Comparison of Solvay process with Others Methods of Production

As mentioned above in the introduction, Leblanc process was the industrial process for the production of soda ash used throughout the 19th century. It involved two stages: Production of sodium sulfate from sodium chloride, followed by reaction of the sodium sulfate with coal and calcium carbonate to produce sodium carbonate. The Leblanc process was a batch process in which sodium chloride was subjected to a series of treatments, eventually producing sodium carbonate.It is also noteworthy that in addition to valuable alkalis, the Leblanc process produced two waste products, hydrogen chloride and calcium sulfide. Acidic hydrogen chloride gas was sent up the chimney, after which it decimated vegetation in the vicinity of an alkali works. Insoluble calcium sulfide was conveniently disposed of in heaps where the vegetation used to be. Unfortunately, when calcium sulfide reacts with rain water it farts out noxious hydrogen sulfide.

Hence alkali manufacturers based on Leblanc process became popular targets for lawsuits and government regulations. The British Alkali Act of 1863, for example, required the absorption of 95% of the hydrogen chloride produced by the salt cake furnace. This was easily accomplished, hydrogen chloride being quite soluble in water; the waste gas was sent up through a stone tower filled with coke; water dribbling down through the tower absorbed the hydrogen chloride, producing aqueous hydrochloric acid.

In addition to hydrogen chloride which can be presently considered as valuable product own its own account and after Henry Deacon in1868 introduced a process for turning waste hydrogen chloride into bleaching powder, which could be utilized in paper and textiles industry, calcium sulfide produced is a problem for soda manufacturers using the Leblanc process. This (calcium sulfide produced) became a persistent problem owing to the twin problems of stinking heaps of tank waste, and the loss of valuable sulfur. It is noteworthy at this point that pure source of Sulfur is not present in Sri Lanka. An alternative to sulfur is sulfur dioxide which was obtained during the latter stages of the soda industry from roasted from pyrites, which alternatively would have to be imported to Sri Lanka. Also whatever the source may be, it will eventually end up as calcium sulfide waste. The only positive solution for the disposal of calcium sulfide lies in the fact that it could be converted into sodium thiosulfate, used by photographers to fix photographs but this cannot be considered as a feasible option. Also at present there is a method of recovering sulfur from tank waste owing to the discoveries of Alexander Chance in 1887. But these slight improvements would adversely affect the efficiency of the overall process.

While the construction of a Solvay plant would be more expensive than a comparable Leblanc plant, it would require fewer raw materials; thus capital investment is higher but operating costs will be less. With lower operating costs, the Solvay process will be able to drop the price of soda produced. It can be said that the Solvay process is cleaner and more efficient where else Leblanc process is crippled by pollution and waste. Also the Solvay process has a distinct advantage over its Leblanc counterpart because of the continuous developments being carried on it, in other parts of the world where else soda production from Leblanc process is one of the diminishing technologies. It is evident from the fact that world production of soda in 1863 had been 150,000 tons, all produced in Leblanc plants. By 1902 world production of soda was at 1,760,000 tons, where over 90% of the amount was be produced using Solvay plants.

A very recent development in the soda ash industry has enabled the co production of vinyl chloride monomer (for production of Poly Vinyl Chloride) and soda ash. Akzo Zoul Chemie Nederland has succeeded in producing (on a small scale) vinyl chloride with soda ash as a co-product. The process uses medium pressure steam and carbon-dioxide instead of more expensive electrical energy (for the electrolytic production of production of chlorine). The overall energy consumption of the new process is about one half that of the conventional method when caustic soda is the co-product. Here soda ash is produced from the reaction of salt with a concentrated aqueous trimethylammine- carbon doxide solution obtained downstream in the vinyl chloride process.

Even though the advantages of the Akzo Zoul Chemie process are attractive, as evident from the above process description soda ash would be a by-product in this process. This would mean that the amount of soda- ash produced would be dependent on the demand for vinyl chloride monomer. That would imply that the soda ash output would be governed by the demand for PVC in the market. Also the capital cost that needs to be incurred for a plant involved with the cogeneration of both soda ash and PVC would be relatively higher than that for a plant involved with only the production of soda ash only. Also this is a relatively new technology which is still being developed and the rights for this plant are restricted by the patent company. Therefore additional administrative charges and royalty from the revenue needs to be bared.

The Dual purpose plant came into being because of the concerns associated with the discharge of solid waste. The dual purpose is a modification of the existing Solvay process. It was developed by a Chinese chemist Hou Debang in 1930s. It is the same as the Solvay process in the first few steps. But, instead of treating the remaining solution with lime, carbon dioxide and ammonia is pumped into the solution, and sodium chloride is added until it is saturated at 40 °C. Then the solution is cooled down to 10 °C. Ammonium chloride precipitates and is removed by filtration, the solution is recycled

to produce more sodium bicarbonate. Hou's Process eliminates the production of calcium chloride and the byproduct ammonium chloride can be used as a fertilizer. The constraints of Dual process are facts that it has to depend on the availability of carbon dioxide gas and ammonia from fertilizer unit and the production of soda ash will be restricted depending on the demand for ammonium chloride in the market.

The New Asahi process in a relatively new modification of the Dual purpose process. This process came into being owing to the developments in the Japanese soda ash industry. The Japanese soda ash industry was based on high cost imported salt. Therefore this new NA process was developed which is essentially dual purpose process on a large scale. Many new modifications were made to incorporate a more efficient and less raw material utilizing process. These modifications include changes in the Carbonation section, cooling system used for cooling the mother liquor from centrifuges, adoption of Ammonium chloride distillation unit, system of raw salt crushing, etc. It also bears the same constraints of the conventional dual purpose process. In addition the initial investment is very high for New Asahi (NA) process. This is because the New Asahi process requires more investment to be made in the lime burning/lime slaking, ammonia recovery and ammonium chloride crystallization sections.

It can be said that the effluent generation from Solvay plants is more compared to other process plants. Owing to the requirement of placing Solvay plants with proximity to the sea, the effluent from process are discharged into sea after some minor treatment which does not affect the ecological balance because the composition of effluent compares favorably with sea-body composition. Generally, a costal soda ash plant should go for the Solvay process whereas inland plant should opt for Dual process near a fertilizer unit.

As discussed above the most likely option for the soda ash production plant is the conventional Solvay process and the Dual purpose method. A comparison of the two processes is given below.

Solvay Process Dual Purpose Process Uses Brine, limestone and coke as raw

materials Uses brine, ammonia, limestone and coke. Uses small amounts of ammonia and carbon

dioxide as makeup gases, while the bulk of the gases are recycled

Uses small amounts of and carbon dioxide as makeup gases, while the bulk of the gase is recycled

Soda ash is obtained as product Soda ash and Ammonium chloride(fertilizer) is obtained as co-products

Calcium chloride is obtained as a by product/waste. Therefore has a waste disposal problem

No effluent disposal problem

Recovers most of the valuable ammonia

Ammonia has to be imported as a raw material if the process is stand alone plant (Not adjacent to a fertilizer production facility)

Production is not constrained

Depend on availability of carbon dioxide and ammonia from a fertilizer unit. But presently in Sri Lanka there is no such production facility

Not a completely closed process Completely close process when fertilizer unit is also present

The production is solely dependent on the soda ash demand in the market

Production of soda ash is restricted by the demand for ammonium chloride

Efficiency is comparatively low Yield is slightly higher

Investment and maintenance costs are low Investment and maintenance costs are comparatively high

Since the raw materials are present at the same location at Puttalam the plant can be easily sited there

Many factors influence the placement of the plant. For example fertilizers produced is in demand at the agricultural zones near the Mahaweli, saturated brine is obtainable from salterns, imported ammonia can be stored at a low cost near a port, etc.

The technology of the Solvay process has undergone several developments and is at a mature state

This is relatively a new technology used mainly in China and India

Table 4.1 a comparison of the Solvey and dual processes

4. 3 Process Selection Conclusions

The conventional solvay process appears to be preferred for a plant producing 50 tonne/day of light soda ash. The capital cost advantage of this process and the ability function independently surpasses the benefits of the high efficiency dual purpose process. Furthermore, the higher temperature (because of the availability of a kiln) and the favorable effect of pressure enable a greater recovery of energy from the process. This choice, made on both economic and operational grounds, can be said as being consistent when we consider the production capacity of soda ash, in the world throughout, from Solvay process over the Dual purpose method.

CHAPTER 05