Tower-still coking

Tower-still coking. For many years, coking was carried out as a batch process in which reduced crude (or other heavy oil) was heated by direct fire in horizontal vessels. Because they were equipped with condenser towers, these coke stills were known as tower stills. The charge was heated until all the volatile products had been driven overhead. The layer of red-hot coke remaining in the still was allowed to cool and was then removed manually by laborers who entered the still. More-economical coking processes were later developed that operate on a continuous basis. 

Shell Stills. At the beginning of the period under review, large quantities of crude oil were processed in tower stills. These were cylindrical vessels 10 to 15 feet in diameter and about 40 feet long. The vessels were mounted horizontally and arranged so that a fire could be applied to the underside (10, 55). The stills were charged with crude oil and the appropriate fractions removed by distillation. The residue was then destructively distilled or coked. Heating was continued until the bottom of the still was at a dull red heat (55). 

The high temperatures applied to the bottom of the tower stills damaged them so that they had to be replaced frequently. This was relatively expensive. As a result, special coke stills were used. Similar to the tower stills but smaller, the bottoms of these stills were cheaper to replace. The crude oil was reduced in the tower still to about 20° to 25° API gravity and transferred to the atmospheric coke still. This still was then heated rapidly to effect the cracking and coking. The entire coking operation required about 40 hours (55). Twenty-five years ago, a major part of petroleum coke was made in such shell stills. 

One method of maximizing the LCO end point is to control the main fractionator bottoms temperature independent of the bottoms pumparound. Bottoms quench ( pool quench ) involves taking a slipstream from the slurry pumparound directly back to the bottom of the tower, thereby bypassing the wash section (see Figure 9-9). This controls the bottoms temperature independent of the pumparound system. Slurry is kept below coking temperature, usually about 690°F, while increasing the main column flash zone temperature. This will maximize the LCO endpoint and still protect the tower. 

Benzene releases in byproduct recovery operations Naphthalene residues generated in the final cooling tower Sulfur and sulfur compounds recovered from coke oven gas Wastewater from cleaning and cooling (contains zinc, ammonia still lime, decanter tank tar, or tar distillation residues)... 

In the 1904 edition there is, for example, a sample calculation of the heat balance on a Glover tower treated as an evaporator, which shows how inefficient it was then ( what a heat waster it is (23)), There is also a discussion on the efficiency of various packings, explaining in terms of surface areas why coke is 1.5 to 2 times more efiBcient than bricks (23, 26), But in general, Davis approach was still empirical the operations are described as procedures of practical utility, and are not based on fundamental physics. Neither the work of Osborne Reynolds nor dimensionless group theory had been assimilated yet into the profession. 

The demister pad is partially coked. Quite often, the demister spray header (see Fig. 13-1) is designed for too large a wash oil flow. At low flow, the wash oil does not distribute to the ends of the spray header, and the peripheral area of the demister dries up and cokes. The sections of the demister still open are exposed to velocities high enough to promote resid entrainment. A high AP across the demister shows that coke plugging is the problem. An examination c the demister before it is removed from the tower will indicate which areas are not being wetted with wash oil from the spray header. This information should then be used to revamp the spray header. 

The bottoms from the pipe still are reheated and further separated in a vacuum tower. Heavy gas oil and lubricating oil cuts are obtained from this tower. The bottoms from the vacuum tower are referred to as reduced crude or residuum and are used for asphalt and coking. Other common terms used for the distillation fractions include middle distillates for light gas oil and heavy distillates for heavy gas oil. 

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