Liquefaction tail gas

Lokhandwala et al. [6] have reported field and laboratory work on the recovery of chlorine from liquefaction tail gas. Results suggest the following ... 

Hydrochloric acid may be purchased or produced internally. It is a widely available commodity, easily obtained in good quality. HCl is available in the anhydrous form as well as in the form of aqueous acid (up to 23° Be or about 37% HCl). The use of aqueous acid is standard in the chlor-alkali industry, and we do not discuss anhydrous HCl here. Byproduct acids are available, sometimes at lower prices, and may be suitable for use in the chlor-alkali process. Their quality should be checked carefully, and testing may be advisable before use. When HCl is produced from chlorine liquefaction tail gas, the absorbing water is the most likely source of impurities. Demineralized water is the standard source when producing acid for use in a membrane-cell chlorine plant. A certain amount of chlorine tends to be present in burner acid. This can be minimized by process control, and a small bed packed with activated carbon (Section 7.5.9.3B) is a useful safeguard. Usually only the acid intended for use in the ion-exchange system need be treated in this way. 

The chlorine remaining in liquefaction tail gas is an intolerable emission. If recovered, it can be a useful economic asset, especially in the larger plants, and many processes have been considered or used for its recovery [77]. TTiere are other sources of dilute chlorine... 

This is a natural application for liquefaction tail gas and other lean or intermittent chlorine vents. A bleach plant is a convenient receptacle for these streams, and so the topic is considered in this chapter. In other cases, bleach is the main product of an operation and is produced from commercial chlorine and caustic soda. A full discussion of these operations is in Section 15.3.2. 

Besides the liquefaction tail gas discussed in Section 9.1.9, there are several other sources of chlorine-containing vapor, most of them intermittent, which must be handled. Silver [77] gave a summary of these streams and methods for their treatment. Some streams are continuous, such as those from air-based dechlorination. Intermittent streams include... 

The most frequent synthetic use of electrolytic hydrogen is the production of HCl, which is covered in Section 9.1.9.2. This is a natural outlet for a chlor-alkali producer, who can use HCl to acidify cell feed brine or depleted brine and to regenerate ion-exchange columns. It also allows profitable use of the chlorine contained in the liquefaction tail gas. The HCl that can be consumed internally, and so the amount of... 

The decomposition voltage for the process is low, as with the Kyoto cell (Section 15.2.2.3). A possible application for this approach is the recovery of chlorine from liquefaction tail gas and other dilute streams. 

Liquefaction of chlorine is always incomplete, because the non-condensable impurities carry chlorine at its vapour pressure as they leave the liquefaction process. This exit gas, or tail gas, is handled in any of several different ways. It is of course an intolerable plant emission, and the contained chlorine must at least be destroyed before the gas is released to the atmosphere. There is also a powerful economic incentive for recovering much of the chlorine in some usable form - Silver s estimate of the value of the chlorine in the tail gas produced in the United States alone in 1981 was 50 million [3]. 

Lokhandwala et al. [6] have already provided costs for the membrane-based upgrading of tail gas for recycle to a liquefaction process. They showed that recovery of 228 kg h-1 (1915 tonnes per annum) of chlorine from a 20% (v/v) tail gas should provide a pay-out time of about 14 months. Other comparisons made by Membrane Technology and Research, Inc. [11] for an 800 tonnes per day plant shown in Table... 

Furthermore, liquefaction efficiency will always be less than 100%. Some of the chlorine produced must remain with the non-condensable tail gas. The relevant factors were addressed in a paper presented at the 1997 SCI London International Chlorine Symposium [3]. In the processing of the tail gas, up to about 4% of the chlorine produced in the electrolysers is diverted to lower value products such as bleach or hydrochloric acid. Small quantities of secondary products such as these materials can also present a marketing problem. A further loss of chlorine product can occur in the storage system, particularly in systems where padding air is employed. 

If a liquefaction efficiency of 97% is assumed, the hypothetical 600 tpd plant will lose 18 tpd to tail gas. It is also assumed that the tail gas is diverted to production of sodium hypochlorite bleach with a sales value of US 50 per ton of chlorine equivalent, while the chlorine feeding the EDC unit has a value of US 180 per ton. The lost chlorine will cost the producer more than US 800 000 per year. Even this figure ignores the facts that an equivalent quantity of caustic soda is consumed and that there will be a marketing cost associated with the bleach product. 

In the first step the chlorine from the tail gas and chlorine feed reacts with the caustic in the jet-loop reactor. The advantage of the jet-loop reactor is that it also acts as a suction device for the gas stream. The residence time of the liquid in step one is dependent on the capacity of the hypochlorite production and liquid level in the tank and varies between 1 and 4 h. A heat exchanger in the loop controls the temperatures in steps one and two. The amount of caustic in the feed-tank of step two is the back-up for failure of chlorine liquefaction. 

Coal liquefaction Fischer-Tropsch synthesis Synthesis of methanol Hydrogenation of oils Alkylation of methanol and benzene Polymerization of olefins Hydrogenation of coal oils, heavy oil fractions, and unsaturated fatty acids Adsorption of S02 in an aqueous slurry of magnesium oxide and calcium carbonate S02 or removal from tail gas Wet oxidation of waste sludge Catalytic desulfurization of petroleum fractions Wastewater treatment... 

Raw materials are diluted hydroxide solution containing some 18—20 per cent of NaOH, and electrochemically produced chlorine (it is also possible to use tail gas, escaping from condensers in the liquefaction of chlorine). Chlorination proceeds in two immediately subsequent stages. At first, chloride and hypo-chlorous acid are formed ... 

