Shell and tube heat exchanger design for production graphene oxide nanoparticles from agricultural waste

Heat exchangers are used to transfer heat energy, using fluids or gases, both hot and cold, from one area to another due to temperature differences. Almost all agricultural wastes serve as carbon products which are essential to produce graphene oxide, it is very possible to get graphene oxide by synthesizing it from agricultural waste which is expected to reduce the amount of waste on the earth. After carrying out the synthesis, it is necessary to test the characterization of the resulting product. The characterization stage requires a sintering process (20 o C to 98 o C) until the product is completely solidified. This research aims to design a heat exchanger that is highly effective in assisting the sintering process. For the design to be well-directed, several things must be done, such as calculating the main components (shell and tube) and designing a heat exchanger. Based on the TEMA standard and the use of simple calculations that refer to the attached calculation formula using Ms. Excel, hot and cold fluids used in this heat exchanger design are ethylene glycol and water. The length of the shell and tube is 5 m, the inner and outer shell diameters are 254 mm and 279.4 mm; the inner and outer tube diameters are respectively 22.09 mm and 25.40 mm. The calculation results show an excellent design effectiveness value of 86.14%. There is a comparison with previous studies using different fluids, when compared to this study it gives a higher effectiveness value.


Introduction
The heat exchanger changes the temperature and phase of a fluid type; this process utilizes the heat transfer of high-temperature fluids to low-temperature. In the design process of the shell and tube, the heat exchanger functions to find out what factors influence the size of the design itself. The shell and tube form is considered to have many advantages in terms of fabrication, cost, and performance [1]. Shell and tube heat exchangers (STHEs) are the most widely used heat exchangers in the industry due to their relatively simple manufacture [2], [3]. A heat pipe (HP) is a simple device for transferring large amounts of heat from a heat source to a cold environment through an enclosed space; it is efficient and does not consume much external energy [4], [5]. This pipe has the basic working principle of a continuous heating process and a condensation process for the liquid filled in it; therefore, the fluid's thermophysical properties significantly influence the pipe's performance [4], [6].
Nanofluids consist of a base fluid and particles or sheets of nanometer dimensions. It is believed that nanoparticles can form Van der Waals forces whose name has a high tendency to form a fouling layer resulting in thermal resistance for a long time with high loads [7], [8]. Nanofluid stabilization requires different physical and chemical treatments [4], [5], [7], [9].
Graphene is a two-dimensional nanomaterial that has heat-limiting and heat-locking mechanisms that differ significantly from one-dimensional zero-dimensional nanoparticles [4], [10], [11]. There is still a need for studies on the effect of GO on PHP performance; therefore, in this study, GO nanofluid is used as a working fluid in PHP. In previous studies, the synthesis of graphene oxide from agricultural waste has been carried out (such as orange peels, bagasse, and rice husks) [12]. Almost all agricultural wastes serve as carbon products which are essential to produce graphene oxide. The topic taken in this study is the design of a heat exchanger for synthesizing graphene oxide from agricultural waste.

Preparation of agro-wastes powder
The agro-waste used is the part that has a lot of fiber. First, the waste is washed clean using tap water, then rinsed using distilled water. The waste is dried and ground to a fine powder. This preparation acts as a carbon source for graphene oxide synthesis [13].

Synthesis/fabrication of graphene oxide (GO)
GO synthesis was carried out using the Hummers method. Mixing was carried out between agro-waste powder, 95 -97% sulfuric acid solution, and 30% sodium nitrate powder until it became homogeneous with a ratio of 9:1 (v/v) using mechanical stirrer. Potassium permanganate is added slowly, then stirred until an oxidation reaction occurs (20 o C). The mixing results were diluted with distilled water, hydrogen peroxide was added, and (98 o C). Then wait for it to separate between the solid and liquid phases. The liquid phase was discarded and take the solid paste phase. For characterization, the paste obtained was dried by the sintering process using 150 o C (GO150) [14], [15], Then as shown in Figure 1.

Mathematical models for HE design
Ethylene glycol and water are used as hot and cold fluids. The data assumptions used to design the shell and tube heat exchanger are listed in table 1. Data collection refers to the standard Tubular Exchanger Manufacturers Association (TEMA), which is used as a design reference and uses Ms. Excel for analysis of simple thermal calculations as shown in the equation. 1-19 [16].
The calculation equation is divided into several parts, namely: Basic parameters (Q; LMTD; A; N; and shell diameter), Tube parameters (at; Gt; Ret; Prt; Nut; and hi), Shell parameters (as; Gs; de; Res; Prs; Nus; and ho), shell and tube parameter (Uact), Heat rate parameters (Ch and Cc), Effectiveness parameters (ε; NTU; Qmax; and Rf). Not all calculations need to be done, in the TEMA standard book there are several parameter values assuming the desired size variation.
The energy transferred (Q) was measured by using equation 1.
where, Q = Energy transferred (W) m = Mass flow rate fluid (kg/s) The Logarithmic Mean Temperature Difference (LMTD) was measured by using equation 2. The heat transfer field area (A) was measured by using equation 3.
where, A = Heat transfet field area (m 2 ) The number of tube (N) was measured by using equation 4. The shell diameter (Ds) was measured by using equation 5.
where, K = Thermal conductivity of the tube material (W/m o C) The Nusselt number in tube (Nut) was measured by using equation 10. Similar formula is used in shell calculations (Res) The heat transfer coefficient in tube (hi) was measured by using equation 11.
The equivalent diameter in shell (de) was measured by using equation 12.
The convection heat transfer coefficient in shell (ho) was measured by using equation 13.
The overall heat transfer coefficient (Uact) was measured by using equation 14.
where, ∆ = Wall thickness (m) The hot fluid rate (Ch) was measured by using equation 15. A similar formula is used for cold fluid rate and the result used as Cmin.
where, Cph = Specific heat capacity (J/kgK) Ch = Hot fluid rate (W/K) The heat exchanger effectiveness ( ) was measured by using equation 16.
where, Qact = Actual energy transferred The number of heat transfer units (NTU) was measured by using equation 17.
The energy transferred maximal (Qmax) was measured by using equation 18.
The convection heat transfer coefficient in shell (Rf) was measured by using equation 19.
where, Uact = Actual overall heat transfer coefficient (W/m 2 o C)

