May I ask, what is the difference between semi-open and closed types of glass-lined reaction vessels? How to tell the difference?

An open-type reactor is one in which the reactor vessel can be divided into a vessel body and a head (or cover), with a high-neck flange of the same diameter as the vessel body in between. A closed-type reactor, as the name implies, is relatively enclosed; it lacks a high-neck flange in the middle, and instead has only a large lid on the top of the reactor. The diameter of this lid is much smaller than that of the reactor itself, and its purpose is to allow for the installation and removal of the stirrer. Under normal circumstances, there is not much difference between open and closed types ； But for equipment where pressure and leakage rates are critical, closed systems are used, as they facilitate sealing and can handle high pressures ； For large-scale equipment, a closed design is used; large-diameter flanges are costly and require high precision in processing. Closed-type equipment requires high standards in manufacturing and incurs high maintenance costs; it is not as flexible as open-type equipment. The selection between open and closed type reaction vessels is based on the following requirements: 1. For open-type reaction vessels, the main consideration is the need for maintenance of the internal components, such as agitators and internal coils ； Second is the cleaning of the materials; when the materials aggregate or solidify severely, extensive cleaning is required, which is why an open-type reactor should be used. 2. If the above conditions are not required, then a closed-type reactor is certainly the better choice. The advantages of a closed-type reactor include good sealing performance, ease of manufacturing by producers, and low production costs. In a closed-type counter-current reactor, if there are components inside the reactor such as agitators or built-in cooling pipes, these should be designed to be removable and accessible through manholes. Basically, reactors with a capacity of 500L or less are all of the closed-type variety, as it is not possible to install manholes on their heads; manholes are designed for allowing people to enter in order to clean or repair the equipment, and they are also used for adding materials. Therefore, a welded flange method is employed, which means an open-type design. For capacities above 500L, closed-type reactors are generally used, as it is possible to install manholes on their heads. Of course, whether an open-type or closed-type design is used depends on the customer’s requirements.

The author says in the last paragraph that systems below 500L are basically closed-type, but that’s backwards; they should be open-type.