Armfield Pilot Crystalliser for Margarine

General Overview

Margarine is a crystallised emulsion of an aqueous phase (containing milk, water and other ingredients) dispersed in a fat phase. The fat phase is usually made of vegetable oils but can also be dairy fat. As a legal requirement, the fat content must not be below 80% in margarine so any product containing less fat is called a soft spread. Soft spreads containing less than 20% fat can be manufactured using modern techniques. The crystallising is achieved in a Scraped Surface Heat Exchanger. This unit employs a

rotating shaft inside a barrel having a concentric jacket which contains circulating secondary refrigerant at reduced temperature. Scraper blades, fixed to the shaft, clean continuously the inside surface of the barrel as crystals are formed, preventing fouling and blocking of the heat exchanger. The semi-solid is made up of crystals whose shape must then be modified to give the required texture in a 'worker' unit. This unit also employs a rotating shaft within a barrel but the barrel and shaft have steel pins arranged along their length in order to impart a beating action to the crystals. Finally, the product is held for a period of time in a resting tube in order to dissipate some of the heat of crystallisation and make the product suitable for packing. The Armfield Scraped Surface Heat Exchanger unit provides each of these facilities including the refrigeration plant all incorporated into a mobile framework. A comprehensive

control console allows versatile operation and efficient process monitoring. All important process parameters are variable including scraper and pin worker speeds, process temperature, barrel pressure and throughput rate.

The unit has been designed to allow faithful reproduction of results obtained on the full sized plant but on a miniature scale in the laboratory. Special attention has been given to the instrumentation incorporated in the design in order

that the user can gain as much information as possible from the process. Also the refrigeration and secondary refrigerant circulation systems are incorporated in the mobile cabinet so that all that is required for operation is connection to a suitable single phase electricity supply.

Description

Where necessary, refer to the drawings in Equipment Diagrams section 8. This manual describes both the Armfield FT25BP and the FT25BBP. The FT25BP has one

scraped surface barrel and a pinworker, the FT25BBP has two scraped surface barrels and a pinworker. Otherwise the units are identical.

9.1 Overview

The Armfield continuous soft spreads crystalliser is mounted on a mobile stainless steel cabinet (1), which also houses the refrigeration system (2) and control panel (3). The feed

pump (6) is a progressive cavity pump with the speed of rotation (hence the product throughput rate) controlled from the control panel. Feed enters the first barrel (7) at the driven end and is subjected to the mixing action of the rotating shaft (sometimes called a `mutator´ or `dasher´). As the mix progresses along the barrel it comes into contact with the cold surface and crystallises. Crystals are continuously removed from the heat transfer surface, which is hard chrome plated, by four sharpened

stainless steel scraper blades coated with titanium nitride. The rotating shaft is driven by a geared motor unit (8). When the mix has reached the exit of the barrel it is in a semi-solid

state. The product is then passed to a second identical scraped surface heat exchanger barrel, which is driven at the same rotational speed as the first. The product discharges from the second barrel through a stainless steel back-pressure valve (9). This valve is used to create a pressure in the barrel, influencing the physical

characteristics of the end product. Barrel operating pressures up to 8.0 bar (116psi) can be used, pressures in excess of this being relieved through spring loaded safety valve (10). After the back pressure valve, the product enters the pin worker (11) at the front end and exits to the resting tube. Sampling valves (12) are incorporated after both barrels and the pin worker. One of the unique features of the equipment is the ease with which the scraper shaft can be

removed from the barrel. After checking there is no residual pressure in the barrel, the end cover is removed by unscrewing the three knurled screws and the shaft carefully extracted. The floating blades are sharp and care is required to prevent scratching of the chromed

surface of the barrel. Freezing temperatures in the two scraped surface barrels are created by a calcium chloride

secondary refrigerant solution which circulates in the annular jacket, counter-currently to the feed, entering at the front (underside) and leaving at the rear (top). Secondary refrigerant circulation and refrigeration systems are contained inside the cabinet. This system employs a reservoir containing the secondary refrigerant circulated by a pump through a plate heat exchanger where it is contacted indirectly with refrigerant R404A vapour. The refrigerant is cooled by a semi-hermetic compressor with a water cooled condenser. Operating secondary refrigerant temperatures as low as -25oC can be achieved. Five semiconductor temperature sensors are located at various relevant positions in the process. (T1) is the feed temperature to the first barrel, (T2) the interim temperature between the barrels (FT25BBP only) and (T3) is the product temperature after the final cooling barrel. (T4) is located at the secondary refrigerant inlet to the barrel jacket and (T5) is the secondary refrigerant outlet temperature from the jacket. (T3) is used as the sensor for the temperature controller, and so this temperature is displayed on the temperature controller itself. A hygienic pressure sensor (11) is located between the pump outlet and the barrel inlet.