Distillation pumparound

The eadiest use of heat-exchange network synthesis was in the analysis of cmde distillation (qv) units (1). The cmde stream entering a distillation unit is a convenient single stream to heat while the various side draws from the column are candidate streams to be cooled in a network. So-called pumparounds present additional opportunities for heating the cmde. The successful synthesis of cmde distillation units was accompHshed long before the development of modem network-synthesis techniques. However, the techniques now available ensure rapid and accurate development of good cmde unit heat-exchange networks. 

It would be possible to remove all of the heat by pumping cold reflux from the distillate drum to the top of the tower and thus eliminate the cost of the pumparound circuit. Where more than one sidestream is withdrawn, however, it is usually economical to withdraw part of the heat in a pumparoimd reflux system farther down the tower. The following economic factors affect the choice ... 

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

There are two ways to remove heat from a distillation tower top reflux and circulating reflux. In this chapter, we call a circulating-reflux stream a pumparound. 

Increasing pumparound heat duty will unload the overhead condenser. This will cool off the reflux drum. A colder reflux drum will absorb more gas into the distillate product. Less gas will be vented from the reflux drum, and this is often desirable. 

Figure 3.13. Crude oil vacuum tower. Pumparound reflux is provided at three lower positions as well as at the top, with the object of optimizing the diameter of the tower. Cooling of the side streams is part of the heat recovery system of the entire crude oil distillation plant. The cooling water and the steam for stripping and to the vacuum ejector are on hand control.

A typical flow diagram of a two-stage crude oil distillation system is shown in Fig. 18.14. The crude oil is preheated with hot products from the system and desalted before entering the fired heater. The typical feed to the crude-fired heater has an inlet temperature of 550°F, whereas the outlet temperature may reach 657-725°F. Heater effluent enters the crude distillation (CD) column, where light naphtha is drawn off the overhead tower. Heavy naphtha, kerosene, diesel, and cracking streams are sidestream drawoffs from the distillation column. External reflux for the tower is provided by several pumparound streams.12... 

McNulty and Chatterjee (1992) discuss the use of nonequilibrium models to design packed bed pumparound zones of crude distillation towers. 

Complex distillation may be defined as a multistage vapor-liquid separation process that includes one or more of the following features multiple feeds, side draws, pumparounds, and side heaters or coolers. 

The separation brought about in a given column section depends on the number of stages and the UV ratio in that section. The UV ratio is a function of the feeds and product rates, heater and cooler duties, and pumparounds associated with the section. The fractionation may be of a distillative or an absorptive or stripping nature (Chapters 7 and 8), although one effect or the other may be predominant in different situations. The various features of complex distillation are described in this chapter. 

Distillation columns and other multistage columns generally require heat addition and/or removal, which normally take place at the reboiler and/or condenser. For a variety of reasons, it may be desirable in certain types of columns to add or remove heat at intermediate trays aside from the condenser and reboiler. Examples of such columns include demethanizers that utilize a side reboiler alongside the bottom reboiler, multi-product columns where intermediate condensers are associated with some of the side products, and absorber intercoolers used for partial removal of the heat of absorption. The method most commonly employed for exchanging heat between a column tray and a heat source or sink is the pumparound, where a fluid is drawn from the tray, sent to a heat exchanger, then pumped back to the column. The following sections pertain to the various applications of side heaters and coolers and the different types of pumparounds. 

It is proposed to utilize a hot process stream as a heat source to supplement the reboiler duty of an existing distillation column. The heat from the process stream would be added to the column via a pumparound, a few trays above the reboiler. How many additional degrees of freedom would result from the proposed modification ... 

Earlier chapters use simplified and binary models to analyze in a very informative manner some fundamentals such as the effect of reflux ratio and feed tray location, and to delineate the differences between absorption/stripping and distillation. Following chapters concentrate on specific areas such as complex distillation, with detailed analyses of various features such as pumparounds and side-strippers, and when they should be used. Also discussed are azeotropic, extractive, and three-phase distillation operations, multi-component liquid-liquid and supercritical extraction, and reactive multistage separation. The applications are clearly explained with many practical examples. 

