Fig. 60. CIP-able bag filter principle
Fig. 60. CIP-able bag filter principle

You are reading the e-book Milk Powder Technology by Vagn Vestergaard.

Powder recovery from spray dryer exhaust systems for food and dairy products has in the past typically been done in cyclones, either alone, where environmental issues have not been in focus, or, where stricter requirements are enforced by the authorities, cyclones combined with wet scrubbers or textile bag filters.

While requirements to environmental protection, energy consumption, noise level, production efficiency and product quality are increasing everyday for the food and dairy industries, powder manufacturers have witnessed a development especially for powder recovery systems. The SANICIP™ CIP-able bag filter is replacing the cyclones and has now reached a point where it is setting the standard for almost all dryers. See Fig. 60.

The SANICIP™ bag filter is of the reverse jet type and made of stainless steel. It consists of a cylindrical bag housing with scrolled air inlet, clean air plenum on top, and a conical bottom with fluidized powder discharge. During operation the product collected on the outside of the filter material is removed by a compressed air jet stream blown into each bag via a special reverse jet air nozzle (patented) positioned above each bag, see Fig. 60a. A jet is formed which draws air from the clean air plenum into the bag as well, thereby saving compressed air. This is an efficient and sanitary solution. The reverse air jet nozzle has furthermore a dual-purpose during CIP as described below.

The bags are clean-blown individually or 4 together, resulting in a very even discharge of powder and using higher air-to-cloth ratios. The frequency and duration of the cleaning sequence can be adjusted to suit actual running conditions.

The bag filter housing is insulated on the cylindrical part below the hole plate and is in a fully welded execution. The supply air system for the fluidizing bottom has a multiple purpose: During production the cone of the bag house is first heated by the warm air, which subsequently is used for fluidizing the powder in the bottom. This ensures an even powder flow out of the bag house. During standstill the air is used for heating of the cone alone. Condensation and risk of mould growth is therefore avoided.

Cleaning-in-place plant
Cleaning-in-place plant
The filter bags are made from a special 3-layer gradient polypropylene material. This material is fully CIP-able with NaOH and HNO3 in 2% solutions at 75ºC and 60ºC, respectively. It is further approved by FDA. It is heat-treated to give a special dust-releasing surface. Each bag is supported on a stainless steel cage and is easily dismantable. In normal bag filters and old design CIP-able bag filters the clean-blowing of the bags is performed by using a Coanda nozzle placed on the top of each bag, a solution that works, however, the area around the nozzle is difficult to clean, and it is designed with a hollow space.

The CIP of the bag filter is divided into the following main items:

  1. The internal bag CIP cleans the bag from the inside towards the dirty side  (out-side). Clean water is injected into the inside of the bag through the reversed jet nozzle and the water is atomized by compressed air. Powder that has penetrated into the bag material is forced out towards the dirty side by the water spray. No recirculation of water in this step.  This feature is extremely important, as it is otherwise difficult, if not impossible, to clean out this entrained powder from the outside only.
  2. The clean air plenum CIP cleans the clean air plenum of the bag filter above the hole plate. No recirculation of water in this step.
  3. The hole plate CIP cleans the bottom side of the hole plate and the snap ring area of the bag using a specially designed nozzle. This nozzle is positioned on the bottom part of the hole plate between the bags, and it also cleans the outside of the filter bags. The nozzle has a dual purpose as well, as it during the drying process is purged with compressed air to keep the hole plate free of deposits. Discolouring/denaturation is thereby avoided. The CIP water is recirculated.
  4. The shell CIP is performed by means of standard retractable CIP nozzles. The CIP water is recirculated.

Advantages of the SANICIP™  Filter:

Low pressure loss across the bag filter and thus the entire exhaust system, i.e. reduced energy consumption and noise emission.

  • Designed for optimum air-to-cloth ratio and powder load (due to one bag being blown at the time)
  • Better utilization of raw materials due to no second grade products
  • The scrolled air inlet offers low mechanical impact on the bags. Shaking is avoided and so is thus wear and tear of the bags.
  • Design with 4 or 6m bags to suit specific building requirements

Reduced space requirements for new installations

  • Easy replacement of cyclones for retrofits without major building changes
  • USDA 3A acceptance
  • Short dry-out time, compared with other CIP-able bag filters

Which system to select as an additional powder separator after the cyclones, depends on how the recovered product - liquid or dry - can be disposed of. In either case the collected product cannot be regarded as first-class. Therefore, the tendency today is to avoid the cyclones and install a CIP-able bag filter.

Comparison of Powder Separators

Cyclone

Cyclone + Bag
Filter

Cyclone +
Wet Scrubber

SANICIP™

Emission

20-400 mg/Nm3

5-20 mg/Nm3

max. 20 mg/Nm3

5-20 mg/Nm3

Pressure loss
Exhaust system
(incl. ducts etc.)

280 mm WG 

340 mm WG

340 mm WG

170 mm WG

Auxiliaries

none

compressed air

liquid circulating
system

compressed air

Cleaning

suitable for CIP

difficult

suitable for CIP

suitable for CIP

Hygroscopic
products 

insensitive

sensitive

insensitive

insensitive *)

Use of separated
product

first grade

first and second
grade

not recom-
mended 

first grade

Maintenance

minimal

service of com-
pressed air sys-
tem and change
of bags

minimal

service of com-
pressed air sys-
tem and change
of bags

Sanitary conditions

good

relatively good

less good

good

*) watch out for permeate, if the humidity in the outgoing air is too high

Read the previous chapter:
Wet scrubber

Read the next chapter:
Pneumatic conveying/cooling