Heat Transfer in Glass-Lined Double-Jacketed and Half-Coil Jacketed Reactors
Thanks to the outstanding characteristics of the Glasteel® composite material, glass-lined vessels are used frequently in the chemical and pharmaceutical industries. In nearly all cases for heat transfer in reactor vessels, the heat transfer between the glass-lined internal vessel side and the outer heating/cooling jacket is vital for ensuring optimal product quality and an economically efficient process. Heating, cooling and thermoregulation of products in the vessel require the supply or removal of reactive heat. For this reason, the design of such vessels includes either a double jacket or a half-coil jacket. This heat transfer of course requires a heating or cooling agent be supplied to the jacket. Thermal oils, which are thermo-regulated in a heating/cooling unit, depending on the respective process requirements, are suitable for precise temperature control. In practice, however, we also observe frequent use of steam for heating and water for cooling.
The amount of heat transferred through a glass-lined wall is generally determined by the available heat transfer area of the vessel, the heat transfer coefficient of the glass-lined steel wall and the median temperature difference between product and jacket. The total heat transfer coefficient (K-value) depends on the heat transfer coefficient between product and jacket, the wall thickness of the steel vessel, the thickness of the glass layer and the thermal conductivity of the steel and glass materials. Glass-lined reactors are special in that the thermal conductivity of the steel wall is 45 times higher than that of the glass layer. The K-value, and as a result the amount of heat transfer, are therefore largely limited by the glass layer. The use of properly designed mixing systems can optimize heat transfer on the product side. Realistically, the heat transfer coefficient can be improved by up to 30% just by designing the mixing system properly. With older reactors, the replacement of the mixing system represents a simple option to optimize heat transfer, reduce process times and save energy.
When designing glass-lined vessels, the question frequently arises whether to give preference to a vessel with half-coil jacket over a vessel with a double jacket for the benefit of efficient heat transfer. The following practical example highlights this question for the case of a large reactor with a volumetric capacity of 55 cubic meters, filled to 80% of capacity with 20% hydrochloric acid. The double-jacketed vessel comes with five DN80 jacket nozzles equipped with agitating nozzles for tangential introduction of the cooling liquid. To prevent excessive pressure drop in the half-coil, the jacket is split into three separate zones thus providing for shorter half pipe lengths. For this analysis of the cooling process we utilize cooling water in both vessel jackets. Based on a flow rate of 100 cubic meters of water per hour, the pressure drop in the half-coil jacketed vessel is at 1.1 bars compared to 0.3 bars in the double-jacketed vessel. Due to this increased pressure drop in the half coil jacketed vessel, the pressure head of the pumps must be significantly higher for the long cooling period. Hence, the pump energy demand during the lifecycle of the vessel is much higher. The unheated spaces between the half-coil pipes lead to a loss in heat transfer surface in the range of 30% compared to the double jacket. Next we analyze the heat transfer coefficient on the jacket side. As a result of the increased velocity, the heat transfer coefficient is more than twice as high in the half-coil jacketed vessel than in the double-jacketed vessel. However, the jacket heat transfer coefficient influences the overall reactor heat transfer coefficient (K-value) between product and heating/cooling agent by only approximately 12%, whereas the glass layer has an influence on the overall reactor heat transfer coefficient of nearly 50%. So, in our example the K-value in the half-coil jacketed vessel is 447 W/(m2 K) and in the double-jacketed vessel 416 W/(m2 K). The difference is so insignificant, that mathematically the cooling period between the two types of vessels differs by merely two minutes. When heating by means of steam at a temperature of 160°C the results are similar. The heat transfer coefficient in the half-coil jacketed vessel is 453 W/(m2 K), compared to 456 W/(m2 K) in the double-jacketed vessel. Mathematically, the heating time for both vessel types is identical, so practically there is no difference.
This example illustrates that the heat transfer advantages of the half-coil jacketed vessel in solid steel reactors are not applicable to glass-lined vessels, because basically it is the glass layer that determines the heat transfer rate. The choice to use a half-coil jacketed vessel is therefore dependent on other criteria, for example, when very high required pressure is required on the heating/cooling side or when it is necessary to separate the sections of the half-coil jacket with incompatible heating/cooling agents (two-way half-coil jacket). The double-jacketed vessel, in most cases, is the more economical and durable construction option, especially due to the use of agitating nozzles for heat transfer fluid supply. In addition to reduced manufacturing costs, the lower required pump head (due to decreased pressure drop) over years of operation will ensure energy savings, which decreases operating costs and increases profit. Also, after a certain operating period, half-coil jackets are prone to crack formations in the half-coil that may lead to leakages which are beyond repair in most cases. Therefore, the lifecycle of the half-coil jacket is frequently shorter than that of a double-jacketed vessel.
Pfaudler will support you in all topics related to heat transfer in glass-lined vessels. By using tried and tested calculation methods and computerized process simulations (CFD), we are able to make reliable predictions which provide high process certainty to the user as early as the project planning stage. Our core competency includes, among other things, the design of reactors and their mixers, which greatly influences the internal heat transfer within the vessel. In addition to the glass-lined reactors, Pfaudler also offers matching heating/cooling units (Thermal Control Units known as TCU’s). We also provide our heat transfer services and retrofits for existing reactors to improve their efficiency. In these instances, it is possible to take advantage of considerable improvement potentials. We will analyze your individual application and support you from the design phase to delivery and assembly of the needed retrofit components. We utilize over a century of worldwide experience in chemical and pharmaceutical processing systems to provide benefits to our customers.
Contact Pfaudler today to learn more.