215- Reform plan for reducing load and costs through pre-desalination using secondary steam vane separation, along with resin desalination, in soda ash multi-effect evaporators – NOVEL

This plan is a technical solution for an upgrade project aimed at reducing the operating costs of the resin desalination process used prior to reusing condensate water in the multi-effect evaporators of soda ash plants, by employing a vane separation pre-desalination technique. Specifically, by adding pre-desalination resin reduction equipment, the number of times water is reused in the feather leaf separator is increased, thereby reducing the frequency of resin desalination and cutting operational costs. The implementation of this plan requires design and adjustment based on specific circumstances to ensure its effectiveness and reliability. -

In soda ash plants, multiple sets of multi-effect evaporators are required to concentrate and evaporate saline solutions, with heat energy coming primarily from boiler steam as well as the secondary steam from higher-stage evaporators (used for the secondary evaporators). The low-grade secondary steam generated by the lowest-stage evaporator is usually cooled using water cooling, air cooling, or a combination of both, to produce condensate water. The attached figure is a flow diagram of the multi-effect evaporation process in a soda ash plant for reference:

Soda ash manufacturers with large production scales typically have multiple production lines, and each line is equipped with evaporators that consist of three, four, or even five stages. The low-grade secondary steam generated by the final stage of evaporation operates at atmospheric pressure or even under negative pressure, and it contains numerous salt crystalline particles as well as droplets and foam of saline deposits; therefore, it is necessary to desalt this secondary steam. Since many soda ash plants built in the early days used traditional structures for their evaporators as well as conventional desalination technologies for secondary steam, the low-grade secondary steam generated by their final evaporators contained a larger amount of salt crystalline particles and saline scale droplets and mist, making it very difficult to desalt this secondary steam.

Here are examples of multi-effect evaporators used in soda ash plants built in earlier periods; excerpts from their images are shown below:

The upper part of this evaporator features a conical design, with the secondary steam outlet located at the apex of the cone, which tapers significantly. Below the secondary steam outlet, a conventional wire mesh demister element is used; the thickness of the wire mesh block is 150 mm, and spray washing pipes are installed below the wire mesh demister to wash the mesh. Due to the significant tapering at the upper part of the evaporator, the steam generated in the middle and lower sections of the evaporator also experiences a greater degree of flow contraction in this tapered area. As a result, most of the area surrounding the existing wire mesh demister is in a state of no flow or weak flow, while the central area is under high flow conditions, which leads to unstable operation of the system. Furthermore, the salting-out crystalline particulates and salt-containing scale droplets carried by the secondary steam accumulate rapidly on the surface of the wire mesh demister and spread to the middle and deeper layers, thereby deteriorating the desalination efficiency of the secondary steam.

The figure below shows another common evaporator structure for early soda ash plants:

This post was last edited by luoli519 on 2023-3-8 at 15:44. The upper part of this evaporator also features a conical design, with the secondary steam outlet located at the apex of the cone as well. Below the secondary steam outlet, a swirl plate demisting element introduced into the country around the 1990s–2000s is used; this swirl plate is not equipped with spray washing pipes for cleaning. The swirl plate demister has been in use abroad since the 1940s; it is primarily used for preliminary separation of air streams and not for the precise desalination and demisting of secondary steam in evaporators. However, once introduced into China, it was treated as a valuable asset, yet its dynamic separation mechanism was not properly understood, leading to its widespread use without proper consideration. The driving force for the operation of the swirl plate comes from the kinetic energy of the secondary steam and the static head, which drive the mixed flow to rotate in the same direction in order to achieve separation based on differences. Therefore, stable operating conditions are required, along with high kinetic energy of the secondary steam and high static head; otherwise, it is difficult to achieve even a basic level of separation. The operating pressure of the multi-effect evaporators in soda ash plants is often low pressure, atmospheric pressure, or mild vacuum; they do not possess the dynamic operating conditions of \"stable operating conditions,\" \"high kinetic energy of secondary steam,\" and \"high static head.\" Furthermore, the swirl plate is formed by the overlapping of outer swirling blades moving toward the center; the dense arrangement of blades in this central area leads to the rapid formation and accumulation of scale and particulates there, which affects its long-term continuous and stable operation. Therefore, when swirl plates are used in evaporators for desalinating saline solutions via secondary steam, a spray washing device needs to be installed.

The secondary steam released from the aforementioned final evaporator is directed to a condenser to produce condensed water, which is then sent to the resin desalination process to yield condensed water that meets the process requirements and is reused as boiler feedwater. Each set of final evaporators requires a condensation unit and a resin desalination unit.

Some units are equipped with secondary steam condensers, in which water is directly sprayed into the condenser to mix with the secondary steam and undergo condensation. Therefore, the source and quality of the matching water affect the composition of the secondary steam condensate. The attached figure shows the diagram of the secondary steam intercondenser in the final evaporator of a soda ash plant:

The owners stated that due to the poor desalination performance of the secondary steam from the final evaporators in their multiple production lines, some of the water used for mixing is also derived from condensate, and the salt content in the condensate produced by the condensers is above the acceptable level. This condensate with excessive salt content enters the resin desalination process, resulting in the fact that the water produced in a single pass also does not meet the standards, necessitating repeated recycling of the water. The resin desalination process is overburdened, leading to frequent resin regeneration, high consumption, and persistently high costs – creating a vicious cycle. The attached figure shows excerpts of the secondary steam condensate data from one of the final evaporators of this device: