Heating of chemical reactors with thermal oil boilers | Pirobloc
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Reactor químico Pirobloc

PROCESSES

Pirobloc provides efficient, safe and custom solutions for heating chemical reactors, employed in a wide range of industries such as:

Chemical

Cosmetics

Phamaceutical

Paint

HEAT TRANSFER IN CHEMICAL PROCESSES

The Chemical and Pharmaceutical sector includes the manufacture of chemical synthesis products such as pharmaceuticals (medicines) and oleochemicals (food, cosmetics), or those products related to the manufacture of polymers (rubber, glue, paint, etc.), as well as laminated films, foils and paper.

Heat transfer from one fluid to another is an essential component of all chemical processes. Either for heating components before setting off a reaction to obtain a final product, or for cooling a chemical substance after it has formed during an exothermic reaction.

It is essential to maximise the efficiency of the heat transfer systems for chemical processes, as the heat transfer step in many chemical processes consumes a lot of energy, so a lack of focus on energy efficiency may increase manufacturing costs unnecessarily.

Understanding how to design an effective and efficient heat transfer system is key to profitability in chemical manufacturing. This involves knowing the key variables in the production process, such as:

  • The physical characteristics and chemical composition of the fluids.
  • Process flow rates
  • System temperatures and pressures.
  • Permissible pressure drop.

The corrosiveness of some chemical products is another point to consider as the equipment will have to be built with corrosion-resistant alloys. This is especially relevant for devices such as the heat exchanger.

SYSTEM PERFORMANCE

Although the thermal fluid system can be used to provide heat to numerous stages in the production process, it is in the distillation stage where it is particularly important to ensure that optimum product quality results are achieved.

During the distillation stage the temperature is used to separate the many elements of a prepared mixture. In this regard, the thermal fluid boiler provides improved control, resulting in more precise process temperature management, as well as heating and cooling using a single fluid.

Both batch and continuous systems are common in this sector. Batch, or discontinuous operations may involve large variations in heat load, due to the high initial demand for maintaining the end-of-batch temperature.

Each consumer has a three-way control valve that enables highly accurate automatic temperature control. By simply selecting the set-point temperature on the operator panel, the system automatically and proportionally regulates the flow rate of thermal oil entering the consumer to achieve the desired temperature.

The valve is fitted with an electric or pneumatic servomotor, which may regulates the position of the valve plug, allowing proportional flow to the straight way or return path. In this way the valve inlet path is completely open at all times, while the straight and return paths are inversely proportional. When the straight way is open at 70%, the return path is open at 30%, the same percentage that returns directly to the boiler. With this simple system, temperature regulation accuracies of +/- 1ºC or better are achieved.

In circuits with several consumers at different temperatures, a secondary unit is also provided by a recirculation pump. With this device it is possible to work within the whole range of temperatures, no matter how different they may be, between the different consumers of the installation.

As in all thermal oil circuits, there is an expansion tank that collects and absorbs the expansion of the thermal fluid without pressurising it. This tank is usually connected to the collection tank. A reversible pump associated with the collection tank allows both filling and emptying of the circuit.

The schematic diagram that is usually followed by this type of application is as illustrated below.

Quimica Pirobloc
Quimica Pirobloc

THERMAL OIL BOILERS

As mentioned above, thermal boilers are used in the different chemical sectors mainly for the heating of reactors, which is carried out by means of “half-round” enclosures, double heating bottoms, jackets or coils ranged inside the reactor.

Reactors are not exclusive to the chemical industry, as they are also used in the food and other sectors, but it is here where we find a wide range of applications: chemicals, fine chemicals, cosmetics, pharmacy, paints, etc.

These applications sometimes concur with the need to heat other consumers such as heat exchangers, distillation columns, evaporators, tanks, etc. The heating of each consumer can be controlled individually by valves that control the flow of thermal oil through the equipment.