The dry gas is usually compressed before use. The level of compression depends on the application. A large fraction of the world s output of chlorine is consumed on site. The production of ethylene dichloride (EDC) is the single largest-volume use. The dry gas supply pressure then is determined by the needs of the EDC process. Merchant chlorine will be liquefied. The choice of compressor output pressure is then an economic/technical balance with the requirements of the liquefaction process. Because some of the impurities in the chlorine gas are noncondensables, a chlorine-containing tail gas always results. Some of this chlorine value can be recovered directly or in the form of various byproducts. A system is also required for safe disposal of any unrecovered chlorine as well as any released from the process during emergencies. 

The presence of noncondensable gases in the chlorine means that liquefaction is always incomplete. These gases leave the process and carry a certain amount of chlorine with them. This tail gas must be treated to remove the chlorine before it can be disposed of by venting it to the atmosphere. The chlorine value can be converted to a salable product such as bleach, HCl, or FeCls. It also can be recovered as elemental chlorine by absorption in a solvent followed by stripping. In some cases, it is simply absorbed in an alkaline medium and then treated for disposal. Section 9.1.9 covers the subject of tail gas handling. 

The other options include enhanced recovery of elemental chlorine and its conversion to another product. We might include, under enhanced recovery, the addition of higher-severity stages to a liquefaction plant. While this is more correctly a reduction in the amount of chlorine present in the tail gas rather than its recovery, it is a viable retrofit technique and one that has been used to replace some of the older recovery plants. A unit quantity of refrigeration becomes more expensive, hydrate accumulation may be more troublesome, and some of the problems of deep liquefaction are exacerbated. TTiese are the normal effects of extending the process, and they add nothing new to the technical discussion on liquefaction. 

Unless liquefaction conditions are mild in order to produce a rich tail gas and a correspondingly greater quantity of bleach, the intermittent streams will probably contribute most of the design maximum chlorine load. The operating temperature therefore will vary and in the absence of intermittent flows will be lower than the specified maximum. 

In principle, a solvent can be used on the whole chlorine gas stream as well as on the tail gas. This can replace a conventional liquefaction system. Its advantage is that it makes possible more complete recovery than can be achieved in a liquefaction system at the same pressure and a reasonably economic temperature. It is not a process for the recovery of the chlorine value of tail gas and in that sense does not belong in this section. However, the motivation for use of this process would largely be the practical elimination of a tail-gas problem. 

Production of HCl uses chlorine that otherwise could be recovered (at a cost) and so detracts from its net production. Where there is a reliable outlet for caustic, however, the best approach may be to increase electrolytic capacity, use HCl liberally in its in-plant applications, and reduce somewhat the severity of liquefaction. This improves the quality of the cell gas and allows more chlorine to appear in the tail gas, which is the raw material for HCl production. Both these changes reduce energy consumption in the liquefaction process. The gross production of elemental chlorine is preserved, all the benefits of acidification are obtained, and more caustic is available for use or sale. 

Clean dry air added to the liquefiers along with the chlorine dilutes the hydrogen and eliminates that hazard. The rate of addition of air is governed by a flow-ratio controller, with the chlorine flow to the liquefier acting as the primary flow (Fig. 11.28). The ratio to be used depends on the concentration of hydrogen in the chlorine and on the depth of liquefaction. It should be reset as required, based on the concentration of hydrogen in the tail gas from liquefaction. The control valve should fail open and have an equal percentage characteristic and a positioner. A small bypass flow of air may... 

Air systems include compressed air as a plant utility and refined versions for more specialized uses. These include instrument air and breathing air. Of particular significance in a chlor-alkali plant is a supply of dry air for use in the chlorine processing section. An alternative to dry air is nitrogen, which also serves as an inert gas in the hydrogen plant and sometimes in chlorine liquefaction and tail-gas handling. For convenience, we include the discussion of nitrogen in the section on air systems. 

Dry air is used when air is to be injected into the process anywhere on the high-pressure side or after the drying columns on the low-pressure side. When dilution of the tail gas is necessary in order to prevent accumulation of dangerous concentrations of hydrogen, the major consumer will be the liquefaction system. Other points of use include compressor seals, purging and maintenance connections, suction chiller bottoms pot connections, and anhydrous caustic processing equipment. 

Section 9.1.9.3 mentioned the production of NaOCl from liquefaction plant tail gas. This is an opportunistic approach to converting a hazardous waste into a useful product. The main objective usually remains the safe disposal of the uncondensed... 

Advantages of the foam include a choice of metals to 1000°C, low weight, compaction and the abihty to be formed in complex shapes. Some potential applications include gas turbine tail gas heat exchangers, hydrogen liquefaction plant and partial oxidation of hydrocarbons to hydrogen. The foam can be catalysed, of relevance to HEX-reactors. Within the area of heat exchangers, the work at ETH was based on aluminium foam of the type shown in Figure 4.11. 

Chlorine can be recovered from the tail gas from liquefaction with a chlorine recovery system. 

Tail gas from liquefaction and chlorine from the plant evacuation system together with the snift compressor and stripper recycle streams are supplied to a snift compressor suction knock-out drum. The gas is compressed by the snift gas compressor to 7.0 kg/cm with a discharge temperature of 85 °C. 

There are many options in the liquefaction of chlorine, depending upon the storage requirements for the liquid and the system used to handle the tail gas. Low-pressure storage is recommended because of the reduced hazard in the event of a spill. However, this requires lower storage temperatures. Low temperatures also reduce the amount of chlorine in the tail gas from liquefaction. Yet lower temperature may not be necessary if a plant also deliberately produces bleach or HCl from the tail gas. 

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