Double-Column Process

This process requires a large number of trays (150-200), resulting in units of about 100 m. The reflux can be condensed with cooling water (column pressure 16-19 bar) or in air coolers (column pressure 21-26 bar). 

For the large throughputs that are common today, the double-column process is preferred over the single-column process, since it does require smaller columns with smaller column diameters, which makes the transportation 

Schematic of double-column process. (Adapted from Ullmann s Encyclopedia of Industrial Chemistry, Vol. A22, VCH Publishers, Inc., New York, 1993.) 

---

Cryogenic distillation has been used extensively ia the processiag of natural gas for nitrogen removal and for helium recovery (22—23). Two basic processes are now used for nitrogen rejection from natural gas— the single-column heat-pumped process and the double-column process. Eadier processes utilized multistage flash columns for helium recovery from natural gas (24). 

Figure 5.4 shows a schematic of the Ciyoplus process [76] for the removal of nonhydrocarbon contaminants, and Figure 5.5 shows a flow diagram of the Costain nitrogen removal process [77], The Costain double-column process is sufficiently flexible to handle natural gas with nitrogen concentrations of 5—80 mol%. Feed natural gas above 27 bar can be directly processed without any compression. 

A schematic of the double column process is illustrated in Figure 9. 

Figure 10 Double column process for high purity O2 and Nj...

The double-column process shown in Fig. 2.3A produces oxygen and nitrogen. There are industrial applications requiring exclusively nitrogen. For these, numerous modifications of the double-column system have been developed, two of which are introduced here ... 

Linde process A high-pressure process for the production of liquid oxygen and nitrogen by compression to about 200 bar (20MN/m ) followed by refrigeration and fractionation in a double column. 

Continuous benzene alkylation was conducted in a reactive distillation column of the type illustrated in Figure 1. The process unit comprises the following principal elements a double column of solid catalyst 32, packing columns above and below the catalyst bed, a liquid reboiler 42 fitted with a liquid bottoms product takeoff 44, a condenser 21 fitted with a water collection and takeoff, and a feed inlet... 

This problem was solved by the introduction of the Linde double-column system. Two rectification columns are placed one on top of the other (hence the name double-column system). In this system, liquid air is introduced at an intermediate point in the lower column. A condenser-evaporator at the top of the lower column provides the reflux needed for the rectification process to obtain essentially pure nitrogen at this point. In order for the column to also deliver pure oxygen, the oxygen-rich liquid (—45% oxygen), from the boiler in the lower column is introduced at an intermediate level in the upper column. The reflux and the rectification process in the upper column produce pure oxygen at the bottom and... 

The nitrogen purity from the Linde double column system is limited to about 5 ppm oxygen. In order to produce a higher purity nitrogen product, additional trays in the low pressure distillation column and some additional complexity is required in the process. 

Mg. 11.1-7 Scheme of a process for air fractionation by distillation. (A) Basic process with two columns. (B) Improved process with Linde double column... 

Figure 6.2.5 Unde process for air separation by distillation with a double column. Adapted from Stichimair and Fair (1998) and Baerns eta/. (2006) E heat exchanger, C column.

One of the most important milestones in the history of industrial air separation was the introduction of the soolled double column for the distiUative separation of air into its components under cryogenic conditions. Even today this principle is stiU applied in numerous variations in most of the cryogenic air separators. A frequently applied process is going to be introduced in the foUowrng by example. It is built this way or similarly by aU commercial vendors of air separation units. Elementary concepts of process technique will be applied to explain the key features of the air separator, such as concentration profiles in the columns, argon production or refrigeration. A separate section describes so-called internal... 

With a double-column, the vapour and liquid flows within the individual column sections are not freely adjustable, unless additional enhancement recycles are applied. This is a speciflc characteristic of the double-column principle. As a consequence, the product yields are also more or less fixed. In particular, it is not possible to withdraw all the gas ascending to the top of the low-pressure column as pure nitrogen. However, a fraction of up to 50% of the processed air can be obtained as pure nitrogen (d) with an 02-content of 1 ppm, if residual gas with an 02-concentration ranging between 0.1-3% is withdrawn some trays below the top. Thereby the liquid/vapour-ratio in the top section, the so called nitrogen section , increases such, that the oxygen can be rectified downwards in this section. 

The two-column process (Fig. 2.9) is based on the classical double column supplemented by a further condenser/evaporator unit (6) at the top of the low-pressure column (5). The column pressures are chosen such, thatthere is a driving temperature difference to maintain the heat flow at the two condenser/evaporator units (6) and (4) ... 

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

---