Thermal fluid boilers provide key advantages in the pharmaceutical industry due to their ability to provide temperatures in the range of -100°C to +400°C, allowing both heating and cooling operations to be carried out. This versatility is especially relevant in the case of exothermic reactions or for multi-step control of temperatures throughout the production process, during which time the system precisely controls any temperature variation in the reactor.

Another advantage of thermal oil boilers is that they can operate at high temperatures while maintaining low pressures.

Pirobloc thermal oil boilers operate by means of a burner that heats, by radiation, the thermal fluid flowing through the coil, as well as the heating devices in other appliances. Several units can be heated with a single boiler by connecting different thermal oil branches for each consumer. The thermal fluid flow rate and pressure drop are adjusted to the needs of the individual consumer and can therefore be configured to meet a flow rate that matches the process requirements.

In terms of maintenance and operation, thermal fluid boilers are easier to manage than other systems, as the risks of corrosion and scaling can be prevented with simple maintenance.

HEAT EXCHANGERS

Shell and tubes

Shell and tube heat exchangers are the most widely used type of heat transfer equipment in the chemical processing industry due to their flexibility in design and their ability to handle fluids with different temperature and viscosity levels. They consist of two parts: the shell and the tubes.

All tube side components must be corrosion resistant and compatible with one or both fluids.

A common design in the chemical processing industry consists of a single-pass exchanger, which means that the fluids only pass through the exchanger once. There are multiple designs of exchangers. The TEMA BEU double head design is one of the most widely used because of its ease of cleaning. This exchanger has a bottom and a fixed tube plate at each end.

The number and length of the tubes, as well as the diameter of the shell is determined by the heat transfer requirement. (In some cases, it may be necessary to restrict the size to fit into a defined space in a chemical plant). The actual design and size can be determined by the end user (chemical company) or recommended by the manufacturer after an analysis of the overall system requirements.

Plates and frames

Plate and frame heat exchangers are used in certain areas of the chemical processing industry as an alternative to shell and tube heat exchangers. Plate and frame designs require less space than a shell and tube exchanger made for similar heat transfer, but also have some limitations with respect to the process fluids and conditions under which they can be used.

Plate and frame heat exchangers are mainly available with stainless steel or titanium plates. The heat transfer is determined by the overall dimensions of the plates as well as the number of plates in the heat exchanger.

Plate and frame exchangers are composed of two end plates, which are designed to hold the plates together, and a series of heat transfer plates between the end plates. Gaskets are required to separate the two fluids passing through the system at each of the inner plates.

The use of plate and frame heat exchangers is limited by the temperature and pressure the gaskets can withstand; typically, this is less than 365°F (185°C) or 360 psi of pressure. These systems are also limited to fluids that have few or no solids because the internal plate channels are narrow and can be easily plugged.

The advantages of plate and frame exchangers include the ability for an experienced equipment manufacturer to add internal plates at any time to increase heat transfer and the overall ease of cleaning the exchanger.

MATERIALS

In the chemical processing industry, many processes require equipment made of highly corrosion resistant materials.

The construction materials ensure suitability for process fluids.

Corrosion resistant alloys commonly used in the chemical processing industry include austenitic stainless steels (300 series), duplex stainless steels, nickel alloys, titanium, zirconium, and tantalum. Each of these metals and alloys has corrosion resistance to certain chemicals and can be used in chemical plants for a prolonged service life.

The construction materials used are usually determined by the chemical company, often after consultation with a metallurgist working for the manufacturer, or a metal supplier. Knowledge of corrosion and understanding of alloys and availability are critical in determining which alloy to use. The overall objective is to ensure a cost-effective system with a long service life.

An essential consideration when determining which alloy to use is to ensure that all aspects of the fluid chemistry are known. Often, as companies change processes, the corrosiveness of fluids changes and alloys that were used in the past may not be as effective today. Switching to a new metal or alloy may be the most cost-effective solution and could extend the life of the equipment.  This is where a metallurgist with sound experience in working with corrosion resistant materials is needed to determine which metal or alloy would be optimal.



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