Steam tube dryers are used to remove both molecularly bound and free moisture from solid industrial products such as soda ash. Soda ash is received as a wet monohydrate cake and must be dried to an anhydrous dry powder. This is an important step in manufacture, since if the product is incompletely dried it will not meet specifications and it will lump or cake, making transfer difficult or even impossible.
A steam tube dryer passes steam into a plurality of tubes in the dryer containing the solid material to be dried. The steam never contacts the product directly; instead the steam heats the tubes and the product contacts the tubes. The steam heats the tubes to a high enough temperature to remove water from the solids. The capacity of the dryer is limited by the temperature difference across the tube walls, the area of the tubes, and the heat transfer coefficient. The heat transfer coefficient is strongly related to the shape of the condensing water film on the steam side of the tubes, and the depth of condensate within the tubes. Condensate within the tubes effectively reduces the amount of tube surface area available for heat transfer. In order to sweep the condensate out of the tubes, excess steam is fed to the dryer to increase condensate removal. This steam is termed excess steam because it is not condensed (its heat value is not used), it simply sweeps water out of the tubes.
Thus steam is fed under high pressure to the dryers through a plurality of tubes. Excess steam is used to increase the capacity of the dryer. Moisture is evaporated from the product and vented to the atmosphere. Excess steam is also vented to the atmosphere. The excess steam results in increased costs of the system due to the high cost of generating steam. Steam traps have been used in the past to recover the condensate leaving the dryer. Steam traps allow water to pass but prohibit the passage of steam. Since the purpose of the excess steam is to sweep through the dryer without condensing, the steam traps had to be bypassed to maintain production rates. The excess steam is now at a lower pressure and temperature than the feed steam to the dryer and it cannot be re-used. Excess steam is at a pressure about 5 psig lower than the incoming fresh steam.
A typical prior art dryer system 10 is shown in FIG. 1. A wet cake product is transferred to a steam tube dryer 12 through a line 13. Steam is passed at a rate of about 30,000 lb/hr via a pipe 14 at 400 psig (600.sup.P F) to the steam tube dryer 12. The product moisture is drawn from the dryer 12 via a pipe 15. A steam trap 16 is connected to the dryer 12 and draws off the condensate through a pipe 17. The excess steam is vented to the atmosphere through a vent 18. For each such steam tube dryer 12 in a drying system, about 3,200 lb/hr of steam is lost through the excess steam vent 18. The dried product is removed from the dryer 12 via a pipe 19.
Complete moisture removal from a wet product is essential to meet specifications and prevent handling problems. Thus, a large flow of high pressure, high temperature steam is required to feed these tube dryers. It might be thought that steam costs can be reduced by reducing the amount of excess steam used during drying by reducing the steam flow rate and thus use more of the latent heat in the steam, producing less excess steam and more condensate. However, we have found that this reduces the production drying rate and bottlenecks plant capacity. If no excess steam is used, we have found that it ultimately becomes impossible to produce dry product. An excess of about 15% of steam is required for efficient moisture removal from the product.
What would be highly desirable is to be able to increase the capacity of the steam tube dryers and simultaneously eliminate the loss of the excess steam without increasing energy costs.