Condensed mode operation

Condensed mode operation. To avoid accumulation of liquid in the reactor the dew point of the gas must be kept above reactor temperature. When operating in condensed mode, higher-boiling components in the recirculating gas are allowed to condense in the heat exchanger, and this liquid is fed back to the reactor where it evaporates. 

Features - particles of growing polymer form in fluidized bed - catalyst residence time 2-4 hours - morphology and psd of catalyst are important - previously (pre-1990s) restricted in range of comonomer that could be used because of emergence of "condensed" mode operation, a wide range of comonomers may now be used... 

In the 1990s, an improvement for the gas phase process was developed called "condensed mode" operation of Unipol reactors (12). This technique greatly expanded capacity of gas phase reactors and product capability by making it more practical to use higher alpha-olefin comonomers such as octene-1. 

The Unipol process, initially developed for polyethylene production, was later extended to polypropylene manufacture. The process consists of a large fluidized-bed gas-phase reactor for homopolymer and random copolymer production, and a second smaller reactor for impact copolymer production. The second reactor is smaller than the first one because only 20% of the production comes from the second reactor. This reactor typically has a lower pressure rating as copolymerization is usually carried out at lower temperatures and pressures. Condensed mode operation is used in the homopolymer reactor but an inert diluent is not required because propylene is partially fed as a liquid. The copolymerization reactor is operated purely in the gas phase. The Unipol process has a unique and complex product discharge system that allows for very efficient recovery of unreacted monomer, but this does add complexity and capital cost to the process. 

Cmde helium (containing 50—70% helium, associated hydrogen and neon, 1—3% methane, and the balance nitrogen) can easily be obtained by minor enhancements to the nitrogen rejection unit, particularly with natural gases containing 0.5% or more helium. For example, by operating the double-column condenser in a partial condensation mode, a stream of uncondensed vapor at about 50% helium concentration can be obtained. This cmde helium stream can be fed directly to helium purification and Hquefaction units. 

Boilers and air heaters can be designed to operate in the condensing mode, that is, the outlet temperature of the products entering the flue can be at or close to the dewpoint. The usual design criteria for flues which attempt to avoid condensation are no longer applicable and it is necessary to incorporate additional features. These will include ... 

The temperature difference between inlet and outlet temperature at the coil(s) of the refrigerant should be smaller than 1 °C (AT < 1 °C), to ensure a uniform condensation on the total coil. On warmer areas no ice will condense until the temperature at the ice surface has increased to the warmer temperature on the coil. For large surfaces it is necessary to use several coils or plates in parallel, each of which must be separately temperature controlled. If the condenser is operated in an overflow mode, the weight of the liquid column should not change the boiling temperature of the liquid at the bottom of the column measurably. 

The plant has been commissioned and the first results on the performance are expected soon. Design data give an efficiency to power of 23 and 27 % respectively for extraction and condensation mode of operation while the overall plant efficiency is 73 and 39 % respectively. 

Note 1 For the pyrolysis case the power section is run in two modes Combined Heat and Power mode during the operation of the sugar mill to provide steam needed to supplement the mill s steam demand, and Condensing mode during the period when the mill is not in operation. ... 

Once the desired conditions have been reached and the planned process rate established, the system can be operated in a steady-state mode. This may take four or five hours from a cold start or two to three in a run where hot char from the previous days operation is in the convertor. During the steady mode operation, feed is added about every 30 minutes, the char grate opened for a few seconds 30 to 9-0 times each hour and the char drums changed every hour. Two char barrels equipped with quick disconnect hardware are used for rapid replacement of the drums. To prevent burning, the char is stored in sealed barrels while it cools. The oil is periodically gathered from the condenser and demister. Typically, the condenser recovers about three quarters of the total oil while the demister recovers the remainder. 

Film condensation is the prime condensation mode in distillation condensers. Dropwise condensation can help improve heat transfer, but it is seldom relied on. Fog formation constitutes an operation problem rather than a desirable condensation mode and is further discussed in Sec. 15.13. The following discussion focuses on film condensation. 

ISO 11907-2. 1995 [121] is a static method of lest used to determine the corrosivity of fire gases. The specimens (600 mg) in the form of granules or chips arc heated with an electric resistance wire (800 C) in a crucible. The effluent is contained within a closed cabinet of 20 liters volume maintained at 50 C and 65% rh. The corrosion detector consists of a resistance etched copper plated laminate, and the corrosivity is assessed as the variation in electrical resistance due to attack on the copper circuit. Two operating procedures are given, one for the condensing mode, in which the corrosion sensor is water cooled to 40 C. and one for the non-condensing mode, without water cooling to the sensor see Fig. 22. 

There could also be a more directly physical (as opposed to chemical) origin. Frohlich " showed in 1968 that an assembly of randomly oscillating similar dipoles could be driven to operate in cooperatively condensed mode if the input power exceeded a certain minimum value. This and a series of stimulating subsequent papers opened a new avenue of cellular studies. " ... 

In addition, modern reactors now operate in condensed mode, which greatly increases their throughput. The Unipol process for LLDPE is now licensed by Univation. 

Heat may be removed fi om the primary coolant system by means of a heat exchanger positioned in the primary vessel and which operates by natural convection to an external heat exchanger in the atmosphere or in a large tank of water within the primary containment. In some cases the system requires valve operation to bring it into service while in others it operates continuously. A valved system may operate in a water solid mode or in a boiling and condensing mode and will need some form of reservoir to prime it. Wffiere the system operates continuously, so that there is no problem of start up in an accident situation, there is a steady loss of around 6% of... 

In recent years the fluidized bed processes have been improved through the addition of hquid hydrocarbons into the polsrmer bed (115,116). These evaporate, adsorbing heat from the bed more effectively than gas alone, and are then condensed in the recycle stream and reused. The result of this condensed mode or super condensed mode approach is increased production rates from the same reactor, or greater tolerance for higher activity catalysts. Of course, it reverses some of the benefits of pure gas phase, since it is a step back toward hydrocarbon diluent. Nevertheless, this innovation has greatly improved operations and production of the gas-phase process, instilling new life into the technology. 

Gas phase process technology is widely available and is offered by several technology providers, namely Univation, BP, Basell, etc. The set up of gas phase processes is, in principle, generic and proprietary information on condensing mode, dual reactor operation, catalyst systems, etc. is protected through patents. 

Hexene reactivity Catalyst reactivity with 1-hexene is important in order to reduce the amount of 1-hexene needed in the reactor. Ziegler-type catalysts based on Ti/Mg compounds exhibit relatively poor reactivity with 1-hexene compared with many other catalysts. Such catalysts can require a 1-hexene/ethylene vapor phase ratio of about 0.12 and higher in a commercial reactor, thus limiting the ethylene partial pressure in the reactor and leading to lower production rates and/or lower catalyst productivity. However, with the super-condensing mode of operation, comonomer fed concerns are rare. Most other catalysts used in a fluid-bed reactor exhibit higher 1 -hexene reactivity and, therefore, require lower 1 -hexene feed rates and fewer operational problems... 

The heat of polymerization can be removed by heat exchangers placed on an externa] recirculation loop. However, low boiling point hydrocarbons and a-olefin comonomers can be introduced into the reactor in the liquid phase to absorb the heat of polymerization by their latent heat of evaporation in an operation procedure called condensed mode. Since most polymerization reactors are limited by their heat removal capability, this technique permits a substantial increase in reactor productivity. 

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