Liquid circulation lines These are required when liquid circulation is performed at startup (Sec. 11.10). Often, a jumpover from the column bottom to the reflux line is needed, but other lines may also be required. In a refinery crude fractionator, it has been recommended (237) to install the jumpover line to the return line of the uppermost pumparound and to size it for 20 percent of the net distillate product rate. 

Direct-contact condensers (Fig. 15.14f). These are used for minimizing pressure drop in vacuum condensation. To accomplish this, the direct-contact zone contains low-pressure-drop internals such as packings, or is a spray chamber. Another common application is intermediate heat removal ("pumparounds ) in refinery fractionators. Here the main purpose is to maximize heat recovery at the highest possible temperature levels. A third common application is intermediate heat removal from absorbers or reactive distillation columns in which an exothermic reaction takes places. In all these applications, condensation... 

Partial drawoffs (commonly utilizing downcomer trapouts) are used when a side draw does not share a pumparound drawoff. Figure 19.116 shows the preferred controls (234). Operator action is required to ensiure that the correct distillate quantity is drawn and to prevent drying of trays below the drawoff. The dryout problem can often be mitigated by drawing the side product from the bottom seal pan (i.e., just above a chimney tray). In both arrangements (Fig. 19.11a and b), note the seal loop in the line from the main fractionator to the stripper. This loop prevents vapor backflow at low liquid rates (Sec. 5.1). 

Understand Process Characteristics H2, H2S, and NH3 and light ends are removed from reaction effluents through a series of separation and flashes, resulting in the reaction products in a liquid form, which goes to the stripper, the feed heater, and then to the main product fractionator. The task of the product fractionation is to separate different products based on their product specihcations such as distillation endpoint, ASTM D-86 T90% or T95% point, and so on. Side draws from the column go to the product strippers where kerosene and diesel products are made. The net draw from the column bottom is called unconverted oil (UCO), which is recycled back to the reaction section for nearly complete conversion. There are two pump-arounds, namely, kerosene and diesel pumparounds, as a main feature of heat recovery from the main fractionation column. 

Figure 2.9. A column with pumparound (nonadiabatic distillation) and hve steam.

Among multisection distillation complexes, only columns with side strippings bring practically the whole heat into the feeding and bring hve steam into the bottom. Application of pumparounds decreases energy expenditures and recuperates withdrawn heat for heating of petroleum before separation. 

Therefore, column with side strippings, with live steam into the bottom, and with pumparounds is the best distillation complex for petroleum refining. An optimum way of designing such column is discussed in Section 7.5.2. 

The only operating problem introduced by this control scheme is a rather drastic reduction in pumparound duty. If the pumparound heat is used to generate steam, this is not much of a debit. However, when adjacent distillation towers are reboiled with the hot pumparound oil, the reduced duty creates a problem. 

Table 8-4 summarizes the product distillations for the FCCU fractionator. These distillations are consistent with the vapor and liquid flows presented in Table 8-1. The degree of separation between the tower overhead and LCO products is represented by the ASTM 5% to 95% gap of —2°F. Based on the Packie method, calculations indicate that there are 10 effective trays above the LCO draw tray. This is equivalent to roughly six theoretical separation stages. Since each pumparound section is usually represented as a single stage, we can see by referring to Figure 8-2 that the naphtha wash section is equivalent to four theoretical separation stages.

Example 12-4 Distillation columns can be cooled by using a so-called pumparound stream in the columns enriching section (see Figure 12-27). Liquid and vapor rates above the pumparound stage are larger than those below because some of the vapor in the pumparound stream is condensed. 

Pumparounds are used to remove heat from the tower and to adjust the vapor-liquid flow in the tower. They condense vapors rising in the tower and create an internal reflux for the fractionation stages below the pumparound. They also reduce vapor loads in sections of the tower above the pumparound. A pumparound takes liquid from the tower, cools it, and returns it higher up in the tower. The liquid condenses the vapors in the pumparound section creating liquid reflux for fractionation lower in the tower. Vacuum pipestills do not use overhead reflux seen in other distillation towers, a top pumparound is used instead. 

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