[Q&A] Catalytic Coking Reformate Blending Version 20170901

1) Increase the heat transfer area A 2) Increase the average temperature difference 3) Increase the heat transfer coefficient K

1. Increase the heat transfer area A； 2. Increase the average temperature difference ； 3. Increase the heat transfer coefficient K.

(1) Increase the heat exchange area (2) Increase the temperature difference (3) Improve the heat transfer coefficient

Increase the heat transfer area, increase the temperature difference for heat transfer, increase the overall heat transfer coefficient

1. Increase the heat exchange area by 2. Increase the heat exchange temperature difference by 3. Increase fluid flow rate by 4. Improve the heat transfer medium

There are three main ways to enhance the heat transfer process, but which one is more effective requires a specific analysis. 1. Increase the heat transfer area A. 2. Increase the average temperature difference. 3. Increase the heat transfer coefficient K

Measures to enhance heat transfer should be considered from these three aspects. I. Increase the heat transfer area A. The heat transfer rate is proportional to the heat transfer area, and increasing the heat transfer area can enhance heat transfer. It should be noted that heat transfer can only be enhanced by increasing the heat transfer area per unit volume of the heat exchanger. This can only be achieved by improving the structure of the heat transfer surface. For example, using small-diameter tubes, or replacing smooth tubes with finned tubes or threaded tubes, can increase the heat transfer area per unit volume of the heat exchanger. After switching from φ25 tubes to φ19 tubes in China’s floating-head heat exchanger series, the heat transfer area can increase by 42% for shell diameters D=500–900 mm, while the metal consumption per unit of heat transfer area can be reduced by 21–31%. Some new types of heat exchangers, such as plate-type and fin-type ones, have achieved good results in increasing the heat transfer area. The heat transfer area per cubic meter in a tubular heat exchanger ranges from 40 to 160 m2, while in a plate heat exchanger, the heat transfer area that can be accommodated per cubic meter is between 250 and 1500 m2. Plate-fin heat exchangers have an even higher value, typically reaching 2500 m2, with some achieving over 4350 m2. II. Increase the heat transfer temperature difference Δtm. Increasing the heat transfer temperature difference is one of the methods to enhance heat transfer. The heat transfer temperature difference is primarily determined by the temperatures of the material and the heat carrier. The temperature of the material is set by the production process and cannot be changed arbitrarily, while the temperature of the heat carrier depends on the type of heat carrier chosen. There are many types of heat carriers with varying temperature ranges, but when selecting one, technical feasibility and economic Rationality must be taken into account. For example, water vapor is a commonly used heating agent in industry. As mentioned earlier, water vapor has many advantages, but when used as a heating agent its temperature generally does not exceed 180°C. When the steam temperature reaches 200°C, an additional atmosphere of pressure is required for every 2.5°C increase in temperature; at 250°C, an additional atmosphere of pressure is needed for every 1.3°C increase in temperature. Using high-pressure steam results in larger equipment, higher technical requirements, lower economic efficiency, and reduced safety. Therefore, when the heating temperature exceeds 200°C, it is necessary to consider using other heating agents, such as mineral oil, biphenyl mixtures, or even molten salts and liquid metals. Since the choice of heat carrier is restricted by certain conditions, the range of temperature variation is limited. If both the material and the heat carrier experience temperature changes, counter-current operation can be employed, which allows for a larger temperature difference for heat transfer. III. Increasing the overall heat transfer coefficient K – To enhance heat transfer, it is essential to increase the value of K. As can be seen from its calculation formula, raising K requires reducing each individual thermal resistance, and efforts should be focused on the areas where the thermal resistance is highest. If the internal and external scale layers constitute the main thermal resistance, measures should be taken to prevent or reduce the formation of these scales, or regular cleaning and other such actions should be employed. If the convective heat transfer coefficients on both sides differ significantly, emphasis should be placed on improving the smaller α value. If the heat transfer coefficients on both sides are relatively low, efforts should be made to increase both α values simultaneously. For fluids that do not undergo phase changes during heat transfer, increasing the flow velocity and altering the flow conditions can both enhance the turbulence level of the fluid, thereby improving the convective heat transfer coefficient. For example, increasing the number of tubes in the tube side of a shell-and-tube heat exchanger and the number of baffles in the shell, using finned tube heat exchangers, and creating various grooves in plate heat exchangers ； But at the same time, the impact on flow resistance as well as cleaning and maintenance should be taken into account. Furthermore, the use of fluids with a high thermal conductivity, as well as heat carriers that undergo phase changes during heat transfer, can both result in a higher α value. In summary, there are various ways to enhance heat transfer, and in practice, economic and reasonable measures should be adopted based on the actual conditions of production. Furthermore, in certain special situations, it is the opposite of enhancing heat transfer – it involves reducing heat transfer. Also known as heat retention or thermal insulation. For example, steam delivery pipes need to have insulation material wrapped around their outer walls, and thermos flasks require various insulation measures. Under normal circumstances, most measures to reduce heat transfer involve increasing the thermal resistance, by using materials with very low λ values as insulation layers, such as asbestos, cork, polyurethane, and similar materials.

Increase the contact area, use counter-current contact, and increase the contact time