analysis and calculations, the pressure drop on the shell is 0.8817 psi and the pressure drop on the tube is 3.549 psi. While the allowable pressure for the shell is 1.45 psi and the allowable pressure for the tube is 7.25 psi. The efficiency of the heat measuring device is 82.7%. So it can be concluded that the heat exchanger used in PT Petrokimia Gresik Department III A Phosphoric Acid Concentration Unit is still feasible to use.