Results and discussion
The law of the conservation of energy states that energy cannot be destroyed. Energy changes form from one form to another. Energy/heat can move in three ways: radiation, conduction, and flow. In industry, many production processes involve heat exchange, such as boilers, superheaters, oil coolers, condensers, Etc. Heat exchangers are prioritized to exchange the energy of two fluids with two different temperatures. Energy exchange can occur through a field separating the two fluids or by direct contact (fluid mixing). Exchanged energy/heat will cause a change in fluid temperature (sensible heat) or sometimes a phase change (latent heat). The transfer rate is affected by: fluid flow velocity, fluid temperature difference, surface properties of the plane separating the two fluids, and physical properties (viscosity, thermal conductivity, specific case capacity, Etc.) [1].
The standard used in heat exchangers is TEMA (Tubular Exchanger Manufacturers Association) from America. TEMA discusses the types of heat exchangers, performance calculation methods and design strengths, the term heat exchanger parts (parts), and the basis for application ownership in industrial processes. This standard is used as a reference to minimize negative impacts on users, such as the danger of damage, operational failure, security, and estimated costs used. Based on TEMA, the types of heat exchangers are divided into two groups: Class R, for heavy work applications such as heavy oil and chemical industries, and Class C, for general use with small size and heat transfer capacity. This study's shell and tube heat exchanger design chosen is shown in Figure 2. This tool is used for the evaporation or boiling process of fluids outside the pipe; this type is also often called the boiler type [18].  The shell is one of the two most essential components in heat exchanger design, located on the outermost covering of the tube. In this design, the shell flows water fluid. The shell material used is ASTM A106 Carbon Steel Pipe Seamless with the advantages of thick walls, making it resistant to high temperatures; no welded joints, which means it is more resistant to corrosion and has an excellent cylindrical shape. The tube is the part that flows the ethylene glycol fluid. The tube is located on the inside of the shell. The selected tube material is Stainless SS304 with the advantages of easy to form (weldable), rust resistant, good quality for deep drawing use, able to absorb and conduct heat well, has a long-term value greater than other metals, and able to withstand extreme temperature variations and high durability. The value specifications used in the shell and tube design are listed in table 2. Also, in table 2, the fluid values' specifications are written. The author tries to illustrate a miniature framework of a small heat exchanger designed on a small scale, shown in figure 3.
(a) (b) (c) Figure 3. Miniature illustration of heat exchanger design on a simple scale The heat exchanger design in this study uses a single tube pass (shell and tube) type. Placement of the tube in the shell uses a square position (45 o ) so that the baffle type is single-segmental. The production of graphene nanoparticles requires heating from room temperature 20 o C to 98 o C. The use of ethylene glycol and water provides the possibility to increase heating effectiveness both in terms of energy and time. In Nandiyanto's research (2022) [16], the design of the heat exchanger used to produce carbon particles with the size specifications listed in table 3, this research can be compared because it has almost the same goal, namely to produce carbon products, this research produces a value The effectiveness is not good because the fluid used has characteristics that are not quite right when used in a heat exchanger with a temperature difference that is not that far. A small effectiveness value is also produced because the Q value used with the heat exchange output is not very profitable resulting in a low effectiveness value. Whereas for this study using ethylene glycol and water fluids in the use of heat exchangers so that in the calculations it produces a Q value that is quite profitable to change the temperature according to design. Values and size specifications refer to the standards used (TEMA). Processing is carried out using calculations 1-19 with Ms. Excel's assistance, and the results are obtained in table 2. After going through all the costs, the practical value of the heat exchanger was 86.145%. This figure is relatively high in world industry, and it can be assumed that the design is optimal enough to support the macro-scale graphene oxide production process.

Conclusions
• The type of heat exchanger is shell and tube.
• The material suggested in this research for tube making is Stainless SS304, and the material for making the shell is ASTM A106 Seamless Carbon Steel Pipe. • The cold fluid used is water, and the hot fluid used is ethylene glycol. Sintering 150 o C graphene oxide nanoparticle

Declaration of competing interest
There are no financial or non-financial interests and competition in any material discussed in this paper.

Funding information
No funding was received from any financial organization to conduct